LAX94LA076
LAX94LA076

On December 22, 1993, at 0800 Hawaii standard time, a Hughes 369D helicopter, N115HD, rolled over at the termination of a autorotation in a mountainous area near Waimea, Kauai, Hawaii. The forced landing was precipitated by a loss of power during an external load operation. The helicopter was operated under the provisions of 14 CFR 133 of the Federal Aviation Regulations by Inter Island Helicopters of Hanapepe, Hawaii. Visual meteorological conditions prevailed at the time and no flight plan was filed. The helicopter sustained substantial damage; however, the certificated commercial pilot, the sole occupant, was not injured. The flight originated at the Port Allen Airport, Hanapepe, Hawaii, on the day of the mishap at 0730 as a local area external load flight.

In a verbal statement to Federal Aviation Administration (FAA) inspectors, the pilot reported that he was moving a load on the end of a line when the helicopter experienced what he believed was a governor failure. The pilot said he released the load and went to manual throttle operations, then the engine quit. The pilot entered an autorotation into a river bed area, contacted a boulder during the touchdown phase, and the helicopter rolled over.

FAA inspectors from the Honolulu Flight Standards District Office responded to the accident site and examined the helicopter. All pneumatic and fuel lines were found to be intact and fuel was evident at the site.

After retrieval of the helicopter from the accident site, FAA inspectors and engine manufacturers representatives examined the helicopter and engine systems. All pneumatic and fuel lines were tested for leaks, with none evident. The engine was started for a test run and it was observed that it would not accelerate to flight idle. The engine and related fuel system components were removed from the helicopter and shipped to the Allison Engine Company, Indianapolis, Indiana, for detailed analysis under the supervision of the FAA Manufacturing Inspection District Office.

The engine was installed in a test cell for a functional test run throughout the normal parameter range of operation. The report of the test procedures are attached to this report.

According to the Allison report of the engine examination, during the engine starting procedure the engine was found to stagnate between 25 and 37 percent N1. The cause of the start limiting was found to be a restricted fuel flow through the fuel nozzle. Disassembly of the nozzle revealed a collapsed fuel inlet screen, with heavy contamination noted on the screen.

Contaminants similar in appearance to the material on the fuel nozzle were also found on the fuel control fuel inlet strainer and the fuel pump filter element. During functional bench testing, the fuel control fuel inlet strainer had a measured pressure drop indicative of a filter being contaminated and in by-pass operation. Excessive leakage was noted through the fuel pump filter bypass valve during functional bench testing of the fuel pump filter and by-pass valve unit.

The fuel control, power turbine governor, and the fuel check valve were found to function within service limits. A new production fuel nozzle was installed in the engine and another test run initiated. Stabilized running was demonstrated throughout and accelerations from flight auto to takeoff, and decelerations from takeoff to ground idle, were conducted. Engine response times were found to be within new engine requirements.

The fuel nozzle inlet strainer, fuel control inlet strainer, fuel pump filter element, and filer paper containing drained elements of the fuel nozzle contents were submitted to a metallurgical laboratory for a determination of contaminant composition. The contaminants were determined to consist of a sodium rich silicon material.

During a telephone interview, the operator reported that the company purchases bulk quantities of fuel from a refinery in Hawaii. The fuel is transported to the operator's base by barge and loaded on a company fuel truck. The operator stated that daily checks of fuel samples and the filter element from the truck are performed.

According to a statement from the operator, he performs a compressor wash on a daily basis in accordance with the instructions contained in Allison Service Letter CSL-1135. The operator's procedure was timed at 31 seconds, and 2.5 quarts of water are sprayed into the engine during the process.

The engine manufacturer publishes procedures to remove all forms of contamination using a daily water rinse in Commercial Service Letter (CSL-1135, Revision 4, dated June 30, 1992). The procedure specifically lists "salt water air" as an example of contamination to which the procedure applies. CSL-1135 identifies the Hawaiian Islands as an area of severe corrosion.

The procedure specifies that clean water should be sprayed at a rate of 1 quart in 9 to 11 seconds into the compressor inlet. The manufacturer indicates the water injection should start about 3 seconds before energizing the starter motor to provide a full water flow rate to the compressor. The starter should be energized for 10 seconds without exceeding 10 percent engine N1 rpm while spraying the water. The water spray should continue during engine coast down after the starter is released.

If during the starter energize sequence the engine N1 rpm starts to exceed 10 percent, the starter should be released and water spray continued. The N1 rpm should be allowed to reduce to 5 percent, and then the starter should be energized again to obtain a full 10 seconds of engine rotation while water is sprayed into the compressor. The rinse procedure may be repeated if exposure to excessive salt has occurred.

CSL-1135 indicates that the engine should be operated for 5 minutes within 15 minutes of the water rinse to purge and evaporate all residual water. The commercial service letter does not address when the 15 minute period begins.

A compressor wash drain test was conducted by the Safety Board. An Allison 250-C20B combustion chamber was installed in a fixture tilted about 45o to simulate the installation in the helicopter airframe. Three quarts of water were poured against the side of the combuster. The water was directed away from the fuel nozzle. The flow rate was approximately 1 quart in 10 seconds. The rate at which the water drained from the combuster through the combuster drain valve was timed with the sweep second hand of a wristwatch.

The first quart of water immersed the fuel nozzle. After the 3 quarts were added, it took 30.5 minutes for the water to drain. The fuel nozzle was immersed in the wash water about 25 minutes. During the test, water was noted dripping from the fuel nozzle inlet. Disassembly of the nozzle revealed that the filter screen was wet and water had seeped past the check valve in the fuel nozzle.

It should be noted the fuel nozzle was checked for proper assembly before the tests.

The test was performed again, but this time the fuel drain check valve was removed. Again, the nozzle was immersed by the first quart of water, but all of the wash water drained in less than 1 minute. Removal of the drain valve is not a part of the Allison compressor wash procedures.

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