Pressurisation Control Failure at High Altitude
A business jet is cruising at FL410 with a cabin altitude of 8000 ft and a differential pressure of 8.6 psi. The cabin pressure controller is in the automatic mode. The outflow valve position indicator begins to fluctuate, and the cabin altitude climbs to 9500 ft over two minutes while the aircraft remains at FL410. The cabin altitude warning is set at 10 000 ft. The crew has oxygen masks available, and the passenger oxygen system is a chemical generator type with a 12-minute duration. The aircraft has two bleed sources, both engines operating normally, and the pressurisation system has a manual mode. The outside air temperature is -55 degrees C, and the nearest suitable airport is 90 nm away with an approach available. The crew must decide whether to don masks, descend, or troubleshoot.
1. Explain the significance of the cabin altitude trend and the differential pressure reading, and state whether the crew should treat this as a slow decompression.
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A cabin altitude rising from 8000 ft to 9500 ft in two minutes at a constant FL410 indicates a controlled or uncontrolled outflow increase, not a sudden structural failure. The differential pressure of 8.6 psi is within the normal maximum differential for this class, so the hull is not necessarily compromised. The rate of climb of about 750 ft per minute is slow, which gives the crew time but also means the warning threshold will be reached in about 40 seconds if the trend continues. The crew should treat it as a slow decompression until proven otherwise, because the outflow valve fluctuation suggests a controller or valve fault. The correct response is to don masks at the first sign of an abnormal cabin altitude, not to wait for the warning. Troubleshooting in manual mode can be attempted only after masks are on and a descent is initiated. The chemical oxygen generators give 12 minutes, which is sufficient for a descent to 10 000 ft from FL410 if the descent is started promptly.
2. Calculate the approximate time to reach the 10 000 ft cabin altitude warning and assess whether the oxygen supply is adequate for a descent to a safe altitude.
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The cabin altitude is rising at about 750 ft per minute. From 9500 ft to 10 000 ft is 500 ft, which takes about 40 seconds. From 10 000 ft to a safe cabin altitude of 8000 ft is not relevant because the cabin is climbing, not descending. The real question is how long the cabin remains above 10 000 ft. If the crew starts an emergency descent at FL410, a typical descent rate of 6000 to 8000 ft per minute will bring the aircraft to 10 000 ft in about 4 to 5 minutes. The cabin altitude will follow the aircraft altitude with a lag, so the cabin will be above 10 000 ft for perhaps 3 to 4 minutes. The chemical generators provide 12 minutes, so the supply is adequate if the descent is started immediately. The crew should not delay to troubleshoot. The judgement is to don masks, start the descent, and then attempt manual control.
3. Describe the manual mode troubleshooting sequence and the conditions under which the crew should abandon troubleshooting.
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After masks are on and the descent is established, the crew can select manual mode and control the outflow valve directly. The sequence is: confirm the bleed sources are normal, select manual, and modulate the outflow valve to arrest the cabin climb. If the cabin altitude stops rising and begins to decrease, the crew can continue the descent to a safe altitude and then decide whether to continue the flight or divert. If the cabin altitude continues to rise despite manual closure of the outflow valve, the crew should abandon troubleshooting, continue the emergency descent, and land at the nearest suitable airport. The presence of any smoke or unusual noise would also terminate troubleshooting. The decision logic is that the manual mode is a diagnostic tool, not a fix, and the crew must not spend the oxygen duration on troubleshooting. The 90 nm diversion is short, so landing is the conservative choice if manual control is not quickly effective.