What happened
On September 15, 2002, a Garlick UH-1H helicopter, registration CC-CNI, was engaged in aerial firefighting operations near Sorribos de Alba, León. The aircraft, operating under a wet lease for a Spanish operator, was transporting a 10-person firefighting crew to combat a nearby forest fire. The aircraft was heavily loaded, with an estimated takeoff weight of 9,300 lb.
During the final approach to a landing site approximately 3 meters above the ground, the pilot attempted to arrest a descent rate. Due to the high weight, altitude, and temperature, the engine could not provide sufficient power to maintain rotor RPM when the collective was increased. This caused the main rotor RPM to drop below minimum limits, triggering a low-rotor-RPM warning. Simultaneously, the loss of power led to a loss of tail rotor effectiveness, causing the aircraft to yaw uncontrollably to the right. The pilot attempted to reduce collective to utilize ground effect, but the aircraft struck the ground on one skid and rolled onto its left side. The main rotor blades struck the terrain and broke. Shortly after the impact, hot exhaust gases from the still-running engine ignited surrounding vegetation, which then spread to the aircraft, destroying the airframe.
The investigation
The investigation examined the aircraft's maintenance history, the flight conditions, and the crew's actions. Investigators found that the engine had recently undergone an overhaul and was functioning correctly at the time of the accident. The investigation also focused on the survival factors of the occupants. While the impact forces themselves were not extreme, the investigation established that several passengers were not wearing seatbelts or helmets, which contributed to the severity of their injuries. The technician in the front passenger seat suffered significant head and chest trauma, leading to a coma, while the pilot sustained a fractured foot.
Findings
- The primary cause of the accident was the execution of a final approach with a high rate of descent and low engine power.
- To arrest the descent, the pilot applied excessive collective pitch, which caused the main rotor RPM to drop below the minimum allowable limit.
- The combination of high density altitude, high temperature, and high aircraft weight created conditions where the engine could not sustain the required rotor speed during the maneuver.
- The loss of rotor RPM resulted in a loss of tail rotor effectiveness, leading to the uncontrolled yaw and subsequent rollover.
- The subsequent fire was triggered by hot exhaust gases from the engine igniting nearby dry vegetation.
- The severity of injuries was exacerbated by the fact that some crew members were not wearing helmets or had unfastened their seatbelts in anticipation of landing.