MCT at cruise [Archive] - PPRuNe Forums

Intruder

26th Sep 2013, 15:49

The OP still refuses to give an airplane or engine type, or tell us whether this is a real or FlightSim question...

If the autothrottles are not keeping up with external influences, whether it be mountain wave, turbulence, wind shear, or something else, then the simple answer is to take over manually, regardless of what someone might "prefer". If the conditions are so extreme that the autothrottles command IDLE as he posited, then "loss of separation" is not likely an immediate factor. If you are pushing the red line AND climbing in a wave, spool-up time should also not be an immediate factor unless you got into a "coffin corner" situation in the first place, whether by bad planning or extreme bad luck.

I do agree with misd-agin that speedbrakes are preferable to excessive thrust reduction at high altitude. However, the question appeared to be regarding the ability to control the thrust. Anyone who "prefers" to use autothrottles to control thrust when a described extreme/limiting/edge-of-the-envelope situation exists is not someone I'd "prefer" to fly with.

ImbracableCrunk

26th Sep 2013, 17:00

A320. It's in the title.

latetonite

26th Sep 2013, 17:24

737ngpilot: you look like a pilot who's been there, done that and knows what the question was.

To you intruder:
I am talking about daily ops, not extremes. And yes, in an airplane, not FS.
Stupid being at that level? You have not flown long range overwater yet, or over parts of the world, where two way comms are difficult or not available. Or maybe you choose by default Fl 280 for a long stretch?

Anyway, feel free to choose another flight if it happens me to be the skipper.

Natstrackalpha

26th Sep 2013, 18:35

wave? overseer?

Altcrznav

27th Sep 2013, 01:35

Intruder

Join Date: May 2000
Location: USA
Posts: 2,299
Just push the thrust levers full forward; or "conversely", just pull them all the way back!

Are you a pilot, or is this another Flight Sim question?

I'm a pilot. How about you? Your answer doesn't make sense.

Altcrznav

27th Sep 2013, 01:40

The reason I ask is that at cruise, the donuts and the max EPR ticks are in line with each other. So does moving out of CLB really give you more thrust?

I say no.

Altcrznav

27th Sep 2013, 01:45

Hey intruder, how about you take it down a notch.

I didn't refuse to clarify anything. I'm not on this board daily it may take me a few days to response to posts.

Calm down.

A320 (check the title)

And I don't know why the engine type has anything to do with it but we have IAE V2500s.

Altcrznav

27th Sep 2013, 02:00

737ngpilot
737ngpilot

Join Date: Aug 2007
Location: USA
Posts: 50
Front range of the Rocky Mountains, every winter we have mountain waves, and company procedure is to use speedbrakes and MCT to mitigate, MCT and Climb trust is the same at altitude.... Let me add I have seen airspeed change 50Knots in seconds, MCT/ Climb to power out is totally acceptable.

That's the question here - with the donuts and max EPR ticks touching, do you really get more thrust?

bubbers44

27th Sep 2013, 02:09

We had a situation in an MD80 on a short flight and my captain got a little slow at 36,500ft so called it out and he used alt hold and MCT to regain speed but it was too late. Using MCT actually lowered our N1 and we started to get buffeting so descended at 500 fpm to not stall. We were in the clouds and had opposite direction traffic at fl350 5 miles in front.

That is the only time in my career I Felt out of control as far as keeping our clearance. It never happened when I was captain even when my FO said we could cross a climb fix when transitioning to the 757. The 727was a slug so never accepted a clearance unless I knew We could do it.

bubbers44

27th Sep 2013, 02:43

ALT, A 737 flt LAS to sfo over the Sierra Nevadas we went from mct to idle with speed brakes to keep flying at fl350. We didn't exceed any limits and had no training. I think most pilots could figure it out too but can an Airbus autopilot do it? Speed brakes in cruise is not in their software is my guess.

latetonite

27th Sep 2013, 06:00

To wrap it all up, the MCT is the A/T limit you select. Nothing else. Ever noticed that during approach, your limit goes to G/A? all a matter of understanding what you are doing. Or are some people again afraid from chief pilots, layers etc., cause it is not in their Vol 1, part 8?

MD83FO

27th Sep 2013, 06:12

i believe that as you climb there is a point at which MCT is not that much greater than CLB

latetonite

27th Sep 2013, 07:27

All depends on selected T/O trust, and ambient conditions, altitude. You do not worry about this. But max continuous, is MAX continuous, it will not be lower then your Climb trust, and can be substantial higher to save the day.

mikedreamer787

28th Sep 2013, 02:01

When I was new on the 320 I regularly used MCT when
instructed to expedite climb - then all hell broke loose in
the flt dept when they found that me and my batch (all
ex Boeing) were doing it. We never ever got a definitive
reason as to why, in pilotage terms, we can't damn well
use it to expedite climb. But its their sand castle so we
just shutup and did what we were told.

Only once did I use MCT in cruise when I was dumb
and silly enough to leave the cockpit for an expended
urgent dunny visit with a 200hr wonder at the helm. We
were in an area of CBs but the hdg I set beforehand
was smooth and told Spanky not to change hdg or alt
unless we lost pressurisation or an engine. We were at
REC MAX - 300ft and about 1000ft over OPT due to
enroute ATC constraints further down the track which
would've duckshoved us to FL270 for the remaining 4
hours.

Cutting a long story short - just before I got back the
space cadet had changed hdg and steered us straight
into a bloody cell where the TAT went up by a large
10* and max CRZ power wasn't enough to keep the
speed from dropping off - so I went to MCT. Young
eager-beaver said he saw a break to get closer to track
but failed to scan beyond 10nm and took us smack in
to a blind alley.

junebug172

28th Sep 2013, 02:58

i believe that as you climb there is a point at which MCT is not that much greater than CLB

I checked closely today while enroute. There is no difference between MCT and CLB.

latetonite

28th Sep 2013, 05:10

[QUOTE]We had a situation in an MD80 on a short flight and my captain got a little slow at 36,500ft so called it out and he used alt hold and MCT to regain speed but it was too late. Using MCT actually lowered our N1 and we started to get buffeting so descended at 500 fpm.

How can any prolonged setting be higher then Max Continuous?

mcdhu

28th Sep 2013, 07:03

Up at FL350, Max CRZ, MCT and TOGA EPR are exactly the same.
Demo'ed many times in the sim (A320 IAE)
mcdhu

ast83

28th Sep 2013, 07:10

I'm 99% sure there was a Boeing bulletin issued which recommended selection of MCT in turbulent cruise conditions. I'm flying later today ill check it out and confirm. (737NG by the way)

de facto

28th Sep 2013, 08:20

you are pushing the red line AND climbing in a wave, spool-up time should also not be an immediate factor unless you got into a "coffin corner" situation in the first place, whether by bad planning or extreme bad luck.

Coffin corner?in a commercial aircraft?:hmm:

I'm 99% sure there was a Boeing bulletin issued which recommended selection of MCT in turbulent cruise conditions. I'm flying later today ill check it out and confirm. (737NG by the way)

Please continue to do so.
It shouldnt be a routine...but if you have to do it then do it,no prob with that..
A proper fmc input (oat at t/c especially)will provide you with a correct max cruise.
I normally like to be 1000 ft below max when possible/forecast turbulence,when flying into a warmer mass...
If you dont push it too much use of mct should be rare..used it twice in a year in cruise obviously.

cosmo kramer

28th Sep 2013, 08:54

It should indeed be routine to select MCT during cruise (B737NG). Check the bulletins section of your volume 1.

And to Chuck Intruder, likewise it should never be routine to go idle in cruise, pr. same bulletin. Boeing go as far as to recommend to guard the auto throttle to prevent it from going below 60% N1.

cosmo kramer

28th Sep 2013, 09:01

I'll save you the time... These are the headlines. For the background, have a look in the manual for yourself:

Subject:
Reduced Engine Response Times

Reason:
This bulletin informs 737NG flight crews of slow engine acceleration following thrust reduction at cruise altitude due to a recent EEC software update. The bulletin provides suggested techniques to help prevent excessive airspeed loss.

Operating Instructions
Pilots may want to use the following techniques to avoid excessive speed loss due to slow engine acceleration:

1. Use the autopilot and autothrottle as much as possible.

2. When established at cruise altitude, manually select either CLB or CONT on the FMC N1 Limit page. This will ensure maximum available thrust.

3. If the airplane experiences a sudden increase in airspeed that causes the autothrottle to reduce thrust, manually guard the thrust levers to maintain a minimum of 60% N1, if possible. If thrust is reduced below 60% N1, a significantly longer time will be required for the engines to spool up if the time at idle thrust is less than 60 seconds.

4. If the airplane experiences a sudden increase in airspeed, consider using smooth extension of the speed brakes to increase drag and to avoid large thrust reductions.

5. No specific crew actions are needed if the thrust remains at idle for longer than 60 seconds or if the descent is to an altitude below FL300. Normal engine acceleration can be expected in these cases.

HazelNuts39

28th Sep 2013, 09:26

Item 2 (post #34) probably means that CLB is equal to CONT for the B737NG at cruise altitude.

That may not apply for different airplanes, different engines, different altitudes, but generally, for high-bypass-ratio jet engines, CLB will be close to max. continuous above a certain altitude.

ImbracableCrunk

28th Sep 2013, 14:01

At cruise, on the B738, they're all the same except CRZ, which is maybe 5% less.

de facto

29th Sep 2013, 04:45

It should indeed be routine to select MCT during cruise (B737NG). Check the bulletins section of your volume 1.
I'll save you the time... These are the headlines. For the background, have a look in the manual for yourself:

Subject:
Reduced Engine Response Times

Reason:
This bulletin informs 737NG flight crews of slow engine acceleration following thrust reduction at cruise altitude due to a recent EEC software update. The bulletin provides suggested techniques to help prevent excessive airspeed loss.

Operating Instructions
Pilots may want to use the following techniques to avoid excessive speed loss due to slow engine acceleration:

1. Use the autopilot and autothrottle as much as possible.

2. When established at cruise altitude, manually select either CLB or CONT on the FMC N1 Limit page. This will ensure maximum available thrust.

Kramer,

What does it mean 'may want to use the following techniques to avoid...'

Would you suggest that slow engine acceleration (understand it as loss of speed and AT correction is not acceptable due to your low speed margin) is an every day problem?are you flying every day at such an altitude that requires you to use this TECHNIQUE?
In cruise,the thrust setting should be CRZ unless you need MCT until you either decide to descend or if acceptable to wait for the turbulence to reduce..

Routine is what it is ...Standard thrust setting for cruise is CRZ.
Non routine technique described in your post.

latetonite

29th Sep 2013, 13:21

DeFacto:

You deleted the SOP part of your last message.

Probably as you looked it up and could not find it.
I have to see the first SOP where it is mentioned to verify Cruise limit on A/T in cruise. And a have seen a few.

A few posts back you talked about "use of MCT". I am not talking about use of MCT, but Selecting MCT as a trust limit. That is different.

ImbracableCrunk

29th Sep 2013, 13:58

I know this started as an Airbus thread, but this is an interesting discussion for Boeing folks, too.

From Jet Transport Performance Methods, Blake, et al 2009.

Thrust Ratings

So what is a thrust rating? Simply put, a thrust rating is a maximum level of engine thrust that is permitted for a specific phase of flight. For example, takeoff has its own specific thrust level, called Maximum Takeoff Thrust. That level of thrust can not be used at any other time during the flight. Also, as you’ll see, it can be used only for a specified number of minutes when taking off.

