# NTSB FTW01LA087: Bell 47G2, N1575, Ardmore, OK, March 22, 2001 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: March 21, 2001, 19:45 local (Mar 22, 2001, 01:45 UTC) - Place: Ardmore, OK - Coordinates: 34.190818, -97.109741 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=34.1908&lon=-97.1097&mi=10 - Phase of flight: [Maneuvering](https://openairdocs.org/ntsb-phases/450) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 2 uninjured - Status: Completed, report July 30, 2001 - Report: Approved by Office Director - Parties: FAA - NTSB case key (mKey): 51965 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=FTW01LA087 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/51965/pdf - Page: https://openairdocs.org/ntsb/FTW01LA087 ## Probable cause the loss of tail rotor control during maneuvering resultant from the failure of the tail rotor drive shaft coupling due to a stripped thread which allowed the grease lubricating the shaft to leak out. A factor was the lack of suitable terrain for the forced landing (autorotation). ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Rotor failure/malfunction, maneuvering - rotor drive system,tail rotor drive shaft coupling - stripped thread (cause) - rotor drive system,tail rotor drive shaft coupling - loose part/bolt/nut/clamp/etc (cause) - lubricant,grease - leak (cause) - tail rotor/anti-torque control - not possible, pilot in command(cfi) (cause) - Forced landing, emergency descent/landing - autorotation - performed, pilot in command(cfi) - On ground/water encounter with terrain/water, landing - flare/touchdown - terrain condition - none suitable (contributing factor) - terrain condition - water ## Analysis During a night flight, the helicopter was in level flight, about 1,400 feet MSL over a lake as the student pilot commenced a turn to the left with a forward airspeed of about 45 to 50 knots. The CFI then "came onto the controls" to assist with the coordinated turn, and noticed that the cockpit turn indicator showed that the helicopter was "slipping." Subsequently, the CFI applied left pedal to correct, and as soon as the left pedal was applied, the airframe shuddered and started to yaw to the right. The airspeed started to decrease and the helicopter began to "spin to the right rapidly." The CFI then applied full left pedal and the helicopter continued to spin to the right. After lowering the collective and reducing the throttle, the helicopter stopped spinning. The CFI then lowered the nose and maneuvered the helicopter toward shallow water near the heavily vegetated shoreline, flaring and pulling up on the collective to cushion the landing. The helicopter came to rest on its left side in the shallow water. Examination of the helicopter revealed that the splined coupling at the aft end of the forward tail rotor drive shaft segment was damaged. Removal of the dust cover for the coupling revealed that the screw and washer, part numbers AN501A10-4 and AN960-10, were loose with several of the threads stripped, thus allowing grease to weep during operation. The forward tail rotor drive shaft assembly, part number 47-644-18-9, was replaced on January 29, 2001. The helicopter had accumulated 70 hours of flight time since the last 100- hour inspection and 23 hours since the last 50- hour lubrication. ## Factual narrative On March 21, 2001, approximately 1945 central standard time, a Bell 47G2 helicopter, N1575, registered to Helicopter Spraying Inc., of Ardmore, Oklahoma, and operated by Versatile Helicopters Inc., of Ardmore Oklahoma, was substantially damaged during a forced landing following a loss of tail rotor drive while maneuvering on approach to the Lake Murray State Park Airport, Overbrook, Oklahoma. The certified flight instructor (CFI) and his student pilot were not injured. Dark night visual meteorological conditions prevailed and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 instructional flight. The flight originated from the Ardmore Downtown Executive Airport at 1935, and was en route to the Lake Murray State Park Airport. The CFI reported the following information to the NTSB investigator-in-charge: About 10 minutes after departing from Ardmore, the helicopter was in level flight, about 1,400 feet MSL over a lake that was adjacent to the airport. The student pilot commenced a turn to the left with a forward airspeed of about 45 to 50 knots. The CFI then "came onto the controls" to assist with the coordinated turn, and