Single-Pack Descent Performance in 47 °C Heat Out of OKBK
You are commander of an A320-200 departing Kuwait International (OKBK) for Delhi at 14:20 local in August, take-off mass 71,000 kg. Outside air temperature is 47 °C, QNH 1002 hPa. Pack 1 is inoperative under an MEL dispatch that permits one-pack operation with an altitude restriction and does not require pack reconnection after take-off. Climbing through FL290, a master caution appears: cabin altitude 7,900 ft and rising at about 400 ft/min, differential pressure 6.8 psi and slowly falling. Pack 2 is indicating high flow, both engine bleeds show normal pressure and temperature, and the outflow valve position indicator reads about 70% open. There is no smoke, no fumes and the cabin crew report nothing unusual. Destination weather is fine; the nearest suitable diversion is 120 nm behind. Fuel is sufficient for destination plus 45 minutes holding at the planned level.
1. What are the two candidate explanations for the rising cabin altitude, and which checks discriminate between them?
Read the worked answer
The two candidates are (a) the single pack cannot supply enough conditioned air, and (b) excessive outflow - valve, controller or structural leakage.
The physics points to (a) but must be proven:
- Pack capacity. At 47 °C the pack is working near its heat-exchange limit; with one pack in high flow at FL290 the mass flow available may simply be less than the leakage plus the cabin demand. Falling differential pressure with the pack at high flow is the signature.
- Discriminating check - take manual control of the outflow valve and drive it closed. If the cabin altitude stops climbing and ΔP rises, the fault is outflow/controller related (the pack is producing air, it was simply leaving). If ΔP keeps decaying with the valve closed while the pack is at high flow, the pack cannot deliver - or the leak is not via the outflow valve.
- Supporting checks: pack outlet temperature (high outlet temperature with high flow suggests the pack is at its limit), bleed pressure and temperature, safety-valve position, and the cabin rate-of-climb trend versus thrust setting.
- Do not spend long diagnosing. The cabin altitude is the limiting parameter, not the cause. Descend first; the fault can be isolated once the cabin is stable.
2. What immediate action do you take at FL290, and what triggers an emergency descent?
Read the worked answer
Descend before the cabin becomes the emergency.
- Request lower from ATC now. Descend toward the level at which the single pack can hold cabin altitude - in practice the AFM/MEL single-pack ceiling, typically around FL250 for this class. Quote the reason ("cabin pressure control, descending to FL250"), not the whole ECAM.
- Monitor against fixed triggers: cabin altitude 10,000 ft, or a continued climb rate that the descent is not containing, or ΔP below the structural/physiological threshold. Any of these → passenger masks, "EMERGENCY DESCENT" call, seat-belt signs on, cabin crew advised by PA, and a controlled descent to 10,000 ft or MSA, whichever is higher, at the approved speed schedule.
- Do not simply stop the climb and wait. With 400 ft/min and rising the cabin reaches 10,000 ft in about five minutes; the descent must start while there is still time to make it gentle rather than explosive.
- Confirm the oxygen system (mask deployment, crew masks, passenger oxygen duration versus descent time to 10,000 ft) before you need it - with one pack you may have to descend further than you expect, and the oxygen supply is sized for that descent, not for a hold.
3. Once the cabin is stable, what governs the decision to continue to destination rather than divert?
Read the worked answer
The decision is a performance and exposure calculation, not a preference.
- Can the cabin be held at an acceptable level? If yes at the MEL ceiling, the flight can continue; if it can only be held at FL200, the whole flight plan is invalid and the diversion is the honest answer.
- Cost of the lower level. A descent from FL290 to FL250 costs roughly 10–15% specific range, which may put the destination plus 45 minutes holding outside the fuel on board. Recompute the trip fuel at the new level before deciding; if the margin disappears, divert to the 120 nm airfield while you still have fuel to do it comfortably.
- Exposure. A single-pack aircraft has no dispatch redundancy for a further pressurisation fault, and a cabin altitude that has already drifted shows the system is at its limit in this temperature. That is a strong argument for landing sooner rather than later.
- Administrative follow-up. An MEL-deferred item that performs worse than assumed is an occurrence: report it, and have engineering look at the pack before the next dispatch - the aircraft should not be released on the same MEL twice without a finding.