Bleed trip and uncontrollable cabin climb at FL310
You are the commander of a twin-engine business jet (MTOM 12,500 kg) cruising at FL310 with six passengers and one cabin crew. The No. 1 engine bleed has just tripped with an overheat warning. The aircraft is dispatched in a single-bleed configuration whose AFM ceiling is FL250. Two minutes later the cabin altitude, which had been steady at 7,200 ft, begins to climb at about 800 ft/min, and the outflow valve position indicator shows it part-open. Selecting the pressurisation controller to manual and attempting to drive the valve closed produces no change in the cabin rate. OAT is -38°C, the minimum safe altitude in the area is FL180, the nearest suitable airport is 14 minutes away and your destination is 48 minutes away. You have 1,900 kg of fuel and burn 780 kg/h at FL310. The passenger masks have not yet deployed.
1. State your immediate actions, in order, and explain why this pressurisation problem cannot be worked out at FL310.
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Immediate actions, in order:
- Crew oxygen masks on, establish crew communications - before analysing anything. The cabin is already at 7,200 ft and rising at 800 ft/min.
- Announce to the cabin so the cabin crew can prepare and secure the galley.
- Initiate the descent, thrust idle, speedbrakes as required, and turn towards the nearest suitable airport - do not wait for a full diagnosis. Target 10,000 ft, or FL180 if terrain dictates, whichever is higher on the day.
- Declare the situation to ATC (PAN or MAYDAY depending on whether the cabin altitude becomes uncontrollable) and request a block descent.
- Only then run the checklist for the bleed trip and the outflow valve.
Why FL310 is not recoverable: the outflow valve is not responding in manual, so the only variable left is the pressure available from the remaining bleed. The AFM already fixes the single-bleed ceiling at FL250 - above that altitude the remaining bleed cannot supply the mass flow needed to hold cabin altitude at FL310, so the cabin will keep climbing no matter how the controller is selected. Any further troubleshooting above FL250 simply burns the time of useful consciousness. The descent reduces the differential pressure required and lowers the cabin altitude, which is the only real fix available in flight.
2. At what cabin altitude will the passenger masks deploy, what is your approximate time of useful consciousness at FL310, and why is "masks on, keep going" not a plan?
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Mask deployment: the passenger oxygen system is automatically triggered at a cabin altitude of about 14,000 ft (usually 13,500–14,000 ft depending on type), with the crew able to release it manually. That will be reached in roughly 8–9 minutes at 800 ft/min from 7,200 ft - likely before you can descend to 10,000 ft.
Time of useful consciousness at FL310: approximately 1 to 1.5 minutes, and less for anyone with a respiratory or cardiac condition; the figure drops to around 30–60 seconds at FL350. Note that TUC is the time to useful consciousness, not to unconsciousness, and that judgement is degraded well before that.
Why "masks on, keep going" is not a plan:
- Passenger oxygen is a chemical/limited-duration supply (typically 12–20 minutes), sized for a descent, not for continued cruise.
- At FL310 there is no physiological reserve: if the cabin altitude exceeds the mask supply capability, or if the descent is delayed, the result is unconsciousness within a couple of minutes.
- The masks are a bridge to a lower cabin altitude, nothing more. The aeroplane must be brought down, and the plan must be built around reaching 10,000 ft (or MSA) before the oxygen runs out.
3. Choose between diverting to the airport 14 minutes away and continuing for 48 minutes. Show the numbers and justify the decision.
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Fuel arithmetic (780 kg/h = 13 kg/min):
- Divert 14 min: about 180 kg, leaving roughly 1,720 kg.
- Continue 48 min at FL310: about 625 kg - but FL310 is not available in this configuration, so the real figure is worse.
The key point is that continuing is not a cruise continuation. With the pressurisation uncontrollable, the aircraft must descend to 10,000 ft. At 10,000 ft the burn rises by roughly 20–30% (call it 950–1,000 kg/h), so 48 minutes at the lower level costs about 760–800 kg, not 625 kg. That leaves roughly 1,100 kg on arrival, with no reserves for a second problem, and it commits the aircraft to a long IMC sector at low level in terrain where the MSA is FL180.
Decision: divert now. A 180 kg diversion leaves about 1,720 kg, ample for the approach and a further diversion if the weather there is unsuitable. Diverting also gives the passengers a shorter exposure to a degraded cabin and puts the aircraft on the ground sooner for maintenance. The commercial cost of a 14-minute diversion is trivial compared with continuing an uncontrolled decompression for the sake of schedule. Inform ATC, request priority, and brief the cabin.