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An interesting point was made by an airline pilot in one of the plane forums. It appears that Max 8 with full trim deflection down doesn't have enough controls

by lsh123 8y ago

An interesting point was made by an airline pilot in one of the plane forums. It appears that Max 8 with full trim deflection down doesn't have enough controls authority to recover (at least in the syms). Thus, the trim issues followed by disabling the trim can lead into a situation when pilots can not recover the plan from steep descend. Of course, this is not necessarily what have happened in real life and we need to see the data.

sokoloff 8y ago
Most airliners with jackscrew activated horizontal stabilizers don’t have enough elevator authority to counter a fully deflected tailplane. I don’t think that’s in any way abnormal to the -Max. What is abnormal is the failure mode that seems like it can deflect to full nose down from stability augmentation.
- FabHK 8y ago
  There's a good article, Do you really understand how your trim works?, that explains how trim works, and in particular how stabiliser trim (what the author calls "trimming tailplane") differs from the trim tab ("conventional trimming") found in smaller planes (that many pilots train with). They operate pretty much the same to first order (for small perturbations), but quite differently for larger deviations from neutral. Bottom line, as you said: You can't overcome bad trim in an airliner by "pulling harder". https://www.skybrary.aero/bookshelf/books/2627.pdf https://www.skybrary.aero/bookshelf/books/2627.pdf
lutorm 8y ago
Even if this is true (which I suspect it's not, I think being able to override trim in all positions is a certification requirement, but I could be wrong), disabling the trim just disables automatic trim. It's still possible for the pilots to manually move the trim wheel to trim back up.
- lsh123 8y ago
  Manual trim is possible but at low near the ground one might not gave enough time.
- inferiorhuman 8y ago
  The 737 can definitely be flown into a situation where the elevator doesn't have authority to overcome the stabilizer. That's been true at least since the 737 Classic. Take a look at the Bournemouth incident.
  - bonzini 8y ago
    > Bournemouth incident Here: https://www.gov.uk/aaib-reports/aar-3-2009-boeing-737-3q8-g-thof-23-september-2007 https://www.gov.uk/aaib-reports/aar-3-2009-boeing-737-3q8-g-...
  - lutorm 8y ago
    Ah, interesting. That's nose-up, which is the opposite direction of what we're talking here, and seems to indicate that it is the combination of max thrust at low airspeed and large nose-up trim that exceeds the elevator authority. It's not immediately clear, at least from my skimming of the report, whether a similar situation exists for nose-down trim, since in that case thrust will help the elevator with pitching up.
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.

by lsh123 8y ago

sokoloff 8y ago

Most airliners with jackscrew activated horizontal stabilizers don’t have enough elevator authority to counter a fully deflected tailplane. I don’t think that’s in any way abnormal to the -Max. What is abnormal is the failure mode that seems like it can deflect to full nose down from stability augmentation.

FabHK 8y ago
There's a good article, Do you really understand how your trim works?, that explains how trim works, and in particular how stabiliser trim (what the author calls "trimming tailplane") differs from the trim tab ("conventional trimming") found in smaller planes (that many pilots train with). They operate pretty much the same to first order (for small perturbations), but quite differently for larger deviations from neutral. Bottom line, as you said: You can't overcome bad trim in an airliner by "pulling harder". https://www.skybrary.aero/bookshelf/books/2627.pdf https://www.skybrary.aero/bookshelf/books/2627.pdf

lutorm 8y ago

Even if this is true (which I suspect it's not, I think being able to override trim in all positions is a certification requirement, but I could be wrong), disabling the trim just disables automatic trim. It's still possible for the pilots to manually move the trim wheel to trim back up.

lsh123 8y ago
Manual trim is possible but at low near the ground one might not gave enough time.
inferiorhuman 8y ago
The 737 can definitely be flown into a situation where the elevator doesn't have authority to overcome the stabilizer. That's been true at least since the 737 Classic. Take a look at the Bournemouth incident.
- bonzini 8y ago
  > Bournemouth incident Here: https://www.gov.uk/aaib-reports/aar-3-2009-boeing-737-3q8-g-thof-23-september-2007 https://www.gov.uk/aaib-reports/aar-3-2009-boeing-737-3q8-g-...
- lutorm 8y ago
  Ah, interesting. That's nose-up, which is the opposite direction of what we're talking here, and seems to indicate that it is the combination of max thrust at low airspeed and large nose-up trim that exceeds the elevator authority. It's not immediately clear, at least from my skimming of the report, whether a similar situation exists for nose-down trim, since in that case thrust will help the elevator with pitching up.

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.