Airplane emergency questions

Factual Questions
1

Inspired by a recent movie (I’ll try not to give any spoilers), I have one question of my own and one bet-settler. All questions refer to a Boeing 747.

My question: If there’s an electrical problem and the plane “loses avionics” (quoting from the movie; I’m guessing this is the electronic flight controls) it’s asserted that it can basically be rebooted. How common is this? Is it possible that a “spot of turbulence” is really the pilot taking the helm manually for a while until the co-pilot can get the system back up?

Bet-settler: How long are oxygen masks designed to work for? Do they supply from on-board oxygen tanks or oxygen generators, or do they pull in outside air somehow? More generally, is the cabin airtight when pressurized, or is there a flow of air in from outside at high speed maintaining the pressure?

As for the second, I’m generally going with what the “common knowledge” is (airtight cabin, low duration masks w/ onboard emergency storage), however I understand that common knowledge may be wrong. Either way, I’m going to want solid cites and references on this one.

Thanks.

2

If a pilot takes the helm manually for a while (I assume if the autopilot is disengaged or shuts off) you probably wouldn’t be able to feel it. A spot of turbulence is a spot of turbulence… It’s hard to impossible to “simulate” turbulence using the controls since turbulence very often causes the plane to go in directions the plane normally wouldn’t go pointed the way it is. After a lot of flying your body kind of knows the types of movements the airplane does normally and things out of spec feel very abnormal.

3

I cannot answer most of this with authority but IIRC oxygen systems on board a commercial jet will last around 15 minutes. Oxygen is generated via chemical oxygen generators ( cite ). I had asked about something like this here awhile ago but the hamsters seems to have eaten the post but it is from there I recall the 15 minute time limit. I thought that short at the time but in the case of a loss of cabin pressure necessitating use of oxygen masks the pilots’ would make it a priority to get the plane down to 10,000 feet or so where oxygen masks would not be needed.

As for pressurized cabins I am near certain that commercial jets are airtight. They do draw air in from outside but I think it is still safe to say it is airtight (just as a sub is water/air tight even though it draws water in from outside). Remember though that a pressurized cabin just means the pressure is higher than the outside pressure. Nevertheless the pressure in a plane cabin is lower than sea level (8000 feet seems standard…this Google searach you can see it in the description [doing it this way because I cannot seem to get Wiki up] cite ).

4

Dunno if this helps:

If avionics are well and truly out then I do not think the pilot can fly the plane as most modern jets these days are fly-by-wire.

Avionics looks like the whole ball of wax when it comes to controlling the airplane. If gone I am guessing it is big trouble. That said I am sure there are redundant systems making a complete failure hard to imagine. Need a pilot for that one though.

5

Airplanes are definitely not “air tight” in the way a submarine is air tight. In fact, commercial airliners leak like a sieve, from what I’m told. If somehow you were able to submerse an airliner in water (without damaging it by, say, crashing it into the water) water would be coming in from many places.

Hollywood would have you believe that if someone were to shoot a window in an airliner at altitude, you would have exposive decompression in the cabin. In reality, the worst you’d get is a loud whistling noise.

The seal in an airliner is “good enough” to maintain a comfortable pressure, but it is not a perfect seal.

J.

6

They’re basically a tube with a controllable opening nearish the front, and another controllable opening at the tail. Just with the plane’s forward motion, air can be compressed and its pressure is controlled by how much the various openings are opened or closed.

On some planes, the engines are an active participant in pressurizing the cabin - air is compressed by the various fans in the jet engines, and some is “bled” or taken off at a reasonable pressure for humans to live in. This air’s also run through a heat exchanger to be warmed up - the air temp at cruising altitude is very cold.

As for the avionics, if the entire system fails, you’re probably pretty well screwed.

It all depends on the system design. Newer jets such as the Airbus A380 use a redundant and distributed processing scheme, as opposed to one box that could be a single point of failure. You may not be able to tug on a steel cable and move flight control surfaces any more, but there are so many layers of distribution and redundancy in the controls, processing, communication and ultimate activation to make it absurdly unlikely to have a catastrophic and complete failure.

7

Are you sure the air is heated up? My understanding is that the air is very cold before compression, but I’d be surprised if compressing it from cruising altitude to 8000 feet doesn’t heat it to an uncomfortable degree. I’d want it cooled.

8

Since the OP mentioned the Boeing 747, it’s important to note that even the 747-400 still uses conventional hydraulic assist rather than fly-by-wire.

9

Well, water is pushing in on your submerged airplane with a bit more pressure than air normall would be pushing out.

I am reminded of an episode of Futurama where their ship is submerged under many atmospheres of water:

Fry “How many atmospheres is this ship designed to withstand?!”
Prof Farnsworth “Well, it’s a spaceship so…between 1 and zero.”

10

The environmental control system on a commercial airliner uses bleed air from the main engines (or an auxiliary power unit) to provide cabin pressurization. This air is cooled, filtered, and reduced in pressure before distribution to the ducts within the cabin. The following paper on the Boeing 767 system is very thorough, and quite readable: Commercial Airliner Environmental Control System (warning pdf).

