What happened
On 12 August 1999, a DHC-8-311, registration G-BRYP, was preparing for a scheduled passenger service from Manchester International Airport to Glasgow. During the pre-flight inspection, the crew identified that the No 1 Hydraulic System fluid level was below the minimum required for dispatch. To rectify this, the commander planned to perform a hydraulic fluid transfer from the No 2 system during the pushback process.
As the pushback commenced, the commander communicated to the ground engineer that the fluid transfer would be performed once the aircraft reached the end of the pushback, requesting that the tug remain attached. During this period, the ground engineer moved to inform the tug driver, causing the tug to momentarily halt. This pause led to the mistaken assumption that the pushback maneuver had concluded.
While the commander was focused on the cockpit gauges to monitor the transfer, the tug driver restarted the movement. As the commander applied the Emergency/Parking Brake to initiate the fluid transfer, the tug continued to apply compressive force against the aircraft. This sudden load caused the nose landing gear to collapse, resulting in the nose of the aircraft settling onto the ground and causing damage to the fuselage and nosewheel bay. There were no injuries to the 50 passengers or 4 crew members on board.
The investigation
The investigation examined the structural failure of the nose landing gear and the mechanics of the pushback. It was determined that the 'A' frame upper drag link had pulled out of its trunnions due to extreme loads. The inspection of the towbar revealed that the three shear bolts had all failed. Because the load was compressive, the telescoping sections of the towbar moved inward until they hit the limit, at which point the full force of the 32,600 kg tug was abruptly transferred to the nose gear.
Investigators also reviewed the aircraft's hydraulic systems and the operator's procedures. It was found that while a method for transferring fluid between systems existed, the practice of performing this during pushback was a non-standard procedure developed by crews to address low fluid levels found during morning inspections.
Findings
- The primary cause of the accident was a breakdown in communication between the flight crew and the ground personnel regarding the status of the pushback.
- The commander's experience with pushback operations was limited, having recently been promoted to commander.
- The application of the parking brake while the tug was still applying compressive force created a sudden, massive load on the nose gear structure.
- A lack of formal Standard Operating Procedures (SOPs) for handling low hydraulic fluid levels during the first flight of the day led to the use of non-standard transfer techniques during taxi/pushback.
Safety action
Following the incident, the operator issued new instructions stating that flight crews must not accept an aircraft if hydraulic fluid levels are below the minimum dispatch quantity. Furthermore, the operator mandated that any necessary hydraulic fluid transfers must only be performed while the aircraft is stationary on a level stand, parked, and properly chocked.