2014 Robin DR400-135CDI Ecoflyer accident in Albertville (F-HBIT)

Flight and Emergency Descent On 25 February 2014, a Robin DR400-135CDI Ecoflyer registered F-HBIT departed Annecy aerodrome at approximately 13:00 local time for a local flight. The pilot was accompanied by two passengers. The aircraft flew over the Col des Aravis and the Mer de Glace, reaching the Dôme du Gouter at about 12,000 ft, then descended toward Mont Charvin. Between Mont Charvin and Ugine, while in level flight, the pilot heard a sharp bang from the engine and FADEC A and FADEC B warning lights illuminated. The engine parameter display showed propeller speed above 2,800 rpm and power fluctuating between 0 and 5%. The pilot moved the power lever without effect and entered a descent at the maximum glide speed of 145 km/h. Using the GNSS receiver, the pilot estimated that Albertville aerodrome was reachable and applied the in-flight engine failure and in-flight FADEC failure checklists. No power was recovered. In the Albertville valley, the pilot found the wind stronger than expected and could not reach the aerodrome. The pilot selected a clear field on the edge of the town, but during the final turn, a downdraft prevented reaching the field beyond a railway line. To avoid crossing the railway because of the catenary, the pilot forced a landing in a bramble field just before the railway, about 5.5 km from the runway. ## Wreck and Engine Examination The wreck was found on open ground near a residential building, a railway line, and a stream. All observed airframe damage was consistent with the collision with the ground. No visible engine damage or leaks were found, and oil and coolant levels were correct. No contamination was noted in the fuel filter. Propeller damage indicated that it was producing little or no power. The engine and FADEC were removed for further examination. ## FADEC Data The F-HBIT was equipped with a Thielert Aircraft Engines GmbH TAE125-02-99 engine and a variable-pitch propeller driven through a reduction gear. The engine and propeller pitch were fully controlled by an electronic FADEC computer. The pilot commanded power through a single lever. The FADEC had two channels, A and B. Normally, channel A managed the engine; if a problem occurred on channel A, management could be transferred automatically or manually to channel B. The computer recorded an event report and sixteen engine parameters for each channel. During data download, the accident flight data were recorded, and the data from both channels were similar. At approximately 13:41:30, the engine speed, which had been stable at about 3,560 rpm, suddenly increased to about 4,700 rpm with many fluctuations. This value was well above the normal maximum engine speed of about 3,900 rpm. A maximum recorded value of 5,647 rpm was logged at 13:42:35. From that point, the FADEC fuel pressure command varied greatly. The actual fuel pressure in the injection rail followed the FADEC command until the end of the flight. Fuel was therefore being supplied to the engine until the collision with the ground at 13:51:30, a glide time of 10 minutes. ## Engine and Damper Findings Engine examination showed that damage was concentrated in the torque and vibration damper. The damper transmits rotation from the engine to the propeller and damps engine shocks, operating on a principle similar to an automobile clutch. The damper disk drives the propeller through the reduction gear via a splined hub. The damper cage, fixed to the flywheel, is driven by the engine and compresses the disk to drive the propeller. The cage is attached to the flywheel with bolts tightened to a nominal value. Eight stacks of washers allow adjustment of this tightening, which is performed in the factory and must not be modified by the user. The splined hub of the friction disk was broken into four pieces. The spring and washer stack housings showed indentations. On a witness hub that had reached its service limit without failure, the indentations were centered on the side surfaces of the housings. On the broken hub, the indentations were deeper and off-center. Microfractographic examination of the fracture surfaces showed that the hub failed by progressive fatigue cracking. The damper cage washer stacks contained thirteen Belleville washers, with alignment ensured by a housing and guide. All stacks on the F-HBIT contained thirteen washers. However, one stack had seven washers attached to the guide that were no longer aligned with the rest of the stack. The marks left by this failed stack on the inner face of the rear plate were different from those left by the other stacks, which were consistent with in-service wear according to the manufacturer. No permanent marking was observed where the failed stack should have been; only a light dust deposit indicated that the stack had initially been positioned correctly. These observations suggested that the damper had operated with the failed washer stack, but did not allow the duration of operation in that condition to be determined. According to the manufacturer, mispositioned washers cause an imbalance of forces in the cage and a higher operating torque. The manufacturer added that it was not aware of a similar event. It also stated that there was no special tool for positioning the stacks during cage assembly, and the operator had to be very careful. Because of these assembly difficulties, this damper model had been replaced by a new model since early 2011, with replacement occurring at end-of-life exchange. ## Loss of Propeller Drive and Aircraft History The propeller speed indicated on the instrument panel was calculated by applying a coefficient equal to the engine-to-propeller reduction ratio of 1.69 to the speed measured at the crankshaft. The power indication was calculated by the FADEC from fuel flow. In the event of loss of propeller drive, propeller speed decreases and engine speed increases up to a certain limit. Initially, the pilot therefore saw an indication of propeller overspeed, which was actually an indication of engine overspeed. The FADEC then reduced fuel flow to regulate engine speed. The pilot subsequently observed a loss of power corresponding to the FADEC regulation. The F-HBIT was built in June 2008. It had three flight hours during the issuance of its airworthiness certificate. Following the bankruptcy of the manufacturer APEX before delivery, it was stored until August 2012. It then underwent a 100-hour/3-year inspection, during which all airworthiness directives were applied. Its airworthiness certificate was renewed in October 2012. The aircraft was then delivered to its owner. At the time of the accident, the aircraft had about 257 flight hours since new. The torque and vibration damper was the original one. The maintenance program called for a 100-hour engine inspection and a 50-hour airframe inspection. In January 2014, a 100-hour engine inspection was performed, and no anomaly was found. Since that inspection, the aircraft had flown 67 hours. On 1 February 2014, a pilot reported a propeller overspeed indication. The maintenance shop could not reproduce the phenomenon, and the aircraft was returned to service. In April 2008, the engine had successfully passed bench tests at the engine manufacturer before delivery to APEX. No intervention on the torque and vibration damper had been recorded since the engine was put into service. ## Glide Performance and Landing Site Selection At the time of the failure, the aircraft was at an altitude of about 7,500 ft and about 18 km from Albertville aerodrome, which is at an altitude of 1,035 ft. The aircraft flight manual indicated that at 145 km/h, the glide ratio was 8. The sink rate was then about 1,000 ft/min.