Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Reducing thrust in cruise for overspeed

Tech Log The very best in practical technical discussion on the web

Reducing thrust in cruise for overspeed

Old 20th May 2014, 22:07
  #81 (permalink)  
 
Join Date: Sep 2008
Location: 41S174E
Age: 52
Posts: 2,767
I just doubt the dramatic numbers stated in the beginning of this thread.
It is over 40 seconds for a CFM to spool up from idle at FL 370 with ISA conditions. I've timed it.
framer is offline  
Old 20th May 2014, 22:24
  #82 (permalink)  
 
Join Date: Jan 2008
Location: farmm intersection, our ranch
Age: 53
Posts: 209
It is over 40 seconds for a CFM to spool up from idle at FL 370 with ISA conditions. I've timed it.
100% correct, longest 40 seconds I have experienced.
flyingchanges is offline  
Old 20th May 2014, 23:08
  #83 (permalink)  
 
Join Date: Jul 2013
Location: Everett, WA
Age: 64
Posts: 2,457
It is over 40 seconds for a CFM to spool up from idle at FL 370 with ISA conditions. I've timed it.
A little good news, my counterpart on the 737NG tells me that 7.B.W EEC s/w will improve the accel characteristics at altitude. Should certify this summer.
tdracer is offline  
Old 20th May 2014, 23:56
  #84 (permalink)  
 
Join Date: Jul 2001
Location: East of West and North of South
Posts: 549
LNIDA:
It just don't get your point here, my margin is not a random personal all I was saying is that if forecast shear and large/rapid temp changes then i feel a little caution is safer than pushing all the way up for often no benefit
Yes caution is in order if in actual turbulence or if it's forecasted ahead.

But, NO, that was not what you were saying before:
LNIDA:
i ALWAYS aim to be at least 1000' below the FMC generated MAX flight level.
But, I am happy if the discussion changed your mind a bit.

LNIDA: FMC MAX ALT 384 you happy with 390 to CRZ after all what the FMC says is just a computers best "guess" based on all of the parameter YOU have entered plus the inputs from the AD
I play with the computer instead of letting it play with me:
http://www.pprune.org/tech-log/52429...ml#post8096769 (post 108 if the direct link doesn't work).
If you don't care to read it, let me ask you: What factors determines the MAX ALT? Which one can we be certain of? And which ones are uncertain? Which ones are factual? And how do they individually affect your max alt?
I can think of one example where I would have no problem climbing to FL390 with max FL384 (or lower) show... It's in the post that I linked to.

Last edited by cosmo kramer; 21st May 2014 at 00:23.
cosmo kramer is offline  
Old 21st May 2014, 08:21
  #85 (permalink)  
 
Join Date: May 2013
Location: London
Posts: 112
Cosmo,
I can think of one example where I would have no problem climbing to FL390 with max FL384 (or lower) show... It's in the post that I linked to.
Do you have a procedure to modify the cruise CG? We don't unfortunately, but when it is changed from the default 15% to probably a much more realistic 19-22% the max alt 'magically' increases significantly

Sorry about this, but from that other thread:
...I select CON when I need to. And I do so routinely due to the way I operate the aircraft...In fact, I always select CON with a <15 knots window.
However, I do not set CON routinely, only when I suspect it might actually be needed inline with the FCTM and the recent Bulletin. I don't see any specific issues with CON mind, just CRZ is the intended thrust limit for cruise and I prefer to operate these strategies with specific criteria in mind rather than get into a routine that is, strictly speaking, a deviation - however trivial or slight (or non-existent in impact) that deviation may be.
Kefuddle is offline  
Old 21st May 2014, 09:43
  #86 (permalink)  
 
Join Date: Dec 2012
Location: Gatwick
Posts: 452
Cosmo

My whole point about crz level was on very heavy aircraft crossing temp/wind zones and the sudden narrowing of the overspeed/under speed margin.

Our EFB provides us with a very accurate MAC because we use allocated seating and know where the passengers are seated re zone a,b,& c we subtract 4% from the MAC for FMC perf page and can re calculate inflight after fuel burn off if necessary

This from Boeing FCTM
Maximun Altitude

Maximun altitude at which the aircraft can be operated, It is determined by 3 basic characteristics (for each model) The FMC max altitude is the lowest of

. max certified altitude

.thrust limited altitude - the altitude at which sufficient thrust is available to provide a specific minimum rate of climb (depending on the thrust rating of the engines), the thrust limited altitude may be above or below the manoeuvre capability

. buffet or manoeuvre limited altitude

Although each of these limits are checked by the FMC , available thrust may limit the ability to accomplish anything other than relatively minor manoeuvring. The amber band limits do not provide an indication of manoeuvre capability as limited by thrust.


