Comment by jmyeet | Hacker News Reader

Comment by jmyeet | Hacker News Reader

Parent Full thread

It's unclear exactly what happened but I've heard some aviation experts talk about this and there are already some leading theories, which come down to two factors:

1. The so-called coffin corner [1] where the minimum safe speed for aerodynamic lift and the speed where the plane will stall narrows to a dangerously narrow window. This is caused in part because the speed of sound decreases at high altitude. Turbulence can throw you out of this window; and

2. It seems likely that control was lost because of ice. In released ATC recordings (which are awful to listen to, be warned) the pilot mentions ice. This is likely because of the bad weather over south Florida right now.

The pilot requested a new flight path and I believe tried to ascend from 35,000 feet to 37,000 feet and ended up in a spiral, banked at ~60 degrees, descending at ~30,000 feet per minute, crashing into the ground.

I'm not a pilot but I suspect there will be some element of this that is pilot error or at least could've been avoided and/or mitigated by an experienced pilot.

[1]: https://en.wikipedia.org/wiki/Coffin_corner_(aerodynamics)

That doesn't match what I'm seeing in the online discussions. I think most people who have a basic understanding of aviation or have been part of it would immediately see this as what happens when you fly a Learjet directly into a thunderstorm. The likely question is whether the first cause of loss of control was ice or something aerodynamic, like pushing the nose down in an updraft, causing the horizontal stabilizer to rip off.

In contrast, the coffin corner doesn't really have anything to do with this the plane would have been operating at standard enroute altitudes and speeds.

For context, I am in fact a pilot. I've read a little about what people are saying about this accident, but I've absolutely no new information or unique insight whatsoever.

I think as a pilot all we can say is “loss of control” but there’s many paths to that given available information.

Staying away from thunderstorms is a good rule of thumb no matter the aircraft and crew capabilities. Severe turbulence is expected within 20 miles of the visible portion of a severe thunderstorm.

AIM 7-1-26 paragraph c is in all caps:

NO FLIGHT PATH THROUGH AN AREA OF STRONG OR VERY STRONG RADAR ECHOES SEPARATED BY 20-30 MILES OR LESS MAY BE CONSIDERED FREE OF SEVERE TURBULENCE.

AIM 7-1-27 can be summarized as stay the fuck away from thunderstorms. This entire section is dedicated to avoidance.

https://www.faa.gov/air_traffic/publications/atpubs/aim_html...

See my other comment. This can't be ruled out yet, particularly because the pilot seemingly climbed to get above the weather where it would've been colder and the air less dense, narrowing the operating window so turbulence or a sudden dive could've spun out of control.

Whatever the cause, it seems like climbing to avoid turbulence in or near thunderstorm conditions when the pilot knows there is ice already was probably unwise. A more prudent course was probably to divert around it and/or to descend to de-ice the plane. If so, would this rise to the level of pilot error? I don't know but maybe.

Im not a pilot or meteorologist but from what others have said it seems that the plane would have been very cold from flying at 30k ft and if you fly into a thunderstorm like that where they have super saturated air just looking for any place to condense on, all that water would turn to ice immediatly on contact with the wings and as soon as the wings lose laminar flow they would stop providing any lift and the ice would increase weight and shift the center of gravity/rotation.

Im not sure how high the top of a thunderstorm can reach but if they could have gotten up and over the top above that 100%+ humidity before it had a chance to condense on the plane then i think that would have been the more prudent choice.

Generally in aviation more altitude is more safe. I think the only mistake was waiting too long to try and climb out of it rather than trying to climb over it to begin with. But im not a pilot so idk.

There will be many questions on why the pilot flew into the tops of what (at least on radar) appeared as a small-ish localized cell at that altitude. Vertical cloud development at those altitudes is a clear sign that whatever is inside is not going to be pretty.

Once the aircraft entered the conditions inside the storm, between the turbulence, icing, and up/down drafts it was likely an unrecoverable event.

yeah but they could/would definitely break during the descent so you can't know if that was the cause or the effect

It wasn't coffin corner. This was a Lear at reasonable altitudes, not a U2 at the edge of space. The death spiral suggest a loss of control authority. It is conceivable that one wing severely iced to the point of unrecoverable asymmetric lift but I think it was something more the icing. It might have lost part of its wing, or the controls on one wing locked into an unrecoverable position.

In the pictures, the flaps appear present but not extended. If they had been extended they would have ripped away at that speed. And they would not have been able to lower them after the overspeed began. I think the answer is in the control system. One or more control surfaces were doing something other than commanded.

Some images seem to indicate a loss/absence of the horizontal stabilizer, but it is hard to tell AI from real these days.

You can't say coffin corner wasn't a factor. As an example here was a Bombardier jet operating at 34,000 feet, well within it's safe operating envelope, that got hit by this because of the wake of an A380 [1].

35,000 to 37,000 feet, combined with turbulence from the weather, means this could come into play. We still don't know and may not know for some time, especially given the severity of the impact and what may or may not have survived. But you can't rule it out.

[1]: https://aviationweek.com/business-aviation/learning-test-pil...

But that isn't coffin corner. The "coffin" is a part of a flight envelope where departure results in near-certain horrible outcomes (ie too fast and wings fall off coupled with too slow and the resulting stall also rips the wings off). A lear or any other business jet is designed specifically to avoid any such things. It should stall, but not in any lethal way. And a lear is small enough that no storm should do things like rip bits off the aircraft. A thunderstorm will toss it around like a tennis ball in a washing machine, but wings/tails should not separate from the aircraft.

> Some images seem to indicate a loss/absence of the horizontal stabilizer, but it is hard to tell AI from real these days.

The descent rate seems the most likely cause of the missing stabilizer.

Icing was mentioned early in the ATC audio, then a request to increase altitude, then a comment that they encountered precipitation, implying the icing could get worse.

IANAP but the mention of precipitation points to conditions that could overwhelm anti-icing systems, which the Learjet was equipped with.

Reply on news.ycombinator.com