What happened
On 7 June 1998, a Lear-24, registration N888TW, was conducting a positioning flight from Cardiff, Wales, to the United States following a freight delivery. While climbing through FL300, approximately 50 nautical miles north of Cardiff, the pilot experienced a sudden and rapid loss of cabin pressure. The pilot reported hearing a loud noise accompanied by a visible mist appearing within the cabin.
In response to the event, the pilot declared an emergency with Air Traffic Control and initiated an emergency descent. The descent was noted to be quite rapid, reaching a rate of approximately 13,000 feet per minute. The aircraft was subsequently vectored back to Cardiff, where it completed an uneventful landing on Runway 30. There were no injuries to the two crew members on board and no damage was sustained by the aircraft.
The investigation
Following the incident, the aircraft's maintenance company performed diagnostic checks and necessary repairs. During the inspection, engineers identified several issues within the pressurisation system, including a dirty outflow valve and a loose inlet air pipe. Crucially, a crack was discovered in the plastic housing of the Altitude Selector unit, specifically at the point where an air pipe union was attached.
Data regarding the cabin environment during the event indicated that the cabin differential pressure had reached approximately 10 psi. This exceeded the aircraft's maximum rated differential pressure of 9.75 psi. After the replacement of the faulty Altitude Selector unit, a ground pressurisation test confirmed the system was functioning correctly. The pilot later confirmed that no further pressurisation issues occurred during subsequent flights.
Findings
- The pilot's perception of a pressurisation failure was caused by the opening of the Safety Valve, which is designed to actuate when pressure exceeds the maximum limit by roughly 0.5 psi.
- A crack in the Altitude Selector unit allowed cabin air to leak into the unit, compromising the control of the pressurisation system.
- The cabin differential pressure reached 10 psi, surpassing the maximum allowable limit of 9.75 psi.