Emergency Descent

[antic81]

Hi all,
Just a quick question out of interest, on cabin depressurisation Pilots don Oxygen masks, which I believe are military style, i.e. pressure breathing apparatus?
Whereas the pax get the pure oxygen trickle feed…not to sure of the correct terminology here!
Now after the alert, I would gather that after masking up, the emergency decent is initiated, since the cabin is no longer pressurized are you limited to a max vertical speed to minimize discomfort and possible injury to passengers?

Or is it just a case of get down to FL140 ASAP?
I have not come across anything in my ATPL's on the subject as of yet, and was reading up on the Easy depressurisation and just got me thinking!

Many thanks!
Anthony

[BuzzBox]

With a rapid decompression, I'd suggest it's a case of get down to 10,000 ft or MSA (whichever is higher) ASAP. Better to blow a few eardrums down the back than have them die of hypoxia (even with the oxy masks).

If there's structural damage (fairly likely in the case of a rapid decompression in a widebody), then you'd probably want to limit your speed rather than accelerating to Mmo/Vmo during the descent. That would certainly reduce your rate of descent, but with some aircraft there's the option of lowering the gear to add some more drag at the lower speed.

[ITCZ]

"Military-style" masks? I think you will find that there is quite a difference between the O2 equipment used in strike aircraft compared to the 'quick-don' masks used in airliners. There are variations within the services -- example, the USAF, RAF and RAAF used diluter demand masks that vary the proportion of O2 delivered depending on altitude. The US Naval air services and USMC use 100% O2 only due to risk that such can supply oxygen if the pilot finds himself underwater (catapult problem, anyone?)

The flight crew masks that are fitted to airliners are designed for airliner use, and most would be of the quick-don, diluter demand type, with the ability to deliver O2 under pressure.

+++++++++++++++++++

The environmental risks of unpressurised flight above FL140 (and above FL250 in particular) far outweigh the risk of discomfort and barotrauma.

Above FL140 the occupants will suffer hypoxia.

Additionally, above FL250 the dissolved gases in your body will start to come out of solution and occupants may suffer decompression symptoms and injuries.

Emergency descents are designed to get the aircraft's occupants out of these danger zones as quickly as possible.

Emergency descent flight profiles are therefore designed to achieve the highest rate of descent possible. Mmo/Vmo, idle thrust, clean, spoilers/airbrakes deployed. One type I flew recommended a moderate angle of bank to increase the RoD. More recent types only specify a turn as a traffic avoidance manouevre only, should that be desireable.

If airframe damage is suspected, airspeed is reduced from Mmo/Vmo to a lower speed, maybe Va or Vb. Once again, a risk management approach -- less likely to tear bits of airframe away that might otherwise be useful, at the cost of slightly longer exposure of your crew/pax to the hostile environment.

Passenger "discomfort" and barotrauma rate very low in comparison to these other risks.

Passenger reactions to emergency descent range from seat gripping fear, to annoyance at not reaching their desired destination. G-force during the manouvres are not noticeably greater or less than normally experienced by the travelling public, deck angles and bank angles may be unusual for a frequent traveller but not extreme.

See if I can find a useful reference for you....

[ITCZ]

From the US Naval Flight Surgeon's Manual...
http://www.iiimef.usmc.mil/medical/FMF/FMFE/FMFEref/fs_man/FS_TOC.html


Physiological Effects of Rapid Decompression



The occupants' primary concerns are hypoxia, gas expansion, decompression sickness, and hypothermia.

Lungs.
The lungs are potentially the most vulnerable part of the body during a rapid decompression. Whenever a rapid decompression is faster than the inherent capability of the lungs to decompress, a transient positive pressure will temporarily build up in the lungs. If the escape of air from the lungs is blocked or seriously impeded during a sudden drop in cabin pressure, intrapulmonary pressure can build up high enough to cause tearing and rupture of the lung tissues and capillaries. If the expanding gas is free to escape from the lungs through an open airway, the risk of lung damage is nonexistent. Momentary breath-holding, such as swallowing or yawning will not cause excessively high intrapulmonary pressure and over expansion of lung tissue.

Ears and Sinuses.
Decompression of a pressurized cabin is unlikely to cause symptoms in the middle ear and paranasal sinuses. It is more likely, however, that individuals will develop pain in the middle ear and paranasal sinuses during the subsequent emergency descent as they will be exposed to a large and rapid increase of cabin pressure.

Gastrointestinal Tract.
One of the potential dangers during a rapid decompression is the expansion of trapped gases within the gastrointestinal tract causing abdominal distress. Abdominal distention, if it does occur, may have several important effects. The diaphragm is displaced upward by the expansion of the trapped gas in the stomach which can retard respiratory movements. Distention of the abdominal organs may also stimulate the abdominal branches of the vagus nerve, resulting in cardiovascular depression, and if severe enough, cause a reduction in blood pressure, unconsciousness and shock.

Hypoxia.
Of all the physiological hazards associated with the loss of pressure, hypoxia is the most important. The rapid reduction of ambient pressure produces a corresponding drop in the partial pressure of oxygen and reduces the alveolar oxygen tension. A twofold to threefold performance decrement occurs regardless of altitude. The reduced tolerance to hypoxia after decompression is due to (1) a reversal in the direction of oxygen flow in the lung; (2) diminished respiratory activity at the time of decompression; (3) decreased cardiac activity at the time of decompression.

