I've had one rapid depressurization over the years — at FL 350 in a Boeing 707 — and believe all the practice in the simulator you can get comes in handy.
We tend to do these sim exercises as a chore but there are things we need to consider:
Everything here is from the references shown below, with a few comments in an alternate color.
Figure: GV Emergency descent, from GV Aircraft Operating Manual, §06-04-00, figure 1.
Figure: Change in average atmospheric pressure with altitude, from PhysicalGeography.net, figure 7d-1.
The atmosphere is 79% nitrogen, 21% oxygen, and a bunch of other things in very small quantities. The percentages are pretty much constant with altitude and don’t really matter at all . . . what matters is pressure.
By the time you get to 18,000 feet, the atmospheric pressure is one-half what it is at sea level. By 35,000 feet, you are at one-quarter pressure.
Most passenger carrying aircraft are pressurized to keep their cabins well below these altitudes. A typical airliner will have a cabin pressure below 8,000 feet, because that is the 14 CFR 25.841 requirement. Most large cabin corporate aircraft shoot for 6,000 feet or less.
[14 CFR 91, §91.211]
If an aircraft spends more than 30 minutes above 12,500' or any time above 15,000', each occupant must have supplemental oxygen or the aircraft must be pressurized.
[14 CFR 25, §25.841(a)] Pressurized cabins and compartments to be occupied must be equipped to provide a cabin pressure altitude of not more than 8,000 feet at the maximum operating altitude of the airplane under normal operating conditions.
This seems fairly cut and dried but it isn't. As aircraft performance increases, the economic advantages of high altitude flight start to outweigh a risk that is seen as decreasing. Certification authorities do often grant exemptions to these requirements. The Gulfstream GVII, for example, is allowed to exceed 40,000 feet cabin pressure altitude for a minute and 25,000 feet for three. More about this: GVII Regulatory.
Figure: Times of useful consciousness versus altitude, from AC 61-107B, figure 2-3.
[AC 61-107B, ¶2-7.e.(3)]
WARNING: Slow decompression is as dangerous as or more dangerous than a rapid or explosive decompression.
WARNING: The TUC does not mean the onset of unconsciousness. Impaired performance may be immediate. Prompt use of 100 percent oxygen is critical.
In the early fifties the United States Air Force subjected a group of highly fit, young test pilots to decompression tests in an altitude chamber and came up with what everyone now relies on to determine times of useful consciousness. These times are for an individual at rest, expecting the decompression. Any exercise or fatigue will reduce the time considerably. A rapid decompression can reduce the TUC by up to 50 percent caused by the forced exhalation of the lungs during decompression and the extremely rapid rate of ascent. What's this mean to us? Few of us can expect to last as long as the table would lead us to believe.
Photo: Eddie takes a hit of O2, from Eddie's collection.
[AC 61-107B, ¶2-9.b.(1)] Regulators and masks work on continuous flow, diluter demand, or pressure demand systems.
The pilot masks on most Gulfstream aircraft are certified to a cabin altitude of forty thousand feet and will automatically switch to positive pressurized flow if the cabin altitude exceeds thirty-five thousand feet. Of course the mask will work up to the airplane’s ceiling, but it is only certified to 40,000' and every Gulfstream since the GII has been certified to 45,000' or higher. What gives? Read on...
As we've seen in our discussion above under Cabin Pressurization, the so-called “4 minute rule” doesn’t exist, at least not to say you have to complete the emergency descent in four minutes or less. If you dig deep, however, you can find four minutes as a criteria:
[14 CFR 121, §121.333(e)] When the airplane is operating at flight altitudes above 10,000 feet, the following supply of oxygen must be provided for the use of passenger cabin occupants:
[14 CFR 135, §135.157(b)] No person may operate a pressurized aircraft—
It is a technical point here, but just to be clear: if you don't have enough oxygen on the aircraft for at least 10 percent of the occupants for the times listed, you need to be able to descend to 14,000 or 15,000 feet (depending on 14 CFR part) in four minutes or less. Part 91? No rule specified.
