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Reducing thrust in cruise for overspeed

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Reducing thrust in cruise for overspeed

Old 19th May 2014, 16:18
  #61 (permalink)  
 
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but I do not recognize the dramatic spool up figures presented in this thread.
Probably because ALT* gives at least 20 seconds of spool up time before ALT is annunciated.
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Old 19th May 2014, 16:19
  #62 (permalink)  
 
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PENKO
I was wondering why did you quote idle factor, even DES mode gives same idle unless it is flying repressurisation or geometric segment. I have quoted an official over speed procedure from FCOM. I am surprised people want to device their own procedures in FBW aircraft without referring it to AI.
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Old 19th May 2014, 16:42
  #63 (permalink)  
 
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So if that idle figure is safe for minor level changes, why is it not ok (according to the opening post) to close the thrust levers during an overspeed situation?

Again, I would be very careful with reducing thrust in high altitude level flight, but I do not recognize the dramatic spool up figures presented in this thread. Planes would fall out of the sky if things were that bad!
For Boeing, I can assure you that the spool up time is in the order of 40 secs plus. The EEC changes the spool up time depending on various circumstance:

Analysis of these reports indicates that this is related to the Electronic Engine Control (EEC) software revisions 7.B.U1 and 7.B.V2. Revision 7.B.U1 reduced the engine deceleration and acceleration rates for specific thrust lever transients at high altitude. Revision 7.B.V2 returned the deceleration rate back to normal but retained the reduced acceleration rate. Both versions of the EEC software reduce engine acceleration rate if the throttles are in idle for less than 60 seconds. Slowing the acceleration rate of a heat soaked engine improves engine operability (stall margin) characteristics.
I would advice against using LVL CHG to descent from FL390 to FL370, as you mentioned as an example. Alone from the fact, that it would cause a high rate of descent which may cause nuisance TCAS alerts. If doing so anyway, the ADFS would anticipate the required time to spool up the engines, to level off on speed.
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Old 19th May 2014, 16:49
  #64 (permalink)  
 
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And as for using speed brakes (not just for overspeed, but also undesired speed increases inside the operating limits), Boeing writes:

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.
I don't, in anyway, share the opinion of Gretchenfrage. There is no problems using speed brakes during cruise what so ever (of course while keeping the hand on the level and the eyes on the airspeed indicator).
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Old 19th May 2014, 16:55
  #65 (permalink)  
 
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Originally Posted by vilas View Post
sonicbum
What happens to Idle factor in open descent?
If you read my previous post carefully, you will get your answer.
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Old 19th May 2014, 19:46
  #66 (permalink)  
 
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Vilas, I did not introduce idle factor in this discussion

Probably because ALT* gives at least 20 seconds of spool up time before ALT is annunciated.
Golden Rivet, what about when you are flying manually and leveling off: Do you really anticipate a twenty+ second spool up?
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Old 19th May 2014, 20:02
  #67 (permalink)  
 
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PENKO, my answer in post 63 was for you. If in manual flight (Boeing) and not spooling up the engines EARLY, when leveling off (and with less than 60 seconds idle time) - you are setting yourself up for troubles, i.e. a serious under speed condition.

If you e.g. keep the throttles closed and aggressively pitch up to stop the descent (passengers and flight attendant won't be happy). and THEN reapply thrust. AFDS doesn't do it that way. It goes into ALT ACQ and slowly reapply thrust and slowly increase pitch, so that everything comes together simultaneously (holding altitude, having correct speed and thrust).

If you are lucky (as most times during the descent you are light), you will still have a big speed range to absorb the under speed, if you have to descent for traffic while still heavy, you are worst case setting yourself up for a stall recovery or and embarrassing request to continue the descent.
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Old 19th May 2014, 20:23
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I fly the NG on some of the longest sectors operated by any Boeing NG operator in the world DBX/OSL & TOS/LPA/TFS we routinely leave the ramp at 79.2T (MTOM 78999) high altitude trips down to the Canary isle will often cross multiple significant changes in OAT/wind direction and we need to get high to get the endurance. My routine is bank angle selector 10/15 and MAX CON on limit page.

I'm not in the least concerned about filing a report for an exceeding a limit, but I'm very nervous about getting slow the FMC generated fast/slow limits are all based the aircraft mass being the 'actual' aircraft mass we often use a mix of charter weights/actual baggage weights/standard baggage weights, we sell an allowance of 20kg bag weight yet use an average of 13kg so a 100 bags could put your actual 700kg above your paper weight, for this reason i will not go above the FMC generated OPT flight level unless I'm very sure the wind and temp are stable and i always aim to be at least 1000' below the FMC generated MAX flight level.

I have only once seen the aircraft come back to the top of the slow box following a temp change and even with 100% we could not get away from slow box and had to descend.

