Aircraft Accident/Incident Report

Bakersfield, California 93305
Monday, May 3, 2004 23:45 PDT

NTSB Narrative Summary Released at Completion of Accident

The helicopter collided with power lines and landed hard during a night autorotation following a loss of engine power. As the pilot initiated a turn, both he and the observer heard a loud boom. While they were discussing the origin of the boom, a second boom occurred followed by an immediate power loss. The pilot lowered the collective and entered an autorotation. During the flare, he realized he was going to overshoot the landing zone (LZ) and lined up with a street for landing. The pilot initiated a flare about 60 feet above the ground and felt a strong jolt with an increased rate of descent and a loss of main rotor rpm's as the helicopter collided with power lines. The helicopter landed hard from about 10 feet above the ground. Review of the maintenance logbooks revealed that the engine had been in service 5 months and 161 hours since a 1,750-hour inspection had been completed. An engine teardown was conducted. Various internal components had excessive heat damage, carbon deposits, and metal splatter. There was a partial loss of airfoils for the number 1 turbine wheel. The gas producer (GP) turbine could not be manually rotated and was locked in place, with the second-stage turbine nozzle diaphragm fractured into several pieces. Paperwork obtained from the engine overhaul facility indicated that the second-stage turbine nozzle diaphragm had been replaced by machining out the old diaphragm and brazing in a new diaphragm. A metallurgical examination of the diaphragm revealed that it had separated from the main portion of the nozzle at the braze joints and it was also fractured 360 degrees around the trailing edge side of the diaphragm. The fracture surface revealed no pre-existing cracks, and the damage was determined to be consistent with tensile overstress. Both the leading and trailing edges of the braze alloy fill were measured. The leading edge side had a fill between 10 and 50 percent of the joint width with an estimated average fill of 30 percent. The trailing edge side had a fill between 50 and 100 percent of the joint width with an estimated average fill of 75 percent. The diaphragm's manufacturer indicated that the braze alloy fill should fill the exposed extremities in a continuous line for the first 20 percent of the joint width, or 1.5 times the thickness of the thinnest member being joined, and have a minimum total coverage of 80 percent of the joint. The braze thickness of the accident diaphragm was measured to be at least 0.009-inch in one location. The manufacturer's specifications for the braze thickness was between 0.002 and 0.004-inches. Review of the overhaul facilities procedures for the repair of the diaphragm included requirements for visual and fluorescent penetrant inspection of the brazed joints after repair completion. The Safety Board metallurgist found that those methods of inspections were not capable of detecting internal voids, unless the voids were connected to the surface. The manufacturer and the overhaul facility reviewed the history and processing of the diaphragm. It was noted that normally the overhaul facility used the hydrogen fluoride cleaning method to clean the majority of the diaphragms prior to brazing. However, the accident diaphragm and seven others were cleaned using the vapor degreasing method. Other diaphragms that had undergone the vapor degreasing method were retrieved and destructively examined. The results showed that these diaphragms contained cracking along the brazed joints. Several diaphragms that had undergone the hydrogen fluoride cleaning method were retrieved and destructively examined. They contained no cracks; however, the braze thickness was noted to be as large as 0.020 inches. As a result of the accident, the manufacturer revised their Parts Repair Procedures Letter (PRPL 2-D004) to reflect changes in the procedures for brazing the second-stage turbine nozzle diaphragm, which included hydrogen fluoride cleaning, and immersion ultrasonic inspection and ultrasonic inspection during and after the brazing process. The overhaul facility rewrote their procedures to reflect the manufacturer's revised procedures.

NTSB Probable Cause Narrative

loss of engine power due to the separation of the diaphragm portion of the repaired second-stage turbine nozzle from the rim portion due to the use of an inadequate cleaning method to clean the joint prior to the braze repair during overhaul by the manufacturer's authorized maintenance center. Contributing to the accident was the lack of an inspection procedure that could verify that the joint met the braze fill requirements.

