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ASN Wikibase Occurrence # 141690
Last updated: 7 December 2019
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Date:01-JAN-2012
Time:14:00
Type:Silhouette image of generic B47G model; specific model in this crash may look slightly different
Bell 47G-2 (Tomcat)
Owner/operator:Sky Horse Helicopters Inc
Registration: N9021R
C/n / msn: CCI-165
Fatalities:Fatalities: 0 / Occupants: 2
Aircraft damage: Substantial
Category:Accident
Location:Lohn, TX -   United States of America
Phase: Manoeuvring (airshow, firefighting, ag.ops.)
Nature:Survey
Departure airport:Lohn, TX
Destination airport:Lohn, TX
Investigating agency: NTSB
Narrative:
The pilot performed a preflight inspection of the helicopter to include checking the tail rotor oil level through its sight glass. He had flown the helicopter for more than an hour when the tail yawed to the left, slowly at first then rapidly. The pilot “cut” the throttle, flew the helicopter “to the ground as fast as possible,”and impacted terrain. A postaccident examination of the wreckage revealed that the tail rotor gearbox did not operate when it was rotated. A teardown revealed that the interior surfaces of the gearbox were covered with a black oily substance consistent with residual lubricating oil contaminated with liberated metal particles. The oil level sight glass exhibited no discoloration or other hindrance to normal oil level detection in service.

Examination of the ring and pinion gears revealed that the teeth on the pinion gear were deformed and worn almost down to the tooth root in some locations. Wear was observed on the top land and drive faces of the ring gear. Wear patterns were noted on the coast faces of the ring and pinion gear teeth. The observation of wear on the coast faces of the gear teeth is most probably an indication that the spacing between the gears was too tight as a result of improper alignment during installation, causing accelerated wear. A logbook entry showed that the helicopter’s last annual inspection was completed about 6 months before the accident during which the tail rotor gearbox was overhauled and installed.
Probable Cause: The improper installation of the tail rotor gearbox by maintenance personnel, which led to accelerated wear of the ring and pinion gears, resulting in the loss of tail rotor effectiveness and subsequent forced landing.

Sources:

NTSB: https://www.ntsb.gov/_layouts/ntsb.aviation/brief.aspx?ev_id=20120103X02043&key=1
FAA register: http://registry.faa.gov/aircraftinquiry/NNum_Results.aspx?NNumbertxt=9021R&x=26&y=7


Revision history:

Date/timeContributorUpdates
07-Jan-2012 00:51 Geno Added
07-Jan-2012 05:38 timpman Updated [Total occupants]
09-Jan-2012 18:02 RobertMB Updated [Aircraft type, Cn, Operator, Narrative]
30-May-2014 19:26 TB Updated [Aircraft type, Source, Narrative]
21-Dec-2016 19:26 ASN Update Bot Updated [Time, Damage, Category, Investigating agency]
27-Nov-2017 17:54 ASN Update Bot Updated [Cn, Operator, Other fatalities, Departure airport, Destination airport, Source, Narrative]

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