By Heidi Pérez-Moreno
Minutes after the oxygen masks dropped down from above the seats, Harrison Hove detected an unnerving burning smell.
Shortly before that, on his American Airlines flight earlier this month from Charlotte to Gainesville, Florida, his ears had mysteriously popped.
Many passengers were confused and unaware of what had happened. Hove said the next 20 minutes felt like hours.
Eventually, pilots announced that the cabin had experienced "a possible pressurization issue," and descended the plane from around 30,000 feet to 10,000 feet in mere minutes, the airline said in a statement.
During this time, Hove said there wasn't aggressive turbulence or hysteria from passengers, but he could feel a quick, downward slope ride.
"It was not like a roller coaster at all - it wasn't uncomfortable. I wasn't thinking the plane was ever out of control," said Hove, who serves as associate chair at the University of Florida's journalism department. "Any 'woos' you would feel in your stomach were very brief and not significant."
Movies such as ‘Air Force One’ and ‘Final Destination’ often portray depressurization scenarios with gaping holes in windows, luggage flying out of overhead compartments or even plane doors being ripped off.
The reality typically isn't as extreme as Hollywood depictions would have you believe, and the most recent malfunction illustrates what happens when an airplane loses cabin pressure.
Why are airplanes pressurized?
Because most commercial airplanes fly more than 30,000 feet above sea level, the air within a cabin is pressurized in order for flight crew and passengers to sustain normal functions.
Cabin air is pressurized at an air flow one would feel at 8,000 feet, according to Federal Aviation Administration standards. It's the same as the Grand Canyon's highest elevation point, which humans can safely access.
Airplanes will pressurize air through their engines, where they will suck in the outside air supply while on the ground and divert any excess to the cabin itself.
This can be seen through the cooling air flooding into the cabin before take-off near the overhead compartments.
Air within the engines is then cooled and humidified, and eventually circulated to the rest of the plane. The aircraft will then maintain its ideal pressure level throughout the flight.
Scott Wagner, who teaches aeronautical science at Embry-Riddle Aeronautical University in Daytona Beach, describes air pressure malfunctions as neither common nor rare.
They can potentially be serious if flight crews don't appropriately address cabin safety protocol before take-off, or if they don't confront the problem correctly once the plane is in the air.
They are also one of the most common issues addressed in training for pilots, which typically involves using simulators.
How do pilots know they're losing cabin pressure?
Trainings at accredited aviation schools and programmes are intended to follow regulations.
They mandate pilot or flight engineer stations to be equipped with instruments that show the difference in air pressure inside and outside an aircraft.
The same applies to devices showing cabin pressure within the aircraft, and the rate at which the pressure changes throughout the flight.
These instruments deliver warnings when the flight's pressure differential or cabin pressure altitude limit poses a risk.
If this alarm goes off, aircrew will be prompted to follow an itemized checklist to remedy the problem.
"I don't want to say it's routine, but it's one of many things that are regularly done in training," said Marvin Frantz, a pilot who works for the Office of Aviation Safety at the National Transportation Safety Board.
At what altitude do planes need to be pressurized?
An altitude of 10,000 feet is the ceiling at which the air pressure allows normal breathing.
When an airplane cabin's air pressure malfunctions, it's standard practice to approach this height to protect those on board.
Fly any higher, and the surrounding air becomes thinner, requiring pressurized oxygen within aircraft cabins for those on-board to survive.
"The normal pressurization in the airplane gives enough oxygen that we breathe normally," said Wagner, who teaches aircraft systems and flight-safety classes.
"If that's exceeded, and altitude is high enough that we don't have enough oxygen in the air, that's where the supplemental oxygen needs to comes in."
On Hove's flight, the plane descended to that altitude sweet spot shortly after pilots were alerted to a pressurization issue.
It took three minutes for the plane to drop nearly 15,000 feet, and another few minutes before the plane dropped an extra 4,000 feet, according to FlightAware, which tracks domestic and international flights.
"We apologize to our customers for any inconvenience and thank our team for their professionalism," American Airlines said in a statement to The Washington Post.
What causes a plane to lose pressurization?
There are various reasons an aircraft might experience depressurization, including structural malfunctions to windows, doors or sealed pressure vessels, as well as incorrectly activating the cabin's pressurization controls or structural malfunctions to the aircraft's overall system.
A pilot may consider deliberately altering the cabin's pressure system in some cases, such as if they need to clear smoke throughout a cabin, although that measure is considered drastic and less common.
A loss in cabin pressure is categorized as explosive, rapid or gradual, depending on how much time passes before depressurization occurs, according to SKYbrary, an online repository for aviation safety guidelines and resources.
Explosive depressurizations, which usually only happen on smaller aircraft flying at very high altitudes, occur in less than half a second.
The quick drop in temperature and humidity changes can cause the cabin air to fill with fog and debris.
Aircrafts, especially larger carriers, more commonly see rapid depressurization, which occurs over several seconds. Those on board will typically hear a bang and sudden fogging in these cases.
Gradual depressurization is the hardest type to identify before pilot crew might be warned of a potential issue. This will typically occur due to a leak in the aircraft's pressure vessel, reductions to the cabin airflow or the plane failing to pressurize after take-off.
In cases where decompression occurs as a result of a structural failure, such as a window falling off, SKYbrary said there is a risk for to experience strong winds, be hit by debris or potentially be sucked out of an aircraft.
It's unclear what type of depressurization occurred on American Airlines Flight 5916.