What happened
On September 29, 2010, a TB10 Tobago, registration F-GGNG, operated by the French Air Force, was conducting an experimental flight mission over the Grand Lubéron mountains. The aircraft departed from Salon-de-Provence with a crew of three, including a pilot, a safety pilot, and a flight engineer.
During a climb between 2,700 and 3,000 feet, the pilot noticed that the manifold pressure indicator was reading 20.7 inches of mercury, significantly lower than the required 25 inches. As the flight continued, the pressure dropped further to 19 inches. Unable to maintain level flight, the pilot transitioned into a descent and performed an emergency landing in a field near a departmental road in Puy-Sainte-Réparade. There were no injuries and no damage to the aircraft.
The investigation
The investigation, conducted jointly by the BEA and BEAD-air, focused on the engine's performance degradation. Investigators examined the engine and found that the air duct connecting the cowling air intake to the carburetor had become disconnected. The hose clamp had slipped along the duct toward the carburetor, causing the end of the flexible hose to pinch and partially or fully obstruct the airflow.
Technical reviews of the maintenance records showed that a 50-hour inspection had been completed the day before the incident. The mechanic responsible for the maintenance admitted that he had been interrupted several times during the engine cowling reinstallation, specifically to answer telephone calls related to his other responsibilities at the airfield.
Findings
- The engine power loss was caused by a failure to tighten the hose clamp on the carburetor air intake duct.
- The lack of proper securing allowed the duct to decouple due to the vacuum effect created during engine operation.
- The resulting obstruction of the air intake caused an overly rich fuel mixture, which degraded engine performance to the point where level flight was impossible.
- The maintenance error was a direct result of a task interruption; the mechanic was distracted by external communications while reassembling the engine cowlings.