Normal climb to altitude has a different thrust level called Maximum Climb Thrust, or MClT. Similarly, cruise has its own rating, called Maximum Cruise Thrust, or MCrT.

For emergency purposes such as driftdown following an engine failure in cruise, there is a specific thrust rating called Maximum Continuous Thrust, or MCT.

A fifth rating exists, called Go-around Thrust, sometimes also referred to as Maximum Inflight Takeoff Thrust. This is a special rating used only at lower speeds and altitudes such as during a missed approach when maximum possible performance may be required.The Five Thrust Ratings
At the beginning of the chapter, we said that there are five separate thrust ratings, each applicable to a particular phase of flight. Three of these are called certified thrust ratings, meaning that they are the basis of airplane performance
data which is governed by aviation regulations such as the United States Federal Aviation Regulations and thus are published in the Airplane Flight Manual (AFM) and have the force of law. Compliance with these certified thrust ratings is mandatory.maximum continuous thrust

This is the third of the “certified” thrust levels.

Maximum Continuous Thrust, or MCT, is a special thrust rating that is only usable in the event of some emergency situation. It may not be used in normal operation.2 MCT is the greatest amount of thrust that can be used in flight, with the exception of takeoff and landing.2. For some engines, the maximum continuous thrust rating is the same as the maximum climb thrust rating; in those cases, this statement does not apply.

In the 737NG, MCT is the same as CLB. Following note 2, it may be used in "normal operation."

latetonite

29th Sep 2013, 14:49

ImbracableCrunk, I thank you for that.
Some pilots can hardly do anything, unless it is written in stone.

de facto

29th Sep 2013, 15:21

Hi Latetonite,

I am not saying that MCT can not be used as a max thrust limit ,you can do it all you want...and the same goes with using MCT when required,i am just saying that the standard has always been CRZ as a limit and thats what your FMC will give you when levelling in vnav.

Now please explain to me why a CRZ thrust limit(CRZ) is set as default rather than MCT?

Lord Spandex Masher

29th Sep 2013, 15:28

We had a situation in an MD80 on a short flight and my captain got a little slow at 36,500ft so called it out and he used alt hold and MCT to regain speed but it was too late. Using MCT actually lowered our N1 and we started to get buffeting so descended at 500 fpm.

How can any prolonged setting be higher then Max Continuous?

It can't. I think he's made up too many war stories and gets them confused.

ImbracableCrunk

29th Sep 2013, 16:21

Now please explain to me why a CRZ thrust limit(CRZ) is set as default rather than MCT?

That's a good question.

maximum cruise thrust
This thrust rating is used as a reference for the thrust available for cruise. It is not a true limit on cruise thrust, as the engines can be advanced to maximum continuous thrust in an emergency situation. It is usually designated as MCRT. It’s usually a slightly lower thrust rating than maximum climb thrust, although in some instances the two are the same. There is no time limit on the use of maximum cruise thrust.

Cruise is normally conducted at a thrust setting somewhat less than maximum cruise thrust. MCRT is the upper cruise thrust threshold for normal operation. Should the pilot wish to cruise at the fastest speed possible, he may advance the thrust levers to the MCRT setting and accept the resulting speed provided, of course, that it doesn’t exceed the maximum certified speed VMO/MMO. These are discussed in the chapter entitled “Speeds”.

Because maximum cruise thrust is not the basis for any performance level mandated by the aviation regulations, the data for MCRT doesn’t appear in the Flight Manual, but rather in the PEM and the QRH.

Just a guess: it's just about economics. Most of the time, you don't need MCT, so if Boeing says 94.6% is the limit, the engine/airframe will be that much more "fuel efficient." That 0.00001% of cruise at MCT costs more fuel. When marketing is digging for every dime, they can sell a less expensive CASM than the other guys.

latetonite

29th Sep 2013, 16:32

De facto:

Think about the days without FMC's and VNAV.

The engines do not know you have it aboard anyway.
Even less they are caring wether you are in climb, cruise, descent or approach.
However, there is a Maximum Continous Trust they are rated for, in short: MCT. You can use this thrust setting until your next oil change.

At the point you will need MCT to maintain cruise altitude, you reached your maximum altitude for that speed.
As this is not an advisable operating, you fly lower.
Based on margin calculated, you will get a lower, Normal, 'max' Cruise setting, below the engines will operate for given weight, speed and ambient conditions.
This setting will also prevent the thrust levers going "al the way"( to what?), just to adapt for normal speed deviations, in which there is in fact no "hurry".
The FMC will do that for you.

Thrust this makes some sense.

HazelNuts39

29th Sep 2013, 17:12

Just a guess: it's just about economics. Exactly. Maximum Takeoff and Maximum Continuous are airworthiness limitations, established during certification of engine and airplane, stated as such in the engine and airplane Type Certification Data Sheets (TCDSs), and in the Airplane Flight Manual (AFM). It is an offence against the law to exceed certification limitations.

Maximum Climb and Cruise ratings are contractually agreed between the engine manufacturer and the airframe manufacturer to achieve commercially acceptable targets of time between overhauls and parts replacement costs.

If CLB is equal to MCT the difference is obviously academic.

tdracer

29th Sep 2013, 19:50

If CLB is equal to MCT the difference is obviously academic.

At least on the Boeing aircraft I've worked, Max Con and Max Climb are equal above 30k. On FADEC engines - there is effectively a big flat above the max climb thrust lever position. They do diverge below 30k.

cosmo kramer

29th Sep 2013, 23:04

de facto,
Yes, I have no problem step climbing to next level, when the FMC shows it as max. And consequently I have no problem selecting CON as thrust limit as pr. Boeing recommendation - routinely.
Now on the other hand, please tell me where Boeing recommends 1000 feet extra margin on the max altitude (which already has plenty of margin built in). :E
At some point one has to stop adding personal margins on margins already built into the systems.

flyingchanges

30th Sep 2013, 00:09

At some point one has to stop adding personal margins on margins already built into the systems.

If we actually weighed what they tell us, I would have no problem using the FMC numbers. Having the low and high speed buffet hooks touching is not my idea of a safe operation, yet, that is where the FMC will drive you if you believe it.

de facto

30th Sep 2013, 05:22

Kramer,
Yes, I have no problem step climbing to next level, when the FMC shows it as max. And consequently I have no problem selecting CON as thrust limit as pr. Boeing recommendation - routinely.
Now on the other hand, please tell me where Boeing recommends 1000 feet extra margin on the max altitude (which already has plenty of margin built in).

Please do it every day..select MCT, i dont give a :mad: and fly at MaX altitude,you can also select flaps at 19 900 ft and select flaps 1 at 249kts while you are at it,you are the captain Sir.

Boeing doesnt recommend a 1000 ft margin...it is a margin i aim for to avoid ROuTiNeLy having to use speed brakes and MCT.:rolleyes:
If we actually weighed what they tell us, I would have no problem using the FMC numbers. Having the low and high speed buffet hooks touching is not my idea of a safe operation, yet, that is where the FMC will drive you if you believe it.

Exactly.

de facto

30th Sep 2013, 05:35

Latetonite,
Think about the days without FMC's and VNAV.

My aircraft is fitted with 2 Fmc and vnav.
At the point you will need MCT to maintain cruise altitude, you reached your maximum altitude for that speed.
As this is not an advisable operating, you fly lower.

Exactly so do you routinely need MCT?
Based on margin calculated, you will get a lower, Normal, 'max' Cruise setting, below the engines will operate for given weight, speed and ambient conditions.
This setting will also prevent the thrust levers going "al the way"( to what?), just to adapt for normal speed deviations, in which there is in fact no "hurry".
Exactly...and what happens when the thrust is kept to a lower max?
Just a guess: it's just about economics. Most of the time, you don't need MCT, so if Boeing says 94.6% is the limit, the engine/airframe will be that much more "fuel efficient." That 0.00001% of cruise at MCT costs more fuel. When marketing is digging for every dime, they can sell a less expensive CASM than the other guys.

Oups ...seems like Crunk nailed it.:E

framer

30th Sep 2013, 11:25

Yes, I have no problem step climbing to next level, when the FMC shows it as max. And consequently I have no problem selecting CON as thrust limit as pr. Boeing recommendation - routinely.
Now on the other hand, please tell me where Boeing recommends 1000 feet extra margin on the max altitude (which already has plenty of margin built in).
Oh my.
I hope you don't ever get the temperature changes or wind changes that I've seen in the cruise over the years! You'll be re thinking your personal operating procedures pretty quickly if you do. The issue with operating at max isn't normally an aerodynamic one, it's more often than not a thrust available issue. If you get a temp change of plus fifteen or twenty degrees while banked at 15degrees you'll be leaving your flight level with the stick shaker going.
That's just my humble opinion, feel free to explain how wrong I am :)

cosmo kramer

30th Sep 2013, 11:47

The issue with operating at max isn't normally an aerodynamic one, it's more often than not a thrust available issue.
Hence, MCT thrust selected on the N1 page.
If you get a temp change of plus fifteen or twenty degrees while banked at 15degrees you'll be leaving your flight level with the stick shaker going.
As for bank, LNAV calculates safe bank angles that doesn't exceed thrust available. If using HDG select, bank angle selector is set to 10, routinely, when reaching cruise altitude... again pr. Boeing recommendation High Altitude Maneuvering (http://www.smartcockpit.com/aircraft-ressources/B737-High_Altitude_Maneuvering.html) (automatic pdf download)

If you operate the aircraft as Boeing recommends, you do not need to add your own personal margins. Common sense should of course prevail, you do get weather charts in your briefing package don't you? And you do study those charts during your briefing I hope? 15-20 deg temperature change should be something you notice...

Then again I am not the one to report moderate turbulence as soon as we hit light chops.

cosmo kramer

30th Sep 2013, 12:02

...and then just to clarify. I never said that I blindly climb to max altitude, every time it's available. I said I have no problem doings so, and do it routinely - that of course if I see an operational benefit in doing so.

Example cruising at FL360 top of clouds in light chops. FL380 becomes available as max altitude, I climb.

Common sense: I do not need to see FL384, 386 or 390 (or whatever people have for personal margin preferences), to initiate the climb.

And I am not stupid, I plan ahead. Which included looking if 380 also will be my max altitude in near future (wx charts, vertical profile etc), the FMC only tell you the current conditions, not what will happen along the route. You could have asked... but you went on to assume that I am stupid based on things that I did not write. De facto even worse, to assume that I also select flaps with 249 knot (why would I do so, based on what I wrote?). CRM...:rolleyes:

Every decision I make in the cockpit is a conscious, considered decision, based on the facts at hand... Not an automated decision based on old wife's tales or "that I always do/need so".

Keep editing because more things pop into mind:
If you foresee that you are unable to keep the speed, a small tip (which I do not use routinely:p) - instead of going "into a stall" (a bit dramatically put by you, lowspeed band comes first you know). Turn your altitude selector down, and use V/S -100 to -200 fpm, until you regained your speed. Then vs back to level and alt selector back to your altitude. Pretty simple and unnoticeable to ATC.

Meikleour

30th Sep 2013, 12:53

Any "greybeards" on here remember the practice of "putting the aircraft on the step"?

