China Eastern 737 Down in Guangxi

[Gabriel]

Wait, so they dropped out of the sky at constant speed? Makes no sense.

[Gabriel]

FWIW: https://www.youtube.com/watch?v=uUkSlxcC8Jc

[3WE]

Wait, so they dropped out of the sky at constant speed? Makes no sense.

I agree that’s strange- Speed COMPONENT across the ground will be reduced in a steep dive, while airspeed would normally climb…thus off-setting but ‘perfectly’ off-setting…a hell of a coincidence, and very questionable.

And, for there being a fire, the supposedly-fresh crash site photos seem short of smoke and burnt trees.

[Evan]

FWIW: https://www.youtube.com/watch?v=uUkSlxcC8Jc

Concerning the timing, TOD is the first stab trim move since arriving at cruise, correct? If there was a jack screw failure, that might be the moment it let go.

The dive, brief recovery and second dive are reminiscent of Alaska Airlines.

[Gabriel]

All that follows is based on my very limited knowledge of the 737 NG systems. I am not super confident that I am getting everything (or anything) correct.

Concerning the timing, TOD is the first stab trim move since arriving at cruise, correct?

Not correct. The trim is making small adjustments all the time.
The way the AP works in pitch, the AP commands the elevator but the trim as a function to zero the force the the AP has to do on the elevator.
So fuel is consumed and fuel is not exactly at the CG? The trim will be compensating frequently.
A pax in the first rows of economy goes to the restroom at the back of the plane? The AP will compensate and the trim will immediately compensate back.
Basically, the trim is making fine-tuning adjustments all the time during cruise.

BUT if the trim is off or stuck for some reason, the AP has ability to compensate without the help of the trim up to the point where it reaches its force limit and gives up (disconnects and lets go).
BUT if the trim is not working AP operation is not allowed (that is procedurally, since there is nothing in the systems that would physically prevent you from using AP).

That said, the force at which the AP disconnects is not that high. The force that the AP was doing up to that point should be relatively easily to be done by the pilot.
A scenario where the trims stuck, the AP is compensating, but suddenly it lets go, and the pilots let the plane dive vertically is hard to accept a priori.

If there was a jack screw failure, that might be the moment it let go.

Even with what I said before, the above is not impossible.

The dive, brief recovery and second dive are reminiscent of Alaska Airlines.

I have to admit that, for no specific reason, Alaska was the first thing that came to mind when I saw this accident.

That said, what brief recovery are you talking about? I still have to see ADS data that shows the plane leveling at any point after the descent initiated. The graphs you posted before don't seem to support that.

[3WE]

…or if the Jack screw simply broke at that time, or etc. Still, Alaska was doing that P word you like…in spite of words to the contrary, it’s a pretty darn stable dive (with constant ground speed…)

[3WE]

I’m worried that this might be too much for the black boxes AND that they may really really be needed…

[Peter Kesternich]

Two things to be considered, especially since references to Alaska Airlines flight 261 came up here:
1. Unlike in the case of Alaska 261, it appears that there was no communication from the plane. What happened that made the pilots unable to call ATC?
2. The accident sequence for Alaska 261 between the first loss of control and the final impact took about 11 minutes and the plane recovered once before the final dive. In the FR24 readout China Eastern appears to be dropping straight down.

[Gabriel]

Two things to be considered, especially since references to Alaska Airlines flight 261 came up here:
1. Unlike in the case of Alaska 261, it appears that there was no communication from the plane. What happened that made the pilots unable to call ATC?
2. The accident sequence for Alaska 261 between the first loss of control and the final impact took about 11 minutes and the plane recovered once before the final dive. In the FR24 readout China Eastern appears to be dropping straight down.

The Alaska 261 guys were "lucky" (for a short time) that they only got a partial failure where the acme nut moved only until certain position (the stop which was NOT designed or intended or supposed to resist the aerodynamic loads), then the stop failed and they got the final dive. Had the stop failed at once the first time it was hit (which is not unreasonable at all to happen) they would have gone in the final negative-Gs dive at once.

The other thing that made Alaska different is that, during the final dive at negative Gs, the pilot had the astonishing presence of mind to roll the plane inverted to make these negative Gs point up, so they were able to keep flying more or less under control inverted (but lacked enough elevator authority to keep the pitch above the horizon, and by the way the engines were starting to fail too since they are not design to fly inverted, there would be all sorts of issues with fuel and oil supply).

Now modify the Alaska scenario where the stop brakes loose the first time it is hit and the plane goes into a steep dive that cannot be compensated by pulling back on the yoke and you can have a scenario that matches perfectly what we know so far about this china Eastern accident. The pilots would be hanging from the seat belt (negative G's), pulling back both of them with both hands as much as they can (still insufficient) and making a mayday call goes way down your priority list. Not to mention that you may have lost your headset and you are not going to release the yoke to grab the mic.

