Incident Overview
On 13 January 2020, a Tecnam P2006T aircraft, registration SP-KEY, was involved in a serious incident at Bydgoszcz (EPBY). The flight was a training sortie conducted under Instrument Rating (IR) training program by ATO Grafprom Aviation. The crew consisted of an instructor and a student.
Sequence of Events
The flight was planned for intermediate and final approach exercises, including 3D and 2D operations. Because the forecast zero isotherm altitude was approximately 3,000 ft AMSL, the instructor requested and received clearance from Tower to operate at 2,300 ft AMSL. Takeoff occurred at 10:45 LMT.
During climb, at about 900 ft AMSL, the instructor activated the carburetor heat and informed the student. The climb ended at 2,300 ft AMSL over VOR BYZ. The flight was conducted in clouds with moderate humidity and an outside air temperature of +2°C, conditions conducive to carburetor icing.
The student performed a holding pattern over VOR BYZ, then began a VOR approach to Runway 26, intended to terminate with a go-around. During the approach, after passing the 3-mile point, the student executed the 'Final Check' checklist but did not move the carburetor heat control to 'OFF'.
At approximately 350 ft AGL, the instructor commanded a go-around. During the subsequent climb, at around 700 ft AGL, the right engine began running unevenly and lost power. The instructor took control, executed a 180° turn, and leveled off. He informed Tower of engine trouble, noting that the right engine was malfunctioning with rough running and detonation sounds. He then shut down the right engine, leaving its fuel valve in the closed position.
Shortly after, the left engine also began running roughly with similar symptoms. The instructor instructed the student to restart the right engine, but it failed to start because the pilot did not notice the fuel valve was closed. The instructor declared an emergency and performed a landing on the grass near the runway threshold, to the right side of the runway.
Investigation Findings
After the incident, a mechanic started both engines and conducted a check; all parameters were normal. On 16 January 2020, a test flight was performed, and engine operation and parameters were normal.
Analysis focused on the use of carburetor heat procedures. From the moment the carburetor heat was activated during climb until the engine shutdown on the ground, the flight was conducted with carburetor heat engaged. The Tecnam P2006T Flight Manual states that takeoff should be performed with carburetor heat off, as icing is unlikely at full throttle. However, the manual describes carburetor heat use during steady flight primarily in relation to icing conditions, but does not specify circumstances where excessive carburetor heating may occur. The manufacturer's Service Information Letter SIL-2017-02 provides more detail, stating that carburetor heat should be minimized to absolute minimum only when flying in icing conditions, and should be avoided when engine power exceeds 75% or when conditions are not conducive to ice formation, to prevent engine damage from detonation. The crew was not aware of this document.
Analysis of carburetor heat use during the flight indicated that the crew acted correctly in conditions conducive to icing, except during the go-around phase. The use of carburetor heat at full power during the go-around caused fuel to boil in the carburetors, leading to detonation of the fuel-air mixture.
Conclusions
The probable cause of the incident was the flight at full engine power with carburetor heat engaged during the go-around procedure, which caused fuel boiling and detonation of the fuel-air mixture. No contributing circumstances were identified.