LAX04TA202 · Bell OH-58A+

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 Paperwork obtained from the engine overhaul facility indicated that the second-stage turbine nozzle diaphragm had been replaced by machining out the old diaphragm and brazing in a new diaphragm.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.

Verbatim NTSB analysis and probable cause from the NTSB dataset