Two out of three ain’t good

The story of how an Australian-registered Boeing 757 lost two of its three hydraulic systems highlights how seemingly minor maintenance decisions can have major consequences in aviation.

On 5 February last year, the aircraft, a freighter bearing DHL colours, was climbing out of Auckland, New Zealand, on one of its regular runs across the Tasman Sea to Sydney. During the climb, the left main landing gear downlock actuator retract hose ruptured, resulting in the failure of the left hydraulic system. The investigation by the Australian Transport Safety Bureau (ATSB) was unable to find cause of the rupture.

The Boeing 757 uses a power transfer unit (PTU) that turns on automatically when the left system hydraulic pressure falls. The PTU’s purpose is to enable the operation of flaps, slats, landing gear and nose wheel steering even if left pressure is low.

But on this occasion, corrosion inside the PTU’s pressure switch meant that the PTU motor ran ‘dry’ without any load from the hydraulic fluid it was meant to pump. This promptly led to overheating and failure of the right hydraulic system.

The crew, faced with two hydraulic system alerts within minutes, declared pan-pan and returned to Auckland where the aircraft landed without incident. It had its remaining hydraulic system for flight control, back-up systems for extending the flaps and landing gear, and manual braking.

The pressure switch that failed was the subject of a 2010 non-mandatory service bulletin. Had this bulletin been implemented on the aircraft it ‘would likely have prevented the failure of the right system’, the ATSB found.

The aircraft’s previous operators had responded to a 2001 service bulletin about the pressure switch, but a 2010 service letter recommending fitting improved pressure switches had not been followed. The Australian operator took over the aircraft in 2013 and checked to see that all mandatory service bulletins had been applied.

The ATSB found the non-mandatory 2010 service letter to modify the susceptible pressure switch had not been actioned, which probably resulted in the failure of the pressure switch due to contamination/corrosion.

The ATSB said, ‘This occurrence highlights that although some service bulletins are not deemed to be safety critical, they can still have an impact on aircraft reliability. While operators are required to implement all airworthiness directives, the ATSB recommended that operators familiarise themselves with non-mandatory service bulletins and consider the potential impacts of delaying implementation.’

Hose failures, whether of fuel or hydraulic hoses, are a safety issue for both large commercial aircraft and small, general aviation aircraft, as Flight Safety Australia has pointed out from time to time.