Hobo

30th Sep 2013, 13:44

In my experience, flying a 737 right up to the limits of the wt/alt curve even in still air is a fairly 'waffly' flight regime.

de facto

30th Sep 2013, 14:17

...and then just to clarify. I never said that I blindly climb to max altitude, every time it's available. I said I have no problem doings so, and do it routinely - that of course if I see an operational benefit in doing so.

Example cruising at FL360 top of clouds in light chops. FL380 becomes available as max altitude, I climb.

Good,and then the temp gets warmer and your max is now below you?descend again or just hope the temp doesnt increase even more? For light turbulence?you find this to be of an operational benefit?would you increase or decrease fuel burn doing so?would you increase or decrease your margins doing so?..
As for bank, LNAV calculates safe bank angles that doesn't exceed thrust available. If using HDG select, bank angle selector is set to 10, routinely, when reaching cruise altitude... again pr. Boeing recommendation High Altitude Maneuvering (automatic pdf download)
You keep on advancing pdfs in this forum and yet all this and rest of what you advance is in one book,its called the FCTM.Have you heard of it?I suggest you have a look at it,it will save you time searching basics on the WWW and maybe learn a thing or two about operating at or close to the max alt.

Turn your altitude selector down, and use V/S -100 to -200 fpm, until you regained your speed. Then vs back to level and alt selector back to your altitude. Pretty simple and unnoticeable to ATC.

On that great idea im done here:D

Lord Spandex Masher

30th Sep 2013, 17:46

Any "greybeards" on here remember the practice of "putting the aircraft on the step"?

Oh yes, but I have a face like a baby's bum. Do I still count?

framer

30th Sep 2013, 21:50

you do get weather charts in your briefing package don't you?
Yes. They are a good guide. I have seen SAT changes of more than 20 C with a completely clear weather briefing. When queried the Met service commented that they are forecasts, not reports, and are as accurate as currently possible. They also informed me that they don't include several weather phenomena that are able to be forecast but deemed to uncommon to be worthwhile.
instead of going "into a stall" (a bit dramatically put by you, lowspeed band comes first you know).
I didn't say anything about going into a stall. I mentioned stick shaker, if that is allowed to progress into a stall then there is a basic stick and rudder issue that needs addressing. As for the low speed band....does it really come first? If you are cruising at your Max of FL 380 as you described, and the SAT increases from -56 to -46, what do you think happens to the 'low speed band' (sic)? The answer is ......nothing, it stays where it was at -56 C. So does it really come first?
If tooling around at Max altitude a very clear understanding of what drives the PFD displays is a good idea. In that same situation, what happens to thrust available?
At -56 degrees the aircraft was capable of a 100fpm climb rate, what is it capable of now that the temp is -46 ?

de facto

1st Oct 2013, 14:27

From boeing and Airbus :

Maximum Altitude
Maximum altitude is the highest altitude at which an airplane can be operated. In today’s modern airplanes it is determined by three basic charac- teristics which are unique to each airplane model. It is the lowest of:
• Maximumcertifiedaltitude(structural)thatisde- termined during certification and is usually set by the pressurization load limits on the fuselage.
• Thrust Limited Altitude – the altitude at which sufficient thrust is available to provide a specific minimum rate of climb.
• Buffet or Maneuver limited altitude – the altitude at which a specific maneuver margin exists prior to buffet onset.
Although each of these limits is checked by modern flight management computers the available thrust may limit the ability to accomplish anything other than relatively minor maneuvering.
The danger in operating near these ceilings is the potential for the speed and angle of attack to change due to turbulence or environmental factors that could lead to a slowdown or stall and subsequent high altitude upset.

cosmo kramer

1st Oct 2013, 19:14

De facto,
I thought you were done here? :E

You forgot to quote on:
Flight crews intending to operate at or near the maximum operating altitude should be familiar with the performance characteristics of the airplane in these conditions.
I am familiar with the performance characteristics, so for to me I don't see it as a big deal.

Coincidentally I had exactly this scenario last night, so I can back up with some numbers too. Cruising at 360 top of clouds with continuous light turbulence (what other usually report as moderate, for some reason). Max 381 with a 9 knots margin between low and high speed buffet (240 to 249, with 249 equal to approx .79). We climbed at it was smooth at FL380. We kept the speed at just below .79 (ECON cost index 55, can't remember the exact speed) and with the variations it was momentary slightly in the high speed buffet band at times (autothrottle acting more fast to correct so not a problem for me), and comfortably above the low speed band, so as to allow me to continue reading the book I brought along and drinking my coffee with out the risk of spilling any on my tie. :8
After 15-20 mins the margin increased to about 15 knots (still flying close to the upper band giving me 13-14 knots to low speed band).

Now where exactly do you see the problem in this?? Just because you are not familiar with the characteristics, doesn't mean that everyone else is wrong.

And the reason for the pdf was the the FCTM doesn't have those nice graphs that the pdf does. Yes, I am of course am familiar with the FCTM - try answering polite, it will get you futher (unless of course you "are done" here again). :(

Hobo, are you referring to 737 classic? If so, then I agree. It is quite uncomfortable if going into the high speed buffet band, but the NG is smooth as long as you are flying straight and level. Again, with a 200nm direct and no WX to avoid on the screen, I have no problem to climb and to let it run a few knots into the upper yellow band with the occasional speed variations.

Framer,
I remember a thread about a year ago, where we discussed correct flare/thrust reduction technique for the 737. It seemed to me at the time, that you were quite green?
Did you encounter an instant un-forcasted +20 deg temp. increase in clear weather? :eek:
Seems just as likely as encountering Godzilla at FL380.

framer

1st Oct 2013, 22:29

Hi Cosmo, I like the way you play the ball and not the man, I generally stick to that as well.
I remember a thread about a year ago, where we discussed correct flare/thrust reduction technique for the 737. It seemed to me at the time, that you were quite green?
I don't consider myself overly experienced, but not green either. I think I fall into the 'journeyman' category but everyone will have different ideas on what experience qualifies someone to comment on this topic. I have 19 type ratings if you include turbo props, (737-300/400 and 800 counting as one). I'm current on the 737-400 and 800.
Did you encounter an instant un-forcasted +20 deg temp. increase in clear weather?
Yes. We were below optimum at the time and managed to maintain our FL. the Met guys were consulted and queried about the forecasts and the result was as I commented earlier. There are quite a few other examples of this happening around the world hidden away in incident reports on the net.
My opinion is that there is a reason that you are in the minority with your Max altitude cruising so I'll try and covey my reasoning:
In an earlier post you talked about adding margins to already existing margins. I put it to you that there is no margin when at Max altitude as you are already at a limitation of 100fpm capability and any degradation of performance puts you outside of that requirement.
You have often referred to the lower min manoeuvre band, I don't think that the lower min manoeuvre band is well understood when the aircraft is operating at high altitude and I think it is potentially giving you a false sense of security, that is why I pointed out that it won't move as the temperature increases, (I would appreciate a response to that part of the conversation). Also on that topic, what is it giving you? Nothing relevant in my mind. It is telling you the speed at which the airframe/wings can handle 40 degrees of bank prior to stick shaker if you had unlimited power......you don't, so it's not very useful. The reality is you probably would struggle to maintain straight and level flight at the top of the bar ( at altitude) as it is lower than min drag and the drag is too high to be overcome by the engines.
Can you answer this for me please?
At -56 degrees the aircraft was capable of a 100fpm climb rate, what is it capable of now that the temp is -46 ?
Cheers.

de facto

2nd Oct 2013, 04:34

I thought you were done here?
Poorly worded..i meant i was done with you:E
Max 381 with a 9 knots margin between low and high speed buffet (240 to 249, with 249 equal to approx .79). We climbed at it was smooth at FL380. We kept the speed at just below .79 (ECON cost index 55, can't remember the exact speed) and with the variations it was momentary slightly in the high speed buffet band at times (autothrottle acting more fast to correct so not a problem for me)

And you are quite fine with that,high speed clacker? I am quite familiar with high altitude flight issues and i really dont understand how you could come here and tell us that having intermittent clacker is operationally wise or professionnaly acceptable...Did you note this excursion into the technical log?

Does you airline monitor your excesses?(QAR)..

Concerning temp increase,and i only flew this type for about 9 years on the left seat,,is that a 1-2 increase in temp can reduce your max by 100 ft...5c is plenty enough to get you in the lower band and be in an underthrust condition.

Again,i have never got into a high speed or low speed on the 737,call me conservative but i rather have my coffee without going into the clacker,save fuel and deal with what you describe as light turbulence.

Maybe your cabin crew tend to overfill your coffee,since your 'light turbulence' at Fl360 wa a potential tie ruining scenario:E

You seem a perfect fit for a cost 55 airline...
:p

framer

2nd Oct 2013, 05:56

I didn't read Cosmo's post as saying he got the over speed warning, just that he went into the upper yellow band.

de facto

2nd Oct 2013, 06:12

If you are cruising at 380(max 381), 4 kts upper margin sounds correct but to MMO ..
If your speed stayed within the upper margin,how close did it come to an overspeed,if you do it again please take a picture and post..im interested.
In any case flying at max will get you in the overspeed one day or the other.

. Also on that topic, what is it giving you? Nothing relevant in my mind. It is telling you the speed at which the airframe/wings can handle 40 degrees o

You must be flying in easa land,the FAA
use 1.2G(33 deg bank to clacker).

macdo

2nd Oct 2013, 08:53

This happened to me 2 weeks ago. A321 c.83tons. Open CLB
Climbing through a long band of Cb around SE Europe at night, passing about fl280 encountered sev turb., a/c stopped climbing and periodically entered descent. Speed decayed to Green Dot -10 in level flight. MCT selected, which unsurprisingly did nothing to improve the situation at this altitude. V/S -300 selected, a/c regained normal speed range in level turbulent flight.On leaving Cb area, Op CLB reselected and back to normal.
MCT or TOGA do not really help at high altitudes, if you enter this situation keeping the a/c under control inside the flight envelope is the key and accepting whatever flightpath that entails, with obvious regard to terrain and other traffic.

Citation2

2nd Oct 2013, 09:35

There is a new procedure available on A320 QRH , it is called "overspeed recovery".

cosmo kramer

2nd Oct 2013, 12:22

Framer,
I would consider a +20 degs temperature change an event comparable to encountering severe clear air turbulence, and would start an emergency descent. I don't plan my flight on the odd occasion that might happen every 50.000 flight. Though I know that you have a more extreme environment down under. I mean when a jellyfish can kill you... :eek:

You wrote the you were even below the optimum and only just managed to maintain altitude. I assume that is was more a coincidence that you were below optimum... hence had you been higher you would have had to descent too?
Therefore, it would be very inefficient always to fly with the worst possible scenario in mind. As an extreme example, then we could stay below 10000 feet too, to be on the safe side in case of a rapid decompression.

That is exactly what I mean, we can't continue to add personal margins on established margins... where will it end? ...that we stay on the ground.
Boeing say it is safe to operate the aircraft at max altitude, so that is what I am doing (with the use of common sense as pointed out previously).

I have no problem adding margins to margins, when it is justified. I happily bring 2 tons extra fuel if there is a reason to do so. But I also fly with min required if there is no reason to bring extra. What I do not agree with is people saying "I don't fly with less than 800 kg extra", "I always want to see +800 feet on the max altitude before I climb" etc. I have only a big WHY? to say to that.

