Cabin Pressure Controller Fault in Icing Conditions
You are flying a pressurised twin-engine business jet at FL350 with 6 passengers and 2 crew. The cabin altitude is indicating 8,500 ft and slowly climbing, although the selected cabin altitude is 6,500 ft. The outflow valve position indicator shows 60% open, which is more open than normal for the current flight condition. The bleed air pressure and temperature are normal. The aircraft is in IMC and the outside air temperature is -40°C. The anti-ice systems are ON. The autopilot is engaged. You have 1 hour 20 minutes of fuel remaining to destination, with an alternate 30 minutes further. There is no terrain above 12,000 ft along the route. You notice the cabin altitude rate is about 300 ft/min climb.
1. What is the most likely cause of the cabin altitude climb, and what immediate actions would you take?
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The most likely cause is a faulty outflow valve or its controller, because the valve is indicating 60% open when it should be nearly closed to maintain 6,500 ft cabin altitude at FL350. Bleed pressure and temperature are normal, so the supply is adequate. The slow climb at 300 ft/min suggests a leak or a controller that is driving the valve open. Since the valve position indicator shows 60% open, the controller may be receiving a false signal or the valve actuator may be stuck.
Immediate actions: (1) Don the oxygen masks and establish communications, as the cabin altitude is above 8,000 ft and climbing. (2) Select the outflow valve to manual mode (if available) and attempt to close it using the manual control. (3) If manual control is ineffective, select the cabin pressure controller to the alternate mode or channel. (4) If the cabin altitude continues to climb, initiate an emergency descent to a lower altitude, typically 10,000 ft or the minimum safe altitude, whichever is higher, and land as soon as practicable. (5) Advise ATC and request priority.
Given the slow rate, you have some time, but you should not delay. At 300 ft/min, it will take about 5 minutes to reach 10,000 ft cabin altitude, at which point oxygen is required for all occupants. You should also consider that the anti-ice systems are ON and may be using bleed air, but bleed pressure is normal, so that is less likely the cause. The decision to descend should be made before the cabin altitude exceeds 10,000 ft, as passenger oxygen may be limited.
2. If manual control of the outflow valve does not stop the climb, how would you manage the descent and the remainder of the flight?
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If manual control fails, you must assume the valve is stuck open or the controller is unresponsive. You should immediately initiate an emergency descent to 10,000 ft or the minimum safe altitude, whichever is higher. During the descent, use maximum rate consistent with passenger comfort and structural limits, but be aware of the increased noise and temperature. Keep the anti-ice systems ON as icing conditions may exist at lower altitudes.
After levelling at 10,000 ft, the cabin altitude should equalise with the outside altitude if the valve remains open. You should then assess whether the pressurisation system can be restored. If not, you must plan to continue the flight unpressurised at 10,000 ft or lower, which increases fuel consumption. You have 1 hour 20 minutes of fuel to destination; at 10,000 ft, the true airspeed will be lower, so you may need to divert to a closer airport. Consider the passenger oxygen supply: if the cabin altitude is 10,000 ft, oxygen is not required by regulation for passengers, but some may need it.
You should also consider the possibility of a leak in the fuselage or door seal. If the valve is stuck open, you cannot pressurise. You should land as soon as practicable, even if it means diverting. Notify ATC, declare an emergency if needed, and brief the passengers. The key is to avoid a situation where cabin altitude exceeds 14,000 ft, which would require passenger oxygen masks to deploy automatically. Since you are already at 8,500 ft and climbing, you should act before reaching that threshold.