What happened
In the early hours of 6 August 2016, a maintenance crew was moving a B787 from a hangar to a designated bay at Singapore Changi Airport. The towing operation required the vehicle to cross Runway 02L/20R via Taxiway SA.
During the movement, the engineer-in-charge contacted Changi Control Tower to request clearance. The runway controller provided an instruction regarding the expected timing for the crossing. However, the engineer misinterpreted the controller's message, believing the instruction was to expedite the crossing within a few minutes. Based on this misunderstanding, the crew proceeded forward.
As the tow tug moved toward the runway, the driver spotted an aircraft approaching from the north. Simultaneously, the airport's microwave barrier detector triggered an alert in the control tower. The crew immediately halted the B787, stopping approximately 85 meters from the runway edge. At that moment, a B737 was passing the northern threshold of the runway and was about to touch down. The arriving aircraft landed safely and vacated the runway without further incident.
The investigation
An investigation by the TSIB established that the red stop bar lights at the taxiway holding position were unserviceable due to damage caused during hangar construction in 2012 and had not been repaired. Furthermore, the MRO had shifted the de facto holding position to a location outside its premises without notifying the aerodrome operator or air traffic service providers.
The inquiry also found that the runway controller had not deactivated the red stop bar lights, and the aerodrome operator had not informed the control tower that the microwave barrier detector's control had been delinked from the stop bar lights. Additionally, the tow tug driver was operating without a Category 1 Airside Driving Permit.
Findings
- Misinterpretation of radiotelephony instructions by the engineer-in-charge led to the unauthorized movement toward the runway.
- The unserviceable red stop bar lights and the lack of updated runway holding position information contributed to the risk.
- A lack of coordination between the MRO, the aerodrome operator, and air traffic services regarding infrastructure changes and maintenance.
Safety action
Following the incident, several safety improvements were implemented:
- The MRO relocated the runway holding position for Taxiway SA to a new location outside the hangar premises.
- The aerodrome operator installed new runway guard lights at the intersection of Taxiway SA and Runway 02L/20R.
- Standardized radiotelephony phraseology was established to prevent future misunderstandings during towing operations.
- The MRO enhanced its towing procedures, including the addition of an Assistant Flight Deck Engineer to monitor communications and aircraft movement.