noticed that the cockpit turn indicator showed that the helicopter was "slipping." Subsequently, the CFI applied left pedal to correct, and as soon as the left pedal was applied, the airframe shuddered and started to yaw to the right. The airspeed started to decrease and the helicopter began to "spin to the right rapidly." The CFI then applied full left pedal and the helicopter continued to spin to the right. After lowering the collective and reducing the throttle, the helicopter stopped spinning. The CFI then lowered the nose and maneuvered the helicopter toward shallow water near the heavily vegetated shoreline. He then flared and pulled up on the collective to cushion the landing. Upon landing, the CFI moved the cyclic to the left to roll the helicopter in the shallow water, and he and the student egressed without difficulty. Examination of the helicopter by an FAA airworthiness inspector revealed that the splined coupling at the aft end of the forward tail rotor drive shaft segment was damaged. Removal of the dust cover for the coupling revealed that the screw and washer, part numbers AN501A10-4 and AN960-10, were loose with several of the threads stripped, thus allowing grease to weep during operation. A review of the maintenance records showed logbook entries indicating that the forward tail rotor drive shaft assembly, part number 47-644-18-9, was replaced on January 29, 2001. The helicopter had accumulated 70 hours of flight time since the last 100-hour inspection conducted on February 22, 2001, and 23 hours since the last 50-hour lubrication. ## Aircraft - Registration: [N1575](https://openairdocs.org/deregistered/N1575-2018-02-08), registration cancelled 2018-02-08, S/N 57 (OpenAirDocs) - Reported make and model: Bell 47G2 - Registry model: [Bell 47B3](https://openairdocs.org/model/bell-47b3) (registry code 1180606) (OpenAirDocs) - Registry engine: [Lycoming TVO-435-GIA](https://openairdocs.org/engine/lycoming-tvo-435-gia) (registry code 41526, turbo-prop) (OpenAirDocs) - Category: Helicopter - Serial number: 57 - Airframe hours: 11736 - Engines: Lycoming VO-435-AID 240 horsepower reciprocating - Damage: Substantial damage - Flight: Instructional, Part 91 - Operator: Versatile Helicopters Inc. - Registered owner: Helicopter Spraying Inc. - Airworthiness certificate: Standard, Normal - Last inspection: 100 hour, February 22, 2001, 70 hours before - Departed: 1F0, Ardmore, OK - Destination: 1F0 ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 37; injuries: none; commercial, flight instructor; instrument ratings: helicopter; medical: class 2, valid medical--no waivers/lim.; last flight review October 3, 2000; seat: right; flight hours: 333 total, 30 in this make and model, 304 as pilot in command, 150 in the last 90 days, 67 in the last 30 days, 4 in the last 24 hours - Student pilot; injuries: none; student, private; airplane ratings: single-engine land; instrument ratings: airplane; medical: class 2, valid medical--no waivers/lim.; seat: left; flight hours: 231 total, 16 in this make and model, 4 in the last 24 hours ## Weather - Conditions: Visual (VMC), Night/dark - Wind: 160° at 5 knots - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 61 °F, dew point 45 °F - Altimeter: 30.00 inHg - Observed at: 1F0 ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Accident 20010321004619A](https://openairdocs.org/incident/20010321004619A), Mar 21, 2001: Bell 47B3, Ardmore, OK, Substantial damage. (-23) Tail rotor drive failure. Failure of the shaft assemby, p/n 47-644-180-9, fig 46-1 and coupling p/n 47-644-130-13, fig 46-4, due to loss of lubrication.… ## Other investigations on N1575 (4) From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. - [NTSB DFW06CA055](https://openairdocs.org/ntsb/DFW06CA055), Jan 13, 2006: Bell 47-B3 (N1575), Dennison, TX, substantial damage. Probable cause: The in-flight separation of the stabilizer bar tube. - [NTSB FTW98LA400](https://openairdocs.org/ntsb/FTW98LA400), Sep 17, 1998: Bell 47-G-2 (N1575), Ardmore, OK, substantial damage. Probable cause: The pilots failure to maintain clearance with terrain and the ensuing loss of control. - [NTSB CHI77FEC47](https://openairdocs.org/ntsb/CHI77FEC47), Sep 18, 1977: Bell 47B3 (N1575), Lake Orion, MI, substantial damage - [NTSB CHI70FDW10](https://openairdocs.org/ntsb/CHI70FDW10), Mar 27, 1970: Bell 47G (N1575), Port Huron, MI, substantial damage From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.