History of the Flight
The aircraft, a US-registered 1972 Piper PA-23-160 (N100MC), had recently returned to airworthy condition after storage. The last annual inspection was completed approximately 13 flying hours and 12 flights prior. During that period, it was refuelled several times with unleaded petrol (MOGAS) from a garage.
On 16 December 2011 at 1500 UTC, the pilot, aged 44 with 374 total flying hours (12 on type), refuelled with about 60 litres of unleaded MOGAS, strained through a filter to remove water. Engine power checks were satisfactory, with magneto drops under 100 rpm on each engine.
The flight was uneventful. After about 35 minutes, the pilot began a descent from 2,500 ft agl to land on Runway 07 at Mount Airey Airfield. Carburettor heat was selected on. At approximately 600 ft, landing gear and flaps were extended. About one nautical mile from the threshold, the pilot noted low cloud and decided to reposition for Runway 25. Carburettor heat was turned off, power increased, and a left turn commenced. At that point, the pilot realized power had been lost on the left engine.
With full right rudder applied, the pilot experienced difficulty maintaining directional control and leveling the wings. Aware that with only one engine producing power, significant climb rate required retracted gear and flaps, but the single engine-driven hydraulic pump on the failed left engine precluded easy retraction. Approaching asymmetric committal height with electrical pylons ahead, the pilot opted for a forced landing rather than manually raising gear and flaps. The pilot flared slightly high, stalling at about six feet, causing the nose and left main landing gear to collapse during the heavy landing. The pilot and passenger were uninjured.
Examination of the Wreckage
Neither propeller had been feathered. Damage and score marks were consistent with the left propeller windmilling and the right propeller driven at low power when they struck the ground. Both engines turned freely; spark plugs on the left engine were within expected colour range. Fuel was present in both wing tanks, but fuel drained from the left engine carburettor inlet pipe lacked the blue tinge of AVGAS 100LL. The nose landing gear detached, left main landing gear collapsed, and structural damage affected the nose section, flaps, and left wing.
Authorised Fuel
The aircraft Flight Manual and engine Type Certificate Data Sheet (E-274) permit operation on AVGAS 91/96 or AVGAS 100LL. The engine manufacturer does not approve automotive fuels. Supplemental Type Certificates (STCs) have been approved by the FAA for automotive fuels meeting ASTM D-4814 on the Lycoming O-320-B engine, but no such STC had been obtained for N100MC. However, an STC was available permitting this aircraft and engine combination to operate with unleaded or leaded MOGAS meeting ASTM D-439 or D-4814.
MOGAS is not intended for aviation use and has different physical properties and quality requirements. Alcohol (ethanol) in MOGAS is prohibited in aviation. Even with an STC, MOGAS must be tested for alcohol before refuelling. MOGAS may suffer octane loss over time, form gum deposits, and its additives can cause corrosion, water absorption, and seal damage. Lead additives control combustion; unleaded fuels use aromatics that can damage piston crowns, exhaust valves, and seals.
Comment
The pilot encountered a loss of power on the left engine at low height during approach. Difficulty maintaining flight was likely due to drag from the unfeathered, windmilling left propeller. A single-engine go-around from low height with gear and flaps extended carries significant risk; the PA-23-160 has minimal single-engine climb performance in that configuration. Approaching asymmetric committal height with pylons ahead, the pilot decided to conduct a forced landing. A large dip in the field may have contributed to misjudging the flare height. The damage was consistent with stalling at low height.
It cannot be determined if the use of unleaded MOGAS contributed to the engine failure. The use of unleaded MOGAS can damage seals in the fuel system and cause long-term engine damage, and makes the engine more prone to carburettor icing, vapour lock, and power loss due to detonation. Alcohol presence can also damage seals and cause power loss.