Reading between the lines, a GV is certified to fly at 51,000' with masks certified to 40,000' because Gulfstream believes the aircraft will never find itself at a cabin altitude above 40,000 feet.
The answer appears to be 10,000 or 12,500 feet cabin altitude, depending on how you read the regulations and where in the world you are.
(a) General. No person may operate a civil aircraft of U.S. registry—
(1) At cabin pressure altitudes above 12,500 feet (MSL) up to and including 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen for that part of the flight at those altitudes that is of more than 30 minutes duration;
(2) At cabin pressure altitudes above 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen during the entire flight time at those altitudes; and
(3) At cabin pressure altitudes above 15,000 feet (MSL) unless each occupant of the aircraft is provided with supplemental oxygen.
126.96.36.199.1. A flight to be operated at flight altitudes at which the atmospheric pressure in personnel compartments will be less than 700 hPa shall not be commenced unless sufficient stored breathing oxygen is carried to supply:
a) all crew members and 10 per cent of the passengers for any period in excess of 30 minutes that the pressure in compartments occupied by them will be between 700 hPa and 620 hPa.
There is no ICAO requirement to use oxygen under normal circumstances. The altitudes are similar to U.S. requirements:
Some aircraft manufacturers (like Boeing) stipulate that exact number, removing all doubt.
[Boeing 737NG Flight Crew Training Manual, pp. 7.5 - 7.7, Rapid Descent] After Level Off: Recheck the pressurization system and evaluate the situation. Do not remove the crew oxygen masks if cabin altitude remains above 10,000 feet.
A sampling of some business jet manuals show very few specify an altitude. The Bombardier BD-700, Cessna Citation, and Dassault DA-900 manuals, for example, don't mention oxygen masks at all once the rapid descent is begun. The Challenger 605 Operating Manual says oxygen masks are "as required" at a "safe altitude." While most Gulfstream manuals do require oxygen be used under "High Airport Elevation" procedures, no procedure is listed for removal of oxygen masks after a rapid descent.
Figure: G450 Oxygen durations, from G450 OMS-02, table iii.
So you've lost all pressurization in the G450, do you have enough oxygen for everyone on board? In a word: no. As a general rule of thumb, you have more oxygen than fuel for the pilots, you have more fuel than oxygen for the passengers. You are going to need to descend to altitude where the passengers can breathe without a mask. More about this:
14 CFR 25, Title 14: Aeronautics and Space, Airworthiness Standards: Transport Category Airplanes, Federal Aviation Administration, Department of Transportation
14 CFR 91, Title 14: Aeronautics and Space, General Operating and Flight Rules, Federal Aviation Administration, Department of Transportation
14 CFR 121, Title 14: Aeronautics and Space, Operating Requirements: Domestic, Flag, and Supplemental Operations, Federal Aviation Administration, Department of Transportation
14 CFR 135, Title 14: Aeronautics and Space, Operating Requirements: Commuter and On Demand Operations and Rules Governing Persons on Board Such Aircraft, Federal Aviation Administration, Department of Transportation
Advisory Circular 61-107B, Aircraft Operations at Altitudes Above 25,000 Feet Mean Sea Level or Mach Numbers Greater than .75, Change 1, 9/9/15, U.S. Department of Transportation
Boeing 737 NG Flight Crew Training Manual, Revision 12, June 30, 2013
Gulfstream G450 Operating Manual Supplement, G-450-OMS-02, Extended Operations (ETOPS) Guide, Revision 2, April 2, 2009
ICAO Annex 6 - Operation of Aircraft - Part 2 General Aviation, International Standards and Recommended Practices, Annex 6 to the Convention on International Civil Aviation, Part II, July 2008
Wikimedia Commons, Public Domain Artwork
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