What happened
On 28 April 1999, a BAe 1s46, registration EI-CLI, was preparing for a scheduled passenger flight from Birmingham to Dublin. While taxiing on runway 33, the crew received instructions regarding a frequency change for when they passed 2,000 feet. The commander, believing this instruction was accompanied by a take-off clearance, initiated the take-off roll. During this period, a Piper PA-38-112, registration G-BOZM, was performing circuit training on runway 06.
As the BAe 146 accelerated, the tower controller realized the aircraft was departing without authorization and issued an immediate hold instruction. The instructor in the G-BOZM heard the controller's urgent command and observed the airliner approaching the runway intersection. The instructor performed an emergency maneuver, applying heavy braking and turning the light aircraft to avoid the path of the larger jet. The left wing of the BAe 146 passed closely over the right wing of the G-BOZM, but no physical contact occurred and there were no injuries.
The investigation
The investigation focused on why the departure clearance was misunderstood and why the safety procedures failed to prevent the unauthorized roll. Investigators examined the Air Traffic Control Services Instruction (ATCSI) in place at the time, which had recently introduced a procedure to combine frequency change instructions with take-off clearances to reduce pilot workload.
It was established that the controller had misinterpreted the wording of the instruction, believing the frequency change and the take-off clearance could be separate transmissions. Furthermore, the commander of the BAe 146 had assumed that the frequency change instruction would inevitably be followed by a clearance. The investigation also noted that the safety pilot in the BAe 146 had experienced an intermittent radio station, which contributed to the confusion during the transmission.
Findings
- The primary cause of the incident was the BAe 146 commencing its take-off roll without receiving an actual clearance.
- The controller's misinterpretation of the ATCSI allowed for the separation of the frequency change and the take-off clearance.
- The pilot's assumption that the frequency change was a precursor to an immediate clearance led to the unauthorized departure.
- The existing hazard analysis for the new ATC procedure failed to consider the risk of a pilot taking off prematurely based on a partial instruction.
- The timely intervention by the controller and the rapid reaction of the G-BOZM instructor prevented a collision.