Rapid Depressurization Over High Terrain at Night
Flight 61, a twin-engine jet, is at FL370 at night in IMC over terrain with a sector grid MORA of 16,200 ft. Forty minutes after top of climb the cabin altitude warning sounds; the crew dons masks and begins an emergency descent. Passenger oxygen is a 12-minute chemical system; crew oxygen is a two-hour supply. Field X is 25 nm off track: 7,000-foot runway, ILS, weather 300 feet and 1/2 SM in fog, ARFF Index B. Field Y is 60 nm further: 9,000-foot runway, ILS, 1,500 OVC and 5 SM, full ARFF. The direct track to X crosses a ridge with a grid MORA of 15,800 ft; the published emergency descent route keeps MOCA at 9,400 ft but adds 8 minutes. Fuel is 6,800 lb, with a burn of 4,000 lb/hr at 10,000 ft.
1. How long does the descent take, and does the passenger oxygen support the plan?
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From FL370 to 10,000 ft is 27,000 feet. At a typical emergency descent rate of 5,000-6,000 ft/min that is 4.5 to 5.4 minutes of descent, plus roughly 1 minute for recognition, mask donning, and turning off the track - call it 5.5 to 6.5 minutes from the warning to level-off.
The 12-minute chemical generator starts when the masks drop, at a cabin altitude around 14,000 ft, and the clock keeps running while the cabin is above roughly 14,000 ft. Six minutes fits inside 12 minutes with margin - but only if the descent actually goes to 10,000 ft. If terrain forces a level-off at 15,800 ft for the ridge, the cabin never gets low enough to stop the oxygen flow, and the generator is exhausted while the aircraft is still over high ground. That is the key constraint: the descent profile is driven by terrain, not by the pressurization problem.
The crew supply (two hours) is not the limiting factor. Passenger oxygen is, and it is measured in single-digit minutes.
2. Field X is close but fogged in with degraded ARFF; Field Y is further but wide open. What should the dispatcher recommend, and why?
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Recommend Field Y unless the crew's condition demands an immediate landing.
The case for X is time: 25 nm and a shorter diversion. The case against X is that every risk converges there. Weather at 300 feet and 1/2 SM is at or below the ILS minimums, so the crew may fly one approach, go missed, and be back in the air with passengers on portable oxygen and a cabin that never fully stabilized. ARFF Index B may be below the index required for this airframe, which matters on a flight that may land overweight or with an unknown structural problem. A single runway with one approach and no usable weather alternate nearby leaves no second option.
Field Y offers 1,500 and 5, a 9,000-foot runway, and full ARFF - an approach that will work the first time, and a runway long enough to absorb a fast or overweight landing. The extra 8 minutes of flight time is well inside the crew's two-hour supply and inside the fuel available (6,800 lb at 4,000 lb/hr gives about 1.7 hours).
The dispatcher recommends; the pilot in command decides. The recommendation should state the reasoning, not just the airport, so the crew can weigh it against what they are seeing in the cockpit.
3. What actions does the dispatcher take in the first ten minutes?
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- Get the declared emergency on the record with ATC through the crew, and confirm the intended destination so the flight plan and ARFF notification go to the right field.
- Alert ARFF and station operations at both X and Y. The cost of a false alarm is far below the cost of an unannounced emergency arrival.
- Provide terrain and routing information: the ridge MORA on the direct track to X, and the published escape route. The crew may not have the grid in front of them at that moment.
- Compute landing weight and advise the crew: fuel at the time of landing, whether an overweight landing inspection will be required, and the landing distance at Y with the expected conditions.
- Notify company: chief dispatcher, maintenance control, and passenger services at the divert field.
- Issue an amended release to the new destination with the fuel state, and keep the original release on file.
- Hand off cleanly if the dispatcher's own duty limits are close - an emergency is exactly the wrong time to be managing two aircraft and a clock.