What happened
On August 31, 2017, a PZL-130 TC II GC prototype, registration SP-YZL, departed from Warsaw-Okęcie (EPWA) for a scheduled flight test mission. The flight profile involved performing various aerobatic maneuvers, including stalls and rolls, within the Okęcie AREA 004 zone.
Following a sequence of maneuvers including two inverted flights, the pilot reported hearing two distinct knocks from the engine. Shortly thereafter, the engine failed, and the cockpit displayed general warning alarms and alternator failure indications. The pilot attempted an in-flight restart at approximately 2,0-00 ft, but the attempt was unsuccessful. Due to the low altitude (approximately 1,370 ft) following the failed restart attempt, the pilot executed an emergency landing in a field in Zambrzyków Stary. The aircraft sustained significant damage to the propeller, engine, flaps, and underside components, but the crew escaped without injury.
The investigation
The investigation, conducted by the PZL Warsaw-Okęcie Factory Commission, examined the crew's competence, the flight program, weather conditions, and the technical state of the aircraft. An examination of the engine, supervised by a representative from the Canadian TSB, confirmed that the engine was in a proper technical state prior to the landing.
However, analysis of the high-pressure fuel pump revealed evidence of running without fuel. Detailed reviews of flight data recorders and the fuel system's functional performance during various aircraft attitudes confirmed that air had entered the fuel system, causing a temporary interruption in fuel delivery to the combustion chamber.
Findings
- The primary cause of the engine failure was a prototype design feature in the fuel surge tank that allowed air to be drawn into the fuel supply system following inverted flight maneuvers. The system lacked an effective method for venting this air.
- The presence of air in the fuel line caused a momentary interruption in fuel flow, which dropped the combustion chamber temperature below the fuel's auto-ignition threshold.
- Performing two inverted flights in close succession (within five minutes) contributed to the accumulation of air in the fuel system.
- The in-flight restart attempt failed because the pilot delayed closing the fuel supply to the engine for approximately three minutes after the flameout, leaving excess unburned fuel in the combustion chamber.
- The single-lever control system (PCL) complicates emergency procedures, as closing the fuel supply requires moving the propeller to the 'feathered' position, and the existing flight manual lacked specific instructions for managing unburned fuel buildup in the chamber.
Safety action
Structural changes are being implemented on the prototype PZL-130 TC II GC to modify the surge tank venting and its integration with the wing tank equalization system. Additionally, the Flight Manual (IUwL) will be updated to include specific emergency procedures for engine restarts involving the single-lever control system, specifically regarding the removal of unburned fuel from the combustion chamber. These updates will also be applied to the standard PZL-130 TC II type.