What happened
On September 23, 2007, a Robinson R44 helicopter, registration EI-TOY, was conducting a private flight from Leopardstown to Weston Aerodrome. While flying over the University College Dublin (UCD) campus at approximately 1,000 feet, the engine began to run roughly and lost power.
Recognizing the danger of the engine failure over a populated area, the pilot lowered the collective pitch to initiate an autorotation. During this descent, the engine stopped running entirely. The pilot successfully managed a power-off autorotation, landing the 0 fatalities and 0 injuries aircraft on a grass playing field within the campus. The aircraft sustained no damage during the landing.
The investigation
The AAIU investigation initially looked into potential fuel contamination, as the pilot suspected the fuel might be tainted. However, laboratory analysis of fuel samples from both the aircraft tanks and the ground bowser confirmed the fuel met all required standards.
Mechanical inspections of the engine, air filters, and fuel systems revealed no obvious defects. During subsequent test flights, the engine performed normally. However, investigators noted that even without manual carburetor heat, the carburetor temperature remained in a dangerous range. Furthermore, the investigation found that the spark plugs were exceptionally clean, suggesting that the manual carburetor heat system had rarely been utilized by the crew.
Findings
Analysis of meteorological data showed that the ambient temperature and humidity at the time of the incident were highly conducive to carburetor icing. The aircraft was operating in a zone where moderate icing could be expected at cruise power, and near the threshold for serious icing at any power setting.
It is believed that ice formed within the carburetor, causing the initial power loss. When the pilot lowered the collective to prepare for landing, the aircraft's automatic heating system engaged. The sudden influx of heat likely melted the ice, sending water into the induction system and causing the engine to stall. The investigation also noted that the lack of frequent use of the manual carburetor heat was a contributing factor.