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Reducing thrust in cruise for overspeed

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Reducing thrust in cruise for overspeed

Old 2nd May 2014, 14:55
  #41 (permalink)  
 
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Originally Posted by Piltdown
At highish levels, like 410, aggresive airbrake usage will easily take an aircraft like an E190 to the other end of the speed tape and beyond, without a significant change in airspeed
I'm not sure I understand that. If you go to the "other side of the speed tape and beyond", your airspeed must be reducing, unless you are referring to the Vmin foot increasing when the boards are extended (as it does in my machine).

I would have thought acceleration rates of all pax jets would be the same (certification requirement). A slam accel from idle to max in an E190 or a 747 should be the same time?
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Old 2nd May 2014, 19:23
  #42 (permalink)  
 
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Hi All
I need to read all posts ,but one is for shure, speed is my friend and overspeed i can live with. Had a 25 kts plus transient clacker overspeed last year over ca 6 seconds in decend ca 30 000 feet never heard a peep.
Stalling after idle on a Cfm 56 will make CNN. 410 feet ca 41 sec pluss....
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Old 2nd May 2014, 22:36
  #43 (permalink)  
 
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Almost Cap'n Bloggs. If the speed brakes are popped at high levels the Vma bar rapidly shoots up to (and if used too agressively, beyond) your current airspeed, without a corresponding reduction in speed. Thus they do little to reduce an overspeed but rapidly give you another one. It's not until you get down to 360 or so you can use them with relative impunity. Therefore, short of a climb (above the certified ceiling) there is little you can do apart other than closing the thrust levers to reduce your airspeed.
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Old 2nd May 2014, 22:56
  #44 (permalink)  
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Originally Posted by Piltdown Man View Post
If the speed brakes are popped at high levels the Vma bar rapidly shoots up to (and if used too agressively, beyond) your current airspeed, without a corresponding reduction in speed. Thus they do little to reduce an overspeed but rapidly give you another one. It's not until you get down to 360 or so you can use them with relative impunity. Therefore, short of a climb (above the certified ceiling) there is little you can do apart other than closing the thrust levers to reduce your airspeed.
So would closing the thrust levers and then adding thrust well before the airspeed comes back down to the target speed make any sense?

Last edited by JammedStab; 4th May 2014 at 03:42.
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Old 17th May 2014, 13:15
  #45 (permalink)  
 
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"Almost Cap'n Bloggs. If the speed brakes are popped at high levels the Vma bar rapidly shoots up to (and if used too agressively, beyond) your current airspeed, without a corresponding reduction in speed. Thus they do little to reduce an overspeed but rapidly give you another one. It's not until you get down to 360 or so you can use them with relative impunity. Therefore, short of a climb (above the certified ceiling) there is little you can do apart other than closing the thrust levers to reduce your airspeed."

Pitdown, I agree with you in relation to SB use. At very high levels it absolutely does not make any sense to use them
But in relation to the thrust levers, why closing them considering the ATHR is engaged?! Why not just monitor the ATHR and see if it is simply doing is job?

What I teach to my students when flying at very high levels is:
1. Don't touch anything (which means, sidestick, thrust levers and SB)
2. Monitor the ATHR (and corresponding thrust reduction)

Last edited by busav8r; 17th May 2014 at 15:43.
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Old 17th May 2014, 15:02
  #46 (permalink)  
 
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If you fly Airbus you will be better off by following Air bus recommended procedures. There is a procedure for over speed prevention and over speed recovery and after recovery. It recommends use of speed brakes. For Prevention


1. Keep AP/ATHR on.
2. Select a lower speed.
3. Monitor speed trend.ar
4. Speed brake as required.
For recovery
1. Keep AP and ATHR
2. Full speed brakes.
3. Monitor idle or set thrust levers to idle.
After recovery
1. speed brakes as required.
2. Select appropriate speed. Keep a margin to VMO/MMO.
if manual recover smoothly and if ATHR is off adjust manually.
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Old 17th May 2014, 15:33
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Vilas, for your info this subject has been on discussion for quite long time now, and AFAIK many companies do not agree with this "new" recommended recovery procedure for the 320F. Airbus knows about this controversial and I believe they are studying it.

