2010 Puy-Sainte-Réparade TB10 Tobago Incident

Incident Overview and Flight Details On 29 September 2010, a SOCATA TB10 Tobago, registration F-GGNG, was operating an experimental flight for the French Air Force. The aircraft departed from Salon-de-Provence aerodrome at 12:40 UTC with three people on board: the pilot in command, a safety pilot, and an onboard engineer. The mission was conducted over the southern slope of the Grand Luberon mountain range. After approximately one hour of flight, while climbing between 2,700 and 3,000 feet, the pilot in command observed that the manifold pressure indicator was reading 20.7 inches instead of the required 25 inches. The pilot decided to level off and move toward the Durance valley. The pressure indication initially remained stable at 20.7 inches before decreasing to 19 inches. Unable to maintain level flight, the pilot landed in a field near a departmental road. ## Aircraft, Personnel, and Conditions The aircraft was owned by the Direction Générale de l’Aviation Civile (DGAC) and operated by the French Air Force. It was a SOCATA TB10, serial number 945, first placed into service on 8 January 1990, with 8,042 hours of use at the time of the incident. The Lycoming 0-360-A1AD engine, serial number L331-79-36A, had been installed on 10 October 2006 and had 1,812 hours of total operating time. The aircraft was within the manufacturer’s mass and center of gravity limits. The pilot in command was a 41-year-old male with 4,800 total flight hours, including 850 hours on type. The other occupants were a safety pilot in the front right seat and an onboard engineer in the rear seat. A ground aircraft mechanic, 54 years old, held a Group 9 aircraft mechanic license and was qualified to perform scheduled maintenance and issue return-to-service approvals. Weather conditions at the site were estimated to include a west-northwest wind of 10 knots, visibility greater than 10 km, clear skies, a temperature of 22 °C, a dew point of 16 °C, and a QNH of 1015. The aircraft was not equipped with flight recorders, which was not required by regulation for this type of aircraft. ## Investigation Findings and Cause Radiocommunications between the F-GGNG crew and another TB10 in the same sector were recorded on 128.425 MHz. The pilot in command first reported difficulties at 13:32 UTC, stating that he could not maintain level flight and that the manifold pressure indicator was reading about 20 inches, then slightly less. He reported an engine speed of 2,500 rpm, oil pressure above the green arc tangent to the yellow arc, and oil temperature in the middle of the green arc. The assisting pilot reported a similar indication on his own aircraft. The pilot landed on a flat rectangular field measuring 400 by 200 meters, located along a busy departmental road and oriented east-west. The surrounding area was clear to the west and east, while adjacent plots had recently been harrowed or plowed and were less suitable for an off-airport landing. The pilot took precautions for the off-airport landing, including preparing the occupants and cabin, transmitting the situation and decisions on the frequency, and setting the transponder to mode A 7700. The occupants exited the aircraft after the engine was shut down. An on-site examination of the engine revealed a disconnection of the duct connecting the cowl air intake to the carburetor, a sliding of the clamp along the duct toward the carburetor, and a pinching of the duct opening on the air intake side. After the fresh air duct and its clamp were reinstalled, engine tests showed no anomalies, and parameters were normal. Checks by the operator in its workshops did not reveal any failure in the engine or accessories. The TB10 F-GGNG was part of a group of similar aircraft leased by the French Air Force from the SEFA. The French Air Force operated the TB10s from Salon-de-Provence air base, while the SEFA was contractually responsible for maintenance and scheduled inspections. The 50-hour scheduled inspections were detailed in the “VP50” section of the maintenance manual. The relevant maintenance manual chapter on engine cowl removal and installation instructed to reconnect the fresh air duct to the carburetor but did not mention the clamp mounted on the duct. The finishing step simply stated that the engine cowl was to be reinstalled. Maintenance records for the F-GGNG’s VP50 inspection were completed and signed, including the finishing item. After observing the loss of engine power, the pilot in command and the safety pilot consulted the operations manual and applied part of the procedures on sheet 6G “Engine failure in flight,” while determining that the procedures on sheet 13D “Forced landing with engine” did not correspond to the situation encountered. ## Safety Lesson The incident was caused by a loss of engine power due to a disconnection of the fresh air duct between the cowl air intake and the carburetor, associated with a sliding clamp and a pinched duct opening. The maintenance manual did not specifically mention the clamp during the reconnection of the fresh air duct, which may have contributed to the condition. The safety lesson emphasized the importance of ensuring that all duct connections and clamps are properly secured and inspected during maintenance, particularly when the maintenance instructions do not explicitly reference specific components.