This article provides guidance for controllers on what to expect from an aircraft which is believed to be experiencing the effects of Loss of Cabin Pressurisation (or decompression) and some considerations which will enable the controller, not only to provide as much support as possible to the aircraft concerned, but also maintain the safety of other aircraft in the vicinity and of the service provision in general. It also gives some details about the effects of decompression and statistical data.
Useful to Know
Decompression is a quite simple physical process meaning the relief of pressure until a balance is achieved. However, in terms of aviation the word is more specific:
Decompression is defined as the inability of the aircraft's pressurisation system to maintain its designed pressure schedule. Decompression can be caused by a malfunction of the system itself or by structural damage to the aircraft.
The speed of pressure loss is predominantly used to characterise the decompression process:
- An explosive decompression is a change in cabin pressure faster than the lungs can decompress. Most authorities consider any decompression which occurs in less than 0.5 seconds as explosive and potentially dangerous. Explosive Depressurisation/Decompression is more likely to occur in small volume pressurized aircraft, such as military jets or VLJs than it is in large pressurized aircraft and can result in lung damage to the aircraft occupants.
- A rapid decompression is a change in cabin pressure where the lungs can decompress faster than the cabin. The risk of lung damage is significantly reduced in this decompression as compared with an explosive decompression.
- Gradual or slow decompression is usually dangerous only when it has not been detected at an early stage. Automatic visual and aural warning systems do not always provide an indication of a slow decompression until its effects have become significant and these warnings have not always been interpreted correctly.
Effective Performance Time (EPT) or Time of Useful Consciousness (TUC) is the amount of time in which a person is able to effectively or adequately perform flight duties with an insufficient supply of oxygen (For more details see the dedicated article: Time of Useful Consciousness - shown are the 'average TUC' at various altitudes.). EPT decreases with altitude, until eventually coinciding with the time it takes for blood to circulate from the lungs to the head usually at an altitude above 35,000 feet. The rate of cabin altitude ascent to equal aircraft altitude directly affects TUC. Faster rates of cabin ascent result in shorter TUC.
Explosive or rapid decompression could be as a result of or cause structural damage with possible catastrophic outcome, however events of such nature are extremely rare. During decompression there are two sets of immediate dangers:
- Cooling & misting
Physical hazards such as listed above are well recognised, but in most decompressions there is very little risk of mechanical injury.
Anticipated Impact on Crew
A wide range of practical problems could arise during decompression and the following emergency descent:
- Mask/headset donning & retention - the time of useful consciousness rapidly decreases as altitude increases. The pilots have relatively small amount of time to remove their headset and put on their oxygen masks. Often the surprise introduces a delay in response.
- Communications - regardless of the mask model, a significant feature of the design is that it fits quite tightly on the face as to prevent oxygen leaks. Despite built in microphones which attempt to compensate for this, it may lead to changes in the sound of speech including a distorted sound spectrum.
- Sick/invalid passengers - The shock and surprise during decompression together with the accompanying formation of mist in the cabin could be quite overwhelming for some passengers. Possible outcomes are cardiac arrest, lost consciousness from improper handling of oxygen masks, and injuries from flying debris.
- Noise - Though rare, damage to the aircraft skin or loss of a door or window can drastically increase noise levels in the cabin
- Not declaring an emergency - Pilots are trained to call ATC as soon as practicable and advise of their intentions. However, during the initiation of an emergency descent, the workload becomes briefly intense and ATCOs should not expect immediate information about the situation. The crew may begin the descent without requesting clearance or warning ATC.
- Heat from oxygen generators - as the chemical reaction in PSU oxygen generators produces their 15-20 min supply, their containers can reach a temperature of up to 260 degrees Celsius. This heat, and the associated fumes or smoke, can be expected to cause a degree of anxiety and perhaps panic in the passenger cabin.
- Issues with the control of the aircraft - During decompression the aircraft could suffer damage to aircraft systems, for example the hydraulic system, or structural damage affecting the aerodynamic characteristics of the aircraft.
- Emergency Descent procedure - descent procedure should be executed in accordance with the company emergency procedures and associated training. Descent will be rapid unless the crew suspect structural integrity, in which event a much less agressive response can be expected with less airspeed and the avoidance of high manoeuvring loads.
Automatic Emergency Descent Systems
Some modern aircraft are equipped with an Automatic Emergency Descent System.
An emergency descent system is provided for automatically performing an emergency descent. The system monitors cabin pressure altitude and if the cabin altitude exceeds a pre-set value, the emergency descent system may direct the autopilot to descend the airplane to minimum safe altitude. The emergency descent system may also communicate with ground facilities informing them of the descent.
