What happened
On January 4, 2013, an Alouette III helicopter, registration VT-EGK, operated by Prabhatam Aviation Pvt. Ltd., was involved in a serious incident at IGI Airport, New and Delhi. The aircraft was scheduled to perform an in-flight Vibrex check to monitor vibration levels following the recent installation of overhauled main rotor blades.
While taxiing from the hangar area toward 'E' taxiway, the helicopter encountered an uneven surface where the maneuvering area meets the taxiway. This transition triggered a state of ground resonance. The pilot attempted to mitigate the vibration by increasing altitude to a hover; however, the oscillations intensified. In an attempt to stabilize the aircraft, the pilot performed a sudden, hurried landing. This rapid descent, combined with the compression and subsequent extension of the landing gear, caused the main rotor blades to strike the horizontal stabilizer and tail boom. There were no injuries to the pilot or the four passengers on board, but the aircraft sustained substantial damage.
The investigation
An inquiry by the AAIB examined the mechanical state of the aircraft, the pilot's actions, and the environmental conditions. Investigators confirmed that the aircraft's airworthiness certificates and maintenance records were valid, and all mandatory airworthiness directives had been complied with. The investigation focused on the mechanics of the fully articulated rotor head, which is inherently susceptible to ground resonance.
Technical analysis revealed that the helicopter was operating near its maximum permissible weight. The investigation also scrutinized the communication between the pilot and ground control, noting that the pilot had initiated the engine start-up in the hangar area despite instructions to wait until reaching taxiway 'E'.
Findings
- The aircraft's fully articulated rotor head was prone to ground resonance.
- The helicopter was operating at approximately maximum all-up weight, which influenced the blade coning angle and angle of attack during the incident.
- The pilot's hurried descent caused the blades to move downward at an accelerated rate.
- A hard touchdown caused the main oleos to compress and then extend, lifting the tail upward.
- The simultaneous downward movement of the blades and the upward movement of the tail caused the physical contact between the rotor and the stabilizer.
- The uneven surface of the taxiway transition contributed to the onset of the resonance.