According to the U.S. Air Force Air Training Command’s “Aerospace Physiology” textbook, the time of useful consciousness can be reduced 30 to 50 percent when the decompression is rapid because of the sudden outward flow of oxygen from the body’s tissue. The Air Line Pilots Association has slightly more conservative estimates on the time of useful consciousness.
The regulations take these factors into account. According to 14 CFR 91.211, when a pressurized aircraft is above FL 350, if one pilot leaves the controls, the other pilot must don and use an oxygen mask. And above that altitude, unless a quick-donning oxygen mask is available, one pilot at the controls must continuously wear an oxygen mask. When above FL 410, one pilot must continuously wear and use an oxygen mask. Period.
Since 14 CFR 135.89 is more conservative than FAR Part 91, it requires one pilot to don and use an oxygen mask if the other pilot leaves the controls when above FL 250. When above FL 250 through FL 350, unless a quick donning oxygen mask is available, one pilot at the controls must wear a mask. And when above FL 350, one pilot must continuously wear and use an oxygen mask, regardless.
The earlier that crewmembers detect a loss of pressure, the quicker they can take appropriate emergency measures to minimize the risks from the debilitating effects of loss of pressurization or hypoxia. Pressure loss can be identified by a number of symptoms. First is a loud, popping noise often called “explosive decompression.” Crewmembers need to be alert to the possibility of flying debris during a rapid decompression. The rush of air from inside an aircraft structure to the outside is of such force that items not secured may be ejected from the aircraft. Second, fogging is one of the primary characteristics of any decompression because air at a given temperature and pressure can hold only so much water vapor. With a loss of pressurization, cabin temperature equalizes with the outside ambient temperature, which significantly decreases cabin temperature, another indicator. The amount of temperature decrease depends on altitude.
on the eve of a trial in which four airline officials, including two Britons, face manslaughter charges after an airliner crash in Greece which killed 121 people.
Helios Airways Flight ZU 522 from Larnaca to Prague smashed into a hillside near Athens in August 2005. Air crash investigators in Greece concluded the plane crashed after its crew and passengers were rendered unconscious, starved of oxygen by a drop in cabin pressure.
Greek military jets were scrambled to intercept the airliner after it failed to respond to air traffic controllers. The military pilots reported that they could not see anyone flying the plane but could see one pilot slumped over the controls. They later saw a member of the cabin crew holding an oxygen bottle while struggling with the controls. The plane flew on autopilot for nearly two hours before running out of fuel and plunging to earth.
Air crash investigators concluded human error was to blame after a vital switch controlling air cabin pressure was set by engineers to manual instead of automatic. Pre-flight safety checks also failed to spot the error, their report claimed. As a result of the crash, criminal investigations were started separately in Greece and Cyprus, with charges being brought against a number of Helios executives and engineers. The first trial involving four airline officials – former chief executive Andreas Drakos, managing director Demetris Pantazis, operations manager George Kikides and chief pilot Ianko Stoimenov – starts in Cyprus on Thursday. But aviation and legal officials last week described the investigation and prosecution process as a disturbing development. They argue it undermines the confidential reporting system which is credited with helping to establish and maintain the aviation industry’s good safety record.
A smaller number of the ASRS reports indicated “slow” or “insidious” losses of pressure. These were often not detected physiologically by the pilots but rather by the cabin altitude warning system.
As time passed, radio communication between the pilot and ATC became increasingly difficult and erratic. At 8:30 p.m., the airplane began to descend from 14,800 feet msl. The rate of descent increased to more than 1,000 feet per minute. At 8:35 p.m., the pilot transmitted the following: “Denver radio, mayday, mayday. I’ve got myself in (unintelligible).” Two minutes later, another aircraft in the area reported picking up a strong but brief ELT signal. The wreckage was discovered a day and a half later by a rancher investigating a column of smoke on his land.
CAA Pressurisation Incidents reported March 2009
15 Mar 2009 B737. Aircraft presented ready for service after configuration from cargo to passenger. Crew noticed pressurisation selector was still in cargo mode and reselected. Passenger masks would not have deployed in an emergency.
14 Jan 2009. 737. Aircraft failed to pressurise and flight made at FL90. Manual mode selected and pressurisation controlled. Some passengers reported discomfort.
02 March 2009, 757. Pressurisation failure in cruise. MAYDAY declared and aircraft diverted. Significant number of passengers experiencedear trauma and a baby became unconscious after landing.
10 Mar 2009. DHC8. Aicraft depressurised at approx 4000 feet in descent. Rear hold blow out panel was dislodged.