Anytime you use the word “emergency” in the context of aviation, people get understandably concerned. A look at what happens during such an event reveals the planning and forethought that goes into aviation. And, as a good friend of mine likes to say, “With knowledge comes comfort.”
In aviation, when a Pilot uses the word “emergency,” it is simply a message, a code word, to the Air Traffic Controller (ATC) that the Pilot has an issue that will require traffic priority. Just using the word as part of the call sign is an efficient way to “declare” that a situation exists on your aircraft that requires priority handling.
Flights have “call signs” that identify them, like license plates on a car. For an airliner, the call sign is the flight number. When a Pilot on, say Southwest Flight #123, calls ATC with, “Center, Southwest 123 emergency, we are losing our cabin and need a descent to 10,000 feet,” ATC knows precisely what that flight needs.
There are fewer situations more important to a Pilot than when the cabin pressure goes too high, requiring descent to a lower altitude swiftly and smoothly. And, to a controller who may have airplanes below the aircraft that must descend, getting those planes out of the way quickly and safely is the overriding issue that must be resolved to allow the aircraft in trouble to descend without delay.
The “emergency descent” profile that a Pilot uses to expedite a descent is actually not much different than a normal descent. The nose is lowered and the speed brakes (spoilers on the wing) are raised to help the plane descend faster. This occurs routinely in normal operations when ATC starts the plane down late due to traffic below. The major difference is the Pilots fly a little bit faster, closer to their maximum allowable speed, to expedite the descent. The rush is to simply get the plane down where pressurization is not required (around 10,000 feet). The onboard oxygen canisters operate long enough to allow the plane to descend to where oxygen is plentiful in the thicker surrounding air, normally between 12,000 and 10,000 feet.
Emergency descents are also used for medical inflight emergencies, when we have to get a Passenger on the ground quickly.