Loss of Pressurization During Flight, Part 1

The Learjet 35 that depressurized. Credit: NTSB

The mere mention of Payne Stewart’s name brings a gloomy recall of the consequences of losing cabin pressurization. In the aftermath, especially after the release of the extensive NTSB investigation, there was heightened emphasis on the prevention and proper reactions in the event of depressurization.

It has been 22 years since that tragic event. This a good time to pause and determine if lessons learned from the past have effectively been adopted into training and flight crew procedures. To help us answer this question we did a lengthy search of the NASA Aviation Safety Reporting System (ASRS) database. In a recent 18-month period, there were 256 reports in the ASRS database in which a loss of cabin pressurization event was experienced in a business jet.

What important insights does this large collection of actual real-world events reveal? For starters, many of the reported events occurred at significant altitudes. Approximately 14% occurred at/above FL400. Roughly 56% occurred at/above FL350, and 97% occurred above FL300. In other words, all of these occurred at altitudes in which the pilot’s ability to remain conscious and functioning for a relatively short duration of time is severely threatened with a loss of pressurization.

How severe were the losses of pressurization in these ASRS reports? More than 40% of the reports in this sample were “rapid” decompressions (defined as being a loss of cabin altitude exceeding 2,000 fpm), and an additional one-third were termed by the flight crews as a “moderate” depressurization rate (defined as a loss of cabin altitude between 500 and 2,000 fpm).

Rapid decompression risks dangerous physiological effects to the body at these altitudes. Nearly all of the flight crews that experienced a “rapid” or “moderate” loss of pressurization indicated that the first symptom of the problem was the immediate change of pressure felt on their eardrums and other internal sinuses. This physiological symptom immediately alerted the flight crew to look at the cabin altitude indicator, which confirmed a loss of cabin pressure. All of the “rapid” or “moderate” losses of pressurization led to an immediate emergency descent by the flight crew.

The physiological effects of a rapid decompression range from trapped-gas expansion--within the ears, sinuses, lungs and abdomen, cold and wind chill--to hypoxia. The gas-expansion disorders can be painful and may become severe, but they are transient. The most serious hazard for the aircrew member is hypoxia. Fortunately, all of the sampled ASRS reports involving “rapid” or “moderate” loss of pressurization were resolved without further harm to the aircraft occupants because the flight crew reactions were nearly immediate.

Crewmembers also 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. 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. The amount of temperature decrease depends on altitude.

According to FAR Part 91.211, when a pressurized aircraft is above FL350, if one pilot leaves the controls, the other pilot must don and use the oxygen mask. Above FL350, unless a quick-donning oxygen mask is available, one pilot at the controls must continuously wear an oxygen mask. When above FL410, one pilot must continuously wear and use an oxygen mask. Part 135.89 is more conservative than Part 91 and requires one pilot to don and use an oxygen mask if the other pilot leaves the controls when above FL250. When above FL250 through FL350, unless a quick-donning oxygen mask is available, one pilot at the controls must wear a mask. When above FL350, one pilot must continuously wear and use an oxygen mask.

Were pilots observing these rules? There were a couple forthright admissions by ASRS submitters that they had not been observing them. Otherwise, the exact wording of the submittals left that open to interpretation. For instance, an event happening while cruising at FL430 and the crewmembers mentioning “we both immediately donned our masks” could infer that one of the pilots wasn’t wearing a mask as required. However, given this lack of certainty I wasn’t comfortable presenting definitive numbers on this question.

The earlier that crewmembers detect a loss of pressure, the quicker they can take appropriate emergency measures to increase survival and prevent the debilitating effects of pressurization loss or hypoxia. Twenty-seven percent 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, which then commanded the pilots’ attention to look at the cabin pressure gauge. Clearly the “insidious” loss of altitude “snuck up” on the pilots and caught them by surprise. In many of the cases it was fortunate that a warning system was operative to warn the pilots of the loss of pressurization, and prior to the onset of the early symptoms of hypoxia. 

A troubling number of the events (roughly 5%) indicated that flight crews were not using oxygen masks while they were troubleshooting cabin pressure problems as the cabin altitude climbed.