The auto throttle logic on the NG is very slow to reduce thrust, much quicker to add thrust and with rapid speed increase due to wind/temp will likely result in an incursion into the barbers pole zone, there is ,if i can find it a Boeing bulletin on this subject with a warning that thrust recovery can be very slow when slowed to F/I at high altitude. It will be interesting to see how the B738MAX fares in this regard given my companies intent to use them on 8 hour + sectors across the ITCZ !!!

Small level changes will not normally result in flight idle thrust, in any event the aircraft pitch will pitch up as it starts to acquire the new altitude and increase thrust so by the time its back in level flight thrust will be at required

NSB 38 covers this subject, in short if thrust is reduced to F/I for less than 60 sec then the EEC software reduces the acceleration rate of heat soaked engines to improve stall margins, guard the thrust levers from reducing below 60% below this figure spool up time will be much longer with a possible speed loss above 20-30 knots, this only applies above FL300, if the thrust levers have been at F/I for longer than 60 sec then acceleration of the engines will be normal

Last edited by LNIDA; 19th May 2014 at 20:59. Reason: Adding NSB 38 info
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Old 19th May 2014, 20:34
  #69 (permalink)  
 
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LNIDA, so I guess this would make you uncomfortable?

I took those pics for this thread, where MAX ALT was discussed to death:
http://www.pprune.org/tech-log/524297-mct-cruise-5.html

I fly 737-800 (26k engines) at MTOW too (long distance and/or often takering sectors fueled to MTOW).


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Old 19th May 2014, 21:53
  #70 (permalink)  
 
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uncomfortable

Been there done that

But as i said it all depends on the day and as i also said i'm less concerned about the overspeed condition, but anyway why push it? you might have someone in the other seat with little or no high altitude experience, so your sat there with a 9 knot margin and you take a toilet break, next minute chummy gets overspeed and out go the speed brakes and back comes the thrust, probably something he/she as never seen before, meanwhile your stood by the door (wrong side of) awaiting chummy to follow the correct procedure to let you back in.

Having looked at the CRZ/thrust/weight tables either our brand new aircraft are heavier than we think or have more drag than Boeing thinks

If you were flying above the FMC MAX ALT in our company and caught at it you'd be off the line, these limits & restrictions are there for the safety of the passengers and crew, in any event flying above OPT is normally counter productive unless your on a speed limit due slower traffic ahead at your OPT level

So never mind me being uncomfortable, why make your colleague feel uncomfortable? CRM is all about team work,with me i think you'd be flying single crew on the next sector, although it sounds like you fly single on all of them
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Old 20th May 2014, 00:16
  #71 (permalink)  
 
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So never mind me being uncomfortable, why make your colleague feel uncomfortable? CRM is all about team work,with me i think you'd be flying single crew on the next sector, although it sounds like you fly single on all of them
Why get personal. I didn't ask you if you are the type that always brings a ton extra fuel "just to be sure", although one might get the impression from "i always aim to be at least 1000' below the FMC generated MAX flight level"... Where did you get this arbitrary 1000' from?

in any event flying above OPT is normally counter productive
Not more counter productive than flying below. And there may be environmental benefits as well, that speak for choosing a level different from optimum, like more favorable wind etc.

It's really off-topic, but I just can't stand such unfounded personal margins. If Boeing wanted you to always operate at max alt minus 1000 feet, and deemed it unsafe to go higher, they would simply have programmed the FMC to show max alt 1000' lower.
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Old 20th May 2014, 07:35
  #72 (permalink)  
 
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Is it not the case that Max Alt is affected by the CRZ C of G% in the FMC. The default is 5% and is the worse case. The actual MAC will tend to be very much more. Inserting the correct number will raise the Max Alt displayed and that is the real aerodynamic Max Alt. Some companies allow the actual to be inserted, others maintain the default 5% for a buffer margin.
Further, is it still the case where FAA & JAA had different 'g' margins at CRZ levels? I think it was the case where FAA allowed 1.3g buffer & JAA wanted 1.4 or 1.5g. Correct if wrong.
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Old 20th May 2014, 09:39
  #73 (permalink)  
 
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Hi Rat 5,
Inserting the correct number will raise the Max Alt displayed and that is the real aerodynamic Max Alt.
You should have the correct number inserted into the FMC from load and balance calculation.

On a previous type we were permitted to reduce the buffet margin from 1.3 (max 30 degs bank) to 1.2 (Max 15 degs bank) provided there was no turbulence forecast ahead. By using fuel distribution graphs, we could insert a new cruise C of G figure which then displayed a new MAX FL.
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Old 20th May 2014, 09:55
  #74 (permalink)  
 
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Cosmo not personal at all, but i spend a lot of time training pilots to adhere to company procedures and along comes someone who makes it up as they go along, there is no doubt your knowledge is deeper than average and to answer your other question i take the fuel required no more no less, the required being what i need to operate the flight safely sometimes that is plog fuel sometimes more and sometimes less (our plots use longest SID/STAR)

If the FMC entered mass is correct i.e. actual then the FMC will normally provide you with the most efficient CRZ level taking into account fuel burn, maintenance, & so on, our company guidance is that a 1000 below OPT is normal preferable than a 1000 above and we have had 2 high level upsets.

