[bencpeters]

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.

[bencpeters]

I wanted to update my explanation ^ a little bit - it's based on how trim works in smaller trainer aircraft (which are all I've flown). On a 737 there isn't a "trim tab" - instead the whole littler wing at the back of the airplane (tailplane) can change its position based on the trim, while the elevator is the adjustable tab at the back of that which moves based on the pilot's inputs.

I think the basic procedure for how to undo the excessive trim based on too much force to move the tailplane still is accurate though...

I just read this great article (https://www.skybrary.aero/bookshelf/books/2627.pdf) linked lower down in this thread for more details.

[clon]

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.

[inferiorhuman]

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.

The only airliner I can think of that uses trim tabs is the DC-9 and its derivatives (MD-80/MD-90/Boeing 717). Some, like the L-1011, went in the completely opposite direction and use an "all moving tailplane" where the functions of the elevator and stabilizer were integrated into one piece.

[SomeHacker44]

An example fully trimmable tailplane in a general aviation airplane is the Mooney, at least the 201 and related models. Look at pictures of it and compare it to, say, a Cessna 310’s plane. You will see the “trim tabs” at the back of the 310’s elevator and none on the Mooney 201.

I seem to recall some tail draggers I flew had no trim tabs too.

[foldr]

Yeah this explanation is completely backwards. The "trim" moves the entire horizontal stabilizer. The elevator control moves the elevator (a moveable section of the back of the horizontal stabilizer).

[nickjj]

In an average case (trained pilot, solid amount of experience, etc.) what is the turn around time between detecting this problem and fixing it in the air?

Like, how long does it take to determine the exact problem, spin the manual trim wheel to get to the point where you can recover from a nose dive in this case?

How does that number (in seconds I'm guessing?) translate to altitude levels?

[clon]

By regulation, when a system fails (as designed) the pilot MUST be able to recover:

- in 3 seconds when in cruise

- in 1 second when on approach

- when in the landing phase - immediately!

Such a system would be certifiable.

Most modern airliners have ~10 "memory items" that are procedures that are to be recalled and applied immediately without consulting any checklists. Runaway stabilizer is such a memory item. But first you need to recognize the issue as such..

It bears to mention that the 737 is riding on it's "grandfathered" certification status from the 60's, getting a free pass on many newer requirements that are subjected to airliners designed today. This is why it doubly makes sense for the bean counters to not design a new aircraft.

[nickjj]

That's really impressive turn around times. I wish I could debug most software errors in 3 seconds or less.

That is pretty frightening about the grandfathered rules.

I don't know much about cars but I do know they have a similar thing. Suddenly because you have a car 1 year before a point in time it can be obnoxiously loud but today it wouldn't be street legal. I almost can't believe the same thing happens with planes.

[SomeHacker44]

If you lose an engine in a small piston twin engine aircraft in the first few hundred feet after takeoff... 1 second might even be generous.

[zanderz]

have you heard that old pilot joke about twin engine aircraft? The second engine serves mainly to get you to the crash site faster.

[nradov]

The situation is actually worse with airplanes. When automobile regulations change, owners can generally continue to drive older vehicles but manufacturers can't continue to sell new vehicles approved under old rules. But with aircraft, once a type certificate is issued the manufacturer can generally continue producing it forever.

[weaksauce]

which regulations did the 737 get grandfathered into?

[toufka]

I think (please correct me if I’m wrong), the plane’s “type” is fixed, and pilots have certifications for that “type”. So theoretically the plane is simialar enough to not require completely new training if a pilot is certified to the type - this new 737 Max is grandfathered into the 737 “type”. If it were different enough, then it would be a new type. If the plane is a new type, pilots, and likely all sorts of other things, mist be retrained, and reissued.

So there’s an incentive to not just make the type backwards compatible to keep the common “type”, but also not introduce too many new features that might bring into question the grandfathered training.

This 737 Max added this new “safety feature” without telling the pilots (because it’s the “same type”). And that feature seems to have an unfortunate interaction with other systems in some circumstances.

[oliveshell]

I don’t know the specifics, but I assume the process to get changes approved to an already-certified type of aircraft (the 737 family in this case) is less rigorous than what you’d have to go through to get an entirely new type certificate.