So, let us assume you are in a simulator for your favorite jet plane, and the instructor gives you a LOFT (Line-Oriented Flight Training) scenario that goes as follows:
- A normal takeoff and climb to cruise altitude, and perhaps thirty minutes of normal cruise flight
- The non-recoverable loss of speedbrake (flight spoiler) functionality due to a mechanical jam, or some other malfunction such as the loss of all SECs in a FBW Airbus.
- Another 30 minutes of time to deal with the first malfunction: run the checklists, plan and initiate a diversion if needed.
- You lose normal cabin pressure for some reason, and have to conduct a rapid descent to get back to 10,000' and complete your diversion.
How do you execute the descent, considering that the rapid descent procedures for most jets rely on using the speedbrakes to achieve a high descent rate without overspeeding the aircraft? To use the 737 QRH checklist for Emergency Descent as an example:
EMERGENCY DESCENT . . . . . . . . . . . . . . . . . . . . . .Announce
The captain will advise the cabin crew, on the PA system, of impending rapid descent. The first officer will advise ATC and obtain the area altimeter setting.
ENGINE START switches . . . . . . . . . . . . . . . . . . . . . . . . CONT
THRUST levers . . . . . . . . . . . . . . . . . . . . . . . . . . . . CLOSE
Reduce thrust to minimum or as needed for anti-ice.
SPEED BRAKE . . . . . . . . . . . . . . . . . . . . . . . . . FLIGHT DETENT
DESCENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Initiate
Target speed . . . . . . . . . . . . . . . . . . . . . . . . . . . Mmo/Vmo
If structural integrity is in doubt, limit speed as much as possible and avoid high maneuvering loads.
Level-off altitude . . . . . . . . . . . . . . . . . . . Lowest safe altitude or 10,000 feet, whichever is higher