11

If by “opening nearish the front” you mean the engine, then right. Otherwise, not so much. Unless maybe you’re trying to partially describe a RAMJET. :slight_smile:

Again - not quite. The engine compressor pressurizes and heats the outside air (for example temperatures, refer to the link I provided above). The air must be cooled before mixing and distribution to the passengers.

12

Many still use oxygen tanks.

Commercial jets are full of holes, and nowhere near airtight. The air is constantly being replaced by outside air.

Utter nonsense.

Name an aircraft that doesn’t use the engines to pressurize the cabin. I can think of maybe one or two.

It varies widely but usually around 120-160 psi. Not a reasonable pressure for this human to live in.

It has to be run through an air cycle machine to be cooled down, since it’s usually 400+ degrees.

13

Yep, you’re right. But, it *is * heated by the engine, one way or another. :stuck_out_tongue:

14

I don’t know where they found that person, but he has no business answering questions about aircraft pressurization.

15

Broomstick and a few other aviation-inclined Dopers usually have a good angle on things like this. Granted, I attended Embry-Riddle, I was a B.S.E.E. and didn’t really get into the aviation side of things.

. . . until I got into RED HORSE, but even that was just paving runways and aprons.

Tripler
Paging Broomstick!

16

Former big-jet airline pilot here …

I’ll try to make fairly short answers to each of the OP’s questions rather than a single magnum opus.
“Avionics” can refer to anything electronic on the aircraft. That includes instruments, autopilots, radios, navigation computers, and on some of the newer aircraft, electronic flight controls.

In general, avionics can’t be rebooted in flight. There’re no controls for that, and for critical components, no way to interrupt all power to them.

For older aircraft and less critical components, like an auxiliary navigation radio receiver, there may well be an on/off switch. For something like that, a common inflight troubleshooting technique would be to turn it off for a couple of minutes & turn it back on.

But that largely went away with the advent of digital systems; their interactions and failure modes are just too complex to let the pilots be monkeying around with switching parts off and on.
The term “electronic flight controls” generally refers to a system design where the pilot’s controls are connected to a computer, and the computer is actually the only thing connected to the physical control surfaces (ruudder, ailerons, horizontal tail, etc).

Airbus aircraft from the A320 and after are built that way. If all the EFC computers quit, the aircraft is a very expensive unguided dart & will be utterly unresponsive to the pilot’s inputs. It will then either impact the Earth or break up in flight in less than a minute.

This same sort of system is common in fighter-type aircraft, pioneered by the US F-16 and used in the F-18 and all subsequent US fighters as well as late-model European & Russian equipment.

The upside of that system is the airplance can have very safe & easy-to-fly handling qualities even if the actual aerodynamics are less than ideal. An aircraft designed for maximum economy (airliner) or maximum manueverablility (fighter) will generally be a bitch to fly. A full-authority EFC will bridge that gap, and provide a good machine for the mission and also a good experieince for the crew & pax.
Conversely, Boeing aicraft use EFC as an adjunct to the pilots’ control of the machine, not as an intermediary. The pilots’ wheels & pedals are connected by braided steel cables to hydraulic valves in the wings and tail which move to direct hydraulic power to push the control surfaces against the airflow. This is the same systems design used back into the 1950s.

The innovation is that the EFC can also push & pull those same cables to assist the pilots by improving the apparent handling qualities of the airplane. It can also push against the pilots if HAL thinks they’re doing something dumb. But Boeing’s design philosophy is to resist the pilots if need be, but never to override them. If worst comes to worst, the pilots’ inputs will carry the day.

And if all the EFCs quit, the pilots can still fly the beast at what will seem to the passengers like completely normal behavior. The crew workload will be higher and there may be some areas, like extreme high or low speeds that would be best avoided, but for typical flight regimes the loss of EFC in a Boeing is an annoyance, not a catastrophe.
Naturally, on all models from all manufacturers, EFC systems are heavily armored. That software and hardware is reliable like nothing you’ll ever see on a desktop. it also has multiple redundant independent power supplies to ensure the juice never runs out.
So, bottom line, NO, a spot of turbulence is NOT the pilots rebooting part of the avionics.

17

Oxygen systems:

Passenger oxygen systems are generally good for about 15 minutes.

On older airplanes (727, early 737, early 747 and IIRC the Airbus A300), they are actually tanks, like giant SCUBA tanks, stored in the belly with plumbing to each oxygen mask cluster above the seats.

Later model airplanes use the the chemical oxygen generators as others have noted. The upside is no dangerous gaseous oxygen stored at high pressure and no cost for periodic replacement of depleted tanks from the inevitable minor leaks. The downside is the generators make a lot of heat, noise, & stink when activated (something the preflight briefing avoids mentioning). Having one generator per seat cluster also eliminates a lot of single points of failure.

Outside air plays no role in the emergency oxygen system.

Having said that, each mask has that infamous bag ("… The bag on the mask may not fully inflate. …") The truth is more like " the bag won’t inflate much at all, period." but that wouldn’t be reassuring to the customers, so the gals don’t say that.

