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Pilot here. Correct, in most aircraft that incorporate a trimmable horizontal stabilizer, elevator authority will be insufficient to counteract the aerodynamic

by clon 8y ago

Pilot here. Correct, in most aircraft that incorporate a trimmable horizontal stabilizer, elevator authority will be insufficient to counteract the aerodynamic effects of a the entire tailplane having deflected through a certain point. Thus it is important to quickly recognize and correct a runaway trim situation. Of even more significance is that is currently unclear if it is even possible for a pilot, once it has disengaged the trim motors (following faulty commands from MCAS) to manually correct the trim as per Boeing procedure [1]. The problem lies in the fact that it makes a lot of sense to haul back on the yoke as hard as you can if the nose starts dropping. Elevator upwards deflection loads the tailplane aerodynamically in such a way that it becomes harder to trim the tailplane in the required direction. Called colloquially a yo-yo maneuver, you are then require to "offload" the tailplane (think - push yoke forward..) in order to be able to manually correct the runaway trim. Plane going nose down, push yoke forward at 500ft? I do not envy the crews at the pointy end of those flights. My heart breaks just thinking about it. The Lion Air pilots must have been pulling back on the sticks until their tendons break, to no effect. I hope there is a special place in hell for Boeing execs. [1] 737 Flight Crew Training Manual, chapter Non-Normal Operations/Flight Controls, sub heading Manual Stabilizer trim: "Excessive air loads on the stabilizer may require effort by both pilots to correct mis-trim. In extreme cases it may be necessary to aerodynamically relieve the air loads to allow manual trimming. Accelerate or decelerate towards the in-trim speed while attempting to trim manually." This was recently brought to the attention of members of a certain pilots forum that does not welcome lurkers, hence not adding a link.

