Individually Owned: 150,657
Corporate Owned: 137,281
Los Angeles International Airport
Aircraft Accident/Incident Report
Los Angeles, California 90045
Wednesday, July 25, 2001 2:50 PDT
NTSB Narrative Summary Released at Completion of Accident
The helicopter was climbing eastbound after liftoff and was passing 300 feet agl when the crew heard a loud squeal and felt a high frequency vibration in the airframe and in the antitorque control pedals. The pilot executed a 180-degree turn and landed back at the helipad. The NOTAR fan drive system was disassembled for examination. The Aft Tube Assembly transmits antitorque pedal inputs to the NOTAR fan blade pitch change mechanism and moves fore and aft through the hollow rotating fan drive shaft. The tube does not rotate. Three distinct areas of circumferential scoring were noted on the tube; one in the center at roughly the middle of the drive shaft, and one at each end of the tube corresponding to locations just within the fan drive gear box on the forward end and just inside of the fan assembly. The pilot reported that during an earlier flight in the accident helicopter, the antitorque pedals seemed to stick or slightly bind with 2 inches of right pedal displacement. The pilot stated that this was an annoying problem that regularly occurred with all the 520N's in their fleet, and that the maintenance department typically cleared it up by cleaning (lightly sanding) a Teflon coated sliding tube in the NOTAR fan assembly. According to the pilot, dirt and oil would get on this sliding tube and cause it to bind in the housing, and the mechanics would simply clean this tube to resolve the problem. Maintenance department mechanics confirmed that the pedal-sticking problem typically involved foreign material on the aft tubes' Teflon coated pitch slider, and that they routinely resolved the discrepancies by removing the aft tube and lightly sanding the Teflon coating with fine sand paper. With the exception of the aft tube and it's associated pitch slider shaft, extensive examinations and tests of the NOTAR system components revealed no anomalies that could be related to the vibratory event. Examination of the pitch slider shaft found numerous, very fine, axial scratches on the surface, and measurements found that the outer diameter had been reduced by up to 0.014 inch from the manufacturer's specified constant 0.818 inch (+0.001 inch). The hand sanding of the liner resulted in axial surface scratches and a variable shaft outer diameter that was smaller than that required by the MDHI specification. The decreased and variable liner OD degraded the ability to isolate the aft tube assembly from drive system vibrations. The bending mode natural frequencies of the aft tube assembly were assessed to be 65, 220, and 450 Hz. Initial MDHI estimates stated that the first bending mode natural frequency of the aft tube assembly was 94 Hz. The operational rotor speed of the MDHI 520N helicopter is 7.95 Hz, and the operational NOTAR fan and drive system rotational speed is 89.4 Hz. While in comparison, the measured first bending mode natural frequency of the aft tube assembly of 65 Hz offers a margin of 24.4 Hz under the 89.4 Hz operating speed of the NOTAR fan drive syste;, the actual natural frequency may be higher and result in a smaller margin. The investigation determined that a maintenance practice of sanding the Teflon coated pitch slider tube in the NOTAR assembly resulted in excessive clearance between the antitorque system components. This excessive clearance allowed a vibration to occur in those components resulting in the noise and airframe vibrations noted by the pilots.
NTSB Probable Cause Narrative
the excitation of the first bending mode natural frequency of the aft tube assembly that is installed through the rotating NOTAR fan drive system. Contributing factors to the incident included the operator's maintenance practice of hand-sanding the pitch slider component of the NOTAR fan system.
Event Information
| Type of Event | Incident |
| Event Date | 7/25/2001 |
| Event Day of the Week | Wednesday |
| Time of Event | 250 |
| Event Time Zone | Pacific Daylight Time |
| Event City | Los Angeles |
| Event State | CALIFORNIA |
| Event Country | -- |
| Zipcode of the event site | 90045 |
| Event Date Year | 2001 |
| Event Date Month | 7 |
| MidAir Collision Indicator | No |
| On Ground Collision occurred ? | No |
| Event Location Latitude | 335700N |
| Event Location Longitude | 1182440W |
| Event Location Airport | Los Angeles International |
| Event Location Nearest Airport ID | LAX |
| Indicates whether the acc/inc occurred off or on an airport | On Airport |
| Distance from airport in statute miles | 0 |
| Degrees magnetic from airport | -- |
| Airport Elevation | -- |
| Weather Briefing Completeness | Not pertinent |
| Investigator's weather source | Weather Observation Facility |