My point about CRM is simply that every time an F/o comes along and says "well I was flying with Capt Kirk and he said the FMC figures are guesses/rubbish/tool/just a guide" and guess what its the same names that keep cropping up, anyway enough drift

We are all agreed that reducing thrust at high FL to idle is not good and that gentle application of speed brake is what the manufacture recommends to address transient unintentional over speeds

Last edited by LNIDA; 20th May 2014 at 13:51.
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Old 20th May 2014, 10:39
  #75 (permalink)  
 
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It's really off-topic, but I just can't stand such unfounded personal margins.
No offence to Linda because my agreement with Cosmo does not reflect on his protagonist, but Where does lack of confidence in one's abilities or the aircraft's capabilities end once on this rocky road?
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Old 20th May 2014, 11:21
  #76 (permalink)  
 
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Unless I'm missing something kida-really big, the take home is to make at-altitude power adjustments slowly and gently, even when planning to descend. Cannot think of a good reason to chop from 93% to idle in one pull. If one needs to descend rapidly, point the SOB down, use speed brakes and reduce power, but gently. Even if one needs to head down quickly, i.e. pressure loss, the last thing you want is loss of one or more pulling gizmos! do it gently, use the speed brake and point it down. It it is still flying, maintain what you've got, slow a bit, point down and consider your options. Except in an RTO situation a zero AGL, I cannot imagine a good reason to chop from high to idle in two seconds. Again, am I missing something?
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Old 20th May 2014, 11:32
  #77 (permalink)  
 
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My point about CRM is simply that every time an F/o comes along and says "well I was flying with Capt Kirk and he said the FMC figures are guesses/rubbish/tool/just a guide" and guess what its the same names that keep cropping up, anyway enough drift
And you know what? I have the exact same problem! When I ask an F/O to request FL390, I have to go into a long explanation why it isn't necessary to ALWAYS have max alt plus 1000', 600' or at the very least 400', before initiating a climb, and explain that Capt Lnida's personal limits have nothing to do with Boeing procedures.

Except for the barber pole and the minimum speed (which is taken from air data), Capt "Kirk" is right (isn't Kirk always right by the way? ). Everything the FMC puts out are best guesses. You see my margin as 9 knots. I see my margin (for the VERY worst case) as 203 -267 = 64 knots (of course fluid with the load factor induced on the aircraft by turbulence).

With "VERY worst case, I am talking about the extremely unlikely event (that will happen 2 times in a career), where due to a strong sheer I end up in deep in the yellow and unable to pull out. To clarify, I never allow the speed to get into the yellow band.

But those few times in a career where it might happen: I will rather say "xxx, descending, unable to maintain altitude, call you leveling off".
...and operate the rest of my thousands of flights efficiently.

I assume you agree that the minimum maneuver speed yellow band, isn't aerodynamically dangerous per se, but when going in there it's just a reduction of your margins? I also assume, that your real concern is rather lack of sufficient thrust? I also assume (given the description of your ops) that you have 26k or 27k engines. Those factors together Boeing addresses with the following:
FCTM:
On airplanes with higher thrust engines, the altitude selection is most likely limited by maneuver margin to initial buffet.
You are founding your personal limits by ONE bad experience. Which wasn't even that bad: You just descended and regained speed and probably climbed back up a little bit later? Sure, you had to take a break from reading your newspaper and your adrenalin level might increased a bit, but was this a near disaster?

It's a bit like someone experiencing a decompression refusing to fly above FL100 again, because he never want to have to do another emergency descend.
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Old 20th May 2014, 13:49
  #78 (permalink)  
 
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Cosmo

Yep you got it in one thrust limited, the problem with your approach is that when do you say that's high enough?? so 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.

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 other than WoW look at me I'm at FL410 and if your sat there at 100% to stay out of the yellow box you'll be burning more of YOUR companies fuel than i will be a few 1000ft below.

Lets leave it at that and agree to disagree.
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Old 20th May 2014, 14:37
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what about when you are flying manually and leveling off: Do you really anticipate a twenty+ second spool up?
PENKO, I anticipate a much longer spool up time. I would not be descending at 3,000 ft per min with 1,000 feet to go. I'd be aiming for around 1500 ft per min or less which would give me at least 40 seconds to level off.

What sort of RODs to you have with 1,000 ft to go?
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Old 20th May 2014, 16:59
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Goldenrivet, with or without the AP? I have no set numbers in mind for ROD. Since we are not allowed to fly manually anyway in RVSM.
Don't get me wrong, I do appreciate that the spool up time is much higher, I just doubt the dramatic numbers stated in the beginning of this thread.

Let's put it differently. Occasionally I've had to arrest the descent instantaneously on request by ATC I can't remember having had any significant troubles with the spool up time vs. level off... but hey, I'll keep my eyes open next time!

Last edited by PENKO; 21st May 2014 at 04:20.
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