Total pack failure
 

Hi,

I'm studying some jet aircraft systems and learning about pressurisation. However, I don't think I've got a total understanding of what happens with a total pack failure (not a rapid depressurisation-door failure etc). My questions are:

I understand cabin pressure will rise, but why can't the outflow valves seal the cabin completely to simply maintain the cabin pressure? Also, will cabin temperature rise or fall?

Thanks

Most aircraft have atleast 2 packs. A dual pack failure or other situation requiring both packs (or both engine bleeds) switched off is unlikely.

In this case, the outflow valve will move to the fully closed position in an attempt to maintain cabin pressure. The aircraft will tend to depresurise as they are not air tight. How fast it depresurises will depend on how much it leaks. It is cabin altitude that will rise, not cabin pressure.

Initially cabin temp would fall with the drop in pressure, although later will heat up quickly if the cabin is full of people and no other action is taken.

However, it is likely that a 2-pack airplane is allowed to dispatch with 1 operable pack. In that case, a second failure is not unforeseeable...

Normal leakage will allow slow depressurization even if the outflow valves are closed.

The ram air valve installed in some (e.g., 747 Classic) airplanes may give a few tenths of a psi of differential pressure, but that's all.

Without a means to pressurize the airplane, cabin pressure doesn't rise; it falls. Cabin altitude climbs. With most aircraft, cabin altitude limiting will drive the outflow valves closed at a certain point, typically around 11,000 cabin pressure altitude...but this doesn't buy much or do much if the packs aren't available.

The problem is that while bleed air is available, it's too hot for cabin use, and the packs are required to deliver air for use in pressurizing the cabin. Without packs or a cooling system (packs aren't used for all aircraft), it's either emergency bleed air (if the aircraft has such a feature; not all do), or a descent to a lower altitude.

At a high altitude, cabin temperature will fall if no bleed air is being admitted to the cabin. In some aircraft that utilize emergency bleed, such as the Learjet, the cabin can get very hot.

An aircraft cabin isn't a tight pressure vessel, like a balloon. It won't stay "inflated," and leaks too much to hold pressure. In some airplanes, even items such as door seals will deflate as soon as bleed air is lost, because it's bleed and internal cabin pressure that inflates, and holds the door seal...meaning that when the bleed goes away, even more leakage occurs.

Pressing the "Ditching" PB will help. :ok:

Loss of packs
 

After the failure of both (or all) packs, with the aircraft sealed as well as it can be, what source of ventilation is there? Five or six hundred lungs pumping away in a sealed tube doesn't sound like a good idea.

True the air will become quite stale. However in order to combat this problem, aircraft are equipped wtih recirculation fans which pushes ambient air through the distribution ducts to the eyeball outlets above the passenger seats. On the older Boeings, this was called the gasper fan.

This following is an exerpt from the Boeing 737 NG Manual.

The recirculation fan system reduces the air conditioning system pack load and the engine bleed air demand. Air from the passenger cabin and electrical equipment bay is drawn to the forward cargo bay where it is filtered and recirculated to the mix manifold. The fans are driven by AC motors. Each recirculation fan operates only if the respective RECIRC FAN Switch is selected to AUTO. In flight, the left recirculation fan operates if both packs are operating unless either PACK switch is in HIGH. The right recirculation fan operates in flight if both packs are operating unless both PACK switches are in HIGH. On the ground, the left recirculation fan operates unless both PACK switches are in HIGH and the right recirculation fan operates even if both PACK switches are in HIGH.

Very true, but in that case we are limited to FL250 max (at least in the 737). As others have said, the outflow valves will close as the cabin altitude starts to rise. In the event of a climbing cabin with both pack lights illuminated and inextinguishable the QRH calls for a descent to 10 000' or the highest safe level. At 10k the outflow valves can be safely opened and the depresured AC can be continued or diverted as decided by the captain - although at 10k, regardless of company SOPs, a fuel diversion would be the likely probability.

RIX

On older aircraft one of the more common occurrences is that if one bleed/pack fails, the remaing bleed will be supplying both ac packs and does frequently lead to overloading of the precooler or temp/press controls on the remaining system resulting in a second bleed failure and press'n loss.

