Drift-Down and Terrain Clearance After Cabin Depressurization
A transport aircraft is cruising at FL370 over mountainous terrain with a minimum safe altitude of 21 000 ft along the route. Gross weight is 58 000 kg. The cabin altitude climbs rapidly past 10 000 ft; the crew don oxygen masks and initiate an emergency descent. The flight engineer's panel shows both bleed sources available but the outflow valve indicating fully open. Outside air temperature is −54 °C. The nearest suitable airport is 90 NM ahead, with an approach available; a second airport is 140 NM behind. The aircraft must descend to 10 000 ft or the lowest safe altitude, whichever is higher, before considering a diversion.
1. Explain the drift-down profile and how the crew should plan the descent to remain above terrain.
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A drift-down is the descent profile an aircraft can achieve with one engine inoperative, or in this case with the cabin depressurized, while maintaining a safe speed and terrain clearance. The crew should first turn to a safe heading away from high terrain if the route permits, then descend at the appropriate speed - typically the engine-out drift-down speed, or a higher speed if structural limits allow - while keeping the aircraft above the minimum safe altitude for the sector. The flight engineer should compute the level-off altitude at which the aircraft can maintain flight at the drift-down speed and compare it with the 21 000 ft minimum safe altitude. If the aircraft can hold 21 000 ft or above, it can proceed toward the nearest airport; if not, it must descend below the minimum safe altitude only where terrain permits, which requires an immediate turn to lower terrain. The crew should use the oxygen supply duration to bound the time available and should not delay the descent for checklists.
2. Assess the significance of the outflow valve indication and what it implies for the depressurization.
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An outflow valve indicating fully open is consistent with an uncontrolled decompression: the valve has opened to its maximum and the cabin pressure is following the outside pressure. The fact that both bleed sources are available suggests the problem is not a loss of pressurization air but a failure of the valve or its control. The crew should not attempt to reseat the valve in flight unless the checklist permits it, because an open valve is the safest state during an emergency descent - it prevents a pressure differential from building and allows the cabin altitude to equalize. Once at a lower altitude, the crew should attempt to close the valve and re-pressurize only if the checklist allows and if the cabin altitude can be controlled. If the valve cannot be closed, the aircraft cannot be re-pressurized and must remain at 10 000 ft or below for the diversion.
3. What reporting and record-keeping obligations arise from this event?
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An emergency descent and a cabin depressurization are reportable occurrences. The commander must file an occurrence report with the authority, normally within the time prescribed for serious incidents, and the operator's safety management system must be notified so that the event can be investigated internally. The technical log must record the depressurization, the emergency descent, the oxygen used, and the suspected cause. The oxygen system must be serviced and the masks inspected before further flight, and the outflow valve must be inspected and functionally checked by maintenance. The flight data recorder and cockpit voice recorder data should be preserved. The flight engineer should also record the drift-down profile and the altitudes achieved, because this data supports both the investigation and the operator's performance analysis.