What happened
During a certification flight test program for an aerial tanker conversion, a Bombardier DHC-8-402 (registration C-FBAM) was performing a series of push-over/pull-up manoeuvres near Tofino, British Columbia. The flight, operated by Cascade Aerospace Inc., involved testing the aircraft's handling characteristics with a water/retardant load. During the final push-over manoeuvre, the aircraft experienced a simultaneous loss of low oil pressure in both engines. This was immediately followed by an uncommanded reduction in propeller speed and a corresponding increase in engine torque.
While the No. 1 engine recovered to its selected speed, the No. 2 propeller entered an overspeed condition, governed at 1060 rpm. The crew subsequently feathered the No. 2 propeller and shut down the engine. Although a subsequent attempt to restart the engine was made, the propeller could not be unfeathered. The aircraft returned to Abbotsford International Airport with one engine inoperative. There were no fatalities or injuries during the event.
The investigation
The investigation focused on why the oil pressure loss triggered the propeller disturbances. Investigators examined the engine's modified oil pressure relief valve (PRV), which had been updated via Service Bulletin 35038 to prevent oil leakage into the bleed-air stream. Data from the flight data recorder (FDR) showed that while the aircraft remained within the load factor limits specified in the aircraft flight manual, the pitch-up angles reached approximately 43° to 46°.
Further testing by the engine manufacturer revealed that the modified PRV configuration caused the low oil pressure condition to persist longer than it did with the original valve. This extended duration allowed the propeller blades to move toward the coarse pitch direction, leading to an underspeed condition. When the pilot advanced the power lever on the No. 2 engine, the combination of torque exceeding 50 per cent and the existing underspeed condition triggered the automatic underspeed protection circuit (AUPC), which locked the propeller in an overspeed state.
Findings
- The simultaneous loss of oil pressure in both engines was caused by the high pitch-up angle and a brief negative g-load experienced during the manoeuvre.
- The modified engine oil pressure relief valve contributed to the incident by prolonging the duration of the low oil pressure condition.
- The activation of the No. 2 propeller's automatic underspeed protection circuit was triggered because the engine torque increased above 50 per cent while the propeller was already in an underspeed state.
- The flight crew was unaware that the AUPC activation would prevent the propeller from being unfeathered, as this limitation was not documented in the aircraft manuals or pilot training materials.