Accident Details
On a night visual flight rules (VFR) departure from Pukatawagan, Manitoba, a Beaver Air Services Ltd. Piper Navajo PA31-310 (serial No. 31-7612107) was en route to The Pas. The pilot had boarded six passengers and a small amount of hand luggage. Shortly after engine start, the pilot configured the aircraft with 15 degrees of flap, the setting used for short field take-offs, and backtracked to the threshold of runway 15. The aircraft accelerated normally and rotated at 85 mph. Immediately after becoming airborne, the left wing dropped sharply. The pilot levelled the wings, but the aircraft was now off the side of the runway, and he lost sight of the runway lights. He noticed the left engine surging and decided to carry out an engine failure procedure. He moved the landing gear lever to the UP position and, concerned about rising terrain to the left of the runway, pulled back on the control column while maintaining a wings-level attitude. The aircraft descended rapidly into the terrain to the left of the runway, struck the ground in a nose-high attitude, and slid about 150 metres through deep snow. The aircraft came to rest about 100 metres to the left of the departure end of the runway. One passenger sustained a back injury.
The runway at Pukatawagan is 2,850 feet long with a downslope towards the departure end of runway 15. At the departure end, there is a deep gully, after which the terrain immediately rises about 100–125 feet, directly in line with the runway about 600 feet past its end. To the left of the runway, across the gully, the terrain also rises quickly about 100 feet. The terrain causes turbulence off the end of runway 15, particularly in warmer months or with strong wind. Due to the rising terrain, the accepted practice for company pilots was to use a short field take-off procedure for the Navajo on this runway, which is authorized in the PA31-310 Approved Flight Manual (AFM).
The pilot reported a high overcast ceiling with about six miles visibility in light snow at Pukatawagan. The wind was about five knots from the south, favouring take-off on runway 15. These conditions were consistent with reports from other aerodromes and from another company pilot awaiting take-off on the taxiway about halfway along the runway. The take-off was conducted in darkness, with no lights on the ground beyond the runway. The waiting pilot observed that the occurrence aircraft crossed the mid-point of the runway at a higher-than-normal altitude, then descend rapidly and crash. Several occupants reported hearing a warning horn after take-off.
Aircraft and Pilot Information
The pilot completed his initial flight training in 1992 and obtained a commercial pilot licence with multi-engine land and sea endorsements, and an instrument rating in 1994. Before joining Beaver Air Services, he had approximately 1,400 total flight hours, with about 900 hours on multi-engine aircraft, primarily the Britten Norman Islander. He completed Navajo training with Beaver Air Services and flew a successful pilot proficiency check (PPC) with a Transport Canada inspector on 21 November 1996, in a Piper PA31-350 Chieftain. For pilot proficiency, the PA31-310 and PA31-350 are grouped together. At the time of the occurrence, his experience on the PA31 (both models combined) was about 70 hours, and he was qualified to fly single-pilot IFR. His last aviation medical was in July 1996.
The pilot indicated that his decision to continue the take-off after detecting an engine anomaly was based on inadequate runway length to land and rugged terrain under the flight path. He did not use significant rudder to stop yawing. He reacted by cycling the landing gear up to reduce drag. Concerned about proximity to rising terrain, he pulled back on the control yoke. He did not try to establish airspeed or attitude and had no recollection of them during the emergency except rotation speed of 85 mph. He maintained full back pressure until he heard the stall warning horn, then relaxed some pressure, but the aircraft struck the ground before further action.
Engine Examination
The aircraft was examined with no faults found on the airframe or fuel delivery system. Fuel was clean and free of contamination. The left engine was removed and examined at the TSB regional facility, revealing no discrepancies except two loose clamps on the turbocharger compressor discharge housing duct. Technical records indicated the exhaust transition assembly on both engines had been replaced about three weeks prior, necessitating loosening of these clamps.
The left engine was mounted in a test cell with a fixed pitch test club propeller, leaving the turbocharger and fuel systems intact. The clamps were left loose. The engine was started and run to normal operating temperatures. After a magneto check, it was run to 2,000 rpm with normal fuel flow. At 2,575 rpm (simulating take-off power), there was a sudden drop of 500 rpm and a drop of five inches of manifold pressure. The engine surged momentarily and then regained rpm. Several further attempts could not reproduce the surge. Subsequently, the engine ran normally at 2,575 rpm with 39 inches manifold pressure, clamps still loose. Due to safety requirements, it was not feasible to simulate an instantaneous air leakage at the duct clamps during the test.
The aircraft engines have compressor bypass doors on the induction housings. The AFM states that in the event of a turbocharger compressor failure, the engine automatically reverts to normally aspirated operation, or about 75% of normal rated power.
Analysis and Causes
The magnitude of the engine surge observed in the test cell likely duplicated the reported surge during the occurrence, indicating partial power during take-off. The drop of 500 rpm in the test cell would not likely occur with the engine and propeller system mounted in the aircraft, but a similar drop in manifold pressure and power would be expected. Partial power is consistent with the pilot not using significant rudder to counteract yaw. With no other fault found, it is possible that the loose clamps allowed an unsealing of the duct, causing an instantaneous change in turbocharger discharge pressure and resulting in the surge. It was not feasible to directly link the loose ducting to the surge, but if such a leak occurred, the engine could have momentarily reverted to normally aspirated mode, reducing power by up to 25% and producing a surge. The loss of thrust likely caused the left wing to drop.
Under the circumstances, with the left engine likely producing partial power, the right engine at full power, and a deep gully along the flight path, the pilot's decision to continue the take-off was likely the best decision. However, the pilot's unawareness of actual airspeed and attitude, and his maintenance of full aft pressure on the control yoke, probably placed the aircraft in an abnormally high pitch attitude. This higher-than-normal pitch attitude, coupled with available engine power, likely caused the higher-than-normal departure path observed. Although engines likely produced sufficient power to continue take-off, the high pitch attitude, slow speed regime, and high drag configuration probably reduced airspeed until approaching aerodynamic stall speed. The stall warning horn heard by pilot and passengers corroborates this. As the aircraft approached stall, it descended rapidly into terrain.
The pilot was certified and qualified for the flight in accordance with regulations.
The left engine surged immediately after becoming airborne. During the test cell run, the engine surged momentarily but could not be reproduced. The two loose clamps securing the duct between compressor discharge housing and engine fuel controller inlet housing could cause a sudden leak leading to a surge. The clamps may not have been adequately tightened during maintenance three weeks prior. After the surge, the pilot pulled back fully on the control column to clear rising terrain. The aircraft was likely approaching an aerodynamic stall prior to impact.
Causes and contributing factors as stated in the investigation: The left engine surged immediately after take-off for reasons which could not be determined. While attempting to continue the take-off, the pilot allowed the airspeed to decrease close to the aerodynamic stall speed. A high rate of descent developed, and the aircraft descended into the terrain on the airfield.