What happened
On 19 May 2025, a Boeing 737-800, operated by Malaysia Airlines with registration 9M-MLL, was taxiing at Singapore Changi Airport for departure on Runway 20C. During the taxi sequence, the Runway Controller (RWC) initially cleared the aircraft to line up on the runway and deactivated the red stop bar lights at Holding Point T2 to facilitate movement.
However, the flight crew subsequently notified the controller that they required an additional two minutes to prepare for departure. In response, the RWC reactivated the stop bar lights and issued a clearance cancellation. The pilot-in-on-command (PIC) provided an incorrect readback of this instruction and continued taxiing toward the runway. Although the controller attempted to instruct the aircraft to hold position, the transmission was truncated, leading the crew to believe they were cleared to line up and wait. This resulted in the aircraft crossing the holding point and entering the runway, triggering a runway incursion alert via the airport's surface movement guidance system.
The investigation
The investigation by the Transport Safety Investigation Bureau of Singapore focused on the breakdown in radiotelephony communication and the controller's subsequent actions. Investigators examined the controller's decision-making process, noting that after the incursion occurred, the controller prioritized directing an arriving aircraft to vacate the runway rather than correcting the 9M-MLL crew's incorrect readback. The investigation also reviewed the controller's use of phraseology, specifically the lack of a direct imperative structure (such as "Hold position") when cancelling the initial clearance.
Findings
- The primary cause of the incursion was the incorrect readback of the clearance cancellation by the flight crew.
- The controller's instruction to hold position was not fully received by the crew due to a truncated transmission.
- The controller failed to correct the crew's second incorrect readback, choosing instead to focus on managing an arriving aircraft.
- The use of non-standard, non-imperative phraseology contributed to the ambiguity of the instructions.
- There was no evidence that the controller's recent use of cough medication caused impairment during the event.
Safety action
Following the incident, the aircraft operator distributed memos to flight crews emphasizing the need for attentive listening and the necessity of seeking verification if communications are interrupted. The Air Traffic Service Provider has since implemented a Radiotelephony Handbook with recommended standard phraseologies. Additionally, a safety recommendation was issued to ensure air traffic controllers prioritize direct imperative structures when using plain language for instructions not covered by standard ICAO guidance.