NOTE to get the most accurate altitude limits from the FMC ensure airplane weight, CRZ CG and temp entries are correct

You provide some interesting and thought provoking comments and i always welcome that from anyone, but you are in my humble opinion acting in a manner that at best will confuse and more likely intimidate your co-pilot, i would be very interested to know if your company procedures allow you to do this and exceeding the FMC derived maximum altitude its is done with blessing of your pilot/training management? the fact that the aircraft may be able to achieve it secondary to the limitations and what do you suppose would happen to you and your co following an high altitude upset where upon the FDM showed that your were above the max altitude? the reason for the upset is not relevant but if there were injuries arising from the recovery then i suspect the lawyers would have a field day, so why Capt Kirk did you think this was a good idea?

Don't get me wrong, i don't doubt that you are a committed and experienced Capt but the fact that your F/o's are having to be re educated by you on the subject of high altitude flights suggests either your training department is at error or dare i suggest it without offending you that perhaps you should reflect upon your own style of operation

Either way fly safe 41'000 is for biz jets or big aeroplanes with big donks

PS the next update of the FMC will provide a thrust limited altitude for manoeuvre
LNIDA is offline  
Old 21st May 2014, 11:34
  #87 (permalink)  
 
Join Date: Apr 2003
Location: Europe
Posts: 2,974
It is over 40 seconds for a CFM to spool up from idle at FL 370 with ISA conditions. I've timed it.
It must be a Boeing thing then. At FL320 on the Airbus it takes more or less 15 seconds. Reducing speed at FL380 from M .78 to .72 commands idle thrust...and the thrust comes back on decelerating through .74
PENKO is offline  
Old 21st May 2014, 13:27
  #88 (permalink)  
 
Join Date: Jul 2001
Location: East of West and North of South
Posts: 549
LNIDA, no I wouldn't go above max alt, as displayed by the FMC.

What I was trying to get at, was which input the FMC uses to decide on these limits. One that is oft forgotten is the selected speed. The FMC doesn't calculate the max altitude, based on the most optimum speed for that level. It uses the speed you have already select (or cost index you entered), assuming you will want to continue with this speed at the next level.

If this speed is very high or very low, the FMC will show you your max alt lower, than what the aircraft is capable of, if you change your speed. Hence, I can get the FMC to SHOW a very low max altitude, but still climb safely above it. Of course I would change the input to show something sensible before actually initiating the climb.

THIS is what my point is, an extra xxx feet becomes arbitrary, if you don't consider what input is behind it. My only concern is instead this:

"How big is my speed window in the new level." In smooth air my personal limit is 10 knots between the low and high speed maneuver band. In turbulent air, I might want to see 30 or 40 knot, which may be an even bigger margin than your 1000 feet. The difference is, I know what I am getting, and you are not.

By using the FMC, like I described in the other thread in post 108, you will always know exactly how big your speed window is, regardless of other factors. Still of course your weight, cg and temp needs to be fairly accurate.

If you are bored in cruse on you next flight, play around with the values to see how much e.g. a ton extra ZFW, moving the CG 5 units etc. affects your margins. If you suspect the aircraft to be heavier, than the values on the load sheet, you will the have a feel for how many knots that actually change on your margins.

A good eyeopener for a new F/O is to increase the ZFW with 10 tons. At first they see the and unbroken yellow band from minimum speed to MMO, they become a little wary. But after a few seconds, when they see the aircraft doesn't care and happily keeps flying, they become relaxed and and realize that what the computer says doesn't aerodynamically affect the aircraft. Garbage in garbage out. It's still just a stupid computer and the reality of nature determines if the aircraft will fly or not. And no, I don't don't do it to bother them. But to increase their confidence in what they see and feel is more real that what the computer calculates.

PS the next update of the FMC will provide a thrust limited altitude for manoeuvre
The FMC already consider thrust limited altitude based on a residual rate of climb of 300 fpm in the climb and 100 fpm at cruise using LRC tables.

