Official Aviation Thread...

[LateFan]

You guys who know what you're looking at in a cockpit will get something out of this piece.....
https://www.nytimes.com/interactive/...h-cockpit.html




Possible first indication of trouble


Pilot appears to be struggling with manual control


Plane plummets

[flyboyx]

I've always believed that, if the plane is trimming down and trying to fly you into the water, don't let it! Pilot controls (at least in Boeing) overcomes automation. Pull back hard, fly the plane, and figure-out why it was doing that later.

Yes, but in a jet airliner, the trim moves the entire horizontal stabilizer as in the entire tail wing surface. There is no way in hell that full up elevator input will overcome a fully pitched down horizontal stabilator.

[varg]

I'm no commercial pilot or fighter pilot, just an engineering student with private pilot training. But it seems to me like an significant oversight to neglect to mention an automatic stall mitigation system that has full authority over stabilator angle in training or the manuals/handbooks. I've never been through it, so I'm not sure what training for a new variant of an aircraft under the same type certificate is like, but from what I've read since I found out about this it is a system that did not exist in any capacity in the older 737 variants and seems worth mentioning.

[hoveringuy]

Yes, but in a jet airliner, the trim moves the entire horizontal stabilizer as in the entire tail wing surface. There is no way in hell that full up elevator input will overcome a fully pitched down horizontal stabilator.

I'm not type rated on the 737 so I don't know first-hand what the relationship between trim and control effectiveness is (plus the elevator feel computer is adding forces), but the elevator trim is connected to the flight deck through cables, so when the trim is being commanded down the big wheel by your thigh is spinning. It's obvious.

It ends up being the exact same thing as runaway trim, which is taught and practiced in the simulators.

[tttoon]

Based on what we know so far, I think these guys got IAS disagree, ALT disagree and AoA flags pretty much after take-off. Throw in a trim runaway while you're doing the memory items for unreliable airspeed and things get interesting very quickly.

[flyboyx]

I'm not type rated on the 737 so I don't know first-hand what the relationship between trim and control effectiveness is (plus the elevator feel computer is adding forces), but the elevator trim is connected to the flight deck through cables, so when the trim is being commanded down the big wheel by your thigh is spinning. It's obvious.

It ends up being the exact same thing as runaway trim, which is taught and practiced in the simulators.

I mostly agree with you 100%. all I am saying is that full nose down trim of the entire horizontal stab can not be overcome with elevator input in flight. the article link latefan posted actually mentions this as well. from an engineering standpoint, think of the surface area of the entire stab compared to the elevators. perhaps like ttoon said, its possible that they had so many bad things going on in the cockpit, that the trim going bonkers was missed because they were busy with other things. maybe the trim slowly rolled to full nose down? maybe they tried to inhibit the trim but the airplane wouldn't allow it?

its possible that they lost the ship because they didn't have the proper training to overcome the problem and were not able to think outside the box. maybe they should have pulled a Denzel Washington and flew upside down? it is also possible that not even Sully could have saved this one. we will just have to wait and see.

I did read somewhere that they thought the airplane hit the water going about 600 miles an hour? I wonder if a dive at 5000' to sea level would attain such a speed without the engines producing full power? at the very least, why not turn off the auto thrust and bring the power to idle? in the end, I guess the only difference between hitting the water at 200 versus 600 is that the pieces will be a little bigger.:(

[hoveringuy]

Unfortunately, the voice recorder is buried in the sea mud, it should answer those questions.

[varg]

Now that I've read more about it and seen that there was a problem with the aircraft on a previous flight I've been thinking about what caused the issue in the first place. Given the caliber of training, servicing, and personnel you'd expect from this airline with a less than stellar record in a developing nation, it would not be a surprise if the AoA sensors or the system itself was damaged in ground handling or maintenance and it was simply not reported, not known, or ignored.

[LateFan]

...from an engineering standpoint, think of the surface area of the entire stab compared to the elevators.(

Oh, I'm just getting all this - the stick only operates the elevators. That trim wheel by your knee runs cables to the horizontal stabilizer. On some planes (maybe only little ones?) I thought that was stationary and the elevators did all the work. So if that big stabilizer is shoving the nose down, you need to know why and disable that pronto. If they didn't even know about this system, what could they do except keep trying to fight it? And what happened at the end where it just dropped?