mywittyname 8y ago
Would it be inappropriate to ask for an explain-like-i'm-5 version of this?
- clon 8y ago
  I guess you can think of it like so: 1. The confuser commands a nose down trim, this means that the leading edge of the tailplane is raised. In actuality, this represents a decreased angle of attack (think of tailplane as a upside-down wing). 2. Pilots haul back with all their might. This means that the elevators (the trailing control surfaces at the trailing end of the tailplane) deflect upwards in order to increase the angle of attack and create more "downforce" from the tail, raising the nose. 3. Think of the trimmable tailplane [1] as moving around a pivot. In reality it is a jackscrew/hinge combination. 4. The elevator deflections (the intuitive response) "creates a twisting force" the tailplane around the "pivot point" and against the desired direction that you want the trim to travel. 5. Unload the elevator pressure to make it easier to trim it. The key is that elevator deflections, despite intuitive are not sufficient. Luckily, it is a trained manoeuvre. [1] https://i.stack.imgur.com/9RQY4.jpg https://i.stack.imgur.com/9RQY4.jpg Note the UP and DOWN markings on the airplane's skin.
- _s 8y ago
  The elevator (smaller wing on the back) points up or down, pushing the air up or down, which in turn raises or lowers the tail/nose of the plane (it's pitch). Some planes have little cutouts on the back of the elevator that move, thus deflecting the wind. Other planes, the whole back wing moves (stabilator). To keep the nose pointed up or down, it can get tiring to constantly apply force on the pitch control. So now we can "trim", which changes the neutral position of the elevator to be higher/lower. Some planes have little cutouts on the elevator or stabilator for trim. Other planes, the whole elevator / stabilator moves for trim. Some times, when the whole elevator / stabilator moves up/down for trim, you need a lot of force to move it back down/up, especially if its neutral position has changed. Occasionally, under high loads due to wind/pressure, you just can't. So you have to go to it's current neutral position (no matter how high or low) so you're not fighting a loosing battle. Then you have to reverse the trim. Then you can apply the opposite force.
- bencpeters 8y ago
  Maybe not quite 5-year-old, but here's my attempt: The elevator is the little wing at the back of an aircraft that tilts up and down to make the nose go up and down. When the pilot is flying, this up-down is what moving the yoke forward/back does. The elevator also has a tab (the trim tab) part of the wing that can move independently from the main part. This trimming movement allows for adjustments to the plane's up/down movement that don't require the yoke forward/back (this is useful to "lock in" the current desired climb/descent/level flight so that pilots don't have to be constantly pushing/pulling on the yoke to get the plane to be climbing/descending/level the way they want it). The 737-MAX has a system that automatically uses this trim tab to pitch the nose of the plane down when it senses certain conditions, without notifying the pilots. In this case (plane inexplicably pitching down), the natural response from a pilot is going to be to pull back on the yoke to counteract. This can cause issues because the act of pulling back on the yoke increases the pressure on the elevator (because physics - the more the elevator deflects in an attempt to change the plane's attitude, the more force the airstream flowing over it exerts. This "catching the airflow" is why it can change the plane's attitude at all). Apparently on this plane if the trim tab is way out of line even if disconnect the erroneous system that was automatically adjusting the trim tab and try to reset the trim to a safe position by hand, the airflow over the "loaded" elevator (which is trying to counteract the position of the trim tab and keep the plane from crashing) is too strong to physically allow the manual control to move the tab. So the "correct" procedure is to push the yoke in (allowing the nose to go down/lose altitude) to reduce the airflow that's hitting the elevator, while frantically spinning the manual trip wheel to get it back to neutral. Then, once you've reset the trim manually, you presumably pull back on the yoke to get the nose up and pull the plane out of the dive. The issue with that is that the ground can get in the way in between when you've let off the yoke and you've spun the wheel enough to get the trim tab back to neutral.
  - clon 8y ago
    Incorrect, this describes a typical General Aviation aircraft with trim tabs. Most (all?) airliners use a trimmable tailplane, meaning the entire tailplane tilts up and down to trim for a specific speed.
    - 8y ago
webXL 8y ago
> Plane going nose down, push yoke forward at 500ft? Is this in anyway akin to drivers counter-intuitively steering into a skid in order to regain traction?
- clon 8y ago
  You are correct in it being counter-intuitive and seemingly opposing the desired outcome. You need to recognize the situation fast and know precisely what you are doing. Unenviable position to be in at 500ft AGL.
  - jacquesm 8y ago
    In this case you are either going into a commanded descent into terrain or the broken computer will do it for you, at that altitude there aren't many chances of getting out of that situation in one piece.
- hammock 8y ago
  More like how you have to steer a bike slightly in the opposite direction, before spinning the wheel towards the way you want to go.
userbinator 8y ago
Cars have power steering, where it's pretty much impossible for steering to require superhuman effort. Why can't the hydraulic assist of a plane be designed the same way?
- anamexis 8y ago
  Airliner control surfaces are 100% hydraulically actuated. It isn't a matter of actuation force, it's a matter of aerodynamics.
  - bronco21016 8y ago
    Not always. See the MD88.
  - clon 8y ago
    The 737 stabilizer trim is actuated by steel cables running from the nose of the aircraft to the tail, guided by pulleys [1]. The same cables also make the wheels spin when the stab trim motors are running. [1] http://aerossurance.com/wp-content/uploads/2015/03/737-controls.png http://aerossurance.com/wp-content/uploads/2015/03/737-contr...
    - anamexis 8y ago
      Wild. Thanks for the correction.
  - userbinator 8y ago
    The comment I'm replying to says Elevator upwards deflection loads the tailplane aerodynamically in such a way that it becomes harder to trim the tailplane in the required direction. so I'm not sure what you mean by "matter of aerodynamics", because that comment suggests that it only gets harder due to the forces exerted by the air on the control surfaces, and so could be overcome by applying more force --- which a hydraulic system could be engineered to do.
koboll 8y ago
>I hope there is a special place in hell for Boeing execs. I'd settle for a special place in prison.
- mikejb 8y ago
  This desire for a 'final judgement' may be the desperate hope for justice when faith in the human court system is lacking.
Rtr_nt 8y ago
From the previous yo-yo recording and now, a structural engineering boundary seems very likely broken at the connection area to elevator motor. Caused by swelling of a screw type plausible, or a punctured surface mounting causing a built up existence of a opposite hold in sudden decent, then lumbering heat.

by clon 8y ago

mywittyname 8y ago

Would it be inappropriate to ask for an explain-like-i'm-5 version of this?