I put it to you that there is no margin when at Max altitude as you are already at a limitation of 100fpm capability and any degradation of performance puts you outside of that requirement.
Aha, there we have the misunderstanding!! You are confusing FMC max altitude with "service ceiling". This is absolutely incorrect. Flying at max FMC altitude is not flying at service ceiling. Because Boeing already build in a margin for you:

Optimum/Maximum Altitude (OPT/MAX)
Also displays the maximum possible altitude based on the selected target speed and the specified maneuver margin.

Values are advisory only. They are provided for crew reference.

So I guess I don't have to answer your question, as it is irrelevant.

As for the flaps up amber band, it doesn't say anything about thrust available vs. thrust required. Hence, with a decreasing speed approaching the top of the amber band it would be a good idea to select CON to have the extra thrust available (if you didn't already do so, routinely, as pr. Boeing recommendation). If the speed continues to show a decreasing trend and getting close, a slight (to ATC unnoticeable) descent may help you out, as pointed out earlier.

Just to point out, despite my "dangerous" operating of the aircraft I was never in the low speed band. Except briefly one time, flying well below max altitude and flying through the edge of a CB with moderate turbulence. Lost about 300 feet altitude in the process with autopilot in CWS.

De facto,
Sorry, with you I really can't keep my eye on the ball anymore. You really have no clue, do you? Clacker for exceeding maximum maneuver speed? :ugh:

Maybe Boeing should have added:

Flight crews intending to operate at or near the maximum operating altitude should be familiar with the performance characteristics, instrumentation, limitation and basic priciples of how airplane works in these and any other conditions. Flight crews should be certified and have been tested in their knowledge prior to operating the aircraft

Been a captain for 9 years? Complacency setting in already. I can almost hear it: "...I have always done so...."

Of curiosity what is a cost index 55 airline? Actually the cost index was 4 on the OFP, what does that then say about my airline?

As we were late and I wanted to bring the aircraft back on time for the next crew to have the min required turn-around time available I had changed the cost index 55, which is equal to LRC. The benefit is that the climb and descent speeds are changed too. For me it is more efficient to change one entry, than having to modify climb and descent pages manually too. :)

cosmo kramer

2nd Oct 2013, 12:27

Just going to quote it before you edit it away. I am not done with you. :ok:

And you are quite fine with that,high speed clacker? I am quite familiar with high altitude flight issues and i really dont understand how you could come here and tell us that having intermittent clacker is operationally wise or professionnaly acceptable...Did you note this excursion into the technical log?

Does you airline monitor your excesses?(QAR)..

Concerning temp increase,and i only flew this type for about 9 years on the left seat,,is that a 1-2 increase in temp can reduce your max by 100 ft...5c is plenty enough to get you in the lower band and be in an underthrust condition.

Again,i have never got into a high speed or low speed on the 737,call me conservative but i rather have my coffee without going into the clacker,save fuel and deal with what you describe as light turbulence.

Maybe your cabin crew tend to overfill your coffee,since your 'light turbulence' at Fl360 wa a potential tie ruining scenario:E

You seem a perfect fit for a cost 55 airline...:p

...and yes, light turbulence is a potential tie ruiner. If there is moderate turbulence (unsecured objects flying around in the cockpit and cabin) I ask for descent.

ImbracableCrunk

2nd Oct 2013, 13:39

You must be flying in easa land,the FAA use 1.2G(33 deg bank to clacker).

I'm in FAA land and my book has 1.3 (40deg).

de facto

2nd Oct 2013, 15:19

Quote:
You must be flying in easa land,the FAA use 1.2G(33 deg bank to clacker).
I'm in FAA land and my book has 1.3 (40deg).

My FCTM always showed the same..
Maximum Altitude
Maximum altitude is the highest altitude at which the airplane can be operated. It is determined by three basic characteristics, which are unique to each airplane model. The FMC predicted maximum altitude is the lowest of:
• maximum certified altitude (structural) – determined during certification and is usually set by the pressurization load limits on the fuselage
• thrust limited altitude - the altitude at which sufficient thrust is available to provide a specific minimum rate of climb. (Reference the Long Range Cruise Maximum Operating Altitude table in the PI chapter of the QRH). Depending on the thrust rating of the engines, the thrust limited altitude may be above or below the maneuver altitude capability
• buffet or maneuver limited altitude - the altitude at which a specific maneuver margin exists prior to buffet onset. This altitude provides at least a 0.2g margin (33° bank) for FAA operations or a 0.3g margin (40° bank) for CAA/JAA operations prior to buffet.

flyingchanges

2nd Oct 2013, 15:42

If there is moderate turbulence (unsecured objects flying around in the cockpit and cabin) I ask for descent.

You might want to check your definition of moderate, that actually would be considered severe.

ImbracableCrunk

2nd Oct 2013, 15:47

Back to our friend
JET TRANSPORT PERFORMANCE METHODS
Walt Blake and the Performance Training Group Flight Operations Engineering
Boeing Commercial Airplanes

regulatory requirements for maneuver capability

At the time of this writing, there are no known regulatory requirements for minimum maneuver capability at cruise altitude.

Many airlines have a policy of requiring a maneuver capability of 1.3 gees or better at the selected cruise altitude, and more if turbulence is expected. Operators are free to require more conservative margins if they wish – we understand that there are some operators who do require maneuver capabilities greater than 1.3.

Flight at maneuver capabilities less than 1.3 should not be thought of as inherently dangerous, but rather that the speed margins to buffet are less. Flight in light buffet isn’t dangerous, but it is a natural aerodynamic reminder that the airplane is approaching its operating limits.
It seems my airline has gone the more conservative 1.3 route, rather than the 1.2. It's interesting that, according to Blake, there is no regulatory requirement, yet FAA and EASA differ.

latetonite

2nd Oct 2013, 15:53

De facto: the FMC predicted max altitude you refer to is dependent on the stuff you feed it.
Again, your wing does not care about the FMC.
If you let the FAA airplanes believe their 25% Cruise CG default in the FMC, you might be in for a surprise. The JAA 8% will safely underestimate the values.

Cosmo Cramer, you take the words out of my mouth.

737Jock

2nd Oct 2013, 16:38

Seems the 737 club took over. As I recall it was a320 question. At high altitude, max CLB, MCT and TO are the same limit. However selecting MCT or TOGA will react much faster then the autothrust on a speed-decay.

On a 320 you are also lucky that the high speed limits are more generous and in general the margin between low and highspeed is much bigger then on a 737.

I also loved that statement that take-off thrust can never be selected in any other phase then take-off.:ugh:

You guys need to get out more instead of watching the SAT. There is also a difference between optimum, max recommended and max cruise altitude.

Max recommended really is not that dramatic, and you can only plan for what you know. Our company downloads winds and temperature for the entire route into the fmgs, so yes the fmgs can foresee what will happen in the future a long as it is forecast.

cosmo kramer

2nd Oct 2013, 17:19

You might want to check your definition of moderate, that actually would be considered severe.
No with severe the are not merely flying around, they are being tossed around :} (Ok, I exaggerated a bit to prove a point). "Dislodged" - happy? :)

The point being, the amazing number of time I hear colleagues reporting moderate turbulence along the exact same route I am flying in light chops. Hence, if I say that I would climb for light turbulence (like the examples previously), it's because it is uncomfortable for the passengers and cabin crew. However, I refuse to use the word moderate, despite other seemingly doing so casually for light turbulence.

latetonite, thanks for the support. :ok: (Framer, I am not alone you see. :) )

framer

2nd Oct 2013, 20:53

You wrote the you were even below the optimum and only just managed to maintain altitude. I assume that is was more a coincidence that you were below optimum... hence had you been higher you would have had to descent too?
Therefore, it would be very inefficient always to fly with the worst possible scenario in mind. As an extreme example, then we could stay below 10000 feet too, to be on the safe side in case of a rapid decompression.

Not really a coincidence, I can't remember how the FL was chosen on that day but I do know that it would not have been much above optimum initially, and then it would have burned down to the situation of being below optimum. Interesting that you mention efficiency because flying at Max is quite inefficient compared to being within cooee of Optimum but that's a different conversation I guess.Aha, there we have the misunderstanding!! You are confusing FMC max altitude with "service ceiling". This is absolutely incorrect. Flying at max FMC altitude is not flying at service ceiling. Because Boeing already build in a margin for you:
Ok, one of us has a misunderstanding here, I don't think it is me but I will happily admit that it is if we can get to the bottom of it and I am wrong.
The FMC compares three limits in finding the Max altitude
1/Maximum Certified Altitude (Structural).
2/ Thrust Limited Altitude
3/Manoeuvre Limited Altitude

We all know the Structural limit. The thrust limited altitude is based on a residual rate of climb of 100fpm. You say Boeing have built in a margin on this but I have never read that anywhere. Can you provide a reference?
The Manoeuvre Limited Altitude is an Aerodynamic consideration and isn't related to the other two.
Most of the time in an NG you will be operating restricted by number 2.
When limited by the thrust the lower amber band provides no guidance as to whether or not straight and level flight can be maintained at that speed, and it does not change with temperature. If you are at ISA + it is even more misleading. So again, if you are at your maximum altitude and the temperature increases, can you maintain your required 100fpm? Unless you can describe the margin that Boeing have built in then I say no.
The ability to maintain the 100fpm is not the big issue here. It just illustrates how limited your options are when operating at Max. But either way we should determine which of us has the misunderstanding so that we can carry on with increased knowledge :)

cosmo kramer

3rd Oct 2013, 00:08

You say Boeing have built in a margin on this but I have never read that anywhere. Can you provide a reference?

I can. :) It's in the note in the brackets below:

The FMC predicted maximum altitude is the lowest of:
• thrust limited altitude - the altitude at which sufficient thrust is available to provide a specific minimum rate of climb. (Reference the Long Range Cruise Maximum Operating Altitude table in the PI chapter of the QRH).

Service ceiling is determined at the speed for minimum drag = minimum thrust required. Minimum thrust required = Max Range Cruise. MRC in the FMC is ECON cost index 0 (no wind). Reference:
Cruise Speed Determination
Entry of zero for cost index results in maximum range cruise.

So how big is that margin? How much better climb rate do you get at mach .75 compared to mach .79? To be honest I don't know, because I never tried to climb above the FMC predicted max altitude. ;)

But it is certainly enough to maintain level flight, even with the odd speed variations due to changing wind and temp. Even more so when you increase your thrust available by approx. 5% N1 by selecting CON on the N1 page, as recommended by Boeing.

I ask again, where is the problem?

Boeing also provides this note in the FCTM, by the way:
On airplanes with higher thrust engines, the altitude selection is most likely limited by maneuver margin to initial buffet.

Anyway, to sum this all up. It seems that most people get scared by "Having the low and high speed buffet hooks touching", as one poster wrote on page 3 (though they really won't, but will give you quite a few knots of space). But as you, Framer, correctly recognized, a much bigger problem is the thrust available. It's a problem, because there is most like not enough available to "pull you out" of the flaps up amber band, should you inadvertently get in there. The upper amber band, on the other hand is not so much of a problem, as long as you are not in a 30 deg bank (which you shouldn't be anyway according to Boeing recommendations).

Since the upper band is not a problem for your thrust available and really doesn't affect the type of maneuvering you are doing, it would be safe to fly in the band temporarily. Hence your total margin would be from top of flaps up maneuvering band, practically to MMO. This means that at max FMC predicted altitude, you have maybe 8-9 knots on each side of your bug speed (when flying just below the upper amber band) of to absorb wind variations. Is this a problem on normal day for you? Or let me ask you another way, how many knots margin do you need on a normal smooth day?

