What happened
During a domestic flight from Toronto/Lester B. Pearson International Airport to Deer Lake, Newfoundland, a Royal Aviation Boeing 727 experienced a number 2 engine fire warning while climbing through 7,000 feet. The flight crew, consisting of three flight crew members and five cabin crew, managed the situation by shutting down the center engine and discharging two fire bottles. Despite these efforts, the fire warning light remained active, prompting the captain to declare an emergency and execute an overweight landing back at Toronto.
Upon landing on runway 2/4L, emergency rescue services met the aircraft. While no active flames were visible upon touchdown, a post-landing inspection revealed residual smoke and fire damage to the structure surrounding the number 2 engine, which is embedded in the aircraft's empennage. There were 0 fatalities and no injuries reported among the 183 passengers and 8 crew members.
The investigation
Investigators examined the engine starter and the electrical systems of the aircraft, registration C-GRYC. The inspection of the number 2 engine starter revealed it had completely seized. A hole was found in the side of the starter gear case, and the air turbine had been ejected through the retaining screen. This component struck and severed a constant speed drive (CSD) oil pressure line, causing oil to spray onto a generator terminal block and subsequently ignite.
Technical analysis determined that the starter failed due to an over-speed condition. Because the engine was already running, the failure required the engine start valve to have re-opened unexpectedly. While the aircraft had been modified to use an engine start valve master switch as an alternative means of compliance with Airworthiness Directive 83-01-05 R2, this specific modification did not provide the crew with a positive indication of the valve's position, nor did it protect the wiring between the cutout switch and the valve from electrical shorts.
Findings
- A short circuit in the engine wiring harness likely caused the number 2 engine start valve to re-open while the engine was operating.
- The unexpected opening of the valve allowed the starter to over-speed and fail.
- The failure of the starter led to the rupture of an oil line and the subsequent engine fire.
- The existing modification to the aircraft's start system failed to protect the circuit from a short circuit and left the crew without a way to detect the unwanted valve operation.