What happened
On 18 August 2002, a Jet Provost T3A, registration G-BVEZ, was conducting a flight test near Humberside Airport to renew its Permit to Fly. The flight was manned by a pilot and an observer, both of whom were using the aircraft's integral oxygen system as the test required flying above 10,000 feet.
While climbing through FL240, the aircraft began to roll increasingly steeply. The observer, noticing the pilot was unresponsive, took control of the aircraft. During this period, the aircraft drifted through controlled airspace and out to sea. The observer managed to descend the aircraft to approximately FL210, at which point the pilot regained consciousness. Although the pilot briefly attempted to climb again, reaching FL270, he lost consciousness a second time. The observer maintained control and continued a descent until the pilot was coherent enough to resume command. The aircraft eventually returned to Humberside Airport and landed without further incident.
The investigation
The investigation focused on why the pilot suffered from hypoxia despite a pre-flight check of the oxygen system. It was discovered that the pilot's oxygen hose had disconnected at the break point located between the ejection seat and the cockpit floor. This specific connection point is difficult to inspect once the seat is occupied and can be obscured by stowed items.
Crucially, the investigation found that a blown fuse in the 'OXY' warning circuit prevented the Standard Warning Panel from alerting the crew to the disconnection. Furthermore, while the aircraft's ejection seats had been deactivated, the emergency oxygen bottles—which could have provided a vital backup—had been removed. The investigation also noted that the pilot had not completed the full oxygen system check prescribed in the Aircrew Manual, which included a specific step to disconnect and reconnect the mask tube to verify the warning system.
Findings
- The pilot experienced hypoxia because of a disconnection in the oxygen hose at the break point between the seat and the cockpit floor.
- A blown fuse in the oxygen warning circuit meant the pilot received no visual indication of the supply failure.
- The removal of emergency oxygen bottles from the deactivated ejection seats left the crew with no secondary oxygen source during the descent.
- The pre-flight oxygen system checks, which could have identified the faulty circuit or the disconnection, were not fully performed.