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Pressurisation Problems: Guidance for Flight Crews
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|Category:||Emergency & Contingency|
Crews should follow company approved emergency procedures and manufacturer's guidance regarding the conduct of the flight and management of aircraft systems when confronted with any Loss of Cabin Pressurisation event. If an uncontrolled increase in cabin altitude is observed before that altitude reaches a critical value, intervention as directed by the manufacturer, such as switching or cycling the cabin pressure controller, may be considered if time permits. However, if control of the cabin pressurisation cannot be regained without delay or if the cabin altitude reaches a critical value, other measures must be taken to ensure the safety of the passengers, crew and aircraft.
This article considers some aspects of airmanship which are applicable to all aircraft and situations.
At the first indication of a pressurisation problem, or symptoms of Hypoxia, the flight crew should don oxygen masks. At cruising altitude, the Time of Useful Consciousness can be less than one minute so it is imperative that, in the event of a explosive or rapid depressurisation, the flight crew don oxygen masks immediately. Although more insidious, gradual loss of cabin pressure also poses a grave risk and should be reacted to as soon as it is recognised.
The aircraft's emergency oxygen supply is finite; in most aircraft, the supply will last little more than 15 minutes, perhaps less.
If the crew cannot immediately correct the pressurisation issue, they should commence immediately an emergency descent to 10,000 ft or the minimum safe altitude, whichever is higher. An emergency should be declared (MAYDAY) and ATC told that the aircraft is in descent. In a high density traffic area, when there may be a number of aircraft in close proximity, it is wise to declare the emergency and ask for descent before commencing descent. However, if that clearance is not immediately forthcoming, descend without it - the aircraft’s oxygen supply may be exhausted faster than you think, so any delay in commencing descent may prove fatal to crew and passengers.
When making the initial emergency call, it is also a good idea to ask ATC for the local Altimeter Pressure Settings - think “Safety Altitude”.
Consider the terrain ahead of the aircraft. If the en route terrain is above 10,000 ft, would it be better to turn around? What is your escape route away from the high terrain? - Many operators conduct a cruise brief at top of climb in which one of the points for discussion is contingency plans regarding sections of the route where the minimum safe altitude is above 10,000 ft. For scheduled routes over extended areas of very high minimum safe altitudes, regulatory authorities often require additional crew oxygen availability, higher capacity Chemical Oxygen Generators for the passengers and pre-established escape routes.
Temperature Error Correction
Check whether you need to apply Altimeter Temperature Error Correction to your calculated safety altitude.
Emergency Descent Profile
The best profile to fly is idle thrust, speed brakes out, and speed as appropriate for the condition of the aircraft. For an aircraft with no structural damage, maximum operating speed (Mmo/Vmo) is generally considered the appropriate target.
- Don't use speed brakes if you are concerned about the structural integrity of the aircraft.
- Emergency descents are normally conducted with the undercarriage up. If structural integrity is a concern, consider descent with the undercarriage down in order to increase drag and enable a higher rate of descent at a lower speed. If the undercarriage is to be extended, don't forget the limiting altitude and speeds.
Use of Autopilot
Crew coordination and communication are more difficult when wearing oxygen masks and a pilot's field of vision is reduced. Most operators therefore recommend the use of the autopilot for the emergency descent. Use of the autopilot frees up the crew to consider the implications of the emergency, run checklists, and coordinate with ATC. Furthermore, if you subsequently lose consciousness then the aircraft will stabilise at the new set altitude and you, or other members of the crew, will hopefully recover before the aircraft runs out of fuel.
- The specific detail on how to manage the autopilot in an emergency descent will, of course, be dependent on the capability of the autopilot and also be aircraft specific. However, the basic principles are the same: get the aircraft into a rapid descent to level off at a set altitude. For this reason, some operators caution against the use of the vertical speed mode.
- Turn away from track i.e. out of the airway to reduce risk of loss of separation from other aircraft.
Explosive or Rapid depressurisation may have been caused by, and/or result in structural damage to the aircraft. If structural damage is suspected, the crew should consider limiting the speed to the IAS at which the event occurred and avoiding high manoeuvre loads.
Having descended to 10,000 ft, there are a number of factors that must be considered.
- What are the captain’s intentions?
- What is the best route to fly to take into account factors such as terrain, weather, and fuel?
- Is it appropriate to continue the flight to destination or should you land at the nearest suitable aerodrome? This decision depends on the circumstances of the depressurisation, the condition of the aircraft, and the condition of the passengers, as well as the planning considerations above.
Fuel burn will be greater at the lower altitude - do you have enough fuel to make the nearest diversion?
- Apply long range cruise techniques,
- Consider the safety and performance implications of shutting down an engine in order to extend the range of the aircraft. It is very unlikely that a cruise engine shutdown would be considered on larger jets, but for some smaller jets and turboprops operating over the oceans it may be worth considering. If you do consider shutting down an engine, first check that the en route terrain clearance can be maintained on reduced power and adjust the route if necessary. It is prudent to configure the shut-down engine for immediate relight in case of a further emergency (cruise engine shutdown drills normally configure the engine for relight).
- Consider flying at 14,000 ft with only the flight crew on oxygen.
Accidents and Incidents involving Depressurisation
- B732, en-route, Maui Hawaii, 1988: On 28 April 1988, a Boeing 737-200, operated by Aloha Airlines experienced an explosive depressurisation and structural failure at FL 240. Approximately 5.5 metres (or 18 feet) of cabin covering and structure was detached from the aircraft during flight. As result of the depressurisation, a member of the cabin crew was fatally injured. The flight crew performed an emergency descent, landing at Kahului Airport on the Island of Maui, Hawaii.
- 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.
- LJ35, Aberdeen SD USA, 1999: On 25 October 1999, a Learjet 35 operated by Sunjet Aviation, crashed in South Dakota following crew incapacitation due to Hypoxia.
- Pressurisation Problems: Guidance for Controllers
- Explosive Depressurisation
- Rapid Depressurisation
- Gradual Depressurisation
- Loss of Cabin Pressurisation
- Aircraft Pressurisation Systems
- Decompression Sickness
- EUROCONTROL Safety Warning Message: Emergency Descent in High Traffic Density Situations