What happened
On 3 March 2017, an Airbus A320-214, registration G-EZTV, was undergoing pushback from Stand 1 at Manchester Airport under dark and wet conditions. During the maneuver, the flight crew heard a mechanical thud and felt a jolt. Although the ground crew performed a visual check, they did not identify that the shear pins on the towbar had fractured. Believing the noise was merely the tow hitch shifting, the pushback continued, and the left engine was started.
As the aircraft was being pulled forward, it began to deviate from its intended path. The tug driver, noticing the deviation, assumed the towbar had separated and attempted to move away. However, because the central retaining pin remained intact, the towbar did not disconnect. Instead, the aircraft was pulled toward the tug, resulting in a collision between the tug and the lower left fuselage. There were no injuries to the 172 passengers or 6 crew members on board, though the aircraft sustained damage to the nose landing gear assembly and the forward lower fuselage.
The investigation
The investigation examined the maintenance history of the towbar, the training of the ground crew, and the technical specifications of the equipment used. Investigators found that a previous failure of a single shear pin had been addressed by replacing only the broken pin, rather than replacing both, which left the remaining pin under increased stress. Furthermore, the retaining pin had been over-tightened, and the towbar lacked a damping system recommended by the manufacturer to reduce impulse loads.
Technical analysis also revealed that the tug used was significantly heavier than the minimum required weight for the aircraft, which increased the potential for loads to exceed the shear pin capacity. Additionally, the investigation noted that the engine start sequence was initiated before the aircraft had reached the designated position for that specific stand.
Findings
- The fracture of the towbar shear pins was the primary cause of the towbar's failure to separate.
- Maintenance errors, including the failure to replace both pins after a single failure and improper lubrication, contributed to the equipment's condition.
- The use of an oversized tug increased the risk of transmitting excessive forces to the pins.
- The lack of a damping system on the towbar contributed to higher impulse loads during the maneuver.
- Inadequate ground crew training regarding serviceability checks and stand-specific engine start procedures played a role in the sequence of events.