What happened
On 22 June 2009, a Boeing 757-236, registration G-OOOZ, was performing a commercial passenger flight from Boa Vista, Cape Verde, to Manchester. Approximately two hours into the journey, while passing the BIMBO reporting point near Lanzarote, the flight crew performed a routine fuel check. This revealed that the total fuel quantity was 300 kg lower than anticipated. Upon further investigation, the co-pilot observed that the centre tank contents had depleted much earlier than expected, and the right fuel tank was losing mass at a rate of approximately 300 kg per minute.
Although an initial visual inspection from the cabin showed no obvious issues, an Engine Indicating and Crew Alerting System (EICAS) message indicated a fuel imbalance. A subsequent visual check confirmed fuel was leaking from beneath the right engine nacelle. The crew declared a MAYDAY, shut down the right engine while descending through FL280, and diverted to Porto Santo. The aircraft landed safely on Runway 36 in good weather, and no injuries were sustained by the 230 passengers or 8 crew members.
The investigation
The investigation, conducted by the AAIB, involved a metallurgical examination of the ruptured pipe and extensive testing by the engine manufacturer. The focus was on a flexible fuel pipe segment located between the fuel cooled oil cooler (FCOC) and the High Pressure (HP) engine fuel pump.
Engine manufacturer testing on a test bed revealed that the HP fuel pump created a pressure ripple. When the pump from G-OOOZ was tested, it produced high levels of pressure ripple. The investigation established that these pressure harmonics coincided with peak acceleration in the fuel tube assembly, leading to high-frequency vibrations.
Findings
- The fuel leak was caused by a rupture in a flexible section of the fuel pipe.
- Microscopic analysis showed extensive inter-braid fretting within the stainless steel sheath of the pipe.
- High-frequency vibrations driven by the HP fuel pump pressure ripple caused the wires in the braided layers to wear and notch, eventually severing the strands.
- This loss of structural integrity allowed the inner PTFE core to burst under pressure.
- The specific pump installed on the aircraft produced higher pressure harmonics than standard, accelerating the wear process.
Safety action
- The manufacturer has initiated a hardware redesign of the fuel pipe.
- A series of engine runs were conducted to evaluate the impact of vibration and pressure ripple on fuel lines.