Depressurization Over the Rockies: Drift-Down or Emergency Descent?
You are the pilot flying a pressurized twin turboprop at FL250 on an IFR clearance from Calgary to Kelowna. Ten minutes after top of climb, the cabin altitude warning activates and the cabin climbs through 10,000 ft at roughly 1,500 ft/min. The outflow valve indicates fully open. Terrain along your route reaches 12,000 ft within 15 NM either side of track, and the minimum IFR altitude on the airway ahead is 16,000 ft. You have 45 minutes of oxygen for the crew at current flow and 20 minutes for the passengers. Nearest suitable airport is 60 NM behind you at 4,500 ft elevation; ahead is a field 90 NM away at 2,100 ft. ATC is aware of your position but has not yet issued a clearance.
1. What immediate actions and descent profile do you choose, and why?
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Memory items first: oxygen masks on, establish communications, then initiate descent. Because the cabin is already above 10,000 ft and climbing, the correct response is an emergency descent to the lowest safe altitude, not a drift-down. Drift-down assumes you can hold an altitude that keeps the cabin below 10,000 ft; with the outflow valve fully open you cannot, so the priority is getting the aircraft (and cabin) down.
Profile: idle power, speedbrakes as appropriate, and a turn off airway if needed to stay clear of terrain. Descend at the aircraft's published emergency descent speed, typically around Vmo/Mmo minus a margin, until reaching 10,000 ft cabin altitude or the MSA, whichever comes first. Because terrain reaches 12,000 ft within 15 NM of track, do not simply descend on heading; request an immediate block clearance or turn toward lower terrain. Squawk 7700 and advise ATC of the descent rate and intended level-off.
2. How does your oxygen endurance influence the choice between the two airports?
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Crew oxygen at 45 minutes and passenger oxygen at 20 minutes is the binding constraint. At 1,500 ft/min the cabin reaches 14,000 ft in under three minutes, so passenger oxygen is the limiting resource. The field 90 NM ahead at 2,100 ft is the better destination: at a 200 kt descent groundspeed that is about 27 minutes, comfortably within crew oxygen and only slightly beyond passenger endurance at high cabin altitude, but the descent will bring the cabin down well before then.
The field 60 NM behind at 4,500 ft is closer in time but higher in elevation, meaning a longer exposure to a cabin altitude above 10,000 ft after level-off. Choose the lower-elevation field ahead, and plan to be at 10,000 ft or below within roughly 10 minutes. If passenger oxygen were shorter, you would still choose the lower field but request priority handling and a direct routing.
3. What ATC requests and reports should you make, and in what order?
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Declare the emergency first: "Mayday, Mayday, Mayday," with callsign, position, altitude, and nature of the problem (cabin depressurization, emergency descent). State your intended level-off altitude and heading. Request a block altitude if terrain clearance requires it, and request direct routing to the chosen field.
Next, request ARFF and medical services at the destination and advise passenger oxygen status. Only after the emergency is stabilized should you pass along details such as souls on board and fuel. Keep transmissions short; ATC needs the descent profile and destination to protect airspace, and other traffic needs to know you are descending off airway.