# Would ground effect and FOD ingestion be factors at high speeds close to the ground?

How could flight 77 into the pentagon overcome the compression lift of the ground effect at 460 KIAS that close to the ground? Also wouldn't the engines ingest FOD at that power setting that close to the ground on the approach?

• At appears that you're thinking that an airplane traveling fast enough could never crash into the ground ... Dec 25, 2018 at 21:59
• If an airplane could not fly close to the ground, it could never land - duh! If you want to fly in ground effect at much more than stall speed, you just need a bit of down elevator. (At least in a single engine prop plane - I can't afford a 757, but I see no reason they'd be fundamentally different.) Dec 26, 2018 at 5:42

The amount of time a plane in a dive spends close enough to the ground to ingest foreign objects and thereby damage its engines is of order ~fractions of a second.

For a plane that is about to strike a building on the ground, the idea of FOD damage to its engines is not even relevant.

How could flight 77 into the pentagon overcome the compression lift of the ground effect at 460 KIAS that close to the ground?

Pitch. If an airplane's nose is low enough, it will descend and eventually collide with the ground.

In the extreme example, with the nose pointed directly down, lift is no longer holding the airplane up at all; the only force keeping the airplane's descent in check is drag. I don't know exactly how fast the plane will end up descending, but it will be pretty dang fast.

But an airplane doesn't need to be pointed straight down to have a violent collision with the ground. A shallower pitch will also work.

• Might want to compare SilkAir Flight 185 (even though that particular case is disputed).
– user
Dec 25, 2018 at 18:57
• @aCVn: Not seriously disputed - every one of the investigators involved agreed that it was a case of pilot suicide, with the exception of the NTSC chairman (who then promptly overruled the rest of his team and declared the cause of the crash "undetermined"). Feb 14, 2019 at 1:25