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ASN Wikibase Occurrence # 143900
Last updated: 20 September 2021
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Type:Silhouette image of generic B06 model; specific model in this crash may look slightly different
Bell OH-58A-BF Kiowa
Owner/operator:Farm Aviation Inc
Registration: N902SF
MSN: 70-15406
Fatalities:Fatalities: 0 / Occupants: 1
Aircraft damage: Substantial
Location:Near Brawley, CA -   United States of America
Phase: Take off
Departure airport:Brawley, CA
Destination airport:Brawley, CA
Investigating agency: NTSB
During takeoff, the pilot pulled up on the collective, and the main rotor mast fractured near the point where it extends from the transmission; the pilot then aborted the takeoff. The fracture was located below the threads for the mast nut and above the bearing journal for the upper mast bearing. Postaccident examination of the fracture surface revealed multiple fatigue cracks that initiated at the outer diameter of the mast and extended through about 50 percent of the mast's cross section. The fatigue initiation areas and the through-the-wall portion of the fatigue were heavily corroded with extensive pitting and dark deposits visible. A dark oily material was found on the mast's outer surface near the fracture, and the mast's surface was severely corroded on both sides of the fracture. Analysis of samples of the dark oily material suggests that it was a mixture of lubricating oil and water.

When the mast is installed on the helicopter, the location where the corrosion and fracture occurred is hidden. To fully inspect the area, the mast assembly must be removed from the helicopter and completely disassembled. Maintenance records indicated that the most recent overhaul of the main rotor mast was completed 2,499 hours before the accident, about 99 hours beyond the helicopter manufacturer's recommended time between overhaul of 2,400 hours.

The NTSB has previously investigated a similar main rotor mast failure on the same make and model helicopter; in that case, the most recent overhaul of the main rotor mast was completed 1,486 hours before the accident. As a result of these events, in February 2012, the manufacturer issued an alert service bulletin that reduced the main rotor mast overhaul interval from 2,400 hours to 1,200 hours. Additionally, in July 2012, the Federal Aviation Administration issued Airworthiness Directive (AD) 2012-14-11, applicable to Arrow Falcon Exporters, Rotorcraft Development Corporation, and San Joaquin Helicopters model OH-58A, OH-58A , and OH-58C helicopters. The AD requires operators of those helicopters to, within 30 days, overhaul the helicopters’ main rotor mast assemblies.
Probable Cause: The failure of the main rotor mast due to fatigue cracks that originated from corrosion pits. Contributing to the accident was the operator’s failure to adhere to the manufacturer’s recommended overhaul interval for the main rotor mast.


FAA register:

Accident investigation:
Investigating agency: NTSB
Status: Investigation completed
Duration: 8 months
Download report: Final report

Revision history:

22-Feb-2012 22:02 Geno Added
23-Feb-2012 04:02 RobertMB Updated [Date, Aircraft type, Cn, Operator, Narrative]
21-Dec-2016 19:26 ASN Update Bot Updated [Time, Damage, Category, Investigating agency]
27-Nov-2017 17:53 ASN Update Bot Updated [Cn, Operator, Other fatalities, Departure airport, Destination airport, Source, Narrative]

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