Escape slide question

[Hartington]

As I understand it transport aircraft doors are designed in such a way that once the pressure inside the aircraft is greater than outside the door is pressed against the fuselage so that even if the open handle is moved the door cannot actually open.

If that assumption is wrong the rest of the question is rubbish!

Moving on.... At what point does the pressurisation have sufficient pressure to hold the door in place? During taxi? I doubt that given the incident at JFK(?) where a flight attendant jumped out. During take off? But what happens in a take off crash, how does the pressurisation know to return itself to the same as outside?

[TRPGpilot]

aircraft are not pumped full of air like a truck tyre. As you climb into the atmosphere, its pressure decreases. To correct this the air conditioning in an airliner will maintain pressure equivalent to a set altitude, of about 8000ft. logically this gradually will require air to be pumped into the cabin as the aircraft climbs, REDUCING pressure inside gradually. ( sounds strange, but at sea level there is more pressure inside than at 8000ft) Anyway this pressure is maintained right up to cruise, so at 35000ft, the inside pressure is still maybe 8000ft. The opposite happens on decent. Its the difference in the inside and outside pressure that plugs the door, hence the door cannot be plugged at sea level.

best way to demonstrate is to drink half a bottle of water at altitude and close the bottle tightly. Once back on the ground, the bottle will be crushed, due to less pressure inside it than outside. so as it climbs, pressure is lost then equalized to 8000ft right to cruise, and on decent its equalized back to greater than 8000ft, sea level in fact

[Hartington]

Thank you trpgpilot. That's a very good start. But at what point does the pressure differential become significant? It's not going to be a specific altitude if only because of high altitude airports (which is a subject on its' own - what happens when you start from somewhere like La Paz Bolivia?). So, at what rate does the cabin altitude increase when the take off is from sea level? Does it just follow the aircraft altitude until somewhere around 8000ft and then stay ay 8000ft or if, for example you've got to 5000ft is the cabin going to be at 5000ft or something lower?

[Pub User]

Generally the cabin altitude will 'climb' at about 500fpm. but the aircraft will climb a lot faster than that, so the difference between aircraft altitude and cabin altitude gradually increases as the aircraft climbs. At what point the differential becomes significant depends on what you call significant, but it will certainly be well above the height at which you may want to leave the aircraft!

High-elevation airfields are dealt with by a gradual adjustment either from the airport altitude if departing from there, or to it if it is the destination. Details vary between aircraft types.

The pressure differential is controlled by an adjustable opening called an 'outflow valve', which would normally be opened, either automatically or manually, if an evacuation was required.

[Hartington]

OK, got it, thank you gentlemen.

[oldtora]

But I drank half a small plastic bottle of wine , and at sea level it was not crushed. Would the bottle be crushed only if it is water ? :cool:

[fergineer]

Hartington, the aircraft are not pressurised on the ground a height above airfield level is set in the pressure controller and as the aircraft climbs through the set height the outflow valve starts to close and the aircraft slowly pressurises. As the differential pressure increases as the aircraft climbs opening the door with very little diff on would be nigh on impossible think of the pressure in pounds per square inch and then think of how big the door is. Again on the descent the aircraft will be fully depressurised before landing and therefore no pressure holding the door closed. Hope that helps.

[DaveReidUK]

Its the difference in the inside and outside pressure that plugs the door, hence the door cannot be plugged at sea level.Though it's worth pointing out, at the risk of confusing the OP, that you can pressurise an aircraft on the ground. That's normal practice if you need to do a pressure test for whatever reason, saves having to get the aircraft airborne.

Remembering, if the doors aren't plug-type, to placard them in case some idiot on the outside opens one during the test - not a good idea ...

[wiggy]

You should be unpressurised during taxi -out/taxi-in, so if somebody trys to open the door during that stage in proceedings it will open ..:ooh:

Can't speak for all types but during take-off some aircraft definitely do start to pressurise to a very small degree during the roll itself (i.e. whilst still on the ground). For example on the 744 the pressurisation outflow valves start to close as you accelerate through about 80 knots, the 777 does much the same - according to it's Flight Crew Operations Manual: "the system supplies a small positive pressurization prior to rotation to cause a smooth cabin altitude transition to the cabin altitude climb schedule".


During take off? But what happens in a take off crash, how does the pressurisation know to return itself to the same as outside?

It doesn't "know" - it gets told :p ... On the 744/777 one of the First Officer's initial actions in the event of a passenger evacuation being required is is to motor the outflow valves fully open in order to ensure the aircraft is depressurised....now of course if there's a hole in the hull...

[TURIN]

Just as an addition to the subject of doors being opened by nutjob pax.
Some of the more modern a/c doors are fitted with flight locks that automatically engage at approx 80kts IAS. They disengage again after landing.