aerotech777
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Posts: 71
Joined: Sat Aug 15, 2009 3:53 pm

Thrust Asymmetry

Sat Aug 31, 2013 8:00 pm

Hi,

I read somewhere that using NW steering is not efficient in case of thrust asymmetry after engine failure especially after certain speed and below Vmcg.

a) Is the statement above true?

Other persons mentioned the use of differential braking in case of thrust asymmetry after engine failure.

b) What is the recommended procedure by aircraft manufacturers (Boeing, Airbus, Bombardier...etc.) and/ or airlines/aircraft operators in case of thrust asymmetry after engine failure?

c) Is it possible/not possible to use voluntary thrust asymmetry in flight? Why it is possible/not possible?

d) What are the cases where it is possible (assuming it is possible) to use voluntary thrust asymmetry in flight?

Please explain your answer (please don't answer by only yes or no).

Feedback appreciated.
 
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Starlionblue
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RE: Thrust Asymmetry

Sat Aug 31, 2013 9:38 pm

Quoting aerotech777 (Thread starter):
a) Is the statement above true?

Yes. If you're moving at speed, the nose wheel is "lighter" due to more lift.

Quoting aerotech777 (Thread starter):
b) What is the recommended procedure by aircraft manufacturers (Boeing, Airbus, Bombardier...etc.) and/ or airlines/aircraft operators in case of thrust asymmetry after engine failure?

Don't know about specific Boeing/Airbus/Bombardier recommendations but if you're on the runway below V1, you must brake. In the air it may be beneficial to decrease thrust on the working engine in order to decrease yawing moment.

Quoting aerotech777 (Thread starter):
c) Is it possible/not possible to use voluntary thrust asymmetry in flight? Why it is possible/not possible?

d) What are the cases where it is possible (assuming it is possible) to use voluntary thrust asymmetry in flight?

Yes. United 232 at Sioux City is a prime example. Degraded aerodynamic controls mean they had the use the engines for directional control.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
aerotech777
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RE: Thrust Asymmetry

Mon Sep 02, 2013 5:44 pm

I am still wondering if there is procedure from the aircraft manufacturer or airline/aircraft operator in case of thrust asymmetry after engine failure on ground. Is it up to the pilot to decide what to do: use the thrust reverser? or use differential braking? use of NW steering?

Can the thrust asymmetry be used in crosswind landing? Is it an approved procedure by the aircraft manufacturer or airline/aircraft operator?

Feedback ppreciated
 
Pihero
Posts: 4237
Joined: Mon Jan 31, 2005 5:11 am

RE: Thrust Asymmetry

Mon Sep 02, 2013 6:35 pm

Quoting aerotech777 (Thread starter):
I read somewhere that using NW steering is not efficient in case of thrust asymmetry after engine failure especially after certain speed and below Vmcg.

You cannot count on the nosewheel steering : it's ineffective above 100 kt on the pedals and greatly reduced with speed on the handle. Plus, the nosewheel is too lightly loaded.

Quoting aerotech777 (Thread starter):
Other persons mentioned the use of differential braking in case of thrust asymmetry after engine failure.

That's the last thing you'd use. If you can't control the aircraft back on the centerline, that means that you are below Vmcg, therefore below V1.
In this case, abort the takeoff : Reducing thrust on the live engine will bring you back a symmetrical airplane.
You have to know that the thrust assymmetry is fierece and violent Think about 30 tons - 300 kN - 10 meters from the CoG : some 660,000 lbs x 30 ft = 1,800,000 lfbsft.

Quoting aerotech777 (Thread starter):
b) What is the recommended procedure by aircraft manufacturers (Boeing, Airbus, Bombardier...etc.) and/ or airlines/aircraft operators in case of thrust asymmetry after engine failure?

Simple : Keep on the centerline, either with rudder if above Vmcg or by reducing the thrust of the live engine to idle... You'd still need to abort if below V1.
See above.

Quoting aerotech777 (Thread starter):
c) Is it possible/not possible to use voluntary thrust asymmetry in flight? Why it is possible/not possible?

