Pilots of the ill-fated Ethiopian Boeing 737 Max 8 allowed the aircraft to fly beyond its maximum operating limit speed, which may have contributed to the difficulties experienced with attempts to trim the aircraft.
The Ethiopian aircraft accident investigation bureau states that the CFM International Leap-1B engines stabilised at 94% of N1 during the take-off roll, and the throttles did not move for most of the flight.
As the crew worked to resolve pitch-stability problems the aircraft stopped climbing towards its cruise altitude and remained below 15,000ft – the equivalent of around 7,000ft above ground.
With the engines delivering high thrust the aircraft accelerated to at least 340kt, according to the captain’s airspeed indicator, or 360kt according to the first officer’s – exceeding the maximum operating limit speed, known as Vmo. The aircraft sounded an overspeed warning.
As part of their effort to address the pitch-stability problems – notably automatic nose-down commands from the aircraft’s Manoeuvring Characteristics Augmentation System – the crew engaged cut-off switches to prevent the MCAS from shifting the horizontal stabiliser.
When they subsequently attempted to trim the aircraft the pilots discovered that the manual trim was “not working”, according to the investigation bureau.
Manual trim is achieved by physically turning a trim wheel. Trim certification requirements from the European Union Aviation Safety Agency state that the aircraft must maintain longitudinal trim in level flight up to the Vmo limit speed, with the landing-gear and flaps retracted.
EASA’s certification of the 737 Max was subject to an equivalent safety finding regarding longitudinal trim at Vmo, according to the aircraft’s type certificate.
Simulations indicated that at certain speeds – notably in the region of Vmo – the pilot’s thumb-switch trim did not have sufficient authority to completely trim the aircraft longitudinally.
This was the result of Boeing’s intentionally setting limits on the thumb-switch trim, says EASA, as part of a measure to increase safety. Use of manual trim, it says, represents an “alternative trim method” to ensure compliance with EASA trim certification requirements across the envelope.
EASA acknowledges that the need to resort to the trim wheel is “considered unusual” because it would only be needed for manual flight at the Vmo limits.
“Additional crew procedures and training material will clearly explain to pilots the situations where use of the trim wheel may be needed due to lack of trim authority with the [thumb swtiches],” it adds.