In my current company, there is a recommendation to not use the SB at very high levels (>350/360) and even at high levels it is recommended to use it with care.

This situation happened to me before and the final result was not brilliant. The use of full SB to a heavy weight 321 caused a small upset and the airspeed decrease was almost negligible.

Good luck.

Last edited by busav8r; 17th May 2014 at 20:45.
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Old 17th May 2014, 17:38
  #48 (permalink)  
 
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Airbus has put this as a procedures in FCOM on 8th Feb 2013. If the issue was before this then obviously they do not seem to agree and not following it could lead to problem with the authorities or is it after this date? What is A320F?
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Old 17th May 2014, 21:27
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My question re: the above learned comments for my tiny little mind is:


Can you not just Select an Airspeed on the FCU which is lower than the barbers pole and leave it there -


only - do it bit by bit say 5-10knots at a time.?

Last edited by Natstrackalpha; 17th May 2014 at 21:42.
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Old 18th May 2014, 03:47
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I hope some of the contributors remember earlier lessons in meteorology where we all learnt that in wind-shear conditions (and that includes high altitudes!) the exedence to one side will most probably be followed by one to the other side.

Now where would you rather be, too fast or too slow, bearing in mind that you have to report either?

Congratulations, 65% of you answered right, they would rather be on the fast side. I can hardly remember one incident where an airliner crashed having been too fast but two recent and spectacular accidents having been too slow.

With some engines a pull back to idle is not a good idea, on others it is less of a problem, so know your aircraft.

But to use speedbrakes at high altitude is a no go on my deck. For how long did you guys experience an incidental overspeed at altitude and level? Isn't the aircraft somewhat autostable and the increase in drag brings you back in due time?
The Mach shock wave increases drag and reduces lift, the autopilot or the pilot will initially keep the altitude, therefore the speed will come back.
Extending the speed brakes will increase drag and reduce lift even more, the speed might come back more quickly, but the chances that the counter-shear will throw you into a subsequent low speed situation are greatly increased:

You are suddenly in an equally t&b-prone situation, but in a much more dangerous one.
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Old 18th May 2014, 05:37
  #51 (permalink)  
 
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The Airbus procedure I quoted exactly says that. I believe that you should not make your own procedures on FBW because you do not have access to the computer logics. Any problems encountered in line operations must be referred to the manufacturer. After all they have the software, the hardware, wind tunnel and test pilots. They are in touch with the operators on a global scale. Serious issue like recovery from jet upset cannot be decided by consensus. Discussion yes by all means, to share what all happens up there. But solutions must involve the manufacturer.
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Old 18th May 2014, 11:51
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Vilas, I understand your point of view, but I believe this time you are not right. The airbus procedure maybe is adequate for lower levels but definitely not for very high levels. Like I said before, this comes from experience.

This situation cannot be "tested" in a simulator and I can assure you I already tried it in very different sim standards.

What puzzles me why worry using SB if the aircraft is in Normal Law (with all its marvelous protections). If the aircraft suddenly exceeds MMo, for sure this should be something related with a sudden change in OAT and/or wind. If the ATHR is engaged, it should do its job and the airspeed reduction should occur accordingly. If the ATHR it is not engaged, simply reduce thrust to T safe (50/60%).

Can you imagine (try to make an effort) if something happens (like what happen to the AF447) and while the SB are extended the plane suddenly downgrades to Alt Law?

