MAMMOTH LAKES, CA, USA
N5752X
Cessna 320
The pilot performed a static engine run-up, and with no discrepancies noted, he commenced the takeoff. Upon reaching between 25 and 50 feet above the runway during the initial climb, the normal 35 inches of manifold pressure (MP) was observed being produced by the left engine. However, the right engine only produced between 18 and 20 inches of MP. The airplane veered right and settled into the rough/uneven terrain near the airport. The airport's elevation is 7,128 feet mean sea level, and the calculated density altitude was over 8,200 feet. An examination of the right engine's turbocharger assembly revealed the absolute pressure controller was set lower than the manufacturer's specifications required. Also, the wastegate butterfly valve was mechanically binding in its housing and would not fully close.
HISTORY OF FLIGHT On April 28, 1996, at 1345 hours Pacific daylight time, a Cessna 320, N5752X, experienced a partial loss of power from the right engine during takeoff from runway 09 at the Mammoth Lakes Airport, Mammoth, California. The airplane collided with rough/uneven terrain adjacent to the airport and was substantially damaged. Neither the private pilot nor the two passengers were injured. Visual meteorological conditions existed, and no flight plan was filed for the personal flight which was originating at the time of the accident. The pilot reported that immediately prior to taking off he performed a static engine run-up in accordance with the pilot's operating handbook. No discrepancies were noted, and he commenced the takeoff. During the acceleration, no unusual engine sounds were heard. Upon reaching between 25 and 50 feet above the runway surface, the normal 35 inches of manifold pressure (MP) was observed being produced by the left engine. However, the right engine only produced between 18 and 20 inches of MP. The airplane veered right and settled into the terrain. AIRPORT AND METEOROLOGICAL INFORMATION The Mammoth Lakes Airport's elevation is 7,128 feet mean sea level. About the time of the accident, the airport's automatic weather observing/reporting system (AWOS) reported that the temperature was 58 degrees Fahrenheit, and the barometric pressure was 30.31 inches of mercury. The density altitude was calculated to be over 8,200 feet. TESTS AND RESEARCH Two Federal Aviation Administration (FAA) inspectors and a representative from Allied Signal Aerospace (the turbocharger manufacturer) performed an on-scene examination of the airplane. The FAA reported that no irregularities were found except that the right engine's wastegate could not be freely turned throughout its full operating range without binding. Specifically, the mechanical binding restricted its movement so as to preclude the butterfly valve assembly from fully closing. With the wastegate partially opened, development of maximum engine power would be reduced under conditions of high density altitude. The right engine's turbocharger and wastegate were examined at Allied Signal's Torrance, California, facilities under the supervision of the Safety Board. The factory representative reported that the turbocharger's fixed absolute pressure controller was found functional but set lower than the specifications required. The controller regulates the amount of oil pressure to the wastegate actuator which opens and closes the wastegate butterfly valve. With insufficient actuating pressure, the wastegate valve's operation could be adversely affected. The representative further indicated that when the wastegate valve is not adequately closed, too much of the exhaust gas energy would be shunted around the turbocharger, thus not allowing the turbocharger to produce the required manifold boost pressure for development of full engine power. ADDITIONAL INFORMATION A review of pertinent maintenance records revealed that the wastegate was "completely repaired" in October of 1990, by Main Turbo Systems, Inc., Visalia, California. The owner of Main Turbo Systems reported that he does not keep work orders past the required 2-year period.
the partial failure of the right engine turbocharger wastegate butterfly valve to fully close due to mechanical binding and the low setting of its associated absolute pressure controller. A related factor was the high density altitude which reduced the engine's ability to produce adequate manifold pressure.
Source: NTSB Aviation Accident Database
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