Yes efficiency is another matter. I reiterate, I do not blindly climb to max altitude. But there can be a number of operational reason why it can be a benefit - "Maintain level 340, sorry level 360 is occupied same destination as you, can you accept level 380? I have an aircraft 80 nm ahead of you level 380, he reported it is smooth", "able level 380" :ok:

cosmo kramer

3rd Oct 2013, 00:32

Ohhh, I missed this pearl:

De facto:
You must be flying in easa land,the FAA
use 1.2G(33 deg bank to clacker).

Can you please tell me what the bank angle has to do with the clacker (MMO)?

de facto

3rd Oct 2013, 04:41

De facto: the FMC predicted max altitude you refer to is dependent on the stuff you feed it.
Again, your wing does not care about the FMC.

I agree,the max altitude calculated to you by the FMC is as accurate as what you input into the FMC.I never said otherwise.
No i tend to write fast and kramer loves to catch on it so from now on i wont jump steps;-)

Kramer,the reason the clacker comes to my mind each time we speak about buffet is that i believe the vmax band is quite close to the max maneuvring one at your max altitude.

The MMO is a fixed speed(yes the clacker is dependent on this fixed value) but going into high speed buffet isnt acceptable to me at least routinely as you seemed to be ok zipping away your coffee...
Now we agree to disagree...:O
(I must admit i would love you requesting fl 380(max alt) from fl 340 and as you climb it isnt smooth as reported,again it seems like a subjective definition ,and the temp going up by a few degrees and now your fmc,passing through fl360,shows you max is 378.....,what would you do ? Ask for descent with the tail between your legs or continue up,(i bet u do that),and try to convince your Fo about all the margins built in....and squeeze your sorry :mad: ?

How often have you exceeded MMO Kramer? Be honest.
I know even if you exceed mmo,theres extra margin,it aint the end of the world..but we are paid to keep the aircraft within given margins,few seem to believe that to be at exactly those margins is acceptable,,i dont.
Hence my earlier analogy to opearting the flaps at 19900 ft or selecting flaps at 249kts...whats the difference Kramer?
You are within limits?and yet if you overspeed the flaps,the aircraft wont fall off the sky...but you respect that dont you?so why not the same with your alitude?

framer

3rd Oct 2013, 06:06

Ok Cosmo, we are starting to make some headway into the 'misunderstanding ' that you pointed out. I agree that there is a level of misunderstanding on this topic but am not convinced that it lies with me.
Every decision I make in the cockpit is a conscious, considered decision, based on the facts at hand... Not an automated decision based on old wife's tales or "that I always do/need so".
You are quick to point out that adding an extra margin is a wives tale but it's genesis may lie in truth. Your conscious, considered decisions, based on facts at hand will be drawing on your knowledge base and the learnings you have made over the years in order to make the decision. It is neccassary to have self confidence in that knowledge base, but also to be prepared to re-jig it if it is found wanting so that your decision making skills evolve as you go. I want to question that knowledge base without it seeming like a personal attack .With that said, I am more than prepared to eat humble pie and rethink my alt selection if you can direct me to where I have misunderstood. Your last shot at directing me there fell short. I'll show you where so that there is no confusion.
Service ceiling is determined at the speed for minimum drag = minimum thrust required. Minimum thrust required = Max Range Cruise. MRC in the FMC is ECON cost index 0 (no wind)
Only two of those three assertions are correct.
The speed for minimum drag does not equal the speed for minimum thrust required in a swept wing Boeing, and, Minimum Thrust Required does not equal Max Range Cruise in a swept wing Boeing. I agree that nil wind CI zero equals MRC.
So shall we sort out those basics before we move on to determining why you and Latetonite seem to be on the fringe of what B737ng pilots think is an acceptable way to operate the aircraft?
I'm glad you mentioned min drag actually because that speed is probably the only relevant speed to you when you are operating at Max altitude, that and Vmo of course. I have one question for you, in your flight the other day when you said Coincidentally I had exactly this scenario last night, so I can back up with some numbers too. Cruising at 360 top of clouds with continuous light turbulence (what other usually report as moderate, for some reason). Max 381 with a 9 knots margin between low and high speed buffet (240 to 249, with 249 equal to approx .79)......what was min drag speed?

cosmo kramer

3rd Oct 2013, 11:44

De facto,

I don't understand why you think MMO would be more easily exceeded at max altitude?

If I fly .79 at FL360 or FL380, I still have .03 margin in both levels, with the difference of the .03 being the yellow band in FL380. Actually, I believe it to be less likely, because the auto throttle will "wake up" as soon as the speed goes into the yellow band. I am sure you are familiar with cruising at well below max, .79 bugged, but seeing the speed go to e.g. .815 and the auto throttle still asleep.

In the previous times, it was normal to fly 340/.820 if late (before the rising fuel prices), and have the clacker come on intermittently. Ask older colleagues, no aircrafts fell out of the sky for that reason. I don't fear the clacker, but I respect it and avoid it, I don't like to fill out unnecessary paperwork (inefficient use of time). I wasn't in the clacker for years, and when I was it was never at max altitude, because I don't go there if it's turbulent.

During my climb from 340 to 380, my max will most likely go up to 382 during the climb. I think the FMC doesn't* consider the fuel burned during the stepclimb, but I can't back that up with anything in writing.

I did have once that on the way up to FL360 the max altitude dropped down to FL359. And yes we continued up (it was perfectly smooth) and within minutes max and selected again agreed.
Like already been said the wing doesn't know what's written on the cruise page of the FMC. This has to be an individual evaluation what to do.

The difference to using flaps in 19999 or 249, is that there is no point to doing so. Give me one objective example of where it could possibly be an operational benefit of doing so? I can't see one. To make your example slightly less extreme, even if fast, I can't see any objective accomplished in selecting flaps 1 at e.g. 240 knots. I have a gear that is much more effective in slowing me down, and it needs to be extended to land anyway. :)

Framer:
The speed for minimum drag does not equal the speed for minimum thrust required in a swept wing Boeing
Aha, but it's the same curve. It's just two different names for the same point on the same curve. So how in would could they not be the same...

and, Minimum Thrust Required does not equal Max Range Cruise
Ok :hmm: So, which speed would give you the max range then? (If you say long range cruise, explain to me why we always fly slower to save fuel).

......what was min drag speed?
I'll maintain ECON cost index = 0. Didn't check what is was at the day, probably around .76.

*edited

latetonite

3rd Oct 2013, 21:35

Do not forget Econ with cost index 0 still corrects for wind. (+, not -).

framer

4th Oct 2013, 12:08

Cosmo, you were right about min drag and min thrust required. I confused min fuel flow with min thrust required. My apologies.
Off to work with me now. I'll have a look at some numbers in the cruise and see if I can bolster my argument with them tomorrow :)

HazelNuts39

4th Oct 2013, 12:52

So, which speed would give you the max range then?Well ... if you assume constant TSFC (thrust-specific fuel consumption), then max L/D speed will give you minimum fuel consumption per hour. Max range speed is minimum fuel per NM and requires a higher speed -1.32 times minimum drag speed for a 'parabolic' drag polar. The actual max range speed will be somewhat different due to Mach effects and variation of TSFC. Hope this helps.

framer

4th Oct 2013, 23:56

Righto, back to it.
Ok So, which speed would give you the max range then? (If you say long range cruise, explain to me why we always fly slower to save fuel).
Cost index zero. It will always have you arrive overhead with the most gas in the tanks.It will give you a speed faster than the minimum thrust required speed. In addition, as I'm sure you know, it will give you a faster speed again if you are into a headwind. LRC is pretty much irrelevant in today's CI environment, it doesn't take the wind into account and simply gives you an extra 5% speed for an extra 1% burn. Handy in the days when you didn't have Cost Index to do the maths for you.
I think I have identified where you and I are at crossed purposes Cosmo. You stated Hence your total margin would be from top of flaps up maneuvering band, practically to MMO. This means that at max FMC predicted altitude, you have maybe 8-9 knots on each side of your bug speed (when flying just below the upper amber band) of to absorb wind variations
You have identified a range of airspeed where you feel safe operating, but the lower limit of this range is incorrect in my mind. The lower limit you suggest is the top of the lower amber band, whereas the lower limit should be min drag speed. The top of the lower amber band is some way from min drag speed. I will give you some cut and paste examples of why I think this;
The Royal Aeronautical Society has this to say.
The lowest point on the total drag curve is known as L/D max (or Vmd-minimum drag speed). The speed range slower than L/D max is known as slow flight, or the “back side of the power-drag curve” or the “region of reverse command.” Speed faster than L/D max is considered normal flight, or the “front side of the power-drag curve”.
Normal flight (faster than L/D max) is inherently stable with respect to speed. When operating in level flight at a constant airspeed with constant thrust, speed-stability ensures that any airspeed disturbance (such as turbulence) is of short term duration and airspeed will eventually return to the original airspeed if the total thrust and attitude have not changed.
So it is obvious from this that we want to stay in the Normal flight regime or we will end up in the slow flight regime which is described here;
Slow flight (slower than L/D max) is inherently unstable with respect to speed and thrust settings. When operating at a constant airspeed, with constant thrust setting, any disturbance causing a decrease in airspeed will result in a further decrease in airspeed unless thrust is increased. The lower speed subjects the aeroplane to increased drag. This increase in drag causes a further decrease in airspeed, which may ultimately result in a stalled flight condition.
The paper then goes on to say this;
Flight slower than L/D max at high altitudes must be avoided, due to the inefficiency and inherent instability of the slow flight speed range.
And Flight slower than L/D max must be avoided in the high altitude environment. Proper flight planning and adherence to published climb profiles and cruise speeds ensures that speeds slower than L/D max are avoided.
So from that, if we are to believe the Royal Aeronautical Society ( and it's American counterparts) we should be cruising above min drag speed. That isn't really up for debate unless you consider that you know better than the folk who collaboratively produced the guidance document. That is why I asked you what min drag was for you example flight. You took a guess at about Mach 0.76. With respect, I say that taking a guess after the flight is complete is not really good enough. The Captain needs to know where the lower limit of safe flight is while he or she is conducting the flight.
Min Drag speed is usually ten knots or more ( sometimes as much as 18kts in the NG) faster than the top of the lower amber band when at high altitude. My experience is that there is usually only a couple of knots lee-way when operating at Max, not the 8-9kts that you described.
Is there a chance that the reason you feel comfortable is that you have erroneously identified a larger safe operating band than many other pilots?
Most pilots that I fly with can't identify the min drag speed if asked, yet it is only two button pushes away. The fact that it is not easily displayed to the pilots is a failing on the manufacturers part in my mind because both the Royal Aeronautical Society and the FAA produced 'jet upset training aid ' warn of not going below it.
I hope that this doesn't come across as an attack on your professionalism, rather a robust discussion and a chance for both of us to learn something. I have learnt already so thanks for that.

cosmo kramer

5th Oct 2013, 05:38

Framer, it is time to admit you are wrong, instread of digging up more and more obscure arguments as I shoot down your previous ones...

So far you have now admitted, that everything I have posted so far is correct. Min drag = min thrust required = cost index 0 (no wind, as I wrote already in the first post mentioning cost index 0).

So I is really time to let it rest.

LRC is pretty much irrelevant in today's CI environment
Yet this is the speed Boeing uses to calculate the thrust limited altitude. Like I have now written several times, this gives you an extra margin.