Event Information

Type of Event Accident
Event Date 5/3/2004
Event Day of the Week Monday
Time of Event 2345
Event Time Zone Pacific Daylight Time
Event City Bakersfield
Event State CALIFORNIA
Event Country --
Zipcode of the event site 93305
Event Date Year 2004
Event Date Month 5
MidAir Collision Indicator No
On Ground Collision occurred ? No
Event Location Latitude 352435N
Event Location Longitude 1185835W
Event Location Airport --
Event Location Nearest Airport ID --
Indicates whether the acc/inc occurred off or on an airport Off Airport/Airstrip
Distance from airport in statute miles --
Degrees magnetic from airport --
Airport Elevation --
Weather Briefing Completeness --
Investigator's weather source Weather Observation Facility
Time of the weather observation 2254 Pacific Daylight Time
Direction of event from weather observation facility (degrees) 310
Weather Observation Facility ID BFL
Elevation of weather observation facility 376
Distance of event from weather observation facility (units?) 3
Time Zone of the weather observation PDT
Lighting Conditions Night
Lowest Ceiling Height --
Lowest Non-Ceiling Height --
Sky/Lowest/Cloud Conditions Clear
Sky Condition for Lowest Ceiling None
Visibility Runway Visual Range (Feet) --
Visibility Runway Visual Value (Statute Miles) --
Visibility (Statute Miles) 10
Air Temperature at event time (in degrees celsius) 26
Dew Point at event time (in degress fahrenheit) 3
Wind Direction (degrees magnetic) 60
Variable Wind Indicator Wind direction could be determined
Wind Speed (knots) 5
Wind Velocity Indicator --
Wind Gust Indicator Not Gusting
Wind Gust (knots) --
Altimeter Setting at event time (in. Hg) 29.91
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) 19902
NTSB Notification Source FAA
NTSB Notification Date May 3 2004 12:00AM
NTSB Notification Time 36
Fiche Number and/or location -used to find docket information DMS
Date of most recent change to record Jun 28 2006 1:30PM
User who most recently changed record NTSB\JOHB
Basic weather conditions Visual Meteorological Cond
FAA District Office WP-SJC-FSDO

Aircraft Involved

Aircraft #1

Aircraft Registration Number N397E
NTSB Number LAX04TA202
Missing Aircraft Indicator --
Federal Aviation Reg. Part Part 91: General Aviation
Type of Flight Plan filed Company VFR
Flight plan Was Activated? No
Damage Substantial
Aircraft Fire None
Aircraft Explosion None
Aircraft Manufacturer's Full Name Bell
Aircraft Model OH-58A+
Aircraft Series Identifier --
Aircraft Serial Number 70-15442
Certified Max Gross Weight 3200
Aircraft Category Helicopter
Aircraft Registration Class --
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 Apr 1 2004 12:00AM
Airframe hours since last inspection 167.7
Airframe Hours 6667.1
ELT Installed Yes
ELT Activated Yes
ELT Aided Location of Event Site No
ELT Type --
Aircraft Owner Name Kern County Sheriff's Department
Aircraft Owner Street Address 1350 Norris Rd
Aircraft Owner City Bakersfield
Aircraft Owner State CA
Aircraft Owner Country --
Aircraft Owner Zipcode 93308
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 Yes
Departure Point Same as Event Yes
Departure Airport Code BFL
Departure City Bakersfield
Departure State CA
Departure Country USA
Departure Time 2226
Departure Time Zone PDT
Destination Same as Local Flt dest & departure same, accident can occur anywhere
Destination Airport Code BFL
Destination City --
Destination State --
Destination Country --
Specific Phase of Flight Maneuvering - turn to reverse direction
Report sent to ICAO? No
Evacuation occurred No
Date of most recent change to record Jun 22 2006 11:10AM
User who most recently changed record JOHB
Since inspection or accident Last Inspection
Event Location Runway Number and Location NA
Runway Length --
Runway Width --
Sight Seeing flight No
Air Medical Flight No
Medical Flight --