| Time of the weather observation | 250 Pacific Standard Time |
| Direction of event from weather observation facility (degrees) | -- |
| Weather Observation Facility ID | LAX |
| Elevation of weather observation facility | 56 |
| Distance of event from weather observation facility (units?) | 0 |
| Time Zone of the weather observation | PST |
| Lighting Conditions | Night |
| Lowest Ceiling Height | 1400 |
| Lowest Non-Ceiling Height | -- |
| Sky/Lowest/Cloud Conditions | -- |
| Sky Condition for Lowest Ceiling | Overcast |
| Visibility Runway Visual Range (Feet) | -- |
| Visibility Runway Visual Value (Statute Miles) | -- |
| Visibility (Statute Miles) | 7 |
| Air Temperature at event time (in degrees celsius) | 17 |
| Dew Point at event time (in degress fahrenheit) | 15 |
| Wind Direction (degrees magnetic) | 270 |
| Variable Wind Indicator | Wind direction could be determined |
| Wind Speed (knots) | 3 |
| Wind Velocity Indicator | -- |
| Wind Gust Indicator | Gusting |
| Wind Gust (knots) | 0 |
| Altimeter Setting at event time (in. Hg) | 29.89 |
| Density Altitude (feet) | -- |
| Intensity of Precipitation | -- |
| METAR weather report | -- |
| Event Highest Injury | None |
| On Ground, Fatal Injuries | -- |
| On Ground, Minor Injuries | -- |
| On Ground, Serious Injuries | -- |
| Injury Total Fatal | -- |
| Injury Total Minor | -- |
| Injury Total None | 2 |
| Injury Total Serious | -- |
| Injury Total All | -- |
| Investigating Agency | NTSB |
| NTSB Docket Number (internal use) | 14787 |
| NTSB Notification Source | |
| NTSB Notification Date | Jul 27 2001 12:00AM |
| NTSB Notification Time | 1000 |
| Fiche Number and/or location -used to find docket information | DMS |
| Date of most recent change to record | Feb 5 2004 4:26PM |
| User who most recently changed record | NTSB\MONR |
| Basic weather conditions | Visual Meteorological Cond |
| FAA District Office | WP-FSDO-LAX |
Aircraft Involved |
|
|---|---|
Aircraft #1 |
|
| Aircraft Registration Number | N953SD |
| NTSB Number | LAX01IA262 |
| Missing Aircraft Indicator | -- |
| Federal Aviation Reg. Part | Part 91: General Aviation |
| Type of Flight Plan filed | None |
| Flight plan Was Activated? | No |
| Damage | Minor |
| Aircraft Fire | None |
| Aircraft Explosion | None |
| Aircraft Manufacturer's Full Name | MDHI |
| Aircraft Model | 520N |
| Aircraft Series Identifier | -- |
| Aircraft Serial Number | LN070 |
| Certified Max Gross Weight | 3350 |
| Aircraft Category | Helicopter |
| Aircraft Registration Class | U.S. Registered/U.S. Soil |
| Aircraft is a homebuilt? | No |
| Flight Crew Seats | -- |
| Cabin Crew Seats | -- |
| Passenger Seats | -- |
| Total number of seats on the aircraft | 4 |
| Number of Engines | 1 |
| Fixed gear or retractable gear | Fixed |
| Aircraft, Type of Last Inspection | 100 Hour |
| Date of Last Inspection | Jul 11 2001 12:00AM |
| Airframe hours since last inspection | 65 |
| Airframe Hours | 5852 |
| ELT Installed | No |
| ELT Activated | No |
| ELT Aided Location of Event Site | No |
| ELT Type | -- |
| Aircraft Owner Name | Los Angeles County Sheriff's Department |
| Aircraft Owner Street Address | 4700 W. Ramona |
| Aircraft Owner City | Monterey Park |
| Aircraft Owner State | CA |
| Aircraft Owner Country | USA |
| Aircraft Owner Zipcode | 91754 |
| Operator is an individual? | No |
| Operator Name | -- |
| Operator Same as Owner? | Yes |
| Operator Is Doing Business As | -- |
| Operator Address Same as Owner? | Yes |
| Operator Street Address | -- |
| Operator City | -- |
| Operator State | -- |
| Operator Country | -- |
| Operator Zip code | -- |
| Operator Code | -- |
| Owner has at least one certificate | None |
| Other Operator of large aircraft? | -- |
| Certified for Part 133 or 137 Operation | -- |
| Operator Certificate Number | -- |
| Indicates whether an air carrier operation was scheduled or not | -- |
| Indicates Domestic or International Flight | -- |
| Operator carrying Pax/Cargo/Mail | -- |
| Type of Flying (Per_Bus / Primary) | Public Use |
| Second Pilot on Board | No |
| Departure Point Same as Event | Yes |
| Departure Airport Code | KLAX |
| Departure City | Los Angeles |
| Departure State | CA |
| Departure Country | USA |
| Departure Time | 248 |
| Departure Time Zone | PDT |
| Destination Same as Local Flt | -- |
| Destination Airport Code | KLGB |
| Destination City | Long Beach |
| Destination State | CA |
| Destination Country | USA |
| Specific Phase of Flight | Takeoff - initial climb |
| Report sent to ICAO? | No |
| Evacuation occurred | No |
| Date of most recent change to record | Nov 21 2003 4:57PM |
| User who most recently changed record | COOS |
| Since inspection or accident | Time of Accident |
| Event Location Runway Number and Location | -- |
| Runway Length | -- |
| Runway Width | -- |
| Sight Seeing flight | No |
| Air Medical Flight | No |
| Medical Flight | -- |