Emergency Descent

- James Albright (a former G450 driver)

Updated: 2017-07-03

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. There are a few things most of us pilots don't understand about the requirements, the priorities, and the process. A better understanding could save your life.

• Requirements: most of us believe the regulations mandate we be able to get the airplane from any cruise altitude to 10,000' in four minutes or less. There is no such requirement. 14 CFR §121.333 and 14 CFR 135.157 stipulate oxygen requirements that become stricter if a descent to 14,000' (121) or 15,000' (135) cannot be made in 4 minutes.

• Priorities: The G450 will get the aircraft headed downhill for you, even if you do nothing. Your priority is to get on oxygen. Take a look at the case of Learjet N47BA for what happens when you don't.

• Process: The first thing you as the pilot need to do is breathe. Rather than fumble with your glasses, headset, or that filet mignon sitting in your lap, pull out the oxygen and put the mask to your face. Take a few breaths, dial the altitude select knob down, ensure the airplane enters a dive. Then, calmly, takeoff your glasses and headset, place the oxygen mask over your head, and get on with the business of saving everyone's lives.

Notice a recurring theme? Don't believe those times of useful consciousness charts, you are not going to last that long. Read Rapid Depressurization to find out why.

Everything here is from the references shown below, with a few comments in an alternate color.

 Procedure

Figure: GV Automatic Emergency Descent, from GV Aircraft Operating Manual, §06-04-00, figure 1.

[G450 Airplane Flight Manual, §04-21-20]

THE FOLLOWING PROCEDURE ASSUMES STRUCTURAL INTEGRITY OF AIRCRAFT.

IF AIRCRAFT STRUCTURAL INTEGRITY IS IN DOUBT, LIMIT SPEED AS MUCH AS POSSIBLE AND AVOID HIGH MANEUVERING LOADS.

• Power Levers . . . IDLE

• Airspeed . . . ESTABLISH MMO / VMO

  NOTE: An initial pitch attitude of 8° to 10° nose down is recommended to commence descent. As speed approaches MMO / VMO, extend the speed brakes to maximize the rate of descent. Adjust pitch attitude as necessary to avoid an overspeed condition.

  Consider leaving the autopilot engaged and using the touch control steering (TCS) button to override its inputs to turn and point the airplane downward while the PM adjusts the altitude select to the desired lower altitude.

• Speed Brakes . . . DEPLOY

• Airspeed . . . MAINTAIN MMO / VMO

  NOTE: It is recommended that the emergency descent be flown manually in order to achieve maximum descent rate.

  You are better off leaving the autopilot engaged — if you pass out the autopilot can get you down to altitude, level off, and push the power levers forward. Even if the speed brakes are deployed, the aircraft will maintain speed until you recover.

• SEAT BELT / NO SMOKING . . . ON

• Transponder Code . . . SET 7700

• Air Traffic Control (ATC) . . . NOTIFY

• Level Off . . . 15,000 FT OR MINIMUM ENROUTE ALTITUDE

 Automatic Emergency Descent

You may have noticed that there are several sections of the AFM and AOM that tell you the Emergency Descent Mode of the autopilot is armed "anytime airplane altitude is greater than 40,000 feet" and a few others that say "at FL400 or higher." Judging by the numbers of references one would guess the former is correct. According to the Gulfstream chief production test pilot, the answer is the latter, which leads me to believe it is probably the former. So, in short, I don't know. (Newer Gulfstreams have lowered the threshold to FL300 or higher. Or is it greater than 30,000 feet?)

[G450 Airplane Flight Manual, §04-21-30] The autopilot has an automatic Emergency Descent Mode (EDM) that is armed anytime aircraft altitude is greater than 40,000 feet with autopilot selected ON. When the red Cabin Pressure Low warning message is displayed on the Crew Alerting System (CAS) and aircraft altitude is greater than 40,000 feet with autopilot selected ON, the following occurs:

• The SPEED target on flight guidance panel changes to 340 KCAS in MANUAL mode.

• The ALTITUDE preselect is set to 15,000 feet.

• The autopilot commands a left turn with a 90° heading change in the HEADING HOLD mode.

• The autothrottle (if engaged) retards power levers to idle. If the autothrottle is not engaged, it automatically engages and retards power levers to idle.

• The aircraft descends at MMO / VMO to 15,000 ft.

  As the nose heads downward try to consider the aircraft's structural integrity and evaluate if a VMO descent with the speed brakes deployed will do more harm than good.

• At 15,000 ft, the SPEED target changes to 250 KCAS.

• The autothrottle sets power to maintain 250 KCAS.

NOTE: The pilot may override EDM by disconnecting the autopilot.

You should leave the autopilot engaged and use the touch control steering (TCS) to get the airplane pointed where you wanted — if you pass out the autopilot can get you down to altitude, level off, and push the power levers forward. Even if the speed brakes are deployed, the aircraft will maintain speed until you recover.

 Oxygen Duration vs Descent Target

How low do you go? Well, how much oxygen do you have? The G450 oxygen chart in the AFM Limitations section only provides crew duration, on the theory that you will get down to 15,000' where the passengers can - legally - go off oxygen:

But if you want to know the right answer, you will have to look at the Operating Manual Supplement OMS 2, Extended Range Operations, where you will find this gem:

 References:

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

Gulfstream G450 Airplane Flight Manual, Revision 36, December 5, 2013

Gulfstream G450 Operating Manual Supplement, G-450-OMS-02, Extended Operations (ETOPS) Guide, Revision 2, April 2, 2009