Airliner cabin pressurization q

Factual Questions
1

I am a small plane pilot and aerospace engineer, but have no experience with cabin pressurization systems. Something happened yesterday on an airline flight that I want to ask about.

We were boarding a Canadair Regional Jet that was sitting in the hot sun. The cabin was uncomfortably warm, and the crew explained that the auxillary power unit (APU) was not working, and it would cool down once the engines were started. The flight attendant tried to joke about it, but I was not amused (I’m sweating in your hot broken airplane, and you want me to laugh about it?).

I noticed some warm air circulating thru the vents. After the cabin door was closed, I noticed the cabin pressure went thru some unususal swings. Normally, I never even notice air pressure changes in an airline flight unless I have a cold or something, but these swings in air pressure were large enough to make me stop reading and clear my head.

So- 1st question - with no APU or engine power, how is the air being moved in the cabin? I mean the warm air in the vents, and the air needed for cabin pressure? Were we getting compressed air from some ground source? Or is there some way to use electrical ground power to move this air? I would not have thought you could pressurize the cabin without at least APU power.

It gets more interesting - as we sat for awhile, it became apparent we weren’t going to start engines very soon. The flight attendant passed out water and made more jokes. Then the cockpit crew summoned her, so I started watching. After they talked to her, she moved to the door (to the outside) and started to open it. I thought whoa- I hope she knows what she’s doing.

BAM!

As she opened the door, a loud bang went thru the aircraft, as the cabin pressure and the outside were clearly not equal. I saw her hair and clothing take a jump, and I felt the surge of air as it moved thru the cabin. Almost everybody on the plane reacted, some people dropping what they were reading. She tried to make more jokes - “you really got my heart going this morning!” and such. I don’t think I would have been amused in the least. I did not see the door, but I have to believe that if she got a finger or her head near the door when it moved, she could have been seriously hurt.

2nd Q - Why would they be fiddling with the cabin pressure at this point in the flight anyway? Am I wrong to think that somebody up front screwed up in asking her to open the door without equalizing the pressure? Let me be clear, I saw her talking to the crew just before opening the door - so I know she wasn’t doing it on her own.

I did not hear any explanation or apology from the flight crew after this, which really suprised me. One of the pilots got out of the aircraft, and talked about something with a ground guy, then got back in. Any inexperienced travelers could have been seriously frightened and alarmed by the situation.

Pilot dopers - any explanations?

2

Fans were probably running off battery power with no engine or APU running.

The pressure differential probably came from the sun heating the plane and the air in it. I’m surprised she was able to open the door at all, aren’t they designed not to open with a pressure differential?

On a side note I flew on a 757 thursday and had nearly finished a bottle of water before they started descent. I went to take a drink when we were near the ground and the cabin pressure change had crushed the bottle to about 2/3 of its full volume.

3

You can certainly move air about in the cabin by using electrically driven aircraft fans, however that’s all it would be, just moving existing cabin air.This should not pressurise the cabin.
It would also be possible to have conditioned air (at least on all the aircraft I’ve worked on) from an external source which would plug directly into the airconditioning system and would then be controlled and distributed by the aircraft system. I don’t think that was happening to you though as you say the air wasn’t conditioned.
Normally you are correct when you say the APU/ or engines would be required for pressurisation (although in the past I have seen maintenance pressurisation checks carried out on an aircraft with no one on board and using external test equipment to supply the air).

It sounds as though the crew had an air pressure problem, but one thing is for sure it was dangerous to ask one of the cabin staff to open a door without first making sure the air pressure inside the aircraft and out were equalised. In my experience the crew would not only check their instruments but actually open the DV window (if there is one fitted, which has always been the case on the aircraft I’ve worked on) to confirm the pressures were equal.
It’s only a small consolation, but there couldn’t have been much of a pressure differential otherwise the crew member would have been unable to open the door.