On March 10, 1991, a Bell 47G-5A (registration N59403) operated by Ag-air, Inc. was involved in an aviation accident near Farmington, CA. Investigators recorded the probable cause as: The reduction in engine power due to clogging of the engine's air filter by liquid fertilizer, which resulted in the helicopter settling into soft terrain and the main rotor blades severing the tail boom. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive; 1 related events involving the same aircraft type or operator are linked below.
A helicopter suffered engine power loss while dispensing liquid fertilizer during a procedure turnaround. The aircraft settled into soft terrain, causing the main rotor blades to sever the tail boom before impacting the ground.
What happened
The incident occurred while a helicopter was engaged in agricultural operations involving the dispensing of liquid fertilizer. During a procedure turnaround phase of the flight, the engine power reduced unexpectedly. As the aircraft lost lift, it settled into soft terrain below. The descent caused the main rotor blades to strike and sever the tail boom. Following this structural failure, the helicopter impacted the ground.
The investigation
A mechanical examination of the aircraft was conducted to determine the cause of the sudden power loss. Investigators focused on the engine intake system, specifically the air filter component. The inspection revealed that the air filter had become obstructed. Analysis confirmed that liquid fertilizer had clogged the filter elements, restricting airflow to the engine and leading to the reduction in power.
Findings
The sequence of events was driven by an environmental factor interacting with the aircraft's maintenance requirements. The primary contributing factor was the ingestion of liquid fertilizer into the engine air intake system. This contamination resulted in a clogged air filter, which compromised engine performance during a critical phase of flight. The subsequent loss of power led to an uncontrolled descent and structural damage to the tail section.
Probable cause
The reduction in engine power due to clogging of the engine's air filter by liquid fertilizer, which resulted in the helicopter settling into soft terrain and the main rotor blades severing the tail boom.