Problems Encountered During Real Events

Among the problems that pilots reported in the sampled NASA ASRS reports were difficulty with donning the emergency oxygen mask (43%), particularly getting entangled with headsets (39%), eyeglasses (33%) or long hair (13%); re-establishing communications with the controller (29%); problems hearing the controllers (25%); difficulties with the communication panel switches (19%); difficulties in obtaining clearance for lower altitudes (12%); and distractions with passenger safety (9%). Several of the ASRS reports included admissions by the pilot that they were in a “brain fog” for several minutes even after donning their oxygen masks.

If there are some pointers to learn from the ASRS reports, it would be reviewing the correct switches to enable the microphone on the oxygen mask, and which switches on the audio panel to use for the overhead speaker. Realize that if you leave the intercom button on “hot mike” and you switch the sound to the overhead microphone that you are going to be listening to each other’s pronounced breathing on the overhead. It’s loud and it distracts from listening to ATC’s emergency instructions.

Began to Experience Hypoxic Symptoms

Roughly 17% of the sampled reports indicated that a member of the flight crew began to experience the symptoms of hypoxia. Many of the aerospace physiology training materials warn of the insidious onset of it.

Hypoxia symptoms are diverse, varying from person to person, depending on variables such as absolute altitude, rate of ascent, duration at altitude, ambient temperature, blood alcohol levels and physical activity. Therefore, it is important to be aware of all the signs and symptoms.

Symptoms may include fatigue, lassitude (state of exhaustion), somnolence (drowsiness, sleepiness), dizziness, headache, mental and muscle fatigue, nausea, breathlessness, hot and cold flashes, tingling, visual impairment and euphoria. Signs of hypoxia to be aware of in another crewmember may include rapid breathing, cyanosis (a bluing discoloration of the lips and fingernails), poor coordination, lethargy and executing poor judgment.

Gordon Cable of the Royal Australian Air Force Institute of Aviation Medicine (Aviation Space and Environmental Medicine, February 2003) examined incidents of hypoxia reported to the Directorate of Flying Safety of the Australian Defense Force (DFS-ADF) for the period 1990-2001. One of the important conclusions from his study is that the most common symptoms were subtle and often involved cognitive impairment or light-headedness. The vast majority (75.8%) of these episodes were recognized by the aircrew themselves (all of whom had received mandatory altitude chamber training), reinforcing the importance and benefit of hypoxia training.

Adrian M. Smith of the Armed Forces Aeromedical Center in Dhahran, Saudi Arabia, studied the question of whether flight crews that experience hypoxia-related incidents were able to recognize hypoxia because of the similarity to symptoms they experienced during hypoxia awareness training. In a report titled “Hypoxia Symptoms in Military Aircrew: Long-Term Recall vs. Acute Experience in Training,” in the January 2008 edition of Aviation Space and Environmental Medicine, Smith surveyed an extensive group of aviators who underwent altitude chamber training to determine whether there was a positive correlation between the symptoms remembered from previous hypoxia training versus the cognitive and psychomotor impairment dominated symptoms reported after an acute hypoxia event. During the acute hypoxia event, 65% of aircrews experienced the five symptoms they remembered to be dominant from previous training; 57% of aircrews remembered from previous training the symptoms that dominated their experience of acute hypoxia. Smith concluded that hypoxia awareness training is an effective method of allowing aircrews to recognize their personal manifestation of hypoxia (their “hypoxia signature”). It is especially important to note that by the time physical symptoms are recognized, dangerous cognitive defects may already have jeopardized the pilot’s ability to take the proper corrective action.

A study in the U.S. found similar results. Carla Hackworth of the FAA’s Civil Aerospace Medical Institute published a study titled “Altitude Training Experiences and Perspectives” in the April 2005 edition of Aviation Space and Environmental Medicine asking professional business pilots about their experiences and perceptions of hypoxia training. Most respondents to the survey agreed that all pilots should receive introductory hypoxia training (92%), recurrent hypoxia training (86%), initial altitude chamber training (85%) and recurrent altitude chamber training (70%).