Wednesday, March 09, 2011

More Pressure

These puppies have pressurized airframes, and so have an entire system to control the coming in and the going out of the air. Like the other posts in this series, this one will concentrate on what I have to know, with little concession to those who don't know what I'm talking about, but I find it most useful to explain things in quirky ways, so it may be interesting anyway. And feel free to ask questions. You may hit on something I ought to know. The pressurization system includes static ports (holes in the airplane used to sense how rarefied the outside air is), bleed air (compressed air from the engines), an outflow valve (a hole to let the air out), a dump valve (a bigger hole to let all the air out if you need to) a controller (a gauge in the instrument panel with a dial and a knob that you use to tell the airplane how much air to keep inside and how much to dump overboard), a couple of safety valves, a bunch of lines and relays and things, and a couple of gauges that tell you how all that is going for you.

The maximum differential pressure between the inside of the airplane and the outside of the airplane is 7.0 p.s.i., giving a sea level cabin at 16,800' and a 4600' cabin at the service ceiling of 25,000'. The various gauges work kind of like an altimeter or a VSI, comparing reference pressure, cabin pressure and static pressure as appropriate to drive indicator needles. Pneumatic relays associated with the controller create pressure differentials that can drive valves. There is more detail available on this system, but all they need me to know is that I turn knobs to set the controller and that the controller commands things to happen according to my whims, such as allowing or not allowing air out the outflow valve in order to hold the cabin pressure at the set value. The outflow controller will also open if the differential between cabin and external pressure exceeds 7.25 p.s.i. or if the outside air pressure exceeds the inside. When these things aren't happening, a spring holds the outflow valve closed. The training manual describes a number of interesting diaphragms and valves that activate the opening and closing of the outflow valve under different conditions, but they want me to say only that it is opened by vacuum and closed by a spring.

The cabin air pressure dump valve has a similar pressure release function, as the outflow valve, both in the overpressure and underpressure case, plus it can be opened with an electric solenoid valve to dump the cabin pressure, equalizing the cabin with the outside atmosphere. Technically the electric solenoid connects the dump valve to vacuum pressure, causing "the dump valve diaphragm to open the dump valve." But I think I would lose points for getting into that detail, and should say that it is opened electrically, using a solenoid that gets its power from the left essential DC bus, transferable to the right. When pressure is dumped, it is dumped into the nose baggage compartment, which happens to be the same place the alternate static source gets its pressure, so a cabin dump should not be performed while instruments are operating on the alternate static source.

There is also a manual control valve that can be used to bypass the pressure controller and directly adjust the opening of the outflow valve. For the manual valve to have an effect, a switch on the console must be moved from auto to manual. This is the sort of thing I expect from pressurization systems, so you can perhaps get an inkling of the sort of things pilots were yelling at the screen during Snakes on a Plane, which if I recall correctly was the movie in which they solved an overpressurization problem with gunfire through the fuselage.

More interesting than the magic boxes and networks of vacuum hoses is how I use it. First some terminology. The cabin altitude is the altitude I'd be at in an unpressurized airplane for the cabin to be at the pressure it's at. The rate of climb or descent of the cabin is the rate of change of the pressure there. With pressurization, the cabin altitude can be different than the airplane altitude, so that we don't have wear oxygen masks above 13,000'. To set the pressure I dial the controller so that the bottom part displays about a thousand feet higher than the altitude I will be flying at. That way the arrow points to the minimum cabin altitude I can maintain for the flight, without worrying about it banging up and down on small altitude deviations. If the world were flat, I could mostly leave it at that, but airports are at all different elevations and this introduces some issues.

To begin with, the airplane is not pressurized on the ground. It's bad for the airframe to land pressurized, there's no point, you'd have to depressurize to open the door, and why pump up the cabin to an altitude lower than the one the passengers walked in the door at? It wouldn't work anyway, as the dump valve is linked to the squat switch and if it's working properly it stays open on the ground. If you set the cabin altitude to sea level then took off from an airport with a 3000' altitude, as soon as the weight came off the wheels, the dump valve would close and the airplane would try to pressurize the cabin down to sea level. That's just weird, and most people don't like the feeling on their ears when they are descending. The whole point of a pressurized airplane is to avoid that as much as possible. So I set the cabin altitude to the higher of what I will be maintaining in cruise and the departure airport pressure altitude plus a thousand. So coming off a 3000' airport I'd set it to 4000' if I was cruising at 20,000' or below, but if I was going to cruise at 25,000', I'd set the cabin altitude to 5000' or so, because the controller says I can maintain a cabin altitude of 4600' at 25,000'. So I'd take off, the dump valve would close, and the cabin pressure would be 3000'. The controller would not start pressurizing the cabin because it was already at a lower altitude than selected. It would let some air leak out as we climbed to 4000' and then it would stop letting air out and start hoarding it, to keep the cabin altitude at 4000' as the airplane rockets on up to cruise altitude. At top of descent I would set the controller to maintain a cabin altitude about 500' above the landing airport, and a descent rate of 500 feet per minute. If the landing airport were at 1000', that would take five minutes, and meanwhile I could be plummeting through the real descent at 3000 fpm without hurting anyone's ears.

The thing to watch is if the cabin is going to take longer to reach the landing elevation than the actual airplane, because you can "catch the cabin." The problem with that is that the cabin cannot be at a higher altitude than the airplane, because that would mean less pressure inside than outside, and the pressure vessel is only designed to take pressure from the inside. The outflow valve will automatically let in outside pressure if it exceeds the inside pressure, so as soon as you catch the cabin, you feel the descent just as you would in an unpressurized airplane. So to prevent this you can increase the descent rate of the cabin, decrease the descent rate of the real airplane, or level off at an intermediate altitude to allow the cabin to get ahead.

The pressure comes from P3 bleed air, from the second stage diffuser. I'll probably have much more to say about this system once I've actually worked with it.

When I need a break from hard studying work, I start filling out all the company forms. I have to put my height and weight on my security pass application. I'm 1.5 kg above my target weight. I omit the half and put myself down at one above. Or I could wait a week and lose those last 1500g and put down what I should be. I look way better now. You don't notice the little layer when it creeps up on you. A little more butt toning and I'll look the way I want to.


Sarah said...

Interesting post - the idea of setting cabin altitude and/or it's rate of change to a target altitude... one more thing to manage.

"...catching the cabin." The problem with that is that the cabin cannot be at a lower altitude than the airplane, with less pressure inside than outside.

I puzzled over this. Do you mean the cabin cannot be at a higher altitude ( lower pressure ) than the airplane?

Aviatrix said...

Sarah: Yes I did. I'll fix that now.

sean said...

I'm loving the technical posts. They appeal to the nerd in me... :)

But "If you set the cabin altitude to seal level then took off from an airport with a 3000' altitude", the seals would be quite uncomfortable ;)

Jez said...

Congratulations on (nearly) meeting your target weight goal!

gmc said...

operational hint: ears seem to more easily withstand a higher rate at high alt. vs. near sea level... in my experience. I'm guessing it comes down to the corrected pressure change rates going on vs. the indicator reading.

So when faced with a late descent, better to be a little aggressive on cabin rate while high, then taper off when lower. Of course in an ideal world...