Suggested Controller’s Actions
Best practice embedded in the ASSIST principle could be followed (A - Acknowledge; S - Separate, S - Silence; I - Inform, S - Support, T - Time) :
- A - acknowledge the decompression, ask for the crews’ intentions when their situation permits, and establish whether the crew is able to control the aircraft;
- S - separate the aircraft from other traffic, prioritise it for landing (allow long final if requested), keep the active runway clear of departures, arrivals and vehicles;
- S - silence the non-urgent calls (as required) and use separate frequency where possible;
- I - inform the airport emergency services and all concerned parties according to local procedures, as a tower controller expect if the aircraft is to land as soon as possible, airport authorities execute a massive contingency plan
- S - support the flight experiencing the consequences of the decompression with any information requested and deemed necessary (e.g. type of approach, runway length and aerodrome details, etc.);
- T - provide time for the crew to assess the situation, don’t press with non urgent matters.
The controller should be prepared to:
- Acknowledge emergency on RTF
- Take all necessary action to safeguard all aircraft concerned
- Suggest a heading, if so required
- State the minimum altitude, if necessary
- Provide separation or issue essential traffic information, as appropriate
- Emergency broadcast if necessary
- After emergency descent and when the situation permits, ask for pilot’s intentions and other important information, such as :
- Aircraft damage
- Consider aircraft still to be in an emergency situation - The aircraft could be ready to continue flight without any further complications after the emergency descent, if no structural damage was inflicted during decompression. However, often the crew is not able to assess during flight neither the exact type, nor the extent of the damage.
- Personal Awareness - The onset of rapid or explosive decompression will be entirely unpredictable. There could be hardly a defence against such an event. However, an ATCO should always be monitoring the course and altitude of traffic in his/her sector. Being constantly aware of any ongoing deviations should provide precious time for vectoring of nearby traffic.
- Adequate Reaction - A controller can expect an aircraft experiencing rapid decompression to begin a sudden descent and for the reduced quality of RTF communication due to oxygen mask use to be obvious. Some of the possible actions: transfer all other aircraft to another frequency (possible message to all stations to increase awareness); leave the emergency traffic on the current frequency; increase the volume of the receiver; have a colleague(a separate pair of ears) to also listen to all transmissions from the aircraft.
- Technological limitations - Try to keep aircraft within radar cover. Have in mind the features of the existing radar system, refrain from attempting to transfer the aircraft to another sector.
- Organisational Awareness - The fast provision of ATCOs during emergency situations should be an objective at administrative level. Periodic training and drills are likely to improve intra-organisational coordination.
Accidents & Incidents
The following events involved loss of pressurisation:
- B733, en-route, northwest of Athens Greece, 2005: On 14 August 2005, a B737 Series aircraft belonging to Helios Airways, crashed near Grammatiko, Greece following the incapacitation of the crew due to hypoxia.
- RJ1H, en-route, South West of Stockholm Sweden, 2007: On 22 March 2007 when climbing out of Stockholm Sweden, the crew of a Malmö Aviation Avro RJ100 failed to notice that the aircraft was not pressurised until cabin crew advised them of automatic cabin oxygen mask deployment.
- B738, en-route, southern Austria, 2010: On 9 May 2010, Boeing 737-800 being operated by Swedish operator Viking Airlines on a public transport charter flight from Sharm el Sheikh, Egypt to Manchester UK and which had earlier suffered a malfunction which affected the level of redundancy in the aircraft pressurisation system, experienced a failure of the single air conditioning pack in use when over southern Austria and an emergency descent and en route diversion to Vienna were made. There were no injuries to any of the 196 occupants.
- A319, en-route, Free State Province South Africa, 2008: On 7 September 2008 a South African Airways Airbus A319 en route from Cape Town to Johannesburg at FL370 received an ECAM warning of the failure of the No 1 engine bleed system. The crew then closed the No. 1 engine bleed with the applicable press button on the overhead panel. The cabin altitude started to increase dramatically and the cockpit crew advised ATC of the pressurisation problem and requested an emergency descent to a lower level. During the emergency descent to 11000 ft amsl, the cabin altitude warning sounded at 33000ft and the flight crew activated the cabin oxygen masks. The APU was started and pressurisation was re-established at 15000ft amsl. The crew completed the flight to the planned destination without any further event. The crew and passengers sustained no injuries and no damage was caused to the aircraft.
- LJ35, Aberdeen SD USA, 1999: On 25 October 1999, a Learjet 35, being operated on a passenger charter flight by Sunjet Aviation from Orlando to Dallas Fort Worth, crashed in South Dakota following loss of control attributed to crew incapacitation following insufficient oxygen intake after an apparent failure of the pressurisation during the climb.