About the last non-sense you posted, this is basic ATPL knowledge:

The lower limit you suggest is the top of the lower amber band, whereas the lower limit should be min drag speed.
No, this is the lowest speed that should be used as cruise speed (the selected cruise speed). Because a speed lower that this speed is ineffective.

As you quoted
Normal flight (faster than L/D max) is inherently stable with respect to speed. (hope my formatting stays, because it dissappeared from my previos posts, the emphasis should be on NORMAL)....
any disturbance causing a decrease in airspeed will result in a further decrease in airspeed unless thrust is increased. (emphasis "unless thrust is increased")

So it is quite obviuous that we should not plan to fly at a speed lower than L/D max (Max range cruise = cost index 0 (no wind). And we don't.. we always plan to fly faster that this speed, because...
due to the inefficiency and inherent instability of the slow flight speed range.
It is inefficient, because we have a higher drag, and fuel flow plus we will have a lot of throttle movement to maintain the speed.

That all together means that if you cruise at a faster speed, a temporary excursions into this regime is acceptable, but must be corrected by increasing the thrust (pretty obvious - the "unless thrust is increased" emphasis).

We will always cruise faster than MRC, because it makes no sense to cruise with a lower speed. And should the speed temporarily go below MRC, towards the top of the amber band, we will correct it (auto throttle too).

framer

5th Oct 2013, 13:08

Righto Cosmo, I see where you are coming from, correct me where I am wrong.
Cruising at speeds planned to be at or faster than min drag is acceptable because if you drop below min drag you will use thrust to overcome the extra drag and drive on out of it?
I can see how that works in theory but have you never experienced changes in wind and/or temperature that result in a situation where your only option is to descend as the thrust available can't pull you out of it? I know I have and I also know of several departures from flight level because of that very reason.

Framer, it is time to admit you are wrong, instread of digging up more and more obscure arguments as I shoot down your previous ones...

The minute I recognised that I was wrong about min drag equaling min thrust required I posted saying just that and apologising. I am yet to see a similar post from you admitting that min thrust required does not give you MRC. I have to admit that I'm not holding my breath.
So far you have now admitted, that everything I have posted so far is correct.
Not at all. There are three basics we were trying to sort out
1/ cost index zero = MRC ...... We agreed from the start
2/ min drag = min thrust required......we now agree after I admitted I was wrong.
3/ min thrust required = MRC......we still disagree, I say MRC is faster.

I can see your thought process though. At the end of the day I think you are simply comfortable with lower margins than me.

cosmo kramer

5th Oct 2013, 23:32

I can see how that works in theory but have you never experienced changes in wind and/or temperature that result in a situation where your only option is to descend as the thrust available can't pull you out of it?
Yes, well below max altitude due to moderate turbulence. At close to max altitude, no, because I don't go there if it to be expected or is forecasted.

I am yet to see a similar post from you admitting that min thrust required does not give you MRC. I have to admit that I'm not holding my breath.
The best source I can dig up on google to be able to provide you with a link is this:
Maximum Endurance, Maximum Range, and Optimum Cruise Speeds (http://www.eaa1000.av.org/technicl/perfspds/perfspds.htm#Figure_1)

Not the best source, but have a look af the lower curve on figure one anyway.

3/ min thrust required = MRC......we still disagree, I say MRC is faster.
Even if your were right, it only makes my case stronger. It would make the margin even bigger. Since Min Drag Speed would be even lower, than cost index 0 speed. But have a look at the link above.

At the end of the day I think you are simply comfortable with lower margins than me.
I am comfortable with lower margins when I don't need them. If I need higher margins I will take them (more fuel, lower altitude etc.). But there has to be a reason....

framer

6th Oct 2013, 02:09

The best source I can dig up on google to be able to provide you with a link is this:
Maximum Endurance, Maximum Range, and Optimum Cruise Speeds

Not the best source, but have a look af the lower curve on figure one anyway.
Hi Cosmo, thanks for the link. That link provides good information for straight wing propellor aircraft only.
In 2009 the 'Performance Training Group Flight Operations Engineering Boeing Commercial Airplanes ' updated their document 'Jet Transport Performance Methods' . It is a great document and explains the Boeing ethos behind all of their Performance Engineering. PAge 32-14 has a nice graph and explanation regarding the relationship between Min Thrust Required and MRC. It says this:
We've drawn in a line of MRC. Here you see clearly that the speed for maximum range cruise doesn't occur at the point for minimum thrust required, but rather at a slightly faster speed.
So here is where we stand, feel free to correct me:
Min drag = min thrust Required
MRC = cost Index Zero
MRC/ CI zero > Min Drag/min Thrust Required

Even if your were right, it only makes my case stronger. It would make the margin even bigger. Since Min Drag Speed would be even lower, than cost index 0 speed.
I see what you are saying and I agree that the margin to min drag is greater than you originally suggested. It is the same as I always imagined because every time I level out in the cruise I make a mental note of what Min drag is and don't accept slower speeds. I am fairly confident though that next time you climb up there and have a look at what your min drag speed is it will be within a knot or two from the speed you are cruising at. It is always ten or so knots greater than the lower amber band ( often 15-18kts) so when you have a gap of 8 or 9 knots like you suggested, you are very close to it. Obviously some are more comfortable with that than others. Anyway, apart from MRC= min thrust we roughly agree on the basics , the difference being that I am more conservative regarding deviations into the slow speed regime. Fair enough, I wish you thirty years of safe cruising Cosmo. Thanks for the conversation.

cosmo kramer

7th Oct 2013, 09:40

I am aware that the drag curves are greatly simplified, most likely they are too simple for a small prop a/c too. However, they are enough for the purpose of explaining something in an easy and understandable way. I guess that is why they are used for learning material for pilots. I don't pretend to be an engineer, I like the KISS principle. I will give you the following.

MRC/ CI zero ≈ Min Drag/min Thrust Required (for practical purposes)

If you say you have a document from Boeing, that states that MRC is slightly higher than Vmd, I believe you (compressibility, TSFC etc.). Still, it has no practical bearing on the discussion about cruise margins (and even strengthens my point of view). Furthermore we can probable put "≈" in every argument in every discussion, starting with why an aircraft flies at all (was it because of Newton or Bernoulli?).

I wish you thirty years of safe cruising Cosmo. Thanks for the conversation.
Thanks, you too. But before you go, I have one more question:

I make a mental note of what Min drag is and don't accept slower speeds.
Not sure what do you mean with "don't accept slower speeds"?

Do you mean that you won't accept a slower speed as selected speed (say an ATC restriction). And, if so, I assume you mean in high altitude cruise flight only? (in which case I from the beginning agreed, as it is ineffective).

or

You will not accept that the speed temporarily decays below Vmd with wind/temp variations during cruise, and for that reason you will fly with such great margins as to make it unlikely to happen (this is how I interpret your statement). If this is the case, I have to ask a big "WHY?" again. Vmd is not a dangerous speed, flying slower isn't dangerous either. The speed that is dangerous, is "V drag too great for the thrust at hand to pull you out". Clearly this speed is slower than Vmd.

framer

7th Oct 2013, 21:40

Do you mean that you won't accept a slower speed as selected speed (say an ATC restriction). And, if so, I assume you mean in high altitude cruise flight only?
Yeah that assumption is correct for how I operate and yes, only at high altitude.
You will not accept that the speed temporarily decays below Vmd with wind/temp variations during cruise, and for that reason you will fly with such great margins as to make it unlikely to happen (this is how I interpret your statement). If this is the case, I have to ask a big "WHY?" again. Vmd is not a dangerous speed, flying slower isn't dangerous either. The speed that is dangerous, is "V drag too great for the thrust at hand to pull you out". Clearly this speed is slower than Vmd.

If I am cruising at high altitude and the speed decays to Vmd and the A/T doesn't wake up I will open the speed window and select a higher speed to wake it up, when the speed recovers to the target speed I close the window.
I agree that Vmd in itself is not a dangerous speed and that the speed at which you no longer have the thrust required to recover to a 'speed stable ' situation is what is important. This is where you and I probably part ways in our assessment of the situation. I am guessing that we have different ideas about how much thrust we have available to us at max altitude to do just that.
My assessment is that we have juuust about enough thrust to pull the skin off a rice pudding ( 100fpm + the margin you have described earlier) and that if we suddenly gain twenty knots of tail, or lose twenty knots of head, or if the temp increases, or we experience wake or other turb, we may find ourselves having to descend when we are not cleared to. I don't expect these things to occur very often, maybe once or twice a decade, but considering that I want to do this for another twenty years I would rather avoid three or four pan calls in the remainder of my career.
Your assessment of the thrust available in those situations is obviously different and that is not surprising, it is a subjective scenario with many unknowns and the call is therefore judicious. Each pilot will assess it slightly differently.
It is probably worth mentioning that under the right conditions I would climb to max altitude. They would be exceptional conditions as the benefit of being up there would have to outweigh the fact that we would burn more fuel and have less margin.I have yet to encounter a situation where every level below optimum has severe turb but it is smooth above. If I am faced with the choice of being inefficient above optimum or being inefficient below optimum I choose the latter most of the time.
So there we have it, different pilots making different subjective decisions, at least we have both thought about it independantly and not just blindly followed our mentors habits.
If we want to throw another spanner in the works we could talk about the BEST SPEED on the holding page, it is often twenty knots below min drag when at altitude as it is literally the BEST SPEED for minimising RATE of fuel flow, it is normally 15-20kts below min drag and results in extra burn in the turns and thrust levers working hard in the turns. Boeing has given the pilot the best theoretical speed for staying in the air the longest time possible but practically it is rarely that :)
The Boeing performance Engineers say this:
In the holding pattern, however, while the emphasis is still on fuel efficiency in this case we want to minimize the rate of fuel flow. Minimizing the fuel flow rate means minimizing the amount of fuel consumed while holding.

“All right,” you say, “we'll just calculate the fuel flow at the speed for minimum drag. Minimum drag means minimum fuel flow, right?”

Sorry, no. It's a little more complicated than that. Actually, the minimum flow will occur at a speed slightly slower than the speed for minimum drag.
They then go on to give an example of a 757 and their slightly slower speed is thisIt will occur at the point where a fuel flow line is tangent to the thrust required line. In the example you see here, the fuel flow at the tangent point is slightly more than 3500 pounds per hour per engine. And notice the speed at the point of tan-gency: about Mach 0.71. What's the speed for minimum drag in this graph? Approximately Mach 0.75, which is 0.04 Mach or 23 knots true airspeed faster than the speed for minimum fuel flow.
This prompted me to select holds in the cruise and compare the BEST SPEEDS for the NG, they too are significantly ( or slightly depending on what school you went to) slower than min drag.

cosmo kramer

8th Oct 2013, 03:04

My assessment is that we have juuust about enough thrust to pull the skin off a rice pudding ( 100fpm + the margin you have described earlier)
Edit: misread it like you meant I had described that we have a +100 fpm margin, I'll leave first part of the reply unchanged anyway:

No, I didn't - on the contrary, I said we have a lot higher margin because the +100 feet is calculated at LRC, where we have a higher drag than the Vmd. Hence, flying at Vmd the residual climb rate would be higher than as calculated for LRC. Plus, the +100 feet is calculated at CRZ thrust limit. With CON you have 4% extra N1.

