Accident to the Boeing 737 registered TF-BBM operated by Bluebird Cargo on 20/11/2022 at Roissy
Uncontrolled movement of aeroplane during push-back, strike with infrastructures
The crew were to carry out a freight transport flight from Paris Charles-de-Gaulle airport bound for Lisbon (Portugal).
The conditions the day of the accident were the following:
- push-back operation with the nose gear steering lockout pin installed. Hydraulic system A was not pressurised (in accordance with operator’s procedure);
- engine No 1 was operating and engine No 2 was shutdown. This meant that hydraulic system B was not pressurised by engine driven pump 2;
- the standby accumulator was empty as discharged since the previous flight and would not be recharged until pressurisation of system B.
In these conditions, only the activation of electric motor pump 1, powered by engine No 1, could pressurise hydraulic system B and thus ensure the correct operation of the braking system (parking brake and pedal braking).
Hydraulic system B was not pressurised during the occurrence as the crew had very probably not activated electric motor pump 1. However, the investigation was not able to exclude a possible intermittent fault on this pump. Hydraulic system A was not activated by the crew in accordance with the operator’s normal procedures.
In the absence of pressurisation in hydraulic systems A and B and with the accumulator discharged since the previous flight, the braking system (parking brake and pedal braking) and the steering control system were not operational.
At the end of the push-back and before disconnecting the towbar, the crew operated the parking brake lever and considered that the latter was operational on observing that the associated red light illuminated. Due to the design of the system, this light only indicates that the parking brake lever is correctly positioned for applying the brake, it does not guarantee that the braking system is pressurised. The crew explained that they did not check that the pressure of hydraulic system B was at a minimum value of 2,800 psi and did not pay attention to the indicator lights indicating low pressure in the various systems and in the accumulator.
When the towbar was disconnected, the aeroplane moved and the crew tried to brake several times without understanding why the braking system was not operating although the red light was on. With the steering lockout pin and towbar still in place, the aeroplane’s path was not controllable and it came to a halt after striking obstacles.