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Comment by salawat | Hacker News Reader

If the reverser could rip the engine off the wing, it wouldn't be safe to engage on the ground. Just thought that was worth pointing out.

A reverser only redirects thrust, and only generates about 60% of max thrust pushing backwards.

It's only on for a limited time on the ground because under a certain speed, you run the risk of blowing FOD into the intake path, putting the engine at risk.

I'm not aware of any reason to outright lockout a reverser in flight other than to hedge your bets against a very, very poor set of configuration choices by the pilot (reversing near stall speed too low to recover). I could foresee scenarios where being able to use reversers in the air could save the aircraft with the right combination of subsystem casualties.

Not a pilot or engineer, but absolutely LOVE aviation.

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The cruising airspeed of an airplane is faster than its landing speed. I don't know if that alone is enough to rip the engine off the wing, but it's something that it sounds like you did not consider in making your point.

[EDIT remove "roughly speaking an order of magnitude"]

>The cruising airspeed of an airplane is roughly speaking an order of magnitude faster than its landing speed.

No. No it is not. 787 approach speed is 145-153 knots. It does not cruise at 1400+ knots.

Thanks for bringing more precision to the conversation (really).

What would you like to call the factor? 3? Call me a weasel, but I was hoping "roughly speaking an order of magnitude" would go down to 3. I suspected it was comfortably above 2. Am I right about that?

> I was hoping "roughly speaking an order of magnitude" would go down to 3.

I'd call that half a order of magnitude, considering log₁₀(3) is around 0.477. So you're off by more than a factor of two.

You know, I considered saying that the "roughly" buys me a factor of two in log space. I didn't want to be that person, but I'm glad you were ;).

It's surprisingly non-trivial to pin down exactly what the 'correct' scale[0] for a given measurement is. I do agree about "roughly" generally being up to a factor of two in the appropriate scale[1], although I'm the sort who thinks a 19% increase (or 16% decrease) should be called a quarter of a factor of two.

0: Uniform, linear and logarithmic are obvious candidates, but depending on the domain you can end up with some really wierd scales (eg floating-point ULPs, which can look logarithmic or linear, but aren't either).

1: hence > So you're off by more than a factor of two.

3 is half an order of magnitude (because it is a good approximation to 10^0.5)

re go down to 3: Use different words and see if you think it was reasonable

roughly 10x going down to 3x would also mean it would go up to 17x. That's a pretty wide range, so I don't think that order of magnitude is going to ever really be similar to 3x of something on the basis of what it means.

but what about a rough order of magnitude? There is definitely some qualifiers you can put on "order of magnitude" to make it include 3.

"within an order of magnitude"

It seems unlikely that a jet engine could rip itself off the wing in any scenario based on thrust alone. The forces involved are not that large.

Your wording makes it where you aren’t factually wrong, but a plane was in fact wripped apart due to the thrust reverser engaging on one engine of a 767:

https://en.m.wikipedia.org/wiki/Lauda_Air_Flight_004

The difference isn't that large in the grand scheme of things. The engine mount would have to be able to fail in it's operational envelope in order for the reverser kicking in to be realistic.

In fact, the reversed kicking in would decrease the loading on the aircraft by decreasing it's airspeed.

This isn't a case of acceleration being able to break the mount from the frame. If it ever could, one wouldn't want it on the plane in the first place.

Doesn't mean you couldn't ruin your day with it, but it isn't an instant catastrophic failure either.

> but it isn't an instant catastrophic failure either.

Read the accident report and follow up on Lauda Air Flight 004. Boeing specifically was forced to issue a statement that it was virtually impossible to overcome a catastrophic failure outcome from a thrust reverser deploying at cruising speed.

I stand corrected. Way to go Boeing. Seems like that should have come up in a design meeting at some point.

Apropos, the Wikipedia article (https://en.wikipedia.org/wiki/Thrust_reversal) mentions number of events where an engine reverser was deployed in flight, one involving a 767 which resulted in the loss of the aircraft.

That counts as a pretty serious thing to avoid :-).

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