Why would you do that ?

Quoting aerotech777 (Thread starter):
d) What are the cases where it is possible (assuming it is possible) to use voluntary thrust asymmetry in flight?

I see none

[Edited 2013-09-02 11:38:10]
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Starlionblue
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RE: Thrust Asymmetry

Mon Sep 02, 2013 7:56 pm

Quoting Pihero (Reply 3):
Quoting aerotech777 (Thread starter):
c) Is it possible/not possible to use voluntary thrust asymmetry in flight? Why it is possible/not possible?

Why would you do that ?

In an emergency where you lost directional control, e.g. UA232.

Quoting Pihero (Reply 3):
Quoting aerotech777 (Thread starter):
d) What are the cases where it is possible (assuming it is possible) to use voluntary thrust asymmetry in flight?

I see none

It has been done. Again, UA232. I will concede that this is a bit of a one in a million thing.  
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
roseflyer
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Joined: Fri Feb 13, 2004 9:34 am

RE: Thrust Asymmetry

Tue Sep 03, 2013 3:33 pm

Quoting aerotech777 (Thread starter):

I read somewhere that using NW steering is not efficient in case of thrust asymmetry after engine failure especially after certain speed and below Vmcg.

In basic airplane design, the rudder is sized to be able to counteract the loss of an engine at takeoff thrust on the runway. The rudder and nose wheel steering move in tandem on the runway (not necessarily taxiway where the steering tiller is being used). There is a transition from nosewheel to rudder authority on the takeoff roll. It happens around 60-80 knots. Above 80 knots the rudder is developing more force than the lightly loaded nosewheel. If an engine is lost on the ground it can take full rudder input to stay straight. Part of the certification process requires that an airplane demonstrate that it has enough rudder authority to remain on a runway with an engine failure at any time in the takeoff roll. 80-100 knots is the most challenging speed because the rudder still does not have much authority due to low speed of the airplane.

Once in flight, full rudder deflection is never needed. The rudder is sequenced to not move as far (either through fly by wire or rudder authority limiter valves) in flight on most planes even if a pilot puts in full rudder pedal deflection. As we saw with the American A300 departing JFK crash, full rudder authority can generate such high forces that it can cause structural failure. Fortunately more modern planes have fly by wire systems and other mechanical control systems to better manage the rudder.

Quoting aerotech777 (Reply 2):
I am still wondering if there is procedure from the aircraft manufacturer or airline/aircraft operator in case of thrust asymmetry after engine failure on ground. Is it up to the pilot to decide what to do: use the thrust reverser? or use differential braking? use of NW steering?

The procedures are relatively clear. If you lose an engine, the rudder pedals are used to maintain the centerline. Rudder pedals are what should be used on the runway for directional control. Differential braking is an option if stopping, but that is a last resort. If the steering and rudder mechanical systems are working correctly, the airplane has enough authority to keep straight in all conditions with them alone as long as you are below the maximum crosswind limit. If you are above the max crosswind limit and lose an engine on takeoff may God help you because your airplane is going to end up somewhere that it shouldn't ever be.

[Edited 2013-09-03 08:42:03]
If you have never designed an airplane part before, let the real designers do the work!
 
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7BOEING7
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RE: Thrust Asymmetry

Tue Sep 03, 2013 5:39 pm

Quoting roseflyer (Reply 5):
The procedures are relatively clear. If you lose an engine, the rudder pedals are used to maintain the centerline. Rudder pedals are what should be used on the runway for directional control. Differential braking is an option if stopping, but that is a last resort

An interesting scenario I've seen many times in the simulator and probably the most difficult to control is the low speed abort -- about 40 kts (rudder pedal steering/rudder minimally effective), instantaneous failure of one engine, good engine approaching or at maximum thrust (115K) -- if you don't act quick (thrust to idle and brakes) you're off the side of the runway. Differential braking may be your only in this case.
 
roseflyer
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RE: Thrust Asymmetry

Tue Sep 03, 2013 6:43 pm

Quoting 7BOEING7 (Reply 6):
An interesting scenario I've seen many times in the simulator and probably the most difficult to control is the low speed abort -- about 40 kts (rudder pedal steering/rudder minimally effective), instantaneous failure of one engine, good engine approaching or at maximum thrust (115K) -- if you don't act quick (thrust to idle and brakes) you're off the side of the runway. Differential braking may be your only in this case.