The bag is needed because the oxygen flow is continuous and your breathing is in-and-out. When you’re between breaths or exhaling we don’t want to waste the oxygen flowing at that moment. So it’s diverted into the bag to supply your next breath.

In most mask designs your exhaled air flows through check valves out of the mask cup and into the cabin, but in some designs some part of that is retained into the bag & rebreathed.

The flight attendants have the same oxygen arrangement at their seats as the passengers do.
Conversely, the cockpit oxygen systems on all big jets are are, AFAIK, based on pressurized tank(s) of O2 and our masks are very different, often full face units like firefighters have, and can last for about an hour depending on conditions. They can also provide oxygen at significant pressure to ram-fill our lungs at altitude. It may not sound fair, but I can guarantee if we don’t make it then you won’t either.
Finally, US military aircraft typically carry a tank of LOX in lieu of the pressurised cylinders. There has been an effort (Google [OBOGS] for more) to replace that with a tecnlology that separates oxygen from the airflow in teh engine & supplies that to teh crew. It isn’t practical to scale up for more than a handful of people, so these systems are used on fighters & bombers, not transports.

18

^
What he said.

(Very flattering to get paged for this, but for the Big Iron LSL Guy and company are much better authorities than I am)

19

Cabin airflow & pressurization …

The cabin is not in any sense airtight. The overall system is designed as a flow though, where a more or less constant supply of fresh air is pumped in from the engines and allowed to leak out a set of valves in the tail.

If the leak is as fast as the inflow, the pressure inside stays the same. If the leak gets bigger, the net flow is outwards and the cabin pressure decreases (or equivalently, the apparent altitude increases).

When the inflow exceeds the outflow, the pressure increases and the apparent altitude decreases.

On a 727 / 737 / A320 / MD80 sized plane the exhause valve is maybe a foot square. On a 747 there are 2 valves each about 12" x 18"

On the ground the outflow valve(s) will be fully open. At low altitude and high power they may be 3/4ths open, while at high altitude while descending at idle they may be 90% closed. They can’t close to 100%, so there is always a hole with air leaking out.

Other than these outflow valves, the airplane is generally almost entirely air tight. Once there is some positive air pressure to seal the doors tightly in their frames, very little air leaks from doors, windows or other potential leak points. But that’s sort of like saying a boat is water tight, except for those open sea-cocks in the bilge.
The input air is supplied by the engines. Air comes into the engine face at ambient temperature (ie < 40F/-40C at altitude, or > +115F/+50C on the ground in PHX in August). The first part of the engine compresses the air to 15-30 atmospheres of pressure, which heats it to 600+F / 300+C. Most of this very hot high pressure air is then mixed with fuel & burned to produce power. And yes, that’s one hellacious fire in there.

A few percent of the HP air is diverted off upstream of the burners. This so-called “bleed” air is ducted to air conditioning units generally installed amidships below the passenger cabin, not far from the main landing gear. These units de-pressurize, cool, and (if needed) dehumidify the air to bring it to human-compatible temps & pressures. The remaining pressure drives the air into the cabin where it is distributed to ducting running the length of the ceiling & sidewalls, as well as outlets in the cockpit, cargo compartments, electronics bays, etc.
The air conditioning system attempts to keep a constant flow rate into the cabin at all times. Often at idle there isn’t quite enough bleed airflow available to maintain the desired cabin inflow, which is a lot of why cabins are too hot during taxi. Conversely, at high power at low altitude the bleed regulation valves may be as closed as they can be and there’s still a little more flow than desired. That’s why some aircraft models have a real noisy cabin during takeoff and initial climb out. But for most of the altitude vs power setting regime, the inflow is a constant volume and cabin pressure and altitude is controlled entirely by adjusting the outflow valves.
As a rough measure of overall input/output balance consider this. If we were cruising along at altitude and all the engine bleed air was then cut off, the inflow would cease immediately. The outflow valve would react to this by closing its little sphincter as tight as it could (perhaps with some company from others aboard). The cabin would depressurize from a normal pressure of ~7000’ equivalent to ambient ~40,000’ in no more than a couple of minutes tops.

In a small airplane like a fighter or a bizjet, total loss of cabin inflow can depressurize an intact cabin in well under a minute. If there is much fumbling around getting oxygen masks on, the crew can be incapacitated before they get on oxygen, with fatal results for all. Likewise, loss of a cabin window in a bizjet will depressurize the cabin to ambient in a matter of a few seconds regardless of the inflow.
At the other extreme, the loss of a single cabin window in a 747 or A380 would not be a big deal, except to those sitting right nearby. The outflow valves would close a bunch to offset the new hole and the cabin would maintain pressurization fairly well. I’d be surprised if the jet could remain normally pressurized at a high cruise altitude in the 40s, but if they descended to the low 30s I’d bet the cabin could hold normal pressure with a window gone. The noise nearby would be horrific, but there’d be plenty of air to breath.

20

I very much hesitate to make a clarification here, but this statement would more accurately describe a turbojet (such as a USNavy T-2 trainer). However, most/all passenger jets (and most modern military jets) use a turbofan, in which case, depending on the bypass ratio, a significant portion of the total airflow does not pass through the combustor, but still generates thrust.