What happened
On March 27, 2007, a Diamond-C DA20-A1, registration PH-ACX, was conducting a local instructional flight from Hilversum Airport under visual flight rules. The crew consisted of an instructor and a student pilot training for a Private Pilot License. During the initial engine warm-up on the ground, the engine experienced three separate instances of roughness caused by carburetor ice, all of which were resolved by activating the carburetor heat.
After takeoff and transitioning to level flight at 1,450 feet, the aircraft maintained a steady cruise. However, while flying southeast of Amsterdam near the PAMPUS beacon, the engine began running irregularly. The instructor took control of the aircraft and attempted to resolve the issue by applying carburetor heat and activating the electric fuel pump, but the engine continued to run poorly.
The crew discovered that reducing engine power temporarily stabilized the engine, but the roughness returned whenever power was increased. Unable to maintain altitude at low power settings, the instructor declared a "mayday" and executed an emergency landing in a nearby field. There were no injuries and the aircraft sustained no damage during the landing.
The investigation
Following the incident, maintenance technicians performed extensive inspections on the aircraft. The investigation included checks of the compression, oil filter, ignition system, spark plugs, and carburetors. While the engine was tested and functioned normally during subsequent runs, investigators explored several potential causes for the instability.
One theory considered was the presence of lead deposits from AVGAS, which could potentially cause valves or throttle slides to stick, though no evidence of lead residue was found. The fuel system, including the tanks, filters, and lines, was inspected for water or contamination, but no abnormalities were detected.
Findings
- The investigation identified that atmospheric conditions near the PAMPUS beacon, with a temperature of 8°C and a dew point of 6°C, presented a serious risk of carburetor ice formation at all power settings.
- While the aircraft had shown sensitivity to icing during ground operations, the specific cause of the in-flight engine roughness could not be definitively determined.
- The engine stabilized only when power was significantly reduced, preventing the aircraft from maintaining its flight path and necessitating the emergency landing.