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MCT at cruise

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MCT at cruise

Old 11th Oct 2013, 12:31
  #101 (permalink)  
 
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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.
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Old 11th Oct 2013, 15:20
  #102 (permalink)  
 
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Pictures for De Facto:
Thanks but i was looking for the picture of your initial fl380 when MAX was FL381( not 383).
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Old 11th Oct 2013, 16:37
  #103 (permalink)  
 
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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.
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Old 11th Oct 2013, 17:16
  #104 (permalink)  
 
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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.
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Old 11th Oct 2013, 18:33
  #105 (permalink)  
 
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Im sure ull make an operation benefit exception for me
Thanks.
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Old 11th Oct 2013, 19:23
  #106 (permalink)  
 
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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.
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Old 13th Oct 2013, 01:33
  #107 (permalink)  
 
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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.
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Old 13th Oct 2013, 12:50
  #108 (permalink)  
 
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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!
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

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.
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Old 13th Oct 2013, 13:54
  #109 (permalink)  
 
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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.
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Old 14th Oct 2013, 06:02
  #110 (permalink)  
 
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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.
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Old 14th Oct 2013, 13:09
  #111 (permalink)  
 
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How can you have min fuel flow at a speed below min drag?
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Old 14th Oct 2013, 14:01
  #112 (permalink)  
 
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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.
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Old 14th Oct 2013, 14:39
  #113 (permalink)  
 
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Derfred,

because TSFC increases with speed (explained in Boeing JTPM).
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Old 16th Oct 2013, 10:42
  #114 (permalink)  
 
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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.
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Old 16th Oct 2013, 11:09
  #115 (permalink)  
 
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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

Last edited by cosmo kramer; 16th Oct 2013 at 11:35.
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Old 16th Oct 2013, 16:22
  #116 (permalink)  
 
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max endurance speed for jets

derfred & cosmo cramer

Have a look at the lower graph in an earlier post.

I give again the link I gave before, which show the basic curves for power and thrust required.
For propeller-driven planes.

Last edited by HazelNuts39; 16th Oct 2013 at 16:25.
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Old 16th Oct 2013, 19:06
  #117 (permalink)  
 
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It doesn't matter if the thrust is created by a propeller or a jet or a turbofan (with the fan being a propeller too). Those graphs, to which I provided a link twice, shows the aerodynamical most efficient speeds.

Why TSFC is relevant is because a jet doesn't run efficiently at the minimum power required. So it's fuel consumption is actually higher at this lower power setting. Hence, the actual speed for max endurance is slightly higher than the aerodynamical one...

A small technicality, that doesn't render the basic principles of physics invalid.

It has nothing to do with TSFC - that only modifies the curves a bit, but the basic physics remain the foundation.
Actually, more accurate - it doesn't modify the curve, but moves the point where the lower fuel consumption takes place a slight bit forward. As nicely demonstrated too, by the graph in the link you posted.

Last edited by cosmo kramer; 16th Oct 2013 at 19:09.
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Old 16th Oct 2013, 21:58
  #118 (permalink)  
 
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cosmo cramer,

Quoting from your source:
Maximum Endurance
(...)
Since the fuel flow is proportional to the power required, the fuel flow will be minimized at the point where the power required is a minimum.
For jet engines it is usually assumed that fuel flow is proportional to the thrust required.

Both are simplifying assumptions, as shown for the PW jet engine in the graphs I referred to.

You are right about two things though: the drag characteristics of a given airframe are not affected by the means of propulsion, and the fuel efficiency of a fan jet engine is somewhere between a straight jet and a turboprop.

Last edited by HazelNuts39; 16th Oct 2013 at 22:22.
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Old 18th Oct 2013, 00:44
  #119 (permalink)  
 
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[Kramer - thread drift, but why don't you think a PAN allows you to deviate from a clearance? PAN = declaration of emergency = do whatever you like]
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Old 18th Oct 2013, 11:57
  #120 (permalink)  
 
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Pan pan is a prefix for an urgent message, nothing more. It was just discussed in this thread too:
http://www.pprune.org/questions/5199...tuation-3.html
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