History of the Flight
The aircraft, a Boeing 737-73V registered G-EZKG, was conducting a scheduled passenger flight from London Luton Airport to Lisbon International Airport. The commander was the pilot flying. Before departure, the commander discussed with the co-pilot and cabin crew the aircraft's previous reported faults, including bleed air trips on the previous sector. The Bleed Air Regulator was replaced, and satisfactory engine ground runs were carried out before dispatch.
Takeoff and initial climb were uneventful, except that passing FL100 the pilots observed left and right duct pressures of 15 psi and 40 psi respectively. Climbing through FL280, the left BLEED TRIP OFF caution message illuminated on the BLEED panel, and AIR COND illuminated on the System Annunciator. The commander requested the co-pilot to carry out the associated checklist. During this procedure, the commander was distracted from monitoring the co-pilot's actions by ATC transmissions and a call from the cabin crew.
After the checklist was completed, the crew discussed whether to cruise at their planned flight level or a lower one. They elected to continue the climb to FL390. A short time later, while still climbing, the right BLEED TRIP OFF caution message illuminated. The commander observed the cabin altitude climbing at approximately 3,000 ft/min. She stopped the climb and asked for an immediate descent from ATC. Initially no response, but after a PAN call, ATC instructed descent and to set the emergency transponder code. The cabin altitude warning horn sounded; the crew donned oxygen masks and initiated an emergency descent near Nantes, France.
After levelling at FL100 and completing the checklist, the crew removed masks and confirmed no injuries. They returned at FL080 to London Luton Airport and landed without further incident.
Crew Actions
The commander noted that the co-pilot conducted the BLEED TRIP OFF checklist alone in a "read and do" manner rather than "challenge and response." She did not monitor the full procedure while issuing "Standby" to the cabin crew. She regarded this as a priority because she thought they might be concerned about potential pressurisation problems.
Co-pilot's Comments
The co-pilot stated that he had incorrectly selected the bleed switch to off instead of the pack switch to off when completing the left BLEED TRIP OFF checklist. Recorded data confirmed this. The bleed system was then configured with one engine bleed supplying two air conditioning packs. The right engine bleed was unable to meet the demand and tripped off, resulting in loss of cabin pressure.
Commander's Comments
The commander commented that the decision to continue the climb to FL390 with a single engine bleed source operating was influenced by a previous simulator training experience where it was strongly suggested that continuing the climb was appropriate.
Engineering Information
As a result of the incident, several components of the bleed system were replaced. The aircraft subsequently re-entered service with no reported recurrence. Six passenger oxygen hoses became detached from their chemical generators when deployed automatically; the operator checked these components on all its Boeing 737 aircraft and applied additional fastenings in accordance with a manufacturer's service letter.
Safety Actions
- The co-pilot subsequently demonstrated to the operator his understanding of the BLEED TRIP OFF procedure and the consequences of selecting the bleed switch.
- The operator recognised that a previous training experience influenced the commander's decision to continue the climb with a degraded pressurisation system; this observation was forwarded to training management.
- The commander underwent training examining all technical and non-technical aspects of this event and completed a supervised flight duty with a training captain to verify confidence and competence.
- The operator advised its training pilots to consider the implications of offering unique responses to situations for which there might be several acceptable solutions.