Decompression Sickness.
In general, decompression sickness does not occur until cabin altitudes of 18,000 feet are reached. The incidence of decompression sickness is small unless the cabin altitude reaches 25,000 to 30,000 feet. As the duration of exposure to the unpressurized environment increases, so does the incidence of decompression sickness. The incidence of decompression sickness following a rapid decompression appears to be only slightly greater than after a slow decompression to the same altitude.

Hypothermia.
When cabin temperatures drop because of a decompression, it is likely that injuries such as frostbite and hypothermia will exist. Again, the extent and severity will be dependent on the altitude and the type of protective clothing worn during the decompression.

[antic81]

Thanks ITCZ,
Nasty business really isn't it??
That was a very interesting read thanks again!

So it is an airframe limiting VS, obviously taking any damage into account, I was chatting to a couple of Lear pilots and they were telling me they could get the aircraft to decent at 16000 feet per minute, incredible!
What type of rates are talking about with a 737/320 type?
Not taking any structural damage into account.

Sorry about my shocking knowledge on crew respirator equip that was pretty much all guesswork on my part!;)

Thanks again!

[rigpiggy]

Most airline QD masks have three setting
1. diluter/demand
2. 100%/demand
3. Emergency/positive pressure

3:this is not to pressurize the lungs but are used to clear smoke from the mask/goggles. can't find a picture, but I believe our's are from scott

[FE Hoppy]

all modern QD masks will increase the O2 mix up to 100% with cabin alt and provide pressure breathing at higher cabin alts @320 and above.

[411A]

all modern QD masks will increase the O2 mix up to 100% with cabin alt and provide pressure breathing at higher cabin alts @320 and above.

Yep, been this way since the B707 days...:}

[BelArgUSA]

Hola Anthony...
xxx
Sure, there are oxygen masks... but their value to "sustain" life, up there is often exagerated. Pilot masks are ok, they can "do the job" at some FL 250/300, but I would not trust one much above that. Passenger masks... I don't want to say what my opinion is...
xxx
Explosive decompression, get down fast. Do your check-lists if you have the time, but first get down... ASAP -
Get your mask ON first...
Speed brakes...
Get down on the barber pole...
xxx
We have (in the 747 at least) 2 different emergency descent procedures. One is with the gear up, the other, is first slow down to get the gear down (gear extension speed), then descend. With the first procedure (clean configuration), if started around say FL370, we will be out of the high levels earlier, reaching FL250 or lower, much faster. In the "gear down descent", we will be down to FL140 sooner, but our time to reach say FL250 will be longer... My idea (and preference) is to get out of the "higher levels" sooner.
xxx
Getting the wheels down, is awfully noisy, the nose down angle will be steeper, all that contributes to the obvious passenger panick, so I am for the clean descent, even if the time is longer to FL140, I will be down to FL250 sooner.
xxx
Further, we recommend to do the descent on autopilot, with FL140 selected on ALT (in case the pilots have O2 supply problem). With the autopilot, we can get 6,000 FPM rate easy... Then when on speed at barber pole, I select speed hold... One thing I do in the simulator, is to enter in a "1 G" 90º bank (!) to initiate the descent, with the idea of getting off the the airway traffic below, and also to avoid getting the passengers to the ceiling of the cabin. I am concerned as to not spill their drinks...
xxx
As to "FL140", we in Argentina, or my Chilean colleagues, have a smile on the face. Crossing the Andes between Mendoza and Santiago, our MEA level is FL250, as we have "granito-cumuluses" to 22,000+feet in that area. After the Himalayas, we are second... The "Cerro Aconcagua" is 22,830 feet...
xxx
As to keeping the O2 when above 10,000 feet of cabin altitude, I assume people who live in La Paz, Bolivia (13,500 ft elevation) have oxygen masks hanging from the ceiling in their housing. They live perfectly well up there. So, don't worry too much below 15,000 feet. Was there a couple of times in my life (with an old 720B, their power was appreciated for takeoff).
xxx
:)
Happy contrails

[A37575]

One thing I do in the simulator, is to enter in a "1 G" 90º bank (!) to initiate the descent, with the idea of getting off the the airway traffic below, and also to avoid getting the passengers to the ceiling of the cabin

Having conducted one emergency descent for real, I found no need to bank and turn in order to momentarily increase the aircraft rate of descent. The danger of a wing-over type turn is in IMC where large bank angles can quickly get out of hand. A gentle easing forward into the initial dive attitude is almost imperceptable to the passengers if the pilot (human or automatic) flies smoothly. Turning to clear an air route is another thing altogether and of course requires fast evalulation of new MORA. Passengers hitting the ceiling because of negative G in a dive is surely caused by only the grossest of poor flying skill by the pilot.

[antic81]

Hi all,

Thanks for all the detailed replies, very much appreciated!

The thought of a very high MSA didn't even cross my mind!
That would be a most inconvenient place to lose cabin pressure, of course there aren't that many convenient places.

So if you were to descend using the auto-pilot, would you use lvl change and set speed to VMO and adjust as you descend? My guess is this would may be the best option, only a guess though.

Thanks again!

[BelArgUSA]

Sorry A37575...
xxx
Bad habit acquired in T-38s...
And works perfect in "stimulators"...
Have you ever tried a "victory roll" in a 747 simulator...?
Works perfect, despite poor roll rate...
Entry speed 350, nose up 15º...
xxx
:)
Happy contrails