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Comment by Johnny555 | Hacker News Reader

As a non-pilot, I'm surprised that a malfunctioning trim system could cause a crash unless it happened close to ground. When I hear "trim", I'm imagining minor control surface adjustments that the pilot can easily override with his control stick.

Does it take significant effort to overcome trim? Or does a bad AOA sensor add to pilot confusing during recovery?

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On a 737, trim adjusts the pitch of the entire horizontal stabilizer. As such, in the extreme, it actually has more control over pitch than the elevator (which is controlled by the pilot's yoke). If the trim runs out to the limit, and the pilot doesn't correct it, the elevator does not have enough authority to override the trim.

Wow. Didn't know this. I can understand why they do it this way--it's probably the lowest drag solution to the problem. But it sure swaps my connotation of what "trim" and "elevator" mean.

There's a common GA plane (M20) that moves the entire tail for trim, both the horizontal and vertical stabilizer as one piece.

Basically conventional (not a canard) stable (not an F-16) aircraft have the CG in front of the center of lift of the main wing so it wants to nose down. The tail then acts as a counter balance, pushing down to keep the nose up. Because airspeed over the smaller surface effects its downforce and the center of thrust is not always in line with the center of drag, they add trim to prevent the pilot from having to maintain constant control pressure. Some do it by biasing the control surface itself, either in the control mechanism or with a tab that aerodynamically moves the surface, or by changing the angle of attack of the horizontal stabilizer.

I’m no pilot, but one of the first things you learn in Kerbal Space Program is that CoG behind CoL is a completely unflyable configuration.

Ha, yea. I was deliberately using that terminology hoping people might be able to draw some parallels with their KSP experience.

Do you have a source for this claim? Two different comments in this thread are stating opposite claims

https://news.ycombinator.com/item?id=18409562

This explains a bit about how the THS works: https://www.skybrary.aero/index.php/Trimmable_Horizontal_Sta...

The Rostov-on-Don crash in 2016 appears to have been caused by downwards trimming for 12 seconds. This caused a -1g dive from which the pilots were unable to recover. It's inconceivable they wouldn't have been attempting to pull up with the elevator. You can't not notice -1g. https://en.wikipedia.org/wiki/Flydubai_Flight_981

Now the final report is not out yet, so it's not clear if the downwards trim was somehow manual (which doesn't really make any sense), or a similar problem to the Lion Air crash. Either way, it's worried Boeing and the FAA enough to issue this AD.

Is the trim setting (pardon my lack of knowledge) easily visible to the pilot?

In a Boeing, absolutely. The designers know it is important, and they devote a slice of cockpit space to a comically large "trim wheel." It has a white stripe to highlight automatic movements, and a large handle to allow manual control.

See: https://www.quora.com/What-are-the-spinning-wheels-in-an-air...

Recent life-threatening runaway trim incident, on a modern jet:

https://50skyshades.com/news/business-aviation/bea-pilots-sk...

Here's one example where loss of an elevator trim tab caused the pilot to experience +17G, leading to incapacitation/unconsciousness and a fatal crash: https://www.ntsb.gov/investigations/AccidentReports/Reports/...

(also a non-pilot, I found the above surprising, but there it is..)

The plane in question was a highly-modified P-51 taking part in the Reno air races. Looking through the report it was routinely pulling 4G in the turns already (at ground speeds of 360-380kts) so quadrupling that in the face of structural failure isn't beyond belief.

It was pulling 4G turns by using the elevators. My surprise was that losing such a tiny piece as a single trim tab would cause a 17G pitchup.

But yes, that was a highly modified plane pushed way past any sensible operating limits, so maybe one shouldn't generalize too much from it.

This is where the NTSB report most directly addresses the issue: "...the loss of the left trim tab’s downward force on the elevator resulted in a concurrent sudden upward deflection of the elevator, a sudden and forceful aft movement of the control stick, the pitch-up of the airplane, and the rapid increase in G."

At this point, the trim tab was still attached, but its link assembly had broken.

In addition, the report notes "two major modifications to the elevators each made the airplane more sensitive in pitch control: the substantially increased elevator counterweights (about twice that of a stock P-51D) that overbalanced the elevators and the substantially decreased elevator inertia weight (less than half that of stock)."

Just from top of my head, a faulty Airspeed Sensor and some quirky AOA sensor behaviour was one of the reasons why the Air France flight crashed into the Atlantic a couple of years ago. These two factors contributed to a loss of situational awareness, a lack of system knowledge, the quirky AOA sensor behaviour, meant the pilots never regained situational awareness.

And trim may sound trivial, but sudden changes of the center of gravity can be instantly fatal. Once you leave the space of small trim adjustments needed, for e.g. fuel consumption, recovery may well be impossible. That's why load planning of planes and cargo handling / stowing are so important. You don' t want to end up with a center of gravity in a place that is incompatible with flight.

The major difference is that the Lion Air crash took place during the day, in clear weather. AF447 crashed at night, in poor weather conditions.

