In the case of explosive decompression at altitude, is the standard procedure emergency descent optimal? According to Wikipedia, the time of useful consciousness at that altitude is only 15 to 20 seconds. Between headsets and glasses, that may not be sufficient. Flying in a two person crew, oxygen masks are not required above 35,000'. When flying single pilot, the oxygen mask must be on above 35,000' at all times, or above 25,000' if using airline regs as guidance..

 

Here is the standard procedure:

---

LOSS OF CABIN PRESSURE (RED CABIN ALT CAS MESSAGE)

1	Oxygen Masks	DON and 100% OXYGEN
2	L and R MIC Switches	OXYGEN MASK
3	Emergency Descent	AS REQUIRED

 

EMERGENCY DESCENT:

 

1	AP/TRIM DISC Button	PRESS
2	Throttles	IDLE
3	Speed Brakes	EXTEND
4	LANDING GEAR Handle	DOWN
5	Initial Pitch Attitude	INITIALLY TARGET 20 NOSE DOWN

 

WARNING: 

THE AUTOPILOT EMERGENCY DESCENT MODE (EDM) CANNOT ACHIEVE MAXIMUM RATE-OF-DESCENT. THE AUTOPILOT SHOULD BE DISENGAGED AND THE AIRPLANE HAND FLOWN IF MAXIMUM RATE-OF-DESCENT IS REQUIRED.

---

 

Why the big warning? If the cabin is depressurized and the autopilot is engaged, an Emergency Descent Mode (EDM) is initiated with a descent to 15,000'. However if the engines are left in the typical CRU position, the rate of descent may be as little as 400' per minute. From 40,000', it may take a full hour to descend to 15,000' by which time everyone on the plane will be brain dead.

 

Basically EDM is useless without retarding the throttles.

 

The relevant section of the AFM states:

NOTE

If the airplane is above 30,000 feet MSL and the autopilot is engaged, it will automatically enter Emergency Descent Mode (EDM) when cabin altitude exceeds approximately 14,500 feet. The autopilot will initiate a pitch-down, left turn (90°) maneuver. The flight crew must retard throttles to idle, extend the speed brakes and landing gear. the autopilot will control the descent near the Mmo/Vmo limit and level off at 15,000 feet MSL. The flight crew must then retract the speed brakes and landing gear and apply thrust to resume normal flight. If the airplane slows to stall warning the autopilot will disconnect.

 

 

Re-iterated on page 412 of the Garmin G100 Mustang Pilots Guide, is this little note about EDM: "Throttles should be reduced to idle and speed brakes extended to achieve a maximum rate of decent."

 

First of all, the oxygen masks should be brought to the pilot's face in order to start the flow of oxygen. After that, since it takes only a fraction of a second to bring the throttles to idle and extend speedbrakes in the same motion, it makes sense to do that before fumbling with and securing the quick don masks. That way if the pilots pass out, at least the EDM can bring the plane down in time to save their brains.

 

In tests attempted in the simulator, the quickest new jet pilots could don the masks from the secure quick don position, including removing headsets and glasses, was longer than 20 seconds. That assumes the pilots knew what was coming and had already identified a RED CABIN ALT CAS Message.  In practice, it will likely be longer. Since that experience, we've decided as a crew it may be advisable to leave the quick don masks in our laps above 30,000'. That way it takes only a second to at least bring the mask to cover our nose and mouth, even if it isn't secure yet. (The 30,000' is also a reminder that EDM is only possible above this altitude.) There will be more experimentation in the simulator, but the best time to don the masks and improve the rate of descent was:

 

1	Oxygen Masks	Hold tightly to face and 100% OXYGEN
2	Throttles	IDLE
3	Speed Brakes	EXTEND
4	Oxygen Masks	SECURE (remove headset and glasses)
5	L and R MIC Switches	OXYGEN MASK
6	LANDING GEAR Handle	DOWN
7	If oxygen stable, AP/TRIM DISC Button	PRESS
8	If oxygen stable, Pitch Attitude	INITALLY TARGET 20° NOSE DOWN

 

And the rest of the procedure is the same.