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Aviation: 787 dual engine shutdown

A bizarre incident occurred four days ago when Boeing 787-8 JA825A, operating All Nippon Airways flight NH985 from Tokyo Haneda to Osaka Itami, suffered a dual engine shutdown on landing. Shortly after the main wheels touched down and the crew selected reverse thrust, the two Rolls-Royce Trent 1000 engines lost power and the aircraft coasted to a stop around 8,000ft own the runway. Despite following the checklists and consulting with engineers on the ground, neither powerplant could be restarted on the runway and after 40 minutes the 787 was towed to a gate. Further tests conducted later in the day found no defect with either engine.

The advanced Trent 1000 engine has suffered more than its share of problems since it entered service on the 787 in 2011, with turbine blades having to be replaced due to premature cracking.

The Trent 1000 engines on All Nippon Airways Boeing 787-8 JA825A both shut down shortly after touchdown at Osaka Itami on January 19. (BriYYZ)

Nevertheless those problems were unrelated to Saturday's incident in Japan. Having both engines on a twin-engined aircraft fail at exactly the same time is an extremely unusual event, and usually points to a common problem such as fuel contamination or exhaustion. But fuel is not believed to be a contributory factor i the events that unfolded at Itami Airport at the weekend.

TCMA suspected?

A definitive determination of the reason for the dual shutdown will come from a detailed investigation, but in the mean time some pilots current on the 787 have pointed an accusatory finger at the aircraft's Thrust Control Malfunction Accommodation (TCMA) system, which is designed to prevent uncommanded high-thrust when an aircraft is on the ground. Simplistically, TCMA intervenes when the aircraft's weight is on its wheels and the engines are producing high levels of thrust, but the throttles are at or near idle. In such an extremely unusual situation TCMA will shut down the engines to prevent an accident. Uncontrollable High Thrust (UHT) is an extremely rare event that full-authority digital engine control (FADEC - the system that electronically links the pilot's throttle movements to the engine, which includes protections to ensure a powerplant cannot be damaged) has reduced significantly. But UHT is still classed as a potentially catastrophic situation and has prompted aircraft manufacturers to also install extra software on new aircraft to prevent it.

The Rolls-Royce Trent 1000 has been in service on Boeing 787s since 2011. (Michael Pereckas)

Boeing introduced TCMA on the 777-300 and continued its use on the 787. Certainly the 787's this and the FADEC will be subject to some attention during the Japanese investigation, which is being supported by the aircraft and engine manufacturers.

Lending weight to pilot's suspicions that TCMA might be implicated in the ANA engine shutdowns, Boeing had already issued a service bulletin for the 787 stating that that selecting full reverse too quickly after landing and before the aircraft has fully transitioned to ground mode could cause the system to activate. Maybe not totally incriminating, but the bulletin certainly points to TCMA being looked at again.

Software dependency

The Aviation Oracle has already discussed the tragic loss of loss of the almost new Boeing 737-8MAX operated by Lion Air. Although the accident investigation has not yet been completed, preliminary information already released suggests that incorrect data produced by the aircraft's angle of attack (AoA) sensors could have led to the on-board computers to believe that the aircraft was stalling, and caused the autopilot to pitch the nose down. It was also revealed that new software, a Maneuvering Characteristics Augmentation System (MCAS), had been installed on the 737-MAX to improve flying characteristics. The MCAS had not been widely documented, and many pilots were unaware of the potential affects of it malfunctioning.

While reviewing the Lion Air crash, The Aviation Oracle pondered whether pilot skills were being eroded by an increasing reliance on automation, and postulated that hand-flying skills might be being lost. Now there's been an incident in Japan that may - repeat MAY - also be software-related. Bugs in the 787 TCMA may have less serious consequences than errant readings being sent from AoA sensors to the MCAS in the 737-MAX, but they may still produce unexpected control inputs that catch pilots off guard.

Is advanced software such as that installed on new Boeing and Airbus airliners de-skilling pilots - and potentially creating problems? (Pete Souza)

Software built into aircraft has made great advances over the last 40 years. It has been a major contributor to flight safety, to the point where the lives of crew and passengers sometimes depend on it now. But it is extremely complex. No software can ever be proven 100% bug free - it can only be tested so exhaustively that the likelihood of a fault is reduced almost to zero. Humans have limitations too, but they also need to understand what the software does and be able to deal with failures or override it when necessary.

Text © The Aviation Oracle

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