What happened
On 6 December 2015, a SilkAir Boeing 737-800, registration 9V-MGM, was prepared for a scheduled flight to Phuket at Singapore Changi Airport. During the ground handling process, a ground service provider crew used an airtug and towbar to push the aircraft from Bay F41 toward Taxiway C2.
As the operation progressed, the aircraft's right engine was started during the initial pushback. During the subsequent phase, where the aircraft was pulled forward to a designated end-of-tow line, the flight crew also started the left engine. As the aircraft moved forward, the airtug driver struggled to maintain control, noting that the aircraft appeared to be pushing the tug and causing it to accelerate and drift. In an attempt to stop the movement before reaching the target line, the driver applied heavy braking. This sudden force caused the towbar to fracture near the airtug hitch. The resulting loss of connection caused the airtug to swing 180 degrees, striking the left side of the aircraft, which had surged forward. The collision caused structural damage to the lower left fuselage near the radome.
The investigation
Investigators examined the mechanical state of the equipment and the environmental conditions at the time of the incident. The investigation found that the asphalt surface was wet due to rain. While the airtug and towbar components met standard specifications and showed no prior defects or fatigue, the towbar failed due to overloading during the braking maneuver.
Data from the aircraft's flight data recorder and the airtug's onboard recording device indicated that the movement speeds were approximately 11 km/h to 13 km/h. The investigation also reviewed the operational manuals of the airline, the ground service provider, and the aerodrome operator. It was noted that while the airline's manual permitted engine starts during pushback, the ground service provider's procedures lacked specific speed limits for the pull-forward phase and did not utilize a dedicated checklist for passenger flight pushbacks.
Findings
- The primary cause of the towbar failure was the excessive load placed on the equipment during abrupt braking.
- The aircraft's momentum, combined with the engine power being applied, created a turning moment that forced the airtug to deviate from its path.
- The lack of a standardized pushback checklist for passenger flights contributed to potential inconsistencies in safety procedures.
- The wet surface conditions may have influenced the driver's difficulty in maintaining traction and control during the maneuver.