Air Pressure and Planes

Factual Questions
1

No, I realise how they fly. I was watching a really bad movie which had a bomb on the plane, in the economy passenger section. They successfully defused the timer on the bomb, but then it had a secondary trigger due to air pressure. Which they said was at 1000m above sea level.

Now my question is, how could the bomb tell the altitude? It was inside the passenger cabin (all of it). I realise there is a pressure difference when flying (ears popping and all that) but is it really that measureable? In the movie they claimed it was a pressure switch. So can this be done? They could walk around with the bomb so it wasn’t attached to anything else either.

And what is the pressure difference (in the cabin) between ground level and cruising altitude?

FloChi

2

I once took an altimeter with me on a flight from London to Edinburgh. At max altitude (somewhere between 20,000 & 25,000 feet at a guess, I dont recall exactly) my altimeter was reading about 4,000 feet (I remeber that as it was roughly the height of Ben Nevis).

3

An airliner can typically be pressurized up to about 8 psi, and long-haul flights can have a “cabin altitude” of up to 8000 feet. When descending, the pressurization will simply be shut off when the delta-P reaches zero, so there’s a smooth transition and no popped eardrums. The “cabin altitude” is under the pilot’s control.

On a short flight like London-Edinburgh, the crew might keep a higher cabin pressure to reduce the discomfort of a steep transition.

4

‘An airliner can typically be pressurized up to about 8 psi’
this has to be overpressure doesn’t it? Or do you mean down to 8 psi? Atmospheric pressure being 15.7 psi at sea level. Or is it that it can have 8 psi overpressure of the surrounding atmosphere (eg up high where it lower pressure)?

So either way, you are saying that the pressure switch wouldn’t work, yes?

5

The 8PSI would be above the outside pressure, sometimes referred to as 8PSIG where the G stands for gage pressure rather than absolute pressure.

6

Right, I meant that the plane can have an internal pressure that is 8 psi higher than the outside pressure. It usually isn’t set that high, though.

And yes, the pressure switch story is a crock.

7

Thanks for that ElvisL1ves. Just wanted to make sure that my thinking parts were up to speed. Not knowing anything about aeronautics.

8

If you know the plane is going to land at a low altitude airport, then the pressure switch gimmick isn’t a crock. You could set it to go off at 2000 feet elevation on the way down, so that when you go up and the pressure drops it arms the bomb, and then when the plane lands BOOM. If you just pretend it is an unpressurized plane flying to 8000 feet and the landing, why wouldn’t a pressure switch work?

Alternatively, you could put the bomb outside the pressurized hull, such as in a landing gear bay.