# NTSB LAX99LA047: Bell 47G2, N9021R, Helendale, CA, December 12, 1998 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: December 11, 1998, 16:45 local (Dec 12, 1998, 00:45 UTC) - Place: Helendale, CA - Coordinates: 34.739288, -117.349319 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=34.7393&lon=-117.3493&mi=10 - Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 1 uninjured - Status: Completed, report March 31, 2000 - Report: Approved by Office Director - NTSB case key (mKey): 45463 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=LAX99LA047 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/45463/pdf - Page: https://openairdocs.org/ntsb/LAX99LA047 ## Probable cause The pilot's failure to follow proper procedures to recover from main rotor blade aerodynamic retreating blade stall. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Loss of control - in flight, cruise - airspeed(vne) - exceeded, pilot in command - retreating blade stall - encountered, pilot in command (cause) - procedures/directives - not followed, pilot in command (cause) - On ground/water encounter with terrain/water, landing - terrain condition - mountainous/hilly ## Analysis The pilot encountered retreating blade stall while flying at 100 mph. He reported that the helicopter began to shake and it felt like an 'out of balance feeling, like when the washing machine gets unbalanced.' The rotor rpm began to decay. When he reduced power and stabilized the airspeed to about 60 to 65 mph, the shaking stopped. The pilot stated that he was not able to maintain lift, so he entered an autorotation over the hilly terrain. The tail rotor blades impacted an embankment and the main rotor blades severed the tail boom. The pilot's manual for the Bell 47G2 indicates that the 'never-exceed speed' (Vne) for the helicopter is 100 mph. According to the FAA's Basic Helicopter Handbook, the recovery procedure for retreating blade stall is to reduce the collective pitch, increase rotor rpm, reduce forward airspeed, and minimize maneuvering. ## Factual narrative On December 11, 1998, at 1645 hours Pacific standard time, a Bell 47G2, N9021R, sustained substantial damage during a precautionary landing near Helendale, California, following the onset of airframe vibration. The student pilot/owner, the sole occupant, was not injured. The personal flight, conducted under the provisions of 14 CFR Part 91, originated at Kramer's Junction, California, about 1630, and was en route to Helendale. Visual meteorological conditions prevailed at the time and no flight plan was filed. The pilot reported that he was flying between 95 and 100 mph at 200 feet agl when he felt the helicopter begin to shake and slide to the right. He described the shaking as an "out of balance feeling, like when the washing machine gets unbalanced." He stated that the rotor rpm then began to decay. He reported that when he reduced power and stabilized the airspeed between 60 and 65 mph, the shaking stopped but it felt as though he had "lost the tail rotor." He stated that he was not able to "maintain lift," so he initiated an autorotation over hilly terrain. During touchdown, the tail rotor blades impacted an embankment and the main rotor blades severed the tail boom. The Bell 47G2 Flight Manual was reviewed and copies of the relevant portions are appended to this file. The manual indicated that the "never-exceed speed" (Vne) for the helicopter is 100 mph at any altitude from sea level to 1,400 feet msl. It further indicated that the airspeed indicator is marked with a red line at the 100 mph mark. According to the "Basic Helicopter Handbook," published by the Federal Aviation Administration, the tendency for the retreating rotor blade to stall in forward flight is a major factor in limiting the helicopter's forward airspeed. Blade stall occurs during powered flight at the tip of the retreating blade, spreading inboard as forward airspeed increases. The book indicated that the major warnings of approaching retreating blade stall conditions in the order in which they will generally be experienced are: an abnormal 2 per revolution vibration, pitch-up of the nose, and a tendency for the helicopter to roll. At the onset of blade stall vibration, the pilot should reduce the collective pitch, increase rotor rpm, reduce forward airspeed, and minimize maneuvering. ## Aircraft - Registration: [N9021R](https://openairdocs.org/aircraft/N9021R), registered now, S/N CCI-165 (OpenAirDocs) - Reported make and model: Bell 47G2 - Registry model: [Bell-Continental Copters 47G2](https://openairdocs.org/model/bell-continental-copters-47g2) (registry code 1181069) (OpenAirDocs) - Registry engine: [Lycoming VO-435-A1F](https://openairdocs.org/engine/lycoming-vo-435-a1f) (registry code 42200, reciprocating) (OpenAirDocs) - Category: Helicopter - Serial number: CCI-165 - Airframe hours: 7898 - Engines: Lycoming VO-435-A1B 270 horsepower reciprocating - Damage: Substantial damage - Flight: Personal, Part 91 - Registered owner: Ray F. Hunt - Airworthiness certificate: Standard, Normal - Last inspection: 100 hour, October 22, 1998, 38 hours before - Departed: KRAMER'S JCT, CA - Destination: HELENDALE, CA ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 44; student; medical: class 3, valid medical--no waivers/lim.; seat: left; flight hours: 800 total, 148 in this make and model, 700 as pilot in command ## Weather - Conditions: Visual (VMC), Dusk - Wind: 10° at 3 knots - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 52 °F, dew point 3 °F - Altimeter: 30.00 inHg - Observed at: PMD, 39 nm away ## 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 19981211033299A](https://openairdocs.org/incident/19981211033299A), Dec 11, 1998: Bell 47G2, Helendale, CA, Substantial damage. (.19)On December 11, 1998, at 1645 hours pacific standard time, a Bell 47G2, N9021R, sustained substantial damage during a forced landing near Helendale,… ## Other investigations on N9021R (3) 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 CEN12LA120](https://openairdocs.org/ntsb/CEN12LA120), Jan 1, 2012: Continental Copters 47G2 (N9021R), Lohn, TX, substantial damage. 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… - [NTSB LAX97LA285](https://openairdocs.org/ntsb/LAX97LA285), Aug 12, 1997: Bell 47G2 (N9021R), Mojave, CA, substantial damage. Probable cause: The student's improper use of the throttle and collective controls and the flight instructor's inadequate supervision of the flight. - [NTSB NYC72FHJ01](https://openairdocs.org/ntsb/NYC72FHJ01), Jul 11, 1971: Bell-Cci 47G2 (N9021R), Amityville, NY, 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.