Friday, September 12, 2014

When the Pressure is Gone

Socata TBM 900 N900KN in better days. Photo: scotch-canadian - www.airport-data.com

Socata TBM 900 N900KN in better days – Photo: scotch-canadian – www.airport-data.com

Throughout the past 35 years, there have been several high-profile cases of aircraft crashes due to cabin depressurization, with one of these being an airliner. Aircraft pressurization is actually a pretty simple process on modern aircraft, and is almost always computer-controlled. Aircraft are pressurized through the use of compressed air that is either supplied by bleed air tapped off of the engines, or, as in the case of smaller piston engine aircraft and the Boeing 787, by a compressor on board the aircraft.

This pressurized air is regulated through the means of outflow valves that help to maintain cabin pressure to the design limits of the airframe. Most commercial aircraft are pressurized to an altitude of 8,000 feet, with the main exception being the 787, which is pressurized to 6,000 feet. In very rare cases, however, things go wrong, as was experienced in the tragic loss of a Socata TBM 900 off the coast of Jamaica on September 5th.

ANA's first Boeing 787 Dreamliner is being delivered on Monday.

The Boeing 787 is designed to be pressurized at 6,000 feet

At roughly 10:00 a.m. Eastern time, TBM 900 N900KN, the first of its type to be delivered, was last heard from by ATC as it made its way from Rochester, NY to Naples, FL. Approximately 40 minutes later, two F-16s from McEntire Joint National Guard Base in South Carolina intercepted the aircraft and found all of its windows frosted over, and according to audio from the fighters, they could see the pilot slumped over in the pilot’s seat, and reportedly he could “see pilot’s chest moving, hopes he regains consciousness as unresponsive plane descends.”

It is not known at this time what exactly happened, but the frosted windows are one indication of a rapid depressurization of the cabin. The aircraft continued on the last heading programmed into its autopilot for roughly 1,200 miles before finally running out of fuel. Tragic as it is, this type of incident is not unheard of, as it has happened before with very similar results.

Flight path of N900KN. image: Flightaware.com

Flight path of N900KN. Planned course marked in red – Image: Flightaware.com

The most well-known incident of inflight depressurization occurred on October 25, 1999. Pro golfer Payne Stewart was a passenger on a chartered Learjet 35 (N47BA) en-route from Orlando to Dallas when ATC lost all contact with the aircraft.

Upon interception by a USAF F-16, the aircraft was found to have all windows iced over, with no sign of the crew or passengers. The plane continued on a straight course across the midwestern United States prior to running out of fuel over South Dakota and crashing. In the accident investigation, the National Transportation Safety Board report detailed that the aircraft had received maintenance many times over the preceding several months for pressurization issues.

The report also noted that post-crash analysis found that the engine bleed air system appeared to be functioning normally. The most logical cause for the incident would have either been a cabin pressure specific component failure, or a breach in the airframe.

According to the incident report, “a period of as little as 8 seconds without supplemental oxygen following rapid depressurization to about 30,000 feet may cause a drop in oxygen saturation that can significantly impair cognitive functioning and increase the amount of time required to complete complex tasks.” In essence, the crew would have had little time to don their emergency oxygen masks prior to succumbing to the effects of hypoxia and being unable to function, although they would still be alive, barely.

The route of Payne Stewart's fatal flight. Planned path in green, actual in red. Image: NTSB

The route of Payne Stewart’s fatal flight. Planned path in green, actual in red. Image: NTSB

Another well-known and documented case of pressurization issues with an aircraft was Helios Flight 522. However, this crash was not a result of inflight depressurization; rather, it was a cascading result of errors and the effects of hypoxia on the pilots.

On August 14, 2005, Helios 522 departed Lamaca, Cyprus for Athens, Greece, and almost immediately the crew received  an aural alarm along with warning lights indicating that the cabin was not pressurized as the plane passed through 10,000 feet. The captain immediately contacted Helios maintenance control to diagnose the problem and correct the issue.

Boeing 737-300 5B-DBY, the aircraft that operated Helios flight 522. photo: Alan Lebeda

Boeing 737-300 5B-DBY, the aircraft that operated Helios flight 522 – Photo: Alan Lebeda

Prior to departure from Cyprus, the plane was worked on by a mechanic for noises coming from a door, and the plane was pressurized on the ground for troubleshooting. In order to accomplish this task, the pressurization had to be selected to manual, and upon completion, the mechanic failed to reset the switch to auto.

When the crew performed their preflight checklist, they failed to catch it. When the captain was put in contact with the mechanic, he was asked if the pressurization control switch was in manual or auto, however, by this time, hypoxia was already taking its toll, and the captain and first officer were sure they had another issue and ignored the question.

The result was nearly the entire crew succumbing to the effects of oxygen starvation, and being rendered unconscious. However, when the plane was intercepted by Greek F-16s, the pilots of the fighters noticed a person working their way forward in the cabin. It was later discovered that a flight attendant was able to don an oxygen mask and slowly make their way to the cockpit. While the attendant happened to be a licensed pilot in England, he was unfamiliar with the 737 systems, and could not recover the plane. The plane crashed in Grammatiko, Greece, following fuel starvation.

Helios 522 flight path. Image: Oona Räisänen

Helios 522 flight path – Image: Oona Räisänen

While issues like these are rare, they tend to make headlines because of the way they occur. In the past 35 years, there has been six major cases of pressurization issues being a major contributing factor in crashes, and only one of those has involved an airliner. For successful stories of incidents like this, one only has to look at the Miracle Landing of Aloha Flight 243.

Just after leveling off at a cruising altitude of 24,000 feet, the Boeing 737-200 experienced rapid depressurization caused by metal fatigue in the fuselage skin, causing a roughly twenty foot-long section of the upper fuselage to rip away from the aircraft. Miraculously, only one one person was killed, a flight attendant who was sucked from the aircraft, and while there were serious injuries to many of the passengers and crew, all the rest survived. It was shear luck and the skill of the pilots that brought the plane to a safe landing.

Aloha 243 following the emergency landing. photo: NTSB

Aloha 243 following the emergency landing – Photo: NTSB

As mentioned earlier, cabin pressurization is supplied by compressed air taken from either bleed air off the engine’s compressor core in the case of a turbine engine, or through an air pump, whether it is engine driven or electrically driven. It is a simple system in principle, and rarely fails with disastrous results, if it ever does.

 Kris Hull – Correspondent 

Kris lives, breathes, and sleeps aviation. An airline insider for the past ten years, Kris has worked nearly every job in the operations of an airline except for being a pilot and flight attendant. Currently, he spends his time as a FAA certified aircraft mechanic, and he has maintained everything from small to large. When not turning wrenches on an aircraft, Kris can be found with his camera at a nearby airport. 

 @krislhull | Flickr | Hull AeroImages

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