Germany's federal investigation office BFU has disclosed that a Vietnam Airlines Boeing 787-9 sustained 27% main-gear brake application on the left side and 17% on the right while accelerating past 80kt during its 15 August take-off from Munich, with rotation finally beginning 12kt below the planned speed.

What the flight-data recorder shows

The Bundesstelle für Flugunfalluntersuchung released preliminary findings into the accident in which the twinjet overran runway 26L, struck approach lighting, and sustained a tail strike during an attempted departure to Hanoi.

The crew had calculated a V1 of 169kt and a rotation speed of 175kt. But BFU's reconstruction of the take-off roll shows the aircraft's behaviour departing sharply from that profile. Take-off mode engaged about 25 seconds after the aircraft was cleared, and "rudder-pedal input" and "brake pedal movement" from both pedals on the left-hand pilot's side appeared almost immediately on the recorder.

By the time airspeed reached 80kt — around 600m into the roll — the recorder indicated the aircraft had begun braking. That braking remained active as airspeed continued to climb. Some 48 seconds after the initial pedal movements, the aircraft reached 168kt, one knot short of V1, and held that speed for roughly 18 seconds.

"Short speed deviations downwards and then back to the original speed were recorded," BFU states. The investigation authority has not yet drawn conclusions on the cause of the pedal input.

How the aircraft got airborne

Tyre tracks from the main landing-gear were identified abreast of the B4 taxiway, situated about 3,000m from the 26L threshold and roughly 1,000m from the end of the paved surface. Rotation began at 163kt as the jet crossed the threshold of the opposite-direction runway 08R. Moments earlier, the right-hand pilot had manually increased thrust from 100% to 101% N1.

The aircraft pitched to 6.88° nose-up, but only after it had already moved onto grass. The tyre tracks ended 240m beyond the 08R threshold. Both main landing-gear and the tail section struck approach lighting, prompting a tail-strike alert in the cockpit.

The twinjet reached V1 only after passing a height of 11ft, at which point the automatic stall-protection system extended the leading-edge slats. Climb-out speed of 180kt was achieved at 76ft, and the landing-gear retracted.

Return and damage assessment

Four pilots were in the cockpit for the Munich departure, with the assigned commanding pilot occupying one of two jumpseats behind the flying pilot and the first officer. After the overrun, the crew elected to return to Munich and burn fuel before the commanding pilot took over to conduct two low passes over runway 26R for visual landing-gear inspection. The jet then landed on 26R.

None of the 287 occupants was injured. Substantial damage was recorded to the aft fuselage and the left-hand main landing-gear.

What comes next in the inquiry

BFU has indicated it will conduct detailed examination of the brake system control units and electric brake actuator controllers as part of the continuing inquiry. The agency's preliminary findings leave open the central question of why pedal inputs produced sustained braking during a phase of flight in which the system is normally inactive.

With substantial damage to the aft fuselage and left main landing-gear, the airframe's return to service will depend on the BFU's findings on causation and on any subsequent work on the braking system.