Engine failures that cause low-mid speed aborted takeoffs or engine failures after V1 require very quick input from the pilot. The airplane is on the edge of being able to control. If you lose an engine, I believe the actions required are steer with the rudder pedals and thrust to idle. Automatic rejected takeoff brakes will be applied by the autobrake which means brakes apply evenly. In your training, do you ever use differential brakes in that scenario? I would think that you would be on maximum braking.
If you have never designed an airplane part before, let the real designers do the work!
 
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7BOEING7
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RE: Thrust Asymmetry

Tue Sep 03, 2013 7:27 pm

Quoting roseflyer (Reply 7):
Engine failures that cause low-mid speed aborted takeoffs or engine failures after V1 require very quick input from the pilot. The airplane is on the edge of being able to control. If you lose an engine, I believe the actions required are steer with the rudder pedals and thrust to idle. Automatic rejected takeoff brakes will be applied by the autobrake which means brakes apply evenly. In your training, do you ever use differential brakes in that scenario? I would think that you would be on maximum braking.


In the low speed scenario the rudder pedals are not effective enough to keep you on the runway and you're below the speed where the autobrake RTO feature is activated so it's a little more difficult than the V1 cut -- idle on the good engine and maximum manual braking (differential as needed to stay on the runway).
 
Pihero
Posts: 4237
Joined: Mon Jan 31, 2005 5:11 am

RE: Thrust Asymmetry

Tue Sep 03, 2013 9:35 pm

Quoting 7BOEING7 (Reply 6):
An interesting scenario I've seen many times in the simulator and probably the most difficult to control is the low speed abort -- about 40 kts (rudder pedal steering/rudder minimally effective), instantaneous failure of one engine, good engine approaching or at maximum thrust (115K) -- if you don't act quick (thrust to idle and brakes) you're off the side of the runway.

  ... Plus it's the only case at which, if F/O flying, the action/decision is not the captain's. I've seen so many "WTF are you doing ?" instances from incensed commanders who hadn't understood the F/O's predicament !

Quoting 7BOEING7 (Reply 8):
In the low speed scenario the rudder pedals are not effective enough to keep you on the runway and you're below the speed where the autobrake RTO feature is activated so it's a little more difficult than the V1 cut -- idle on the good engine and maximum manual braking


  

Quoting roseflyer (Reply 5):
In basic airplane design, the rudder is sized to be able to counteract the loss of an engine at takeoff thrust on the runway.


Yes.But there is such a thing called Vmcg, under wich the rudder hasn't the directional authority to keep the airplane on the runway.It's a required abort and, once again, the only - repeat *the only* - cure is retarding the live engine to idle... and you won't need differential braking.

Quoting roseflyer (Reply 5):
If you are above the max crosswind limit and lose an engine on takeoff may God help you because your airplane is going to end up somewhere that it shouldn't ever be.


There should be a qualifier here. AFAIK, crosswinds are not taken into account in these cases... and it depends on the side it is : if it's blowing on the live engine side, weathercoking effects would tend to straighten up a yawing moment toward the dead engine... Plus you shouldn't be anywhere near a runway close to your FCOM Xwind limit.( Most modern aircraft do not have a Xwind limit, just a *maximum demonstrated* - meaning *maximum encountered* Xwind during testing).
Remember also that if you do not have directional control, even at or slightly above Vmcg, the reasonable line of action is still an abort.

Quoting Starlionblue (Reply 4):
It has been done. Again, UA232. I will concede that this is a bit of a one in a million thing.


It's more like one in a billion occurrence. The best demonstration, IMO, is the badly shot-up DHL A310 in Iraq.
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