AF447 was a bad airspeed sensor and a switch of the control system from "normal law" to "alternate law" as a response to the momentary bad inputs. https://en.wikipedia.org/wiki/Air_France_Flight_447

From memory; pitot tubes froze over and airspeed became unreliable. Autopilot disengaged and they had a stall warning. One pilot went full nose down, the other wanted full nose up. The latter probably believed the flight envelope protections were still in place which would not let you stall even if you did full nose up. But they weren't because the flight computer switched to alternate law when airspeed was unreliable so they had full control of the airplane and (probably) didn't realize it. Additionally when presented with two different control stick inputs the flight computer just averaged them, and since there was no force feedback there is no tactile sense that you're arguing with the other pilot over what you should be doing. Also only one person is supposed to be in command.

My memory is also that the senior captain was in the third seat and didn't have a full grasp of the situation either.

I did read the air accident report quite a while ago so this might be wrong, but in summary, it's not exactly arguable that the aircraft did the wrong thing (it did as designed minus the freeze issue which is suboptimal but something that can happen) and the pilots didn't exactly do the wrong thing either since they may have little to no experience flying the plane in alternate law (certainly not at high altitude and speed), and it's not unreasonable to be fully aware that they had full control (as opposed to the flight computer mediating and not letting them do something stupid like stall it all the way down to the ocean).

Lastly, when in cruise it's not completely trivial to fly like a Cessna at 1,000ft. At cruise on a big heavy jet you can't go much higher as the air is too thin to fly. You can't go much faster without braking the sound barrier. You can't go much slower as you'll stall. I believe it's called "coffin corner", and while commercial passenger planes do it every day it's not quite as simple as you might think.

> flight computer switched to alternate law when airspeed was unreliable so they had full control of the airplane

Alternate law still enforces bank angle protection. With full control (direct law) it could have entered spin. (or maybe they could realized situation earlier)

Stall warning was confusing because pulling up even more removed warning temporarily because extreme AoA was interpreted as sensor failure.

it's not exactly arguable that the aircraft did the wrong thing

I still think it's arguable that the "right thing" is for the plane to say "Hey, I don't know how to fly this thing anymore, so I'm giving up! Oh and since I'm so confused, the plane is in alternate law, so all of those nomal-law protections you usually have aren't there." and then handing over control to the pilot that hasn't been actively flying the plane and expecting him to suddenly figure out what's been happening and why the autopilot gave up.

Since the pilots did pretty much the opposite of what they should have done, maybe the plane could have done better itself, even with the unreliable airspeed?

There's no excuse for secretly averaging widely divergent inputs, especially in alternate law (which the plane wouldn't be in unless something is going wrong). It should have picked one and indicated to the other pilot that his/her input was being ignored. IIRC this isn't an issue on the Boeing design, only Airbus.

One the contributing factors was that the stall warning cut out when the nose was pointed higher up, giving the copilot the impression his attempt to full nose up was the right decision. When the nose lowered the stall warning kicked back in. IIRC this was because the computer saw the full nose up as an "impossible" condition and decided not to issue the stall warning. As the nose lowered certain values came back in-range so it issued the warning.

Without these two wildly incorrect behaviors the plane probably wouldn't have crashed.

It didn't secretly average inputs. The plane gave alerts both visual and audible (DUAL INPUT) that the pilots were providing conflicting inputs. I agree that the force feedback of the Boeing FBW system is better, but the pilots should have understood the alerts provided, and either communicated or made use of their override switches.

Solving the problem by ignoring one of the pilots would be an interesting engineering problem. How do you choose?

IIRC they hit the ocean so fast, that death must have been instantenous.

Also IIRC the pilots didn't know what's going on until they hit the ocean.

> the pilots didn't exactly do the wrong thing

Wasn't the one pilot pulling back fully on the stick and not communicating that he was doing that? And weren't the pilots aware that they were in a stall scenario? Which means that pulling back fully was the exact wrong thing to do?

My impression is basically that it was almost entirely the fault of the pilot that was pulling back the whole time. Of course, it sounds like Airbus' controls could provide _a lot_ more feedback to pilots, especially when there are divergent inputs.

All cintrolls, invl. Boeing, are fly by wire now. And yes, pulling back was exactly the wrong thing. I would have to look up the rport again, but if I remember well pulling up silenced the stall warning. So nose down -> stall warning, nose up -> no stall warning.

If memory serves me well, Air France revised their training afterwards to cover that exacz scenario, that pulling up silenced the stall warning was expected system behaviour. Complex systems require in depth system knowledge and quick reaction times. That is one of the points I took away from that incident.

An uncorrected out of trim condition can be quickly fatal. On the light planes (piston and jet) I fly the arm force required to overcome an out of trim condition is considerable even for a modest amount out of trim. A failure of the electric trim control is a highly acute emergency and you should know at least two ways of immediately killing that system.

Countering trim forces was the first scary thing I learned about go-arounds, the first time I did one, my instructor let me figure it out all on my own, and battling heavy nose down trim when you're trying to make best climb is stressful in a training aircraft in a non emergency, I can't imagine what the forces are like in a large aircraft in an emergency.

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