So, "Trim" = horizontal stabilizer. Why would you ever change the trim in flight? For weight distribution? Isn't there an ideal attack angle that doesn't change? When you rotate the nose up at take-off, is that just elevators doing that? I always feel like that steep angle is asking for a stall, but I don't understand all the physics.

[tttoon]

You need the trim because civil airplanes are designed to be speed stable. The tail actually produces downforce, and together with the lift of the wing, adds up to zero moment around the center of gravity of the airplane.

If you reduce airspeed, the change in wing lift and tailplane downforce will cause a pitch down, as the airplane tries to regain its trimmed speed. So you're actually only "in trim" for one airspeed and configuration. Airplanes without trim would require constant force on the elevators at all but one speed, if that makes sense.

We actually also have rudder and aileron trim, but those are not used that often. Some military planes (F16 springs to mind) are aerodynamically unstable, which makes them more manoeuvrable, but they have "artificial" stability due to their fly by wire system.

[varg]

Oh, I'm just getting all this - the stick only operates the elevators. That trim wheel by your knee runs cables to the horizontal stabilizer. On some planes (maybe only little ones?) I thought that was stationary and the elevators did all the work. So if that big stabilizer is shoving the nose down, you need to know why and disable that pronto. If they didn't even know about this system, what could they do except keep trying to fight it? And what happened at the end where it just dropped?

So, "Trim" = horizontal stabilizer. Why would you ever change the trim in flight? For weight distribution? Isn't there an ideal attack angle that doesn't change? When you rotate the nose up at take-off, is that just elevators doing that? I always feel like that steep angle is asking for a stall, but I don't understand all the physics.

Think about what a horizontal stabilizer does and adjustable trim will make perfect sense from a physics standpoint. The trim must be adjusted based on configuration, speed, weight and balance, it is something that requires a wide degree of adjustment. Flaps and slats can change the center of lift/center of pressure of the wing, drag from landing gear can impart a moment on the airplane that must be countered, CG location can shift as the aircraft burns fuel, maintaining the higher angle of attack necessary at slower speeds, all of these things require trim changes.

Elevator trim is a universal thing in modern airplanes. Today, fixed trim tabs exist almost exclusively in the realm of ultralights and other very simple, very small airplanes. In the past it was something that existed more widely but that changed quickly because there is no single trim position that is optimal for the entire flight envelope of an airplane. Electric trim is widely used in commercial and even GA aircraft, and in any aircraft with electric trim dealing with a runaway trim situation is a necessary part of the training that familiarizes you with that aircraft in your transition.

[flyboyx]

oddly, the airbus has no electric elevator or aileron trim adjustment in flight. the computers take care of all of it. you can only set elevator for takeoff based on the aircraft weight and balance loading. we do have a manual wheel for elevator but that is only if all the computers fail.

the only trim adjustment that is overridable is the rudder. the only reason why the pilot is allowed to fuck with that is in the event of an engine failure. the 1986 technology computer will adjust for it eventually, but the pilot can trim the plane up so much faster which would be critical in the event of an engine failure on takeoff.

we don't even really have a procedure for elevator trim runaway. I was sitting around the other day trying to figure out what I would do in that situation. the only thing I can come up with would be to reach up to the overhead panel and start shutting off computers.

[hoveringuy]

I instructed in a GlasAir GlaStar the other day. Cool little airplane with a stick and electric trim, except the trim switches were on the panel and I spent the entire flight wondering why the fart the trim switches weren't on the stick...

[LateFan]

Airplanes without trim would require constant force on the elevators at all but one speed, if that makes sense.

It does.

The trim must be adjusted based on configuration, speed, weight and balance, it is something that requires a wide degree of adjustment. Flaps and slats can change the center of lift/center of pressure of the wing, drag from landing gear can impart a moment on the airplane that must be countered, CG location can shift as the aircraft burns fuel....

Good explanations both, thanks.

[FLYNAVY]

Out of curiosity, do Airbii auto trim to 1G flight in the absence of autopilot usage? In the three FBW jets I have spent much time flying, the aircraft FCS software is designed to do that if not in landing configuration........in other words, you barely touch the pitch trim switch at all in flight. If you were to bring the throttle(s) to idle, the jet would simply remain pretty much in 0 VSI level flight as the jet slowed, got more and more cocked up/high AoA, the leading and trailing edge flaps would schedule down as you got real slow, and then after a bunch of angry tones, if you still didn't add power, it would ultimately settle while still trying in vain to seek 1G flight.