What happened
On May 23, 2008, at 17:50 UTC, a Lockheed L-1011-500 Tristar, registration CS-TMP, operated by LUZAIR, S. A., was performing a non-scheduled positioning flight from Lisbon International Airport to Manchester. The aircraft had been dispatched with air conditioning pack #1 inoperative, a condition permitted under the Minimum Equipment List (MEL).
During the climb, upon passing 4,000 feet, the flight crew identified that air conditioning pack #3 had also ceased to operate. The commander continued the climb while performing the appropriate checklists, anticipating that the system might be recovered before reaching 10,000 feet. However, because the aircraft could not maintain adequate pressurization with only one functional pack, the maximum allowable flight level was limited to FL250. Consequently, the commander decided to abort the flight and return to the departure airport.
The investigation
The GPIAAF investigation established that neither the aircraft nor the crew was in any immediate danger. The failure of the second pack occurred before the aircraft reached 10,000 feet, the altitude threshold below which the aircraft can safely operate in an unpressurized state. The investigation confirmed that the pilot's decision to continue the climb was an attempt to resolve the issue via checklists to maintain the original flight plan. The investigation also noted that the technical issues were resolved through the replacement of flow control valves on pack #1 and pack #3 in the days following the incident.
Findings
- The primary cause of the incident was the operational insufficiency of the aircraft's pressurization system, which prevented the aircraft from climbing above FL250 using only a single functional pack.
- The aircraft was operating under a valid MEL for the inoperability of pack #1 at the time of departure.