After departing Hartsfield-Jackson Atlanta International Airport (ATL) on Friday, November 21, Southwest Airlines Flight WN-171 had to return to base as its cabin pressurization malfunctioned. The passengers bound for Chicago’s Midway International Airport (MDW) were transferred to a new plane after returning to base.

The aircraft in question was a Boeing 737-800, registration N8691A, according to the Aviation Herald. A replacement Boeing 737-700 ferried the unlucky flyers to Chicago. That jet’s registration is N204WN, and it arrived with a delay of about 2 hours and 30 minutes.

Aborted Departure For Southwest

Flight WN-171 took off from Atlanta’s runway 26L and began climbing until it reached 5,000 feet. At that point, the aircrew called air traffic control (ATC) and reported that the cabin altitude was not performing normally. The pilots made the call to stop their climb and return to Atlanta. About 25 minutes after lift off, tail number N8691A touched back down on runway 26R and landed safely.

From the incident report found on the Aviation Herald, it appears that the malfunctioning 737 was inspected and repaired within 36 hours of the aborted flight, and returned to service on Sunday, November 23. According to data from FlightAware, N8691A landed at Las Vegas International Airport (LAS) last night and has resumed its routine of making the rounds through the Southwest Airlines network.

It appears to have been a minor problem as the aircraft was only down for a short time, and has proceeded to complete several flights since its maintenance inspection was able to resolve the issue. The jet is a little over 9 years old and has served Southwest Airlines exclusively since it was completed at Boeing’s Renton Factory, according to Planespotters.net data.

A Leaky Boeing

Cabin pressurization is a very important system. As such, it is highly redundant and resilient, but when it does fail or experience an issue with high risk, safety comes before the schedule. If cabin pressure is not sustained at high altitude, everyone aboard can lose consciousness in the worst-case scenario, including the pilots. That danger, called hypoxia, makes it one of the most important systems on the plane.

Fortunately, N8691A does not seem to have required a significant overhaul because of any major system failures, and it’s back to cruising the friendly skies over America as usual already. The plane is less than a decade old, which means it still has at least 20 or 30 more years of service left in it before it’s ready for retirement, so it should be free of any significant system degradation barring exceptional circumstances.

The risk of hypoxia in a passenger plane is extremely low during normal operations due to strict regulations and advanced, redundant safety systems. The cabin is pressurized to an altitude of no more than 8,000 feet, which provides adequate oxygen for healthy passengers.

Flying In Thin Air

Aviation safety regulations and systems are designed to mitigate the risk of hypoxia. If the cabin pressure altitude exceeds a set limit (typically around 14,000-15,000 feet), oxygen masks for all passengers and crew deploy automatically from the ceiling panels. Critical pressurization systems have redundant components to ensure a single failure does not result in a total loss of cabin pressure.

Pilots undergo training to recognize their personal symptoms of hypoxia and are trained to respond immediately to a decompression event by donning their oxygen masks and initiating an emergency descent. Once masks are donned, they provide sufficient oxygen until the pilots can perform an emergency descent to a safe altitude, typically below 10,000 feet, where supplemental oxygen is no longer required.

The potential consequences of unmitigated hypoxia at cruising altitudes are severe. The actual risk to passengers on a commercial flight is exceptionally low due to robust safety regulations, built-in emergency systems, and mandatory crew training.