Hence, my assessment is that you greatly underestimate the thrust available.
Again I quote the FCTM:
On airplanes with higher thrust engines, the altitude selection is most likely limited by maneuver margin to initial buffet.
---
if we suddenly gain twenty knots of tail, or lose twenty knots of head, or if the temp increases, or we experience wake or other turb, we may find ourselves having to descend when we are not cleared to. I don't expect these things to occur very often, maybe once or twice a decade
What is "suddenly"? If you mean instantly - then you would with 99% have had forewarning. The wind doesn't change 20 knots instantly in smooth conditions. Or like I said before, it is just as likely as meeting Godzilla at FL380.

I do maybe 250-300 flights pr. year or up to 3000 flights in a decade. Will I make all 3000 flights is FL100, because of the odd chance I might experience a pressurization problem? Of course not, to me this is similar in likelihood.

Even if I had to descent, a quick call to ATC, "unable to maintain speed, require descent", may often be in time to avoid having to make a mayday call (pan call - urgent message- will get you nowhere, and does not allow you to deviate from your clearance - but that's a different story :E ). Remember, you do not need to make an emergency descent with 4000 fpm sink rate, a shallow descent to the non standard level 1000 below will with all likelihood be enough to regain your speed. Or maybe 1-200 feet will even be enough and you can do that on your own, without even bothering the ATC controller.

---

And now for something completely different (as Monty Python would say):
The Boeing performance Engineers say this:
“All right,” you say, “we'll just calculate the fuel flow at the speed for minimum drag. Minimum drag means minimum fuel flow, right?”
To whom are they talking? Who is this "you"? Clearly, it is someone who didn't do his ATPL:

Min thrust required = Max Range Cruise
Min Power required = Max Endurance

This is really basic ATPL knowledge and I really don't know why you bring Max Endurance into the picture. But I will follow your lead...

Boeing has given the pilot the best theoretical speed for staying in the air the longest time possible but practically it is rarely that :)
On what do you base this assumption?

What is "BEST SPEED"? I have never heard of "BEST SPEED" before, "best speed" for what?

I have a target speed on the holdings page with is "Max Endurance" or "Min Power Required" (at least ≈ , to avoid making it too complicated). If indeed you mean target holding speed from the FMC holdings page, it just shows how much thrust you mostly have available, since it will be just a few knots above the amber band. I wouldn't personally want to fly at that speed at high altitude though, since a 1-2 knots margin to the amber band is on the low side for me (yes I know I am being conservative and that I still have 1.3G margin to stick shaker). ;)

...but the again, who holds at FL380? :hmm:

framer

8th Oct 2013, 07:56

What is "suddenly"? If you mean instantly - then you would with 99% have had forewarning.
Not really instantly, I'm thinking more over a time frame of three or four seconds.
I do maybe 250-300 flights pr. year or up to 3000 flights in a decade. Will I make all 3000 flights is FL100, because of the odd chance I might experience a pressurization problem? Of course not, to me this is similar in likelihood.
Everything we do is commercially driven at the end of the day. If it had no economic impact then yes, you would.
What does it cost you to cruise at 10,000 ft? Lots
What does it cost you to fly within cooee of optimum? Nothing.

Even if I had to descent, a quick call to ATC, "unable to maintain speed, require descent", may often be in time to avoid having to make a mayday call (pan call - urgent message- will get you nowhere, and does not allow you to deviate from your clearance - but that's a different story ).

I will deviate from my clearance as I need to to fulfill my obligations. If nothing on the TCAS Pan will suffice. It is an urgent situation but there is no immediate threat to life - but that's a different conversation :E

To whom are they talking? Who is this "you"? Clearly, it is someone who didn't do his ATPL:
Clearly. The document is written largely in a conversational tone. It's quite well done really. It is aimed at pilots who have an ATPL. You should read it, I reckon you'd dig it.
On what do you base this assumption?
Practical observation. I explained it in my last post. The thrust levers work hard.
What is "BEST SPEED"? I have never heard of "BEST SPEED" before, "best speed" for what?
Bottom right hand corner of the HOLD page. It says BEST SPEED and gives a number. The FCOM says this about it
BEST SPEED

Displays computed best holding speed based on present altitude and conditions.

Note: May exceed maximum speed permitted by regulatory agency.
Does your FMC not have this?
...but the again, who holds at FL380?
I do on a regular basis. Believe me it's not by choice.
Righto, off for a two sector eleven hour day. Have fun.

cosmo kramer

8th Oct 2013, 11:31

A change of windspeed of 20 knots over 4 seconds, is what I would call instant. It would be foreseeable by the presence of at least light turbulence in the area you are flying. And no, if you descent without clearance and you are only calling pan-pan, you are breaking the rules. To deviate from any clearance you have to call mayday.

Bottom right hand corner of the HOLD page. It says BEST SPEED and gives a number.
Thanks for the pointer. Didn't pay much attention to that speed, it's the same speed as LSK1R (predicted target speed), if no manual entries are made.

So as you said yourself:
This prompted me to select holds in the cruise and compare the BEST SPEEDS for the NG, they too are significantly ( or slightly depending on what school you went to) slower than min drag.
So there you have it with your own conclusion above: Your margin to your "V drag too great for the thrust at hand to pull you out" is significant. :ok:

I actually wrote a longer reply to your practical holding observations, but deleted that again as it really doesn't have anything to do with the topic. I'll just leave it at saying: be careful about unfounded assumptions.

HazelNuts39

8th Oct 2013, 11:32

framer,

It's quite well done really. It is aimed at pilots who have an ATPL.
Caveat: It's about Jet Transports, doesn't discuss propellers.

cosmo kramer

9th Oct 2013, 22:48

Pictures for De Facto:

Beautiful day for flying:

http://s23.postimg.org/ebmv7gqcb/IMAG0146.jpg

And nice to pass over those white fluffy things...
(not the dangerous kind, but still more comfortable)

http://s23.postimg.org/nuwk0xduj/IMAG0148.jpg

...in FL370, efficiently 1100 feet close to optimum, 10 knots less headwind compared to 350, and with 300 feet to spare... Max was 372 as climb was initiated, pictures taken a few mins later (when this thread sprung to mind).

http://s9.postimg.org/soqbuyuj3/IMAG0144.jpg

...and a whooping 18 knots margin... :eek:

http://s23.postimg.org/p3kfmk5sr/IMAG0143.jpg

...actual weight 72 tons at the time (forgot to take a picture), still NOT thrust limited (26k engines). As can be seen from the holds page, with a 240 knots recommended holding speed, which is actually 1 knot below the amber band (was changing back and forth between 240 and 241, assume it's a rounding issue and it really wanted to be just at the top at the amber band):

http://s23.postimg.org/wyzkf44t7/IMAG0149.jpg

Questions? :ok:

misd-agin

10th Oct 2013, 00:35

cosmos Kramer wrote -
Coincidentally I had exactly this scenario last night, so I can back up with some numbers too. Cruising at 360 top of clouds with continuous light turbulence (what other usually report as moderate, for some reason). Max 381 with a 9 knots margin between low and high speed buffet (240 to 249, with 249 equal to approx .79). We climbed at it was smooth at FL380.
...
After 15-20 mins the margin increased to about 15 knots (still flying close to the upper band giving me 13-14 knots to low speed band).

In 15-20 minutes your margin increased by 6 kts?

Looked at 737NG buffet charts. 20,000 lbs change(9000 kg) changes buffet speed by 18 kts. That's a lot longer than 15-20 minutes.

framer

10th Oct 2013, 01:29

Thanks for the photo's Cosmo.
I will ask a question.
What do you think the result would have been had you opened the speed window and selected either 240kts or the equivalent Mach number?
Cheers
edit: PS the reason for my question is to challenge the idea that you had an 18kt margin to operate in.

cosmo kramer

10th Oct 2013, 10:45

misd-agin, yes as far as I recall. The pictures are from a different day but show pretty much the same.
On the very first picture, in the post above, in the distant horizon is a long flat cloud. As we reached that I did another few pictures, this is the same cloud:

http://s24.postimg.org/btdca8jpx/IMAG0150.jpg

Didn't take a lot of minutes to get there, from the first photos. And look how fast the margin increased by 3-4 knots:

http://s17.postimg.org/4p7xpyg9r/IMAG0151.jpg

Framer, I think nothing would have happened. The speed would have been 240 knots, and with the natural variations it would temporarily be above 240, and temporarily below 240 (which would then be in the amber band). What do you think would happen?

framer

11th Oct 2013, 06:02

I think that if you reduced to cruise at M0.77 your N1 would have been lower, then if you slowed to M0.76 it would have had to increase to counter the extra drag, more again at M0.75 and by the time you got to M0.74 most likely you wouldn't have had the thrust available to maintain the speed and commence even a 5 degree bank angle turn.
Do you think that the top of the amber band means you can cruise there and have 40 degrees of bank angle to the stick shaker or initial buffet? I'm assuming you know better than that but thought it worth asking seeing as the FCOM makes that statement.

cosmo kramer

11th Oct 2013, 12:31

Well with the thrust you are wrong. And the proof lies in the holding speed. Of course there would be thrust enough to maintain 240 knots, otherwise the FMC wouldn't display it as a holding speed. If I would fly that speed or not is another story, the point is that if flying e.g. .785, like in the pictures, I have plenty of time to react and increase thrust if the speed should decay for one reason or the other. Even if I had bad luck and it decayed to 240, I would still have enough thrust to pull me back.

This goes for this particular instance, other instances may be thrust limited (like maybe with a standard + 15 degs temperature, in the case of the pics it was pretty close to standard).

I don't fly with 40 degs bank at high altitude, max 15 degs as pr. Boeing recommendation. I am sure you have a copy of the FCTM too, so there is really no point for me to quote what the flaps up amber band means, when you can have a look yourself.

de facto

11th Oct 2013, 15:20

Pictures for De Facto:

Thanks but i was looking for the picture of your initial fl380 when MAX was FL381( not 383).

cosmo kramer

11th Oct 2013, 16:37

Very demanding! ... I'll keep the camera ready next time, it's not like it's every day there is an operational benefit in climbing as soon as the max hit the next level. ;)

flyingchanges

11th Oct 2013, 17:16

Your operation must be much better about your actual weight than ours. If we were showing a 300' difference between cruise FL and max FL, there would only be a few knots between the amber bands.

de facto

11th Oct 2013, 18:33

Im sure ull make an operation benefit exception for me:p
Thanks.

latetonite

11th Oct 2013, 19:23

Of course nothing would happen. And if it would scare some people, change your cruise CG to 25%.

For the ones that never get scared, change it to all the way fwd, now you are flying in the yellow band, one way or the other. But apart from that, no, nothing happens..If you can get there in the first place, you can stay there..

Again, how exactly is your weight today? How correct is your load sheet? And now you have your engine degradation of a few percent. Your flight plan takes it into consideration for fuel calculation, but your FMC has still the same old database.
This database determines your so called Cruise Trust Limit.

All this leads me to select MCT as a limit, when I deem appropriate, as Pilot In Command.

