Incident Overview
On 24 November 2008, at approximately 10:48 UTC, a Socata TB-200 aircraft (registration CS-DEG) operated by Academia Aeronáutica de Évora was completing a dual instruction flight near Évora, Portugal. The aircraft was on the base leg of the circuit for runway 01 when the instructor reported to the tower that the engine was losing power and that the aircraft would not reach the runway.
The instructor assumed control, shortened the turn to final, performed appropriate procedures, and planned a landing in an open field parallel to the runway. The landing was uneventful. Both occupants—an instructor and a student pilot—exited the aircraft without injury. The aircraft sustained no damage.
Weather and Flight Details
The early morning at Évora was cloudy with a cloud base at 3,000 feet. Relative humidity was 70%, wind from 310° at 18 knots with gusts of 20 to 25 knots, temperature 13°C, dew point 7°C, and QNH 1020 hPa. The aircraft had departed at 09:00 UTC for a local training flight with a planned route of LPEV/Arraiolos/LPEV, estimated time en route 1 hour 30 minutes, endurance 3 hours 30 minutes, and cruising altitude 4,000 feet.
The flight was conducted in dual command with a student pilot under the supervision of an experienced instructor. Both crew members held the required qualifications, and the student's flight log showed good continuity of training.
Aircraft and Engine Information
The aircraft was a single-engine EADS SOCATA Tobago TB-200, serial number 2067, manufactured in 2001. Total airframe hours were 4,650. The engine was a Lycoming IO-360-A1B6, four-cylinder horizontally opposed, rated at 200 BHP at 2,700 RPM, with a Hartzell constant-speed propeller. The engine had accumulated 151 hours since overhaul. The last inspection on all components was conducted on 20 November 2008.
The engine was equipped with an alternate air intake system, mechanically controlled, to provide backup in case of filter blockage. The flight manual recommended applying power every 1,500 feet of altitude loss during descent to prevent excessive engine cooling and spark plug fouling.
Post-Incident Engine Test
After the incident, the engine was run on the ground at various power settings. No power loss or other malfunctions were detected.
Analysis
The investigation considered two hypotheses: 1. The aircraft may have flown through an area of carburetor icing not identified by the crew. The descent from 4,000 feet to circuit altitude (1,000 feet) occurred under conditions conducive to icing: temperature at 4,000 feet was estimated at 5°C (calculated from surface temperature), and the relative humidity and dew point were within icing ranges. The aircraft was operating in cloud at 4,000 feet. The manufacturer's procedures prohibit flight in icing conditions and require selection of alternate air when encountering precipitation or clouds at or below 4°C. Use of alternate air reduces power by about 15%. 2. Spark plug fouling due to prolonged low-power operation was considered possible. The tailwind increased groundspeed, necessitating early descent and possibly low power settings. When the aircraft turned onto base leg, crosswind required increased power, at which point the engine did not respond adequately.
The investigation noted that many power losses occur when pilots increase power after descent, often related to fuel supply issues or icing combined with pilot technique.
Conclusions
The incident, a landing on unprepared terrain, resulted from a partial engine power loss attributed to possible ice formation in the fuel filter and/or spark plug fouling.