Last edited by cosmo kramer; 21st May 2014 at 13:43.
cosmo kramer is offline  
Old 21st May 2014, 15:33
  #89 (permalink)  
 
Join Date: Dec 2012
Location: Gatwick
Posts: 452
I think your speed spread dependant upon conditions is entirely reasonable and i have also played around with the ZFM and as you say it is interesting to see the yellow bands close.

I want the FMC input data to be as accurate as is reasonably possible.

I am aware of an aircraft that departed with the correct v speeds from an EFB but a -10000kg figure in the FMC which then told them they could go straight to FL410 needless to say despite trying they didn't get there oops

The FMC does consider thrust limited altitude, but the amber band limits do not provide an indication of manoeuvre capability as limited by thrust. The next software release will correct that.

My last comment on the subject.....promise
LNIDA is offline  
Old 21st May 2014, 21:36
  #90 (permalink)  
 
Join Date: Jul 2001
Location: East of West and North of South
Posts: 549
My last comment on the subject.....promise
Thanks for the discussion, I enjoyed it.
cosmo kramer is offline  
Old 22nd May 2014, 22:05
  #91 (permalink)  
 
Join Date: Jan 1999
Location: Gatters.......
Posts: 1,308
"I'd rather write up an exceedance than use speedbrakes in cruise."
Could not agree more.
Have seen quite a few overspeed-in-the-cruise-reports for the A320 over the years, and people intervening - reducing thrust, speedbrake, lifting nose etc. almost invariably make the situation worse.
The A/C is a completely different beast at cruise level, and people are not used to it.
As earlier mentioned no inspection is required if exceedance is less than 20 kts (and I have never seen one higher) , so let the A/P+A/T sort itself out, and write the ASR........
OSCAR YANKEE is offline  
Old 22nd May 2014, 22:12
  #92 (permalink)  
 
Join Date: Jul 2001
Location: East of West and North of South
Posts: 549
As earlier mentioned no inspection is required if exceedance is less than 20 kts
Of curiosity, do you need to make a tech log entry? And if so, can you sign it out yourself.

On Boeing it's a mandatory tech log entry, and only a certified maintenance technician can sign it off. Hence, an AOG item when flying to a remote airport with no maintenance.

P.s. On Boeing it's a non-issue to use speed brakes.
cosmo kramer is offline  
Old 24th May 2014, 05:41
  #93 (permalink)  
 
Join Date: Jun 2007
Location: Wanderlust
Posts: 2,332
OSCAR YANKEE
The autopilot ATHR may not sort it out itself and could lead to more complications. Less than 20KT no inspection is required but crew must report it. The procedure mentioned in my earlier post is given in FCOM:


OVERSPEED PREVENTION

Ident.: PRO-ABN-10-00014874.0001001 / 08 FEB 13

Applicable to: ALL

A318/A319/A320/A321 FLEET PRO-ABN-10 P 11/20

FCOM ← D to E 06 MAR 14
OVERSPEED RECOVERY

Ident.: PRO-ABN-10-00014875.0001001 / 08 FEB 13
Applicable to: ALL
FCOM page F

QRH only gives the recovery procedure.
Abnormal emergency procedures ABN80.06A
vilas is offline  
Old 24th May 2014, 10:24
  #94 (permalink)  
 
Join Date: Jun 2000
Location: last time I looked I was still here.
Posts: 4,510
I want the FMC input data to be as accurate as is reasonably possible.

I suggest it is time to consider increasing the, IMHO, rather ludicrous low ICAO standard pax/baggage weights. It could well be that on a full 189 seater a/c you could be 2 tonnes heavier than the load sheet tells you. If all male, full baggage at 20kgs, it will be much more.

Regarding the CRZ MAC, it seems some companies allow you to insert the actual number, others forbid it and stay with the default 5% (B737NG) for a safety buffer. Going up to absolute max alt and reducing to 1.2g if no turbulence is forecast is really taking TEM management to the limits. The forecasts are made hours in advance by computer models and differ depending on the meteo centre who made them. Their accuracy is unreliable. I say this because I've been through areas of forecast turbulence with nothing, not even a cough. If that is the case then the forecast of no turbulence must also be questionable; although the met forecasts in all cases e.g TAF's etc. seem to be always pessimistic. I thought we were in the risk avoidance business.
RAT 5 is offline  
Old 24th May 2014, 12:50
  #95 (permalink)  
 
Join Date: Jan 1999
Location: Gatters.......
Posts: 1,308
@vilas

I am well aware of the advice in the FCOM.