And that was this topic all about.

framer

13th Oct 2013, 01:33

I think that my statement of you wouldn't have had the thrust available to maintain the speed and commence even a 5 degree bank angle turn
may be correct and I'll explain why below. It would be interesting if someone with access to a sim climbed up to max at a weight of 70T and then slowed to the top of the amber bar and tried a few turns and reported back to us. Anyone?
Cosmo, you said I don't fly with 40 degs bank at high altitude, max 15 degs as pr. Boeing recommendation.
I didn't think you would, I just wanted to know if you were mislead by the statement in the FCOM that says the lower amber band provides you with 40 degrees bank prior to buffet because as you know, that is an aerodynamic consideration and is only true if the thrust is available to do the turn, and at max you won't have the thrust available to both maintain speed and the 40 degree bank at the top of the amber band.
If we look at what Boeing use to determine the max altitude in any given situation and then apply it to your photo it might help strengthen my argument.
1/ The altitude for 1.3 g margin to initial buffet.
In the picture this speed is 17 or 18 kts slower than the speed you are cruising at and is not limiting you, it is an aerodynamic consideration and not a thrust consideration.
2/The service ceiling ( altitude for 100 fpm climb rate at max climb thrust)
This is a thrust consideration and may be limiting you but maybe not.
3/ Max cruise thrust limit ( altitude for 0 fpm climb capability at cruise thrust)
This is a thrust consideration and is sometimes lower than the service ceiling limit and may have been limiting you.
4/ normal accel to initial buffet at optimum altitude + 2000 feet.
An aerodynamic consideration and was not limiting you, if it was the selected speed would have had you sitting on the bottom of the upper amber band.
5/bank angle to max cruise thrust at opt alt + 2000 feet, ISA+15
Another thrust consideration and what I think was most likely the limiting consideration. Your opt was 359, so why wasn't the max 379? Because when a bank angle limit of 15 degrees was applied to 379 it failed to meet the requirement ( maybe the thrust limit was reached at 10 degrees) so the calculation s were done to where the aircraft could comply and your max was calculated at 373 where a 15 degree bank could be sustained with the thrust available. If you slowed to a speed where drag was less, the bank angle capability would have increased, but as you slow beyond that the ability to sustain the minimum required bank angle would have diminished. I maintain that at 240kts it would have been less than the 15 degrees required.
What we really need is someone with sim access to go and do it and report back to us.
Another way of looking at it:
Yesterday in the cruise I determined the max for three different speeds
.76
.77
.79 ( LRC)
The max altitudes were
FL385
FL 386
FL382
So if I had a great desire to fly at FL 386 I would have to cruise at 0.77, if I then changed my speed to 0.76 I would be 100ft higher than the max altitude calculated for the new speed. Which limit am I not complying with? Who knows but at my new speed I no longer have the margin of one of the 5 limits written above. If I used 0.77 to climb up there and then accelerated to 0.79 I would be 400ft above where one of those limits kicked in.

cosmo kramer

13th Oct 2013, 12:50

If I used 0.77 to climb up there and then accelerated to 0.79 I would be 400ft above where one of those limits kicked in. Which limit am I not complying with? Who knows
Finally! :E
I know: you would be maneuver margin limited, but by the high speed band. I actually did exactly what you did to comfort my FO that we had plenty of margin (some are worried, because other captains tells them they alway need 600 feet or more to climb, I try to tell them exactly what we have been discussing now for 109 posts). I selected cost index back to 4 and the FMC showed 376 as max. With 55 it was the 373, that is show in the pictures (actually 372 before the climb). The highest max you get with ECON cost index 0. Had I put in .81 as speed, it would probably have shown max as 350ish. So now: Do you still think I was thrust limited?

Am I cheating? I quote myself from post 54:
Every decision I make in the cockpit is a conscious, considered decision, based on the facts at hand... Not an automated decision based on old wife's tales or "that I always do/need so".

The FMC is just a stupid computer. It has no idea what you are really going to do. Garbage in = garbage out. That's why I object to people saying "I always want +600/800/1000 feet to max altitude. If I have a 10 knots window between the lower and upper amber band I am happy, because it is more than enough to absorb natural wind variations in smooth air (flying at the top of the window and including the amber high speed band, since this is not really a problem to use this range to absorb speed variations too). I really don't care what the FMC is telling me max altitude is, that is my point!

So how do I know if I have 10 knot or not? Simple, use the FMC as a tool instead of being tooled by the FMC:

Hit ALT HOLD, put in desired level in the FMC and execute. Put in 200 knots. FMC now tells you e.g. MIN SPEED 242 KNOTS. Put in 300 knots. FMC now tells you e.g. MAX SPEED 256 KNOTS. So in this example I know the range between low and high speed buffet margin (with 40 degs bank) is 14 knots, completely independent from what I will actually select as cruise speed when I get to that level.

If the margin is around 10 knots or more and it is desirable to climb, I climb :ok:

If you were to fly an old machine with conventional instruments and no speed tape, would you be comfortable climbing if I told you your stall speed was 200 knots, your MMO was 270 and you would have enough thrust to keep you flying? Probably...

I maintain that at 240kts it would have been less than the 15 degrees required.
The holding speed is calculated that you are actually able to do turns in the holding. So I don't think so. Even in the unlikely event that you are in a turn at the very same time that the wind is changing (which would maybe exceed you thrust available, depending on the strength of the sheer), nothing prevents you from hitting HDG SEL, level the wings and accelerate back to acceptable speed before completing the turn. Remember we agreed already that we have no intentions flying at this speed, because it is ineffective - so when I started the turn I would have had .76-.79, if the speed starts to drop back, and keeps dropping even with MCT, there is still the option of delaying the turn.

This is again what I mean, there are so many things you can do, and so many margins on margins that adding your own ones on top makes no sense, when it is not justified. I quote myself from post 68:

I have no problem adding margins to margins, when it is justified. I happily bring 2 tons extra fuel if there is a reason to do so. But I also fly with min required if there is no reason to bring extra. What I do not agree with is people saying "I don't fly with less than 800 kg extra", "I always want to see +800 feet on the max altitude before I climb" etc. I have only a big WHY? to say to that.
If I have, or expect, turbulence I might want to see a 20+ knots window between the low and upper amber band.

And like latetonite, I select CON when I need to. And I do so routinely due to the way I operate the aircraft (de facto's objection against routinely, was what got me involved in this thread, although it is completely in line with the recommendations from Boeing). In fact, I always select CON with a <15 knots window.

And that was this topic all about.

Hobo

13th Oct 2013, 13:54

C-K, ref you comment on my post 56, Hobo, are you referring to 737 classic? If so, then I agree. It is quite uncomfortable if going into the high speed buffet band,

Yes, the classic. It was also very easy to get aileron flutter at just on (or over) the upper limit with any bank - which is another reason I never went there.

framer

14th Oct 2013, 06:02

I think that you are correct Cosmo.
I had another look at your photos and a read of the books and I now concede that you were most likely limited by the high speed buffet margin. My mistake was in thinking that if you were, your selected speed would have been coincident with the bottom of the upper amber band which it probably would have been if you had climbed the extra 300ft to max. My apologies.
A question for you for my own learning;
If you were at max altitude and the selected speed was below the upper amber band, how would you know which of the three thrust based limitations were ' active' so to speak?
The holding speed is calculated that you are actually able to do turns in the holding. can you provide a reference for that because the JTPM written by Boeing has lead me to believe that BEST SPEED is nothing more than the speed at which the engines would have the minimum rate of fuel flow and can be a speed well below min drag. Seeing as Max alt is sometimes limited by the ability of the engines to power you through a 15 degree banked turn I can't see how that speed would allow 15 degrees with the extra drag of being below min drag.
And like latetonite, I select CON when I need to. And I do so routinely due to the way I operate the aircraft (de facto's objection against routinely, was what got me involved in this thread, although it is completely in line with the recommendations from Boeing).
I have no argument with this practice but can see why pilots coming off other Boeings might, in some Max Con is only to be used in emergencies. If the Climb N1 and the Max Con N1 are the same there is no restriction and that applies to the NG.

Derfred

14th Oct 2013, 13:09

How can you have min fuel flow at a speed below min drag?

s_bakmeijer

14th Oct 2013, 14:01

thats the md80.
cant fly that high in imc where youll be using the eai.
performance penalty is significant.
drop 2000' or even 4000' until clear.

HazelNuts39

14th Oct 2013, 14:39

Derfred,

because TSFC increases with speed (explained in Boeing JTPM).

cosmo kramer

16th Oct 2013, 10:42

If you were at max altitude and the selected speed was below the upper amber band, how would you know which of the three thrust based limitations were ' active' so to speak?
I skipped over the limits you posted in your last post, because they didn't make sense to me. I thought we agreed long ago, that we are e.g. not service ceiling limited at FMC max altitude. I know of only 2 limit: 100fpm at LRC speed and 300 fpm during climb.

can you provide a reference for that because the JTPM written by Boeing has lead me to believe that BEST SPEED is nothing more than the speed at which the engines would have the minimum rate of fuel flow and can be a speed well below min drag.

Only from the FMC User Guide. I downloaded the JTPM, and it is a lot of general observations and theory. I would be careful to apply that as a manual for a specific aircraft model. The FMC User Guide, although being unofficial, deals specifically with 737. Further more, there are no warnings in any of the official documents, that the FMC suggested holding speed may be below thrust limit to actually do a hold. 1) Would it make sense to let the FMC suggest such speed? 2) Wouldn't Boeing warn about it if it did?

Seeing as Max alt is sometimes limited by the ability of the engines to power you through a 15 degree banked turn I can't see how that speed would allow 15 degrees with the extra drag of being below min drag.

I assume that the suggested holding speed would increase to Vmd if that was the case. I will play with the gross weight on the perf init page on the next flight to have a look, but don't know if this speed is entry or sensor based. As I said some posts back, this speed never interested me, as I never hold at high altitude (European ATC apparently better than Australian :)).

I have no argument with this practice but can see why pilots coming off other Boeings might, in some Max Con is only to be used in emergencies. If the Climb N1 and the Max Con N1 are the same there is no restriction and that applies to the NG.
In Boeing CON and CLB are not the same at low altitude. Say at FL200 you may have 95% CLB and 100% CON. I use CON too, if asked if I can make an altitude restriction. Why should MCT be limited to emergencies, for ANY aircraft?
MCT = Max Continuous Thrust. You can fly continuously all day with that setting if you like. Max takeoff thrust is more of an "emergency" thrust setting, as you are limited to 5 or 10 mins.

cosmo kramer

16th Oct 2013, 11:09

How can you have min fuel flow at a speed below min drag?
Derfred, drag and thrust are forces acting on the aircraft. A force does not say anything about what happens over time. Fuel flow is measure in unit of mass pr. time unit. It's a rate.

For that reason the lowest fuel flow does not occur at the min drag/thrust. It occurs at the lowest power required.

Power = force x (distance/time) = force x speed = Drag x TAS.

At min drag speed (Max Range), your True Airspeed is higher than at min power required (Max Endurance). Hence:

Lower TAS x higher drag < Higher TAS x lower drag

You are fighting a higher force, but you are doing it at a slower rate. Rate of fuel flow is therefore lower. You are not getting very far (lower speed = lower distance covered over time), great for holding, but not so great for cruise.

Hope this helps refreshing what is basic (European) ATPL knowledge. It has nothing to do with TSFC - that only modifies the curves a bit, but the basic physics remain the foundation.

I give again the link I gave before, which show the basic curves for power and thrust required. And yes basic physics applies to all aircrafts, no matter if they have propellers, jet engines, straight or swept wings, obviously the real curves may not be so nice and linear because we do not live in a perfect world, anyway:
Maximum Endurance, Maximum Range, and Optimum Cruise Speeds (http://www.eaa1000.av.org/technicl/perfspds/perfspds.htm#Figure_1)