The gust lock at Osnabrück

When German investigators analysed what appeared to be a straightforward error by a first officer, they discovered a web of human, organisational and mechanical factors that came close to turning a simple omission into an accident

It was the first officer’s first flight on type as pilot under supervision – a Jetstream 32 flying a passenger service from Münster/Osnabrück Airport to Stuttgart, in Germany.

It was 8 October 2019 and there was only one passenger, but the flight was delayed by slot allocation, much to the captain’s disgust. As the 19‑seat regional turboprop accelerated, the captain, who was line training instructor for the flight, steered the aircraft using the nose wheel, then handed control to the first officer at 70 knots.

About 6 seconds later at 108 knots, the captain said, ‘V1, rotate’. The first officer moved to pull the yoke back – but nothing happened. He called, ‘Cannot pull … the steering wheel’.

Soon after, 2 things happened simultaneously. The captain rejected the take-off (about 130 knots) and the aircraft veered right, oblivious to the captain’s attempts to control it with the rudder.

While slowing, the aircraft went off the runway for about 530 metres and crossed a taxiway, passing alarmingly close – at more than 100 knots – to where a general aviation aircraft was lined up and waiting. With the captain reverting to nose-wheel steering, the Jetstream returned to the runway, slowed to taxi speed and returned to the apron.

There was no mystery about what had happened: the captain told the tower they had rejected take-off because the gust locks (which lock the ailerons, rudder and elevators when the aircraft is parked) had been left on. There was slight damage to the aircraft and some runway signs.

The German Federal Bureau for Aircraft Accident Investigation, the BFU, (Bundestelle für Flugunfalluntersuchung) nonetheless investigated the serious incident.

The BFU was interested in how the Jetstream had been able to attempt take-off despite the gust lock being engaged – the mechanism is designed to lock the thrust levers at idle if not released. The BFU found the fork end of a rod designed to lock the thrust levers was bent and could no longer stop the levers from being moved. The gust lock had been checked 7 months earlier, on March 19 2019, when no problem had been found.

The BFU found 4 similar occurrences on Jetstream aircraft where the rod had bent. In 1992 near the end of the Jetstream’s production run, the manufacturer, British Aerospace, had developed a stronger version of the rod’s fork end and encouraged operators to fit it. But the occurrence aircraft was still using the original fork end design.

The investigation mentioned a similar case in the US in 2014, where a business jet crew of 2 pilots who regularly flew together had got out of the habit of using pre-take-off checklists. They too left the gust lock engaged and, like the Osnabrück case, the aircraft’s locking system had deteriorated and allowed the throttles to be pushed forward.

Instead of aborting the take-off like the German crew, the 2 business jet pilots tried to work around the problem during take-off by resetting the hydraulic system. This failed and the last words on the cockpit voice recorder (CVR) as the jet ran off the runway were ‘Oh, no, no!’ All 7 occupants died in the fire that followed the crash.

The BFU also found that although the manufacturer’s checklist specified disengaging the gust lock in 2 checklists, the operator’s checklist had no specific item to do this. On the operator’s checklist the gust lock was included as a sub-item of the autopilot/flight director test, which only took place before the day’s first flight. However, checklist items for free and full control movement should have detected locked controls.

A test of the stick shaker/pusher stall protection system should also have indicated the locked controls, but this test was not done before take-off. Stall protection was not even switched on, in contravention of both manufacturer’s and operator’s checklists. The CVR recorded the supervising captain as saying, ‘Leave it, not so important.’

The BFU found another potentially dangerous practice had developed, at variance to the checklist. ‘According to the operator, it was common practice to keep the gust lock engaged during taxi [for] preventing the flight controls from moving,’ the report says.

The BFU discovered the first officer had a total flying experience of 157 hours, of which 47 minutes were on type. He had not flown on type in the 90 days before the incident. Frustration and disbelief at this state of affairs are evident, even in the measured and translated words of the report.

‘The BFU is of the opinion that due to the time gap between finishing his type rating and the first flight, intensive attention of the line training instructor would have been required to prevent asking too much of the co-pilot and compromising crew resource management,’ the report says.

It was common practice to keep the gust lock engaged during taxi.

The BFU is of the opinion that the inexperience and the large age and experience gradient within the flight crew made an equal flight-safety-improving CRM difficult.’

The investigation listed 6 organisational contributing factors:

• insufficient supervision, support and monitoring of the line training instructor
• inexperience of the young co-pilot and a long time gap between type rating and the first commercial scheduled flight
• insufficient crew resource management of the flight crew
• pressure of time created by the crew between engine start-up and take-off
• non-stringent application and erroneous completion of the checklists
• checklist items, procedures and choice of wording in the checklists of the operator which did not completely correspond with those of the aircraft manufacturer.

A mechanical deficiency in the gust lock system, which allowed the engine power of both engines to be increased simultaneously, was also a contributing factor.

The BFU referred to ‘training, compliance with and adherence to procedures, checklists and mutual monitoring’ in its conclusion. It emphasised the importance of a simple but too often ignored check: ‘Ultimately, only a complete check of flight controls prior to a take-off run can ensure that all flight controls are full and free.’