Sequence of Events
On 6 August 2016 at approximately 0358 local time, a tow tug began towing a Boeing 787 aircraft from a maintenance hangar on the west side of the southern end of Runway 02L/20R at Singapore Changi Airport. The destination was Bay 300, requiring a crossing of Runway 02L/20R via Taxiway SA onto Taxiway W9. The towing crew consisted of three aircraft maintenance engineers, a headset man, and a tow tug driver from an aircraft maintenance, repair and overhaul organisation (MRO). A ground service provider (GSP) replaced the MRO's tow tug and driver at a point just beyond the MRO's gate on Taxiway SA.
At about 0355, the engineer-in-charge (EIC) and two other engineers entered the B787 cockpit and contacted Changi Apron Control for clearance to cross the runway. The request was relayed to the runway controller, who responded on frequency 121.9 MHz: "... expect to tow across 20R in about two … correction, in about three minutes' time." The EIC believed he heard "expedite to tow across the runway in 2-3 minutes' time" and read back, "... we'll cross the Runway 02L in two minutes' time." The runway controller acknowledged with "Affirm." The red stop bar lights on the Airfield Ground Lighting Control and Monitoring System remained active.
As the towing column moved forward, the tow tug driver spotted an aircraft approaching from the north. He informed the headset man, who told the EIC. The EIC and engineers in the cockpit also saw the aircraft and instructed the driver to stop. The EIC reported to the runway controller that they saw an aircraft approaching. The runway controller saw the towing column apparently not moving and stated that he had not given clearance to cross. The arriving aircraft, a Boeing 737, had just passed the northern threshold and was about to touch down. The microwave barrier detector on Taxiway SA triggered an alarm in the tower, alerting the runway controller. The B737 landed safely and vacated the runway via Taxiway W8. The towing column had stopped about 115 meters from the runway centerline (85 meters from the edge).
Personnel Information
The EIC was a 32-year-od licensed aircraft engineer with nine years of experience and a valid Aircraft Radio Operator's Approval. The tow tug driver, aged 31, held an Airside Driving Permit (ADP) but not a Category 1 ADP, which would have allowed operation on runways and taxiways. The runway controller, aged 29, held an Aerodrome Control (Changi) rating and had 1 year 5 months of experience as an aerodrome controller.
Meteorological Information
The weather was clear and the ground was dry at the time of the incident.
Radiotelephony Communications
The EIC held an Aircraft Radio Operator's Approval authorizing him to obtain clearance for moving aircraft on ground. Tow tug drivers were required to hold an ADP but not a Category 1 ADP; during towing, a driver without a Category 1 ADP had to work with a qualified person for air traffic control communications.
Aerodrome Information
The airport's runway incursion alerting system comprised microwave barrier detectors at all taxiways leading to runways. The MBD on Taxiway SA, located about 85 meters from the runway edge, was integrated with the Airfield Ground Lighting Control and Monitoring System. When red stop bar lights at a holding position were turned off by the controller, the MBD would deactivate automatically and reactivate after about 60 seconds. In this incident, the red stop bar lights were not deactivated.