When the left engine of Southwest Flight 1380 broke apart last week, shattering a window of the aircraft and causing the Boeing 737 to lose cabin pressure, the pilots pushed the nose of the plane down and zoomed from 32,500 feet to 10,000 feet in about eight minutes.
The abrupt dive led some passengers to describe the change in altitude as a free fall. But the pilots appear to have executed a perfect emergency descent.
The reason for the steep drop was straightforward: Without enough oxygen, everyone aboard the plane could lose consciousness.
At 35,000 feet, the duration of useful consciousness—the length of time pilots can perform their duties efficiently while deprived of adequate oxygen—is no more than 60 seconds. At 30,000 feet, they can hang on for as long as three minutes. At 10,000 feet, the air is breathable.
Oxygen masks provide pilots with pressurized air that could last several hours, but passengers receive only 10 to 20 minutes of oxygen with no pressure. At more than 30,000 feet, even with the air flowing, they’re in an emergency situation, so when an airplane’s cabin depressurizes, the goal is to quickly descend to an altitude where people can breathe.
“You can’t do it slower,” said Chris Manno, a former Air Force pilot who now flies 737s for American Airlines. “You have to get to a habitable altitude as soon as possible.”
Although it’s uncommon for two abnormal events to happen simultaneously, the Southwest Airlines Co. pilots were able to respond coolly to engine failure and cabin depressurization because, like all commercial pilots in the U.S., they regularly train for the malfunctions.
“Landing with a single engine is one of the main things you practice,” said Pat Anderson, director of the Eagle Flight Research Center at Embry-Riddle Aeronautical University. Emergency descents are also a fundamental skill that pilots are trained to execute.
During a flight, pilots are aided by checklists that provide step-by-step instructions, with some duties performed by the pilot flying the plane and others by the nonflying pilot.
Because of the investigation into what befell Southwest 1380, neither Boeing Co. nor the airline would provide information about their checklists. But Mr. Manna, Dr. Anderson and James D. Brooks, a former Delta Air Lines engineer and senior researcher at Georgia Tech’s Daniel Guggenheim School of Aerospace Engineering, outlined the procedures.
Ordinarily if an engine is lost, they said, pilots would declare the situation to air-traffic control and be cleared to descend in what is referred to as driftdown—a gentle glide compared to an emergency descent—while disengaging the plane’s autothrottles and autopilot, shutting off fuel and other combustibles to the damaged engine and adjusting the power of the surviving engine.
“You fly at a higher power setting than if both engines are running,” Dr. Anderson said, “but the plane is controllable.”
The aircraft will yaw, or turn, in the direction of the failed engine, which the pilots will correct by adjusting the rudder—a hinged device attached to the vertical fin on the tail of an airplane that controls side-to-side motion.
According to the National Transportation Safety Board, when Southwest 1380 lost its engine, the airplane banked left at about 41 degrees.
Although losing an engine is a terrifying prospect, as a safety measure, the 737 and other commercial jets are designed to fly with only one engine long enough to take off or land safely.
“When the required calculations are done to determine the maximum allowable takeoff weight of any commercial aircraft from any runway that is served, it is done assuming the loss of one engine,” Mr. Brooks said.
That precaution ensures a plane hurtling down a runway can become airborne rather than risk skidding off the pavement or clipping trees. In some ways, losing an engine while up and away is less dramatic than losing one on takeoff.
“There aren’t immediate things you can hit,” Dr. Anderson said.
But in the case of Southwest 1380, a piece of the lost engine broke a window, draining the cabin of its pressurized air and causing the death of one passenger.
Cabin depressurization has its own checklist, which includes donning oxygen masks, announcing the emergency descent to the passengers, advising air-traffic control and descending to 10,000 feet before proceeding to the nearest acceptable airport.
As pilots execute the procedure, it’s possible they may not know what led to the depressurization.
“A broken window is not going to manifest in cockpit,” Dr. Anderson said.
But the procedure is the same no matter what, and in the case of Southwest 1380, the pilots appear to have been fully in control of a chaotic situation.
“By all reports,” Mr. Brooks said, “it was a by-the-number flight crew response.”
Write to Jo Craven McGinty at Jo.McGinty@wsj.com