What happened
On 17 August 2012, a Boeing 747-4H6, registration 9M-MPL, was operating a passenger flight from London Heathrow to Kuala Lumpur. During the initial climb, the crew noted significant vibration in the No 2 engine. As the aircraft climbed through FL170, fuel flow to the engine dropped sharply, and oil pressure exceeded limits. Following an audible bang, the engine failed, prompting the crew to shut it down and initiate a return to Heathrow after jettisoning fuel.
While preparing for a planned autoland on Runway 09R, the aircraft's electrical system experienced a major disturbance. At approximately 3,000 feet, the three autopilots disengaged, cockpit lights and displays began to flicker continuously, and numerous fault messages appeared. The loss of power to AC Bus 2 caused several systems to degrade or fail entirely. The commander transitioned to manual flight to manage the approach. Due to the flickering displays and the loss of radio altimeter callouts, the crew was unable to properly flare the aircraft, resulting in a hard landing.
The investigation
The AAIB examined the aircraft's engine, electrical architecture, and flight data. Investigators found metallic debris in the No 2 engine tailpipe and on magnetic chip detectors. The investigation focused on why the electrical system failed so extensively during the approach.
Testing revealed that a mechanical fault in a Bus Tie Breaker (BTB 3) caused voltage oscillations. While the aircraft was configured for autoland, the electrical system's reliance on specific power sources made it vulnerable to these fluctuations. The investigation also noted that the loss of AC Bus 2 power led to the degradation of the right flight control computer and other critical systems.
Findings
- The primary cause of the electrical instability was a latent mechanical failure in Bus Tie Breaker 3.
- The severity of the electrical failure was compounded by the fact that the No 2 engine was already offline, meaning the remaining generators had to support the autoland configuration.
- Nuisance difference current protection trips occurred due to the faulty hardware in the bus tie breaker.
- The hard landing was caused by the crew's inability to accurately judge the aircraft's height due to the failure of the radio altimeter indications and the continuous flickering of cockpit displays.