Unforecast Icing and a Pressurisation Anomaly at Altitude
You are at FL280 in a pressurised twin turboprop on an IFR flight, 40 minutes from destination, when the cabin altitude climbs from 8,000 ft to 12,000 ft over about two minutes. The outflow valve indication is erratic. Simultaneously, the outside air temperature is -20 °C and you have begun to pick up rime ice on the windscreen and leading edges despite anti-ice being on; the aircraft's anti-ice system uses engine bleed air. Forecast did not mention icing. You are IMC; the minimum safe altitude is 9,000 ft. Your oxygen masks are available; you have 30 minutes of oxygen at the current cabin altitude.
1. Prioritise the threats and decide the sequence of actions for the next few minutes.
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Two threats interact: pressurisation loss and airframe icing. The pressurisation problem is the more immediate because it affects the crew's ability to think and act. At 12,000 ft cabin altitude, hypoxia degrades judgement and coordination; the first action is to don oxygen masks and establish communication. Then run the pressurisation checklist: confirm the outflow valve position, try manual control, and select a lower cabin altitude if possible.
Simultaneously, the icing is a performance and control threat: it increases stall speed, reduces climb performance and can cause airframe buffet. Do not reduce power or lower the nose aggressively while iced; maintain a safe speed and use bleed air as available.
Sequence: oxygen on, masks and comms; identify whether the cabin altitude is still climbing; if it cannot be controlled, begin an emergency descent to a level where you can breathe without oxygen (below 10,000 ft) and where icing is less likely, coordinating with ATC. If the cabin altitude stabilises, continue at a safe level and plan a diversion. Never descend below the MSA without knowing terrain clearance.
2. You are IMC with ice accreting. What descent profile and speed would you use, and how do you manage the icing during the descent?
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If an emergency descent is required, use the aircraft's published emergency descent profile: idle power as appropriate, speed at or below the icing-limited maximum, and a controlled rate that keeps the airframe within limits. Do not exceed the maximum speed for the ice-contaminated configuration. Expect a higher stall speed and reduced manoeuvre margin, so keep a healthy margin above stall and avoid abrupt control inputs.
Icing management: keep anti-ice on, use bleed air as available, and descend to a level where the temperature is above freezing or where the air is clear of visible moisture. If the ice is not shedding, avoid climbing into colder, wetter air. Advise ATC of the icing and request a lower level or a route change to exit the conditions. If the aircraft is handling poorly, treat it as an emergency and say so.
Terrain: the MSA is 9,000 ft, so if you must descend below that, confirm terrain clearance with ATC and your charts before doing so.
3. What would you tell ATC, and what would trigger a MAYDAY versus an urgency call?
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Call ATC promptly with the essentials: callsign, position and level, the pressurisation problem, the icing, your intentions (emergency descent or diversion) and what you need (a lower level, a heading, a priority approach).
Use MAYDAY if: the cabin altitude cannot be controlled and you must descend immediately; the aircraft is handling abnormally due to ice; or any situation threatens immediate safety. Use PAN PAN if the situation is serious but under control and you need priority or assistance without immediate danger.
In either case, state your intentions clearly and keep the transmission short. If you declare an emergency, ATC will clear the airspace and offer priority; accept the help and keep them informed of changes. After landing, file the required occurrence report and note the cabin altitude, oxygen use and icing encounter.