VERY IMPORTANT DISCLAIMER: I am not proposing or even suggesting that this is what happened. Actually it is probably something else. Just showing how an Alaska-like scenario could be compatible with the little we know about this accident which (shall I say it again?) is little.

[3WE]

The accident sequence for Alaska 261 between the first loss of control and the final impact took about 11 minutes and the plane recovered once before the final dive. In the FR24 readout China Eastern appears to be dropping straight down.

Concur.

Fuel starvation from being inverted was another factor that (sort of) ultimately doomed Alaska.

[Peter Kesternich]

All good points...

On a very different level, one thing that struck me browsing through the comments on AvHerald. One user commented that "to this day not a single B737NG had a fatal accident due to technical reasons." (on AvHerald By AndyL on Tuesday, Mar 22nd 2022 14:42Z)... I checked the Wikipedia article on 737NG accidents and none of the previous accidents are attributed to a technical problem involving the plane.
Can anybody confirm that?

[Gabriel]

All good points...

On a very different level, one thing that struck me browsing through the comments on AvHerald. One user commented that "to this day not a single B737NG had a fatal accident due to technical reasons." (on AvHerald By AndyL on Tuesday, Mar 22nd 2022 14:42Z)... I checked the Wikipedia article on 737NG accidents and none of the previous accidents are attributed to a technical problem involving the plane.
Can anybody confirm that?

I didn't go through all the list but it is never that clear cut.
The same guy contradicted himself in the same post when he acknowledged that a person died in that Southwest flight after the engine failure from which debris flew about and severed a window. That is a fatal accident doe to technical reasons.
So he moved the goal post and said "but it was not a crash". But then acknowledged the PIA accident in Amsterdam annotating that it was "a minor fault that the crew should have been able to manage". Well so were the 2 MCAS accident (not NG, just showing how an unclear idea can be modeled the way you want).

So let's try to be more specific: "An accident that resulted in hull loss and fatalities, in which the crew could have done nothing to avoid it". And you have VERY few of those across all the worldwide aviation fleet regardless of type. Even in the Alaska one you can say that there was nothing wrong with the plane itself but with how the maintenance team mishandled the maintenance. And then you can still say that if the pilots would have not tried repeatedly to unlock the trim by resetting circuit breakers several times and actuating the primary and secondary trim motors several time, the acme nut would have most likely remained stuck where it got stuck initially during the climb and they could have landed the plane in that partial mistrim condition, instead of playing with the stuck trim until they manage to make it fail totally (once again, not my thought, just showing how you can play with ambiguous concepts).

Short of TWA 800 and very few others, there are very few accidents where only the plane and nothing else was at fault. At the same time, there are very few accidents (out of intentional acts) where it was 100% human error and the design could not have been made better to prevent the crash. Take AF 447, a glaring pilot error to use mild words, and I wonder why does the AP and AT have to say "I'm, out, your plane" the instant that there are unreliable speeds? The memory items for unreliable airspeed (a set initial pitch and power setting) and the following fine tuning (pitch and thrust look-up table based on weight and altitude) would be very easy to program in a fly-by-wire system as complex as that in the A330 so the AP can temporarily handle it for some several seconds until the pilots get their wit together, diagnose the issue, run the checklist, and eventually disconnect the AP.

But yes, the 737 NG is a very safe plane. But then what transport category airplane isn't?

[Evan]

The other thing that made Alaska different is...

I'm not suggesting that this crash is a carbon copy of Alaska 261. Only—as you seem to be—that a sudden jackscrew brake failure scenario might fit. In the video you just posted, Blancolirio states that the ADS-B reveals that the dive was briefly interrupted by an 8000ft climb. I see this on the plot, but I have questioned whether it is reliable or a data anomaly. It seems quite impossible given the timeframe. But if it is reliable, it might correlate to a momentary recovery of pitch control, followed by a repeated failure or structural failure due to recovery forces. The latest report states that there are two distinct wreckage impact areas, which would confirm some structural failure in the late stage of the dive event.

The reasons giving weight to a jackscrew failure: the sudden, catastrophic nature of the upset, the coincidence of TOD and the fact that a sustained dive such as we are seeing on the videos (assuming a non-inverted, intact airframe) would require sustained nose-down stabilizer forces.

[Gabriel]

In the video you just posted, Blancolirio states that the ADS-B reveals that the dive was briefly interrupted by an 8000ft climb.

He says the exact opposite to that. Watch again.

I see this on the plot

Where? I look at the plot and I don't see it even when I actively try to.

[3WE]

He says

I keep reading of the climb…but it’s not obvious on the graph. I do get a kick out of the flight radar display- it’s playing math games to anticipate where planes actually are…I’ve seen a lot of sudden back pedaling (a bicycle term) of plane icons on runways.