What happened
On 20 November 2022, a Bluebird Nordic Boeing 73 and 400, registered TF-BBM, was involved in an accident at Paris-Charles de Gaulle airport during a freight transport operation bound for Lisbon. The aircraft was undergoing push-back procedures when it began moving forward uncontrollably after the towbar was disconnected from the tug.
Because the aircraft's APU was inoperative, the crew had started the left engine first using a ground power unit. Following the operator's specific procedures for push-back with a steering lockout pin, the crew had depressurized hydraulic system A. At the conclusion of the push-back, the ground operator requested the crew apply the parking brake to allow for the removal of the towbar. Although the crew observed the red parking brake light illuminate and confirmed the brake was applied, the aircraft began to accelerate down a downward slope of the tarmac.
As the aircraft gained speed, reaching a maximum of 12 knots, the crew found themselves unable to steer or brake the aircraft. The presence of the steering lockout pin and the towbar prevented any corrective maneuvering. The aircraft eventually struck a lamppost and a jet blast barrier, resulting in substantial damage to the left wing.
The investigation
The BEA examined the aircraft's flight data recorder, cockpit voice recorder, and ground surveillance footage. The investigation focused on the state of the hydraulic systems and the braking mechanism. Testing of the aircraft's braking system and hydraulic pumps performed in coordination with Boeing showed that the engine-driven pumps and electric motor pumps were functional. However, data indicated that the hydraulic systems were not pressurized at the time of the event.
The investigation revealed that while the parking brake lever mechanism worked correctly, the red light associated with the lever only indicates that the lever is in the correct position; it does not confirm that the hydraulic system is pressurized. Furthermore, the investigation found that the hydraulic accumulator had been discharged since the previous flight and had not been recharged because system B was not pressurized.
Findings
The investigation established that the lack of braking capability was due to the absence of hydraulic pressure in both systems A and B. The primary contributing factors included:
- The crew's erroneous understanding that the illumination of the red parking brake light guaranteed the availability of braking pressure.
- The crew's failure to monitor hydraulic system pressure indicators or low-pressure warning lights.
- The operator's simplified push-back procedure, which required the systematic depressurization of system A without requiring the crew to verify that system B was sufficiently pressurized.
Safety action
Following the accident, the operator updated its "Before push-back" procedures to include a mandatory instruction to check the pressure in the hydraulic systems.