737 stabilizer runaway
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737 stabilizer runaway
Hi,I recently read a book, when it refers to the stabilizer runaway event,
I quote part of it,"Control column movement opposite to the direction of the aerodynamic runaway will aggravate the situation. Use electric trim to counteract the trim runaway !"
I have three questions
1/In the event of a dual brake failure, air load can drive the stabilizer to its mechanical stops, which side will the stop be? all the way to nose up or down? or depend on the aircraft`s attitude when the brakes fail?
2/how is the Control column movement opposite to the direction of the aerodynamic runaway will aggravate the situation? when use the yoke to the opposite direction,the elevator still can counteract the pitch force produced by the runaway stabilizer.
3/can the electic trim be used for the runaway? how? do I need to recover the stab cutout switches which were cut out by the QRH?
Thank you in advance~
I quote part of it,"Control column movement opposite to the direction of the aerodynamic runaway will aggravate the situation. Use electric trim to counteract the trim runaway !"
I have three questions
1/In the event of a dual brake failure, air load can drive the stabilizer to its mechanical stops, which side will the stop be? all the way to nose up or down? or depend on the aircraft`s attitude when the brakes fail?
2/how is the Control column movement opposite to the direction of the aerodynamic runaway will aggravate the situation? when use the yoke to the opposite direction,the elevator still can counteract the pitch force produced by the runaway stabilizer.
3/can the electic trim be used for the runaway? how? do I need to recover the stab cutout switches which were cut out by the QRH?
Thank you in advance~
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You read the MRG 
1) That depends on the AOA on the stabilizer, it will go in whatever direction the relative airflow pushes it, if for instance you have more nose down trim than necessary in the current flight situation the stabilizer will 'fly up' (leading edge up relative to the aeroplane fuselage)
2) If the stabilizer is actually mechanically loose (like in that nasty MD80 crash) the tiny elevator tab itself is certainly not enough to counter the effect of the massive stabilizer, let alone reverse it - imagine the above case, the stabilizer 'flies away' into a massive nose down trim position (positive pitch relative to the fuselage) you trying to lift the nose with full elevator up (with the tab moving upwards relative to the stabilizer) will not help you in the least, au contraire, the elevator will act like a trim tab and assist the stabilizer in moving to positive pitch...
3) Depends what kind of runaway you are experiencing, if you can rule out an aerodynamic runaway - as far as I know thats in the MRG as well - you can try either autopilot or manual trim, depending on what was in use when the runaway started... Sure, you would need to reset one of the cutout switches for that - and thus act outside the QRH on your own risk
Please correct me if I wrote absolute tosh, just came back from duty, can't be sure I got it together correctly
1) That depends on the AOA on the stabilizer, it will go in whatever direction the relative airflow pushes it, if for instance you have more nose down trim than necessary in the current flight situation the stabilizer will 'fly up' (leading edge up relative to the aeroplane fuselage)
2) If the stabilizer is actually mechanically loose (like in that nasty MD80 crash) the tiny elevator tab itself is certainly not enough to counter the effect of the massive stabilizer, let alone reverse it - imagine the above case, the stabilizer 'flies away' into a massive nose down trim position (positive pitch relative to the fuselage) you trying to lift the nose with full elevator up (with the tab moving upwards relative to the stabilizer) will not help you in the least, au contraire, the elevator will act like a trim tab and assist the stabilizer in moving to positive pitch...
3) Depends what kind of runaway you are experiencing, if you can rule out an aerodynamic runaway - as far as I know thats in the MRG as well - you can try either autopilot or manual trim, depending on what was in use when the runaway started... Sure, you would need to reset one of the cutout switches for that - and thus act outside the QRH on your own risk
Please correct me if I wrote absolute tosh, just came back from duty, can't be sure I got it together correctly
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thank you very much STBYRUD！I know what it feel like after the flight, thank you again for your reply.
but I need some time to digest what you said for my No.2 question.
let me think~~
and I feel good about we have the same book!
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Hi, STBYRUD,I figured it out!
cause the aerodynamic shape of the stabilizer is different to the wings,it produce the tail down force for balancing the pitch down force which cause by the center of lift after the CG.
1/if the stab runaway all the way to pitch up position,means the trim wheel rotate to backward,then push the yoke forward,try to stop the runaway,but it won`t, push the yoke will cause the angle of stab chord line between the relative wind become smaller, that produce less tail down force.that is kind acceptable~
2/if the stab runaway all the way to pitch down position,means the trim wheel rotate to forward,then try to pull the yoke backward,to stop the runaway,but it still doesn`t work, pull the yoke will cause the angle of stab chord line between the relative wind become larger, that produce much more tail down force than needed,it will accelarate the aircraft to the ground,that is totally not acceptable.
That`s only my guess,sounds make sense
please correct me if I am wrong.
Thanks a lot~ STBYRUD
cause the aerodynamic shape of the stabilizer is different to the wings,it produce the tail down force for balancing the pitch down force which cause by the center of lift after the CG.
1/if the stab runaway all the way to pitch up position,means the trim wheel rotate to backward,then push the yoke forward,try to stop the runaway,but it won`t, push the yoke will cause the angle of stab chord line between the relative wind become smaller, that produce less tail down force.that is kind acceptable~
2/if the stab runaway all the way to pitch down position,means the trim wheel rotate to forward,then try to pull the yoke backward,to stop the runaway,but it still doesn`t work, pull the yoke will cause the angle of stab chord line between the relative wind become larger, that produce much more tail down force than needed,it will accelarate the aircraft to the ground,that is totally not acceptable.
That`s only my guess,sounds make sense
please correct me if I am wrong.
Thanks a lot~ STBYRUD
