# NTSB CHI91DER13: Bell 47G-3B-1, N73998, Maple Grove, MN, August 9, 1991 - Type: NTSB accident investigation - Classification: Accident · No injuries · Final report - Date: August 9, 1991, 07:50 local (12:50 UTC) - Place: Maple Grove, MN - Coordinates: 45.130474, -93.450279 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=45.1305&lon=-93.4503&mi=10 - Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 1 uninjured - Status: Completed, report April 23, 1993 - Report: Approved by Office Director - NTSB case key (mKey): 14859 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=CHI91DER13 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/14859/pdf - Page: https://openairdocs.org/ntsb/CHI91DER13 ## Probable cause THE PILOT'S FAILURE TO MAINTAIN DIRECTIONAL CONTROL OF THE HELICOPTER DURING THE FORCED LANDING. RELATED FACTORS ARE THE LOSS OF ENGINE POWER, LOSS OF ROTOR RPM, THE ERODED COMPRESSOR BLADES, THE PILOT'S OPERATION WITH KNOWN DEFICIENCIES IN EQUIPMENT AND INADEQUATE AIRCRAFT PREFLIGHT, AND THE BLOCKED SPRAY EQUIPMENT. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Loss of engine power(partial) - mech failure/malf, cruise - compressor assembly,blade - eroded (contributing factor) - rotor rpm - not possible, pilot in command (contributing factor) - annunciator panel light(s) - inoperative - operation with known deficiencies in equipment, pilot in command (contributing factor) - spray/dusting equipment - blocked(total) (contributing factor) - aircraft preflight - inadequate, pilot in command (contributing factor) - directional control - reduced (contributing factor) - Forced landing, descent - emergency - aircraft control - not maintained, pilot in command (cause) - Roll over, landing - roll ## Analysis THE PLT SAID HE HAD COMPLETED AERIAL APPLICATION OF MOSQUITO CTL TREATMENT & WAS RTRNG TO THE LOAD SITE. THE HELICOPTER WAS IN CRUISE FLT AT 300 FT ABV THE GND WHEN THE PLT '...NOTICED IT WAS REQUIRING MORE RIGHT CYCLIC CONTROL TO KEEP THE HELICOPTER LEVEL, ALSO BEGAN TO EXPERIENCE A LOSS IN ALTITUDE.' THE PLT NOTED THE ROTOR RPM HAD DROPPED TO 90%, BUT THERE WERE NO WARNING LIGHTS OR HORNS. THE ENG OUT AUDIO WARNING WAS PLACARDED 'INOPERATIVE' & THE PLT WAS AWARE THE OIL CHIP WARNING LIGHT WAS INOP. THE PLT MADE A FORCED LANDING IN AN ALFALFA FIELD. HE STATED '...DUE TO THE RIGHT CYCLIC INPUT I HAD TO LAND WITH A DRIFT TO THE RIGHT...THE RIGHT SKID GEAR SNAPPED, ROLLING OVER TO THE RIGHT.' AN INV REVEALED THE LEFT HOPPER WAS BLOCKED & STILL CONTAINED AN ESTD 100 TO 300 LBS OF MOSQUITO CONTROL AGENT. THIS LOAD RESULTED IN AN IMBALANCE WHICH WAS COUNTERED BY EXTREME CYCLIC INPUT. EXAM OF THE ENG REVEALED 'SEVERE EROSION' OF THE COMPRESSOR BLADES, AND HORSEPOWER PRODUCTION 'SIGNIFICANTLY BELOW PRODUCTION TEST SPECIFICATIONS.' ## Aircraft - Registration: [N73998](https://openairdocs.org/aircraft/N73998), registered now, S/N 2902, built 1964 (OpenAirDocs) - Reported make and model: BELL 47G-3B-1 - Registry model: [Bell 47G-3B-1](https://openairdocs.org/model/bell-47g-3b-1) (registry code 1181028) (OpenAirDocs) - Category: Helicopter - Serial number: 2902 - Airframe hours: 6015 - Engines: ALLISON 250-C18C 260 horsepower turbo shaft - Damage: Substantial damage - Flight: Aerial application, Part 137 - Operator: Churchill, Scott - Registered owner: Churchill, Scott - Airworthiness certificate: Standard, Normal - Last inspection: Annual, April 15, 1991, 122 hours before - Departed: MIC, MINNEAPOLIS, MN - Destination: NONE, MN ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 49; flight instructor, commercial, private; airplane ratings: single-engine land; instrument ratings: airplane; medical: class 2, valid medical--w/ waivers/lim; seat: left; flight hours: 1,522 total, 1,142 in this make and model, 144 in the last 90 days, 60 in the last 30 days ## Weather - Conditions: Visual (VMC), Day - Wind: 180° at 5 knots - Visibility: 7 statute miles - Lowest ceiling: none - Temperature: 75 °F ## 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 19910809033109A](https://openairdocs.org/incident/19910809033109A), Aug 9, 1991: Bell 47G3B1, Minneapolis, MN, Substantial damage. Low rotor RPM and reported power loss. Right cyclic needed to level. Rolled landing. Left hopper load right empty. ## Other investigations on N73998 (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 CHI94T#A04](https://openairdocs.org/ntsb/CHI94T%23A04), Jun 10, 1994: Bell 47G3B1 (N73998), Medina, MN, substantial damage - [NTSB DEN79FQA37](https://openairdocs.org/ntsb/DEN79FQA37), Jul 6, 1979: Bell 47G (N73998), Socorro, NM, substantial damage - [NTSB DEN76FAQ06](https://openairdocs.org/ntsb/DEN76FAQ06), Jul 23, 1975: Bell 47G3B1 (N73998), Belen, NM, substantial damage - [NTSB LAX65A0071](https://openairdocs.org/ntsb/LAX65A0071), Feb 27, 1965: Bell 47G3B1 (N73998), Bishop, CA, destroyed 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.