Yreka, CA, USA
N1682U
Cessna 207
The single engine airplane experienced a loss of engine power after encountering icing conditions while on an instrument cross-country flight at night. The accident airplane was modified with a Soloy engine conversion, which replaced the Continental TSIO-520 reciprocating engine with a Rolls Royce-Allison 250-C20S turbine engine. The engine was not equipped with an auto-ignition or continuous ignition system. The commercial pilot was utilizing the engine deice prior to encountering icing conditions, and selected alternate engine air once he perceived snow. The pilot reported that the engine lost power shortly after the engine alternate air was selected. The pilot-rated passenger feathered and secured the engine prior to establishing a best glide speed. The pilot attempted to restart the engine on numerous occasions; however, he was unable to obtain the 12 percent N1 required for engine start. As the airplane entered visual meteorological conditions, the pilot selected a gravel area for the forced landing. Examination of the engine revealed that the engine's anti-ice line was separated from the compressor case. Metallurgical examination of the anti-ice line revealed that its mounting pad separated from the compressor case as a result of fatigue cracking. The fatigue cracking originated from the braze joint, which displayed oxidation. According to the engine manufacturer and the engine conversion representative, the broken anti-ice line would have prevented heated air from entering the engine inlet guide vanes even with the engine deice selected. When the pilot selected the engine alternate air, relatively warm air was introduced into the engine inlet, dislodging any accumulated ice, and subsequently flaming out the engine. According to maintenance records, the engine underwent its last 100-hour inspection approximately 29 hours prior to the accident. A review of the engine operations manual revealed that due to thermal differences, the gas producing section of the engine may lock up after an in-flight engine shutdown, requiring the flight crew to wait up to 10 minutes before a successful restart.
On April 16, 2000, at 2052 Pacific daylight time, a Cessna 207 single engine airplane, N1682U, experienced a loss of engine power and made an off-airport, forced landing near Yreka, California. The aircraft sustained substantial damage and the commercial pilot and his pilot-rated passenger received minor injuries. The aircraft was registered to Southern Cross Leasing , LLC, of Medford, Oregon, and was being operated under 14 CFR Part 91 as a personal flight by the pilot when the accident occurred. The flight originated in Santa Barbara, California, at 1845, and was destined for Medford. Dark night, instrument meteorological conditions prevailed at the accident site and an instrument flight rules (IFR) flight plan was filed and activated at the time of the accident. The accident airplane was modified with a Soloy engine conversion, which replaced the Continental TSIO-520 reciprocating engine with a Rolls Royce-Allison 250-C20S turbine engine. The engine was not equipped with an auto-ignition or continuous ignition system. The pilot reported that he had been in night IFR conditions at 12,000 feet mean sea level (msl), when he detected falling snow with his landing light. With the engine deice switch already in the "on" position, the pilot selected the engine alternate air source as a further precaution for icing conditions. The pilot stated, "the engine ran for a very short time and then quit suddenly. There was no surging, no power fluctuations, and no indication that anything was amiss prior to the flameout." As the engine spooled down, the prop went into flat pitch and the pilot-rated passenger feathered it. The passenger then shut off the fuel flow, pulled the throttle back to flight idle, and established best glide airspeed. According to the pilot's written statement, he attempted to restart the engine using the restart checklist (both from memory and by referencing the pilot operating handbook), but was unable to restore engine power. The pilot indicated that they did not obtain N1 rotation during any of the engine restart attempts, and "on all start attempts, the cockpit lights would dim when the starter switch was engaged." The pilot attempted restarting the engine using different control positions such as "power lever in ground and flight idle, anti-ice on and off, [and] alternate air open and closed," all to no avail. As the descent continued down to about 8,000 feet msl, the airplane entered visual meteorological conditions; and the pilot selected what