What happened
On January 3, 2007, a Raytheon Hawker 800XPC, registration CS-DFY, was performing a ferry flight from Lisbon, Portugal, to Marrakech, Morocco. During the climb, while passing FL203, the crew observed amber warnings for Main Valve 1 and Main Valve 2 on the Master Warning System. Shortly thereafter, a red cabin altitude warning light illuminated, indicating that the cabin pressure had dropped to levels corresponding to an altitude above 10,000 feet.
In response to the depressurization, the crew donned oxygen masks and requested a descent to FL150. Although the pilots attempted to resolve the issue by switching the pressurization system from 'AUTO' to 'OVRD' (override) as per the abnormal checklist, the cabin altitude continued to indicate a pressure deficit. The commander subsequently decided to return to Lisbon. Upon landing on runway 03, the aircraft's thrust reverser failed to operate, though the landing was completed safely without further incident.
The investigation
The GPIAAF investigation focused on the electrical and pneumatic components of the pressurization system and the Squat (Weight-on-Wheel) logic. Investigators examined the Squat WOW logic switch located on the left main landing gear strut and the electrical terminal situated in the fuselage above the left wing.
Technical analysis revealed that the aircraft had recently undergone de-icing procedures during a flight to a Nordic country. Investigators determined that moisture or de-icing fluid likely contaminated the electrical terminal, causing high electrical resistance and a voltage drop. This-induced error caused the Squat WOW logic switch to send a false "ON GROUND" signal while the aircraft was in flight, which triggered the opening of the outflow valves.
Findings
- The primary cause of the incident was a malfunction of the Squat (WOW) Logic Switch, which incorrectly signaled that the aircraft was on the ground while in flight, leading to cabin depressurization.
- The contamination of the electrical terminal by moisture or de-icing fluid was the likely trigger for the electrical anomaly.
- The crew's attempt to restore pressure was hindered by the low engine power setting during the descent, which provided insufficient airflow to rapidly repressurize the cabin.
- The failure of the thrust reverser during landing was attributed to the same faulty Squat WOW logic signal, as the reverser system also relies on this sensor for operational logic.
- The investigation confirmed that cleaning the electrical terminals and the Squat WOW logic switch resolved the issue, as subsequent flights showed no recurrence of the anomaly.