This just in...

[Steve]

I don't think anyone answered my question in the other thread about how much harm would be done to a person during a rapid decompression followed by an emergency descent if they were not wearing an O2 mask. Obviously they would pass out but an emergency descent only takes about 5 minutes so assuming they didn't hit their head on something in the lav would there be any permanent effect?

I'm doubtful that everyone in the cabin would be able to don their mask successfully anyway if it was that rapid a decompression.

From http://www.skybrary.aero/index.php/Time_of_Useful_Consciousness

Important Notes

• The physical condition of individuals influences the TUC. The TUC will be less for people with existing health conditions.
• The TUC is less if the person is engaged in any kind of physical exercise, such as moving around the cabin.
• In the case of explosive decompression, the TUC may be halved because of the effects of the sudden outflow of oxygen from the body's tissues.

From http://www.braininjury.com/coma.html

The common cause of coma is oxygen deprivation. Anoxia refers to a complete absence of available oxygen, while hypoxia describes someone who had available oxygen but at reduced levels for a period of time. Anoxemia describes when a person's blood supply (rather than lungs) lacks oxygen. Oxygen deprivation lasting longer than five to ten minutes can be fatal. Almost all persons surviving five minutes or more of complete oxygen depravation or 15 minutes of "substantial" hypoxia sustain permanent brain damage (J.N. Walton, 1994). Those who do not end up in a coma typically have impaired learning ability and retrieval problems. Visual defects are not uncommon. PET studies and CT scanning can show damage in the area of the cerebellum and basal ganglia in severely impaired patients.

Draw your own conclusions. I just don't have much faith in the "5 second rule" myself (FAR 121.333) in getting a plane down under control in time.

 

[Everskyward]

Draw your own conclusions. I just don't have much faith in the "5 second rule" myself (FAR 121.333) in getting a plane down under control in time.

Fair enough but if you don't think the pilots can get their O2 masks on and do the emergency descent then that doesn't bode well for anyone in the airplane not just the person in the lav. The pilots' O2 masks are easier to don and much more effective than the passenger masks plus they have had practice donning them, so if the pilots can't get them on in time I doubt anyone else in the back will be very successful either.

I can't speak for airliners but some newer airplanes have an emergency descent mode. In the CE-680 emergency descent mode is activated if the aircraft flight altitude is above 31,000 feet; the autopilot is engaged; and the cabin altitude reaches 14,500 feet. The pilot needs to reduce the throttles and extend the speedbrakes but the airplane will make a 90 degree left turn and descend on its own at Vmo/Mmo -10 kts to 15,000 MSL. It seems that if an airplane had autothrottles (the 680 doesn't) that they could be programmed to go to idle themselves.

 

[Steve]

Fair enough but if you don't think the pilots can get their O2 masks on and do the emergency descent then that doesn't bode well for anyone in the airplane not just the person in the lav. The pilots' O2 masks are easier to don and much more effective than the passenger masks plus they have had practice donning them, so if the pilots can't get them on in time I doubt anyone else in the back will be very successful either.

Removing the equipment guarantees they won't.

 

[Everskyward]

Removing the equipment guarantees they won't.

I would say it's guaranteed anyway. If the airplane stays at altitude, even if passengers are wearing masks they won't be able to get through the cockpit door to do anything about it. In any case, those passenger masks aren't designed for sustained use at cruise altitude.

 

[Steve]

I would say it's guaranteed anyway. If the airplane stays at altitude, even if passengers are wearing masks they won't be able to get through the cockpit door to do anything about it. In any case, those passenger masks aren't designed for sustained use at cruise altitude.

Might as well do away with all cabin O2 with that logic.

I still feel the requirement to remove the lav 02 is about as effective as the 1 hour no potty break before landing rule in thwarting terrorism.

 

[Trapper John]

...the airplane will make a 90 degree left turn and descend on its own at Vmo/Mmo -10 kts to 15,000 MSL. It seems that if an airplane had autothrottles (the 680 doesn't) that they could be programmed to go to idle themselves.

What's behind the 90 degree turn?

 

[Everskyward]

Might as well do away with all cabin O2 with that logic.

I think that cabin O2 is good for a less catastrophic situation where the passengers would have more time to put on their masks. In that situation the FAs would be able to get to the people in the lav with portable O2 before anything happened to them. Besides, we are talking about 200 people instead of 2. In some ways I think the O2 masks in the cabin are somewhat of a panacea. If the decompression is so fast that the pilots aren't able to put on their O2 masks and do the emergency descent I don't think the passengers will be successful, and even if they are they will just be able to wonder what's going on for a while.

I still feel the requirement to remove the lav 02 is about as effective as the 1 hour no potty break before landing rule in thwarting terrorism.

I'm not worried about terrorism in the lav either but I also wouldn't be reluctant to use the lav because I was worried about not having an O2 masks in the very unlikely event of a decompression.

 

[Everskyward]

What's behind the 90 degree turn?

You would need to talk to the people who designed the system.

I always figured it was to get the airplane off the airway, that is if it was on one.

 

[bigred177]

What's behind the 90 degree turn?

All of the lift is going into the turn instead of keeping the plane up. The plane can keep the speed below Vne and lose altitude in a big hurry.

That is a fun maneuver in the Cherokee, full flaps, crank it over into a steep bank, keep the airspeed in the white arc. You can peg the VSI on the bottom like that.

 

[jesse]

All of the lift is going into the turn instead of keeping the plane up. The plane can keep the speed below Vne and lose altitude in a big hurry.

That is a fun maneuver in the Cherokee, full flaps, crank it over into a steep bank, keep the airspeed in the white arc. You can peg the VSI on the bottom like that.

Not all of the lift and I doubt the bank is really that steep. Plus it's just one 90 degree turn not a continuous bank.

The purpose of the 90 degree turn is to draw the attention of the controller that something is up - with the potential that the crew may not even be continuous. Plus as Mari states it'll get you off the airway if you're on one.

 

[bigred177]

Not all of the lift and I doubt the bank is really that steep. Plus it's just one 90 degree turn not a continuous bank.

The purpose of the 90 degree turn is to draw the attention of the controller that something is up - with the potential that the crew may not even be continuous. Plus as Mari states it'll get you off the airway if you're on one.

Ah, I didn't know it was a turn and not a 90* bank. In that case I take back what I said.

But it is a good bit of the upward component of lift gone in the Cherokee and even though I'm not anywhere near 90* it sure is fun.