What happened
On December 21, 2011, a Cessna 172S, registration OK-COK, was performing a takeoff from runway 23L at Prague Letňany airport. Approximately halfway down the runway, the pilot noticed a sudden drop in engine RPM to 1800 RPM. Following standard emergency procedures, the pilot aborted the takeoff and brought the aircraft to a stop before the end of the runway. The aircraft was then taxied to the apron. There were no injuries to the pilot and no damage to the aircraft.
The investigation
Following the incident, a technical inspection of the engine was conducted. Investigators discovered a foreign object inside the engine's intake manifold, located upstream of the throttle valve. The object was identified as a piece of crumpled, waxed paper measuring approximately 150 x 80 mm, which appeared to be the backing paper from an adhesive sticker.
As part of the investigation, the maintenance history was reviewed. The aircraft had undergone a 200-hour inspection and airworthiness extension on December 16, 2011, which included the replacement of the air filter element. During this maintenance, the engine was tested with the engine cowlings removed and the air filter element not installed. While the paper was consistent with the type of material used for sticker backings from a filter manufacturer, investigators could not definitively prove the paper originated from the maintenance process, noting that the engine had functioned normally during the flight from Vodochody to Letňany.
Experimental testing demonstrated that the intake manifold's design, which features a narrowing cross-section and a spring-loaded intake flap, could allow such an object to move and obstruct airflow under high-power settings. It was determined that while the engine could still reach near-maximum RPM with a blocked intake, the sudden movement of the object during the high-power takeoff phase caused the observed power loss.
Findings
- The primary cause of the incident was a reduction in engine power during takeoff caused by the presence of a foreign object in the engine intake manifold.
- The foreign object, a piece of waxed paper, moved within the intake tract during the takeoff roll, obstructing the airflow.
- The pilot's pre-flight engine check did not detect the issue because the engine performance at lower inspection RPMs was not significantly affected by the obstruction.