Fuel Exhaustion Leads to Stall

While on final to the Toronto City Centre Airport, the pilot of a Piper Aztec lowered the landing gear, extended full flaps, and slowed the aircraft to 90 kt in order to sequence his aircraft behind a DHC-7. Because he was too close, he applied full power, initiated a go-around and, at the tower controller’s suggestion, started a 360º turn to increase the spacing from this traffic. The landing gear and flaps were not retracted. During the left turn, the left engine quit and the propeller stopped turning. The pilot noted that the airspeed was low and that he was descending, so he maintained full power on the right engine and decided to ditch the aircraft into the Toronto Harbour. The pilot, uninjured, exited the aircraft before it submerged and was rescued by members of the Toronto Police Marine Unit. This synopsis is based on the Transportation Safety Board of Canada Final Report A98O0313.

The pilot had 355 hr. of flying time, approximately 40 hr. on multi-engine aircraft, 35 of which were on Piper Aztecs. His last multi-engine aircraft flight was three months before the occurrence in the same aircraft. The pilot reported that before his flight from Toronto City Centre Airport to Centralia and back, the inboard fuel tanks were approximately half full and the outboard fuel tanks appeared to be full. He did not fuel the aircraft before departing. When full, the aircraft’s inboard tanks had a combined capacity of 260 L of useable fuel, while the outboard tanks had a combined capacity of 411 L of useable fuel. The pilot performed the engine run-ups, takeoff, and flight to Centralia with the outboard tanks selected, and he stated that he logged 1.2 hr. total for the flight. The return flight from Centralia was flown with the inboard tanks selected; the flying time was approximately one hour.

The pilot reported using a power setting of 24 in. of manifold pressure and 2400 rpm for his cruise power setting throughout the flight to Centralia and the return flight. The Piper Aztec Manual indicates that the combined fuel consumption at that power setting is approximately 115 L per hour, under ideal conditions.

The critical engine for an aircraft is defined as the engine whose failure would most adversely affect the performance or handling qualities of an aircraft. For the Aztec, the critical engine is the left engine because the right engine produces more asymmetrical thrust. The loss of the hydraulic system with the left-engine failure further complicates operation of the aircraft, especially with the landing gear and flaps extended, because hydraulic power is not available to quickly retract the landing gear and wing flaps.

Minimum control airspeed (V_mc) is defined as the lowest indicated airspeed at which the airplane can always be flown safely after the failure of the critical engine. In the case of the PA-23-250 Aztec aircraft with the flaps retracted, V_mc is 70 kt at the maximum gross weight of 5200 lb. Stalling speed for the same aircraft is 61 kt with the landing gear and flaps extended and wings level; however, the stalling speed of an aircraft in a turn is increased in proportion to the angle of bank. For level turns using 30º and 45º of bank, the stall speeds would be approximately 63 kt and 70 kt respectively. The Aztec is equipped with an audible stall warning horn to warn the pilot of an approaching stall. Before descending to the water, the pilot transmitted to the tower that he had experienced an engine failure and was ditching the aircraft. The aircraft’s stall warning horn was heard in the background during the transmission.

The aircraft was recovered and examined by the TSB, and no pre-impact mechanical discrepancies were identified with the engines or any of the aircraft’s systems. Both fuel selectors were selected to the inboard tanks. The left wing-tip fuel tank separated from the aircraft on impact, and the left fuel cells contained only water. The fuel system on the right side was not compromised, and the inboard tank contained approximately 150 mL of fuel, which was drained from the tank. The right outboard tank contained a considerable amount of fuel and, using the aircraft’s cross-feed system, the fuel from the right outboard tank was fed to the left engine. The engine was started and run for approximately 15 min before the fuel was exhausted.

The TSB determined that the left engine quit during the left turn because of fuel exhaustion and the propeller stopped turning because there was insufficient airspeed to keep it windmilling. Because the hydraulic pump is installed on the left engine, it was not operating after the engine stopped turning, and the pilot was unable to retract the landing gear and flaps. This contributed to the airspeed decreasing quickly. With the airspeed below V_mc, the power from the right engine steepened the aircraft’s turn, and the aircraft stalled. There was insufficient altitude to recover from the stall before the aircraft struck the water. The slow speed of the aircraft and the pilot’s shoulder and lap restraints probably contributed to the survivability of the impact.