ERA21FA233 · BHI H60 Helicopters LLC UH-60A

According to the operator, a new water tank and snorkel were installed on the helicopter to facilitate firefighting operations. Several days of ground testing and calibration were performed before the accident flight, which was the first flight after the water tank was installed. The purpose of the local flight was to check the operation of the fire tank system. The helicopter made six uneventful passes in front of the operator’s hangar at the airport and dropped water that was picked up from a lake adjacent to the airport. During each of these passes, the snorkel was observed to be stable. However, during the seventh pass, the snorkel was swinging from the helicopter. The helicopter then began to hover, released the water from the fire tank, and transitioned to forward flight, gaining altitude and airspeed. The snorkel continued to make large and slightly erratic oscillations as the helicopter climbed. Afterward, witnesses heard a loud bang, pieces of the main rotor blade and tail section separated, and the helicopter descended vertically to the ground. A postimpact fire ensued.

Given the sudden change in behavior of the snorkel from a consistently stable condition in normal flight to one with large and erratic oscillations, it is likely that the tank snorkel support structure was compromised and allowed the snorkel’s oscillations to increase in such a way that the pump assembly at the end of the snorkel hose contacted the main rotor blade. The contact imparted enough energy to fracture the rotor blade at the contact point, which resulted in an imbalanced rotor system and a subsequent in-flight breakup of the helicopter.

The supplemental type certificate (STC) application for the water tank and snorkel had been approved by the Federal Aviation Administration (FAA) about 1 year prior to the accident. A review of the STC application documentation revealed that the FAA had not classified the water tank and snorkel system as an external load. The structural analysis of the tank used the incorrect weight of the snorkel hose and pump combination and did not account for operational loads that would be imparted into the tank by the snorkel as called for in the certification basis in the project-specific certification plan (PSCP). Testing of the snorkel and pump loads did not incorporate the water tank structure to which the snorkel was attached. The system safety analysis did not address the hazard of the snorkel contacting the main rotor system. Increased consideration in any of these areas could likely have identified design insufficiencies.

In addition, the production tank that was used during flight testing was examined after the accident. A manual load test was performed with the snorkel attached to the tank snorkel support structure. When the snorkel was pulled manually from the tank, the tank structure between the hose coupler and the tank face deformed between 0.03 and 0.05 inches. These manual loads represented only a small fraction of the loads that the tank snorkel support structure would experience during normal operation. Thus, the documentation that supported the FAA’s approval of the STC was insufficient because it failed to consider the failure scenario that occurred during the accident.

Verbatim NTSB analysis and probable cause from the NTSB dataset