As soon as the first failure happens, selecting the affected bleed/pack off will avoid the cascade of overloading the remaining bleed/pack. A single bleed/pack will normally keep the aircraft pressurized.

Not a problem on the Boeings... as only one bleed source is permitted to operate one A/C Pack. Don't know about the Douglas or Lockheed, or Airbus.

SNS3Guppy

In the GLEX, we have emergency pressurization just as in the LR. It would only come into play until both packs are inop, both channels on both pressurization controllers were inop and operation above 10,000 were absolutely necessary. From the testing, I understand cabin temp will stablize at around 85-90 degrees F at flight levels. Bearable, but uncomfortable.

GF

Not all Boeings...

In the 747 (both Classic and 400) 2 packs can be run on a single bleed source, even the APU. Also, in normal operation, bleed sources are not isolated, so all 4 engines supply air to a common manifold.

True statement about the APU while on the ground, and your point about the 747. When the APU is being used pneumatically, and if it's threshold temperature is reached the APU load control valve will modulate close to reduce air load to protect the electrical system.

On some aircraft lectrical loads will be shed automatically during engine start if the APU's threshold temperature is reached.

In the Lear 35, the temperature goes up substantially; it gets very hot, and also loud. The lear doesn't have a "pack" per se, but a cooling system comprised of a hot air bypass valve and flow valve, along with a heat exchanger. It's not really a pack, and there's no ACM. Some aircraft are equipped with freon air conditioning.

Yes, I understand, probably like emergency pressurization on the CE-500.

GF

Could be. I'm not familiar with the Citations. The system in the Lear is fairly primative compared to most turbojet aircraft, but it's also an early system, lightweight, and for what's required of it, works well.

Loss of packs
 

Perhaps I didn't express myself clearly enough. My question is , with the outflow valves closed and the aircraft effectively sealed, is there enough oxygen in the recirculating atmosphere in that fuselage to sustain life for 3 or 4 hundred people on what may be a long leg to the nearest suitable airfield? After the cabin is depressurised below 10,000' through normal leakage, maybe the outflow valve(s) could be opened to encourage some flow but where is the air inlet in this situation? Ram air? where from? DV window???

Even in a NEW aircraft, it is never effectively sealed. In other words, it will leak and depending on the inherent leak-rate and altitude, this will determine the level of the emergency. In all likelihood it will cause an emergency descent but of course this rather aggressive maneouver can be modified to suit the cabin depressurization rate. In simulator scenarios, I have seen anywhere from 500fpm to 2000fpm cabin altitude climb rate, solely based on losing pressurization capability. (ie, no explosive decompression, faulty outflow valves, or hole in aircraft). You have to get below roughly 14K' before the O2 masks auto-deploy in the cabin. Then you need to get to 10K' for any sector over 30 minutes.

It is not the lack of O2 that will get you, it is the partial pressure of the O2. There is probably enough oxygen in the cabin, but actually getting it to pass from a person's lungs to his bloodstream, is the problem. A certain amount of atmospheric pressure is required.

Ram air is recommended in most circumstances below 10K' or less than 1psi differential pressure. The outflow valves should then partially open to allow for adequate airflow. Most large aircraft have a ram air inlet for such situations, as well as smoke removal procedures.


Oh yes, you asked if cabin temp will rise or fall. It will fall. Since pressure decreases, temp falls... just like in the earth's atmosphere. Naturally, it will fall anyhow, since at altitude temps can be as low as -72C (coldest I've seen). Even at 10K' in a standard atmosphere, the temp is -5C, so with the Ram air inlet open, it will get cold.

Many thanks for the replies, I have learnt alot from this thread! I think the gaps in my understanding were to do with the leakage of the cabin and the partial pressure of oxygen. My original statement of the cabin pressure rising was a typo :O.

Cheers.

From air test experience on the A321 and A320. Both packs off and outflow closed, the rate of cabin leakage is about 300fpm (cabin alt)

DTG