clon 8y ago
I guess you can think of it like so: 1. The confuser commands a nose down trim, this means that the leading edge of the tailplane is raised. In actuality, this represents a decreased angle of attack (think of tailplane as a upside-down wing). 2. Pilots haul back with all their might. This means that the elevators (the trailing control surfaces at the trailing end of the tailplane) deflect upwards in order to increase the angle of attack and create more "downforce" from the tail, raising the nose. 3. Think of the trimmable tailplane [1] as moving around a pivot. In reality it is a jackscrew/hinge combination. 4. The elevator deflections (the intuitive response) "creates a twisting force" the tailplane around the "pivot point" and against the desired direction that you want the trim to travel. 5. Unload the elevator pressure to make it easier to trim it. The key is that elevator deflections, despite intuitive are not sufficient. Luckily, it is a trained manoeuvre. [1] https://i.stack.imgur.com/9RQY4.jpg https://i.stack.imgur.com/9RQY4.jpg Note the UP and DOWN markings on the airplane's skin.
_s 8y ago
The elevator (smaller wing on the back) points up or down, pushing the air up or down, which in turn raises or lowers the tail/nose of the plane (it's pitch). Some planes have little cutouts on the back of the elevator that move, thus deflecting the wind. Other planes, the whole back wing moves (stabilator). To keep the nose pointed up or down, it can get tiring to constantly apply force on the pitch control. So now we can "trim", which changes the neutral position of the elevator to be higher/lower. Some planes have little cutouts on the elevator or stabilator for trim. Other planes, the whole elevator / stabilator moves for trim. Some times, when the whole elevator / stabilator moves up/down for trim, you need a lot of force to move it back down/up, especially if its neutral position has changed. Occasionally, under high loads due to wind/pressure, you just can't. So you have to go to it's current neutral position (no matter how high or low) so you're not fighting a loosing battle. Then you have to reverse the trim. Then you can apply the opposite force.
bencpeters 8y ago
Maybe not quite 5-year-old, but here's my attempt: The elevator is the little wing at the back of an aircraft that tilts up and down to make the nose go up and down. When the pilot is flying, this up-down is what moving the yoke forward/back does. The elevator also has a tab (the trim tab) part of the wing that can move independently from the main part. This trimming movement allows for adjustments to the plane's up/down movement that don't require the yoke forward/back (this is useful to "lock in" the current desired climb/descent/level flight so that pilots don't have to be constantly pushing/pulling on the yoke to get the plane to be climbing/descending/level the way they want it). The 737-MAX has a system that automatically uses this trim tab to pitch the nose of the plane down when it senses certain conditions, without notifying the pilots. In this case (plane inexplicably pitching down), the natural response from a pilot is going to be to pull back on the yoke to counteract. This can cause issues because the act of pulling back on the yoke increases the pressure on the elevator (because physics - the more the elevator deflects in an attempt to change the plane's attitude, the more force the airstream flowing over it exerts. This "catching the airflow" is why it can change the plane's attitude at all). Apparently on this plane if the trim tab is way out of line even if disconnect the erroneous system that was automatically adjusting the trim tab and try to reset the trim to a safe position by hand, the airflow over the "loaded" elevator (which is trying to counteract the position of the trim tab and keep the plane from crashing) is too strong to physically allow the manual control to move the tab. So the "correct" procedure is to push the yoke in (allowing the nose to go down/lose altitude) to reduce the airflow that's hitting the elevator, while frantically spinning the manual trip wheel to get it back to neutral. Then, once you've reset the trim manually, you presumably pull back on the yoke to get the nose up and pull the plane out of the dive. The issue with that is that the ground can get in the way in between when you've let off the yoke and you've spun the wheel enough to get the trim tab back to neutral.
- clon 8y ago
  Incorrect, this describes a typical General Aviation aircraft with trim tabs. Most (all?) airliners use a trimmable tailplane, meaning the entire tailplane tilts up and down to trim for a specific speed.
  - 8y ago

webXL 8y ago

> Plane going nose down, push yoke forward at 500ft? Is this in anyway akin to drivers counter-intuitively steering into a skid in order to regain traction?

clon 8y ago
You are correct in it being counter-intuitive and seemingly opposing the desired outcome. You need to recognize the situation fast and know precisely what you are doing. Unenviable position to be in at 500ft AGL.
- jacquesm 8y ago
  In this case you are either going into a commanded descent into terrain or the broken computer will do it for you, at that altitude there aren't many chances of getting out of that situation in one piece.
hammock 8y ago
More like how you have to steer a bike slightly in the opposite direction, before spinning the wheel towards the way you want to go.

userbinator 8y ago

Cars have power steering, where it's pretty much impossible for steering to require superhuman effort. Why can't the hydraulic assist of a plane be designed the same way?

anamexis 8y ago
Airliner control surfaces are 100% hydraulically actuated. It isn't a matter of actuation force, it's a matter of aerodynamics.
- bronco21016 8y ago
  Not always. See the MD88.
- clon 8y ago
  The 737 stabilizer trim is actuated by steel cables running from the nose of the aircraft to the tail, guided by pulleys [1]. The same cables also make the wheels spin when the stab trim motors are running. [1] http://aerossurance.com/wp-content/uploads/2015/03/737-controls.png http://aerossurance.com/wp-content/uploads/2015/03/737-contr...
  - anamexis 8y ago
    Wild. Thanks for the correction.
- userbinator 8y ago
  The comment I'm replying to says Elevator upwards deflection loads the tailplane aerodynamically in such a way that it becomes harder to trim the tailplane in the required direction. so I'm not sure what you mean by "matter of aerodynamics", because that comment suggests that it only gets harder due to the forces exerted by the air on the control surfaces, and so could be overcome by applying more force --- which a hydraulic system could be engineered to do.

koboll 8y ago

>I hope there is a special place in hell for Boeing execs. I'd settle for a special place in prison.

mikejb 8y ago
This desire for a 'final judgement' may be the desperate hope for justice when faith in the human court system is lacking.

Rtr_nt 8y ago

From the previous yo-yo recording and now, a structural engineering boundary seems very likely broken at the connection area to elevator motor. Caused by swelling of a screw type plausible, or a punctured surface mounting causing a built up existence of a opposite hold in sudden decent, then lumbering heat.