GRACE, ID, USA
N70410
Bell UH-1L
During log slinging operations the restricted category UH-1L rotorcraft's T53-L-13B military engine experienced a total loss of power. The pilot dropped the log load and autorotated into a small, upsloping, cleared area within the surrounding wooded terrain landing hard. Postcrash examination of the engine revealed that the centrifugal impeller had separated due to fatigue cracking in one of the drain holes. An Airworthiness Directive (AD) had been issued relative to the -08 impeller but did not include the -07 impeller (installed in the accident engine). The -07 and -08 impellers are essentially identical with the exception that the -07 impeller is found on military variants of the engine whereas the -08 is found only on civilian variants. As the -07 impeller was not included in the AD, the operator was not required to conduct the appropriate inspections described in the mandated service bulletin. Additionally, the AD did not require a cycle count for the impeller as addressed in the service bulletin, thus rendering the service bulletin unusable even if it had been mandated for the -07 impeller.
HISTORY OF FLIGHT On September 18, 1995, approximately 1200 hours mountain daylight time, a Bell UH-1L, N70410, registered to/operated by Mountain West Helicopters, LLC., and being flown by a commercial pilot, was destroyed during a hard landing in an auto-rotative descent following the total loss of power while engaged in aerial logging near Grace, Idaho. The pilot was seriously injured and a small post-crash fire was quickly extinguished by ground personnel. Visual meteorological conditions existed and no flight plan had been filed. The restricted category rotorcraft, which was engaged in logging operations, was to have been operated under 14CFR133 and originated from a staging location near the accident site approximately 8 minutes previous to the accident. As the pilot was commencing a log lift cycle the ground crewman heard an explosion and observed flames issuing from the rotorcraft's tailpipe. The pilot radioed that he had a "serious problem" and detached the log load. The aircraft was then maneuvered downslope toward a landing site. The pilot executed a hard landing in a small, upsloping cleared area within the surrounding wooded terrain (refer to photograph 01). Post crash examination of the Allied Signal/Lycoming T53-L-13B engine revealed that the centrifugal impeller had been ejected from the compressor section. Several portions of the impeller were recovered near the ground impact site. AIRCRAFT INFORMATION According to the aircraft engine log the centrifugal impeller was removed from the engine on November 26, 1990, and replaced with "a new unit" part number 1-100-078-07G, serial number M346379 (refer to ATTACHMENT MR-I). The logged engine total time (time since new) at this event was 3310.7 hours. The Component Record Card for the impeller also documented 0.0 hours part time since new at the time of its installation (refer to ATTACHMENT MR-II). The engine was subsequently overhauled on April 29, 1993, at a total time of 3920.4 hours. The engine's total time at the accident was 6468.1 hours, thus the impeller's total time was 3157.4 hours. There was no record of cycles maintained for the impeller or the engine nor was there a requirement under the ex- military aircraft's type certificate issued by the FAA. A Textron/Lycoming Service Bulletin issued November 26, 1985, and subsequently revised on May 25, 1994, required that "To inform operators of a special inspection required for (the) impeller" "On T53-L-13B series engines within 200 engine operating cycles on impellers that have accumulated 4,600 cycles, and at each 500 cycle interval thereafter" conduct a "fluorescent penetrant inspection