appeared to be a "gravel bar along a river" for the ensuing forced landing. During the landing, the left wing impacted a clutch of small trees and separated from the aircraft. The pilot indicated that he turned off the electrical switches and exited the aircraft. The Federal Aviation Administration (FAA) inspector, who responded to the accident site, noted that the left side engine anti-ice line's mounting pad was separated from the compressor case. According to the Soloy engine conversion representative, the anti-ice line provides bleed air from the compressor to the inlet guide vanes. With the line separated, ice would be able to accumulate on the inlet guide vanes even with the deice switch activated. Once the pilot selected alternate air, relatively warmer air was introduced into the engine inlet. If the inlet guide vanes had indeed collected ice, the warmer alternate air could have dislodged the ice allowing it to travel into the engine resulting in an engine flameout. A fully charged substitute battery was connected to the aircraft system at the accident site to test whether the engine would accelerate to the required 12 percent N1 for fuel introduction and ignition. According to an engine representative, who examined the wreckage with the FAA, the engine "accelerated normally." The engine was removed and sent to Air Services International, Scottsdale, Arizona, for an engine test run, and the battery was retained for further testing. The engine was installed on the test stand and tested to overhaul specifications. All engine starts and accelerations were found to be "normal and the engine performed to specifications, meeting all operating parameters." A metallurgical examination of the failed anti-ice line mounting pad was conducted by the engine manufacturer. According to the metallurgical report, the compressor case sheet metal failed "from high cycle fatigue from the braze fillet around one anti-ice port (fitting)," and "prior oxidation of the braze joint is considered a probable contributor to the initiation of the cracking." Wear marks on the fracture surface "indicates that the fitting was separated for some time." Review of the engine and aircraft maintenance records revealed that the engine had obtained approximately 1,606.0 hours since its last overhaul. According to the engine maintenance records, the engine underwent its last 100-hour/annual inspection in accordance with the "Allison & Soloy Maintenance Manuals" on October 28, 1999, at an engine time since overhaul of 1,577.55 hours (28.45 hours prior to the accident). Review of the Soloy Service Manual Supplement revealed that mechanics were to "Use the Rolls-Royce Checklist in conjunction with this section of the Soloy Inspection Checklist." Item 12-1 of the supplement indicates that the mechanics are to "inspect the engine and Turbine Pac for loose or missing bolts, broken or loose connections, security of mounting accessories and broken or missing lock wire." Review of the Rolls-Royce Scheduled Inspections Checksheet revealed that under item 10, mechanics are to "inspect compressor inlet guide vanes and visible blades and vanes for foreign object damage." Under item 14, the checksheet instructs mechanics to "inspect the compressor scroll for cracks or breaks at the anti-ice air valve and customer bleed port . . ." Item 17 of the checksheet instructs mechanics to "check anti-ice valve for security, worn parts and proper operation . . ." According to representatives from the engine manufacturer and the engine modification owner, the battery was tested. It was noted that the battery "cells all tested normal under load and the battery maintained voltage per specifications." A caution is noted in the Allison 250-C20 series Operation and Maintenance manual, which indicates that "DUE TO THERMAL CHANGES WITHIN THE TURBINE, THE GAS PRODUCER SECTION OF THE ENGINE MAY LOCK UP AFTER AN INFLIGHT SHUTDOWN. THIS IS A TEMPORARY CONDITION WHICH EXISTS AFTER THE ENGINE HAS BEEN SHUT DOWN FOR APPROXIMATELY ONE MINUTE AND WHICH MAY CONTINUE FOR UP TO TEN MINUTES FOLLOWING THE SHUTDOWN. THEREFORE, EXCEPT DURING AN EMERGENCY, AIR STARTS SHOULD NOT BE ATTEMPTED DURING THE TIME PERIOD BETWEEN ONE MINUTE AFTER SHUTDOWN AND TEN MINUTES AFTER SHUTDOWN."
the total loss of engine power while in cruise flight due to the fatigue failure of the engine's anti-ice bleed air line, which resulted in the pilot's inability to utilize the engine anti-ice system. Contributing factors were the icing conditions and the lack of suitable terrain for the forced landing.
Source: NTSB Aviation Accident Database
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