Pressurization Failure at Altitude With a Passenger on Oxygen
You are flying a pressurized twin turboprop at FL250 with 14 passengers and 2 crew. A passenger with a documented respiratory condition is using a portable oxygen concentrator (POC) approved for the flight. At 14:05 the cabin altitude warning activates; the outflow valve appears stuck and the cabin climbs at roughly 1,500 ft/min. You don't have a quick-don mask; the first officer does. Outside air temperature is -35°C. Nearest suitable airport is 65 nm ahead with a 9,000 ft runway; a second airport is 40 nm behind with a 5,000 ft runway. You have 45 minutes of cruise fuel plus reserves.
1. What is your immediate priority sequence and why?
Read the worked answer
The first priority is to don oxygen masks and establish communications. As pilot flying, you should don a mask immediately; if you don't have a quick-don mask, the first officer must take control while you secure your own mask. Then set the transponder to 7700 and declare an emergency with ATC, stating the nature of the emergency and your intentions.
Second, initiate an emergency descent. Target a cabin altitude of 10,000 ft or below as quickly as practicable. A descent rate of 1,500 to 2,000 ft/min in a turboprop is reasonable; use idle power, speed brakes if available, and a turn away from terrain and traffic. The cabin is climbing at 1,500 ft/min, so the sooner you descend, the sooner the cabin altitude stabilizes.
Third, address the passenger on the POC. Portable oxygen concentrators deliver oxygen only when the user breathes, and their output is less reliable at altitude. If the cabin altitude exceeds 10,000 ft, the POC may be insufficient. Switch the passenger to a continuous-flow oxygen mask from the aircraft supply if available, and monitor for hypoxia.
Do not troubleshoot the outflow valve until the descent is established and the cabin altitude is under control.
2. Which airport do you choose and how do you manage the passenger and fuel?
Read the worked answer
Choose the airport 65 nm ahead with the 9,000 ft runway. The longer runway gives more stopping margin, better options if you need to land at a higher-than-normal speed, and more room for an emergency approach. The 40 nm airport behind has only 5,000 ft, which is tight for an emergency descent profile and leaves little margin for a long landing or a go-around. Fuel is not the deciding factor: 45 minutes of cruise fuel plus reserves is sufficient for 65 nm at turboprop cruise speeds, and you can declare minimum fuel or an emergency if needed.
For the passenger, switch from the POC to continuous-flow oxygen as soon as the cabin altitude exceeds 10,000 ft, and monitor breathing and consciousness. Brief the cabin crew to prepare for a possible emergency landing and to secure the cabin.
For fuel, plan a direct routing, request priority handling, and be prepared to land with less than normal reserves. Declare an emergency if fuel becomes a concern. Do not divert to the shorter runway to save fuel - the safety margin of the longer runway outweighs the fuel savings.