Once again, good luck.

p.s. A320F = A320 family aircraft
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Old 18th May 2014, 12:58
  #53 (permalink)  
 
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While not trying to deny anything that you experienced I want tell you that there was a detailed presentation by Airbus during Flight Operations and Training Seminar of 2013 to discuss events reported by operators such as yours. It discusses the phenomenon in light of some causes, stage of flight and aircraft protections. Then they have explained the procedures. There two different stages. First is prevention when over speed is likely to occur, the next is recovery from actual over speed. In prevention speed brake is as required. Only in recovery from exceedance full speed brake is recommended. One reason for pilot intervention is limited auto pilot authority for passenger comfort. None of this is my suggestion so I am not right or wrong. My other point is about the authority of the procedure because should something seriously go wrong you can be nailed for not following the procedure. Also procedure that could affect safety should only be changed after consultation with the manufacture more so in FBW. I am fully aware simulators only simulate what is known thus far. So it is not going to help.
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Old 18th May 2014, 15:19
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45 seconds spool up time? Ok...
So what happens if you are at FL390 and ATC instructs you to descend to 370, will you never select OPEN DESCENT or LEVEL CHANGE? This will lead to idle thrust, Boeings and Airbuses do this on a daily basis, none of them fall out of the sky after trying to level off due to the thrust not responding.
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Old 19th May 2014, 10:20
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PENKO - Hi. you are right.


The FMGS will have programmed a spool up at that new lower level - and as you can see it would be bang on the numbers.
Also Thr/Idle (if that's the right one . ..) would command, as you know not total idle but thr idle. (a bit of thrust left in)


Airbussy logic prioritises airspeed over most other things, except RoC or RoD if Selected Vertical speed - therefore airspeed for your 2,000 descent is already in the bag in Managed Mode - usually, on that basis.


I think what the others are referring too - slowing a fast one down when in the past much older types have responded interestingly when a) taken out of autopilot and flown manually by the unawares of high speed high level flight


and


b) Sticking airbrakes up at high airspeed at high altitude.


Whats wrong with that? Well- as you already know when you are high your stall speed is higher - you go fast and you can come nearer to Mach crit, especially true of the old fogies like the 707 and the like not because they are Boeing, that great company but because of their old age design like other aircraft of various manufacturers of that time. also - other fun things at high altitude and high speed include consideration of Longitudinal Stability. Cruising high and fast is a time for being very aware and vary cagey - constantly monitoring the flight and staying away from the envelope edges.


Don`t know. Next time you are at Max Recommend consider trying it out.
before you do that - take a look down at the ground - just to see how high you are and then up at the Stratosphere - I am not being funny - I am being, on the ground, safe, with a nice cup of coffee.


I wouldn`t. - and you can ask me why - if you want . . . but rather take a look at this link - its quite cool.


http://www.skybrary.aero/index.php/H...Considerations

Last edited by Natstrackalpha; 19th May 2014 at 11:04. Reason: day off
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Old 19th May 2014, 12:08
  #56 (permalink)  
 
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Why do you think FMGS would not command total idle?
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Old 19th May 2014, 13:09
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I agree that idle is higher in the upper cruise levels than lower down. However, one would think that that solves the problem originally posted in this thread! Right? No longer spool up time because of the higher idle?

Anyway, airmanship, dare I say the word, dictates that you do not fully close the thrust levers when faced with a transient overspeed condition. That would be a gross overreaction (and not SOP on top).
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Old 19th May 2014, 14:06
  #58 (permalink)  
 
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Originally Posted by Natstrackalpha View Post
PENKO - Hi. you are right.

Also Thr/Idle (if that's the right one . ..) would command, as you know not total idle but thr idle. (a bit of thrust left in)

NAT you are confusing between managed descent with Idle factor and open descent. In open descent you aways get thrust idle.
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Old 19th May 2014, 15:35
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What happens to Idle factor in open descent?
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Old 19th May 2014, 15:43
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Regardless of the idle factor, when you pull open descent (or push managed descend when high on profile) you will get the same idle as when you manually chop the thrust levers!

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!
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