My opinion is strictly personal, but is based on having read the reports, and seen the flight data from those reports where people have used speedbrake, idle thrust, lifting the nose etc.

Airbus has to give advice like this, because they cover any overspeed.

I dont think my company has seen any excursions higher than 6 knots in the cruise, and the observations were quite clear.
People exacerbated the situation by intervening, and you were left with the sensation that they would have been better off just leaving the A/P+A/T to sort itself out.
(However you cannot know of course, it is speculation, albeit qualified.....)
OSCAR YANKEE is offline  
Old 26th May 2014, 05:51
  #96 (permalink)  
 
Join Date: May 2005
Location: middle of nowhere
Posts: 348
My opinion is strictly personal, but is based on having read the reports, and seen the flight data from those reports where people have used speedbrake, idle thrust, lifting the nose etc.
OY, I fully agree.

But we have to realise that we are of a dying species, the one with experience and some common sense left. We are overwhelmed by the new generation who religiously follow the magenta line, the FCOM and SOPS without filter or questions.
Remember the former UAS by Airbus, that was amended after some incidents? It shows that not everything straight from the A/B horse's mouthes is incontestable and of final wisdom.
Gretchenfrage is offline  
Old 26th May 2014, 07:00
  #97 (permalink)  
 
Join Date: Jun 2007
Location: Wanderlust
Posts: 2,332
Gretchenfrage
"Vanity is my favourite sin" Al Pacino (Devil's advocate) You will love this movie. You read my profile and decide for yourself about my generation. However as I have mentioned it before that armed only with FCOM developing procedures on FBW aircraft is asking for trouble. If experience on line differs from a procedure I agree you have to do something to salvage the situation but after that get in touch with the manufacturer. For all the experience on line you do not become a test pilot nor do you get any insight into the computer logics. You may have noticed many things suggested on PPRuNe vary from highly professional to downright dangerous. None of them can replace SOPs.

Last edited by vilas; 26th May 2014 at 07:18.
vilas is offline  
Old 26th May 2014, 07:48
  #98 (permalink)  
 
Join Date: May 2005
Location: middle of nowhere
Posts: 348
vilas

Well, if you throw a stone at a herd of sheep, it's the one who gets hit that yells ....

On another note: I don't quite get you:

One one hand you say ...

However as I have mentioned it before that armed only with FCOM developing procedures on FBW aircraft is asking for trouble
... on what i could agree with you, only to hear you state ...

You may have noticed many things suggested on PPRuNe vary from highly professional to downright dangerous. None of them can replace SOPs
... which puts you right back to the pilots i fear, citing solely FCOM, QRH and ABNs, as in your post below.

Now what is it?

You can recite whatever you want, i will stand firm that on a high speed excursion at high altitude the chances that you will expose an aircraft to a dangerous situation are very much slimmer than falling into a very much more dangerous low speed situation when extending speed-brakes.

Once more: The last decades saw no aircraft crash due to high speed at altitude, but several crashed due to high altitude stalls.

Your choice where common sense lies.
Gretchenfrage is offline  
Old 26th May 2014, 07:57
  #99 (permalink)  
 
Join Date: Jun 2000
Location: last time I looked I was still here.
Posts: 4,510
But we have to realise that we are of a dying species, the one with experience and some common sense left. We are overwhelmed by the new generation who religiously follow the magenta line, the FCOM and SOPS without filter or questions.
Remember the former UAS by Airbus, that was amended after some incidents? It shows that not everything straight from the A/B horse's mouthes is incontestable and of final wisdom.


Slight thread creep, but I have to admit I always taught Boeing incipient stall recovery from stick shaker was, 1st elevator then a split second later add thrust. Guess what it is now. Aerodynamics on a basic aeroplane had never changed. I did, also, demo a stick shaker recovery from low level flight with A/P engaged by just adding power. It was always interesting, so they saw both sides of the coin. It wasn't going to work at medium levels or above, but….
Sadly, now, the syllabus I have to use does not include time for such demos. The knowledge transfer is much less than it was. Time is too squeezed.
RAT 5 is offline  
Old 26th May 2014, 17:57
  #100 (permalink)  
 
Join Date: Jun 2007
Location: Wanderlust
Posts: 2,332
Gretchenfrage
All I am saying is involve the manufacturer. They are in touch with operators across the world and are aware of incidents that happen. They have all the resources. You cannot device something on your own. If there is a problem they are answerable. You cannot replace a procedure listening to someone.
vilas is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.