French investigators believe the crew of a Bluebird Cargo Boeing 737-400 freighter did not activate an electric pump to pressurise hydraulics on the aircraft, leaving the brakes non-functional, before the aircraft rolled into obstacles at Paris Charles de Gaulle.
The cargo jet (TF-BBM) had been pushing back from parking station I81 for a service to Lisbon on 20 November last year.
Normal procedures involved starting the right-hand CFM International CFM56 engine first.
But French investigation authority BEA says the auxiliary power unit was out of service, and a ground-power unit was connected to the right side, so the crew first started the left-hand engine, while the right engine remained off.
BEA adds that a locking pin was engaged in the 737’s nose-gear.
The aircraft has two hydraulic circuits, A and B, each pressurised either by a mechanical pump from one engine or an electric pump from the other.
Since the right-hand engine was not running, the B circuit had to be pressurised with the electric pump from the left-hand engine. The aircraft’s normal hydraulic braking system uses the B circuit.
After the pushback ground personnel asked the crew to apply the parking brake in order to remove the tow bar.
But although the crew confirmed the parking brake had been activated, BEA says the braking system was not pressurised. A cockpit indicator light only showed that the parking brake control had been set in the correct position.
When the tow-tractor was unhooked, the aircraft started rolling forwards with the tow-bar still attached to the nose-wheel axle. The tractor could not be reconnected and the nose-gear locking pin meant the crew could not steer the jet.
The aircraft accelerated, travelling at up to 12kt, and struck a lighting pole and blast barrier before coming to rest.
It sustained substantial collision damage to its left wing, including the leading-edge, flap-tracks, and skin. The accident also led to a fuel leak. Although the inquiry says the risk to personnel was “significant”, neither the pilots nor the ground staff were injured.
BEA says the crew had “most likely” not activated the electric pump to pressurise hydraulic circuit B, but it adds that it cannot rule out a possible intermittent pump failure.
Loss or pressure in the normal braking system would usually result in a switch to the alternate system. But this is pressurised through hydraulic circuit A, which was not activated by the crew, in accordance with the operator’s normal procedures.
BEA adds that an accumulator can provide temporary pressure to the normal and alternate systems, but it had discharged since the previous flight.