Description

On 17 November 2007 a Boeing 737-700 being operated by Virgin Blue on a scheduled passenger service from Coolangatta to Melbourne experienced a failure of the right hand engine bleed during the take off. The flight was continued but subsequently, during the climb in night IMC to FL350, the left air conditioning pack failure and an emergency descent followed with passenger oxygen masks deploying as cabin altitude exceeded 14000 feet. A diversion was made to Brisbane .

Investigation

An Investigation was carried out by the ATSB.

It was found that the initial fault has occurred below 80 KIAS during the take off roll. An attempt at reset after becoming airborne but when still below MSA was not successful and with APU air not available above FL170, it was initially decided to cruise at FL250, a lower level than planned. Once there, however, icing conditions were encountered and it was decided to continue the climb to FL350 so as to be able to cruise clear of cloud.

Then, at FL318 during the climb, about 135 nm south west of Coolangatta, the left air conditioning pack failed and an emergency descent was made to 10000 feet. During this descent, cabin altitude exceeded 14000 feet, which led to the passenger oxygen masks deploying automatically. Thereafter, the diversion to Brisbane was achieved by making a track reversal to an airport near to but larger than the departure airport.

It was noted that maintenance action on the incident aircraft immediately prior to departure had cleared a reported fault of an open left hand air conditioning ram air door made by the previous operating flight crew. Although this subsequently turned out to be a symptom of the split in the flexible hose that contributed to the failure of the left hand bleed supply en route, it was considered equally possible that the reported open ram air door might have been a result of a fault with the door actuator, as diagnosed by maintenance and the reason which had led them to lock the ram air inlet door open as a way to permit continued operation of the aircraft.

It was noted also that the Master Caution during take off activated at less than 80 kts and that the failure to reject the take off was therefore contrary to Operator SOPs and had increased the risk of the underlying aircraft problem having an effect during the flight. Similarly, the decision to deal with the bleed failure soon after take off instead of waiting until above Minimum Sector Altitude, was considered to have increased the risk of the crew being distracted from their primary flying tasks at a critical phase of the flight.

It was observed that a bleed trip-off was less likely in the 737 NG types than 737 Classic types because of system design changes which was the reason why the possibility of using engine anti-ice to facilitate system reset after a bleed trip off was not documented for the 737 NG aircraft.

Given that the aircraft was certified for operation up to a maximum ceiling of FL410 on a single air conditioning pack and the bleed-trip occurred post dispatch, it was notes that the Minimum Equipment List (MEL) maximum operating altitude of FL250 was of no relevance to the decision on an appropriate cruise altitude. However, the climb to FL350 had placed additional load on the left, degraded, air conditioning pack which was "not capable of operating at the maximum ceiling" and this was considered to have increased the chances that a the single pack might trip off.

The following Findings were formally documented:

Contributing Safety Factors

• During a No Engine Bleed Takeoff, a defective high-stage valve allowed a pressure increase in the aircraft’s right bleed air system that triggered the system’s overpressure switch to activate the right bleed trip off. Attempts by the flight crew to reset the system were unsuccessful.
• The flight crew continued the flight and climbed above FL250, which was the aircraft’s minimum equipment list-specified ceiling with a single air conditioning pack operating.
• The left and only operating air conditioning pack tripped off due to a reduced air flow over the system’s heat exchanger due to a damaged flexible hose. As a result, the aircraft depressurised.

Other Safety Factors

• The action to continue the takeoff was contrary to the operator’s procedures and increased the risk of the (at that time) unknown underlying aircraft problem having effect during the remainder of the flight.
• The action to address the right bleed system fault soon after takeoff increased the risk of the flight crew being distracted from their primary flying tasks at a critical phase of the flight.
• Following the no engine bleed takeoff, the flight crew did not reconfigure the air system controls in accordance with the supplementary procedure, inadvertently isolating the right wing anti-ice system from the operating bleed air source, and putting the aircraft at risk of asymmetric wing icing.
• The flight crew did not activate the aircraft’s engine anti-ice systems when operating in icing conditions, increasing the risk of an engine icing event.
• The cabin crew displayed an inconsistent knowledge of the operation of the cabin oxygen system, increasing the risk of reduced cabin staff performance or passenger injury.

Other Key Findings

• A published procedure to reset bleed trips in earlier model B737 aircraft, and that may have been of use in this case, was not published for the B737 NG.
• The cabin supervisor exhibited very good situational awareness, acting to secure the cabin without specific advice from the flight crew as to the nature of the emergency.

Safety Action taken during the course of the Investigation by the Aircraft Operator was noted to have included the amendment of manuals to provide further guidance to crews on oxygen mask operation and a modification to the pre-take-off passenger safety demonstration to include advice that oxygen would flow through passengers’ masks even though individual bags may not have inflated.

The Final Report of the Investigation was published on 29 April. No Safety Recommendations were made.

Further Reading

• Damage Tolerance
• Pilot judgment and expertise (OGHFA BN)
• Adherence to SOPs (OGHFA BN)