What happened
On 17 March 1998, a Boeing 73MAN 737-300, registration G-ODSK, was parked at Stand B6 at London Heathrow Airport. While the crew was performing departure preparations for a public transport flight, a baggage belt loader vehicle was manoeuvring near the aircraft's forward cargo door. During this manoeuvre, the vehicle collided with the aircraft, causing structural damage to the area of the forward cargo door and also damaging the baggage belt loader itself. There were no injuries to the 76 passengers or 8 crew members on board the aircraft.
The investigation
Investigators examined the baggage belt loader to determine why the driver was unable to stop the vehicle. The driver reported that the brakes failed to function as the vehicle approached the aircraft. A technical inspection of the vehicle revealed that the forward left-hand disc brake caliper unit had detached from the assembly. This detachment had severed the hydraulic pipes responsible for the brake system.
Upon inspecting the caliper unit, investigators found that the lower of the two screws used to secure the caliper to the stub axle had unscrewed during service. This left the entire braking load to be supported by a single top lug, which subsequently failed, likely due to fatigue. As the caliper rotated with the disc plate, the rigid brake fluid pipes snapped, leading to a total loss of brake fluid and rendering the vehicle's braking system inoperative.
Findings
- The primary cause of the collision was the mechanical failure of the vehicle's braking system caused by the detachment of the brake caliper.
- The attachment screws used for the caliper were threaded along their entire length, which is unsuitable for carrying the shear loads present in this application; plain shank bolts should have been utilized instead.
- There was insufficient thread engagement; only 3 threads were engaged in the stub axle, far below the recommended 8.5 threads (1.25 times the diameter).
- The use of multiple layers—including a steel washer, a spring lock washer, and a mild steel bracket—created a joint prone to loosening due to relative movement between the components.
- Although the operator had identified a tendency for these screws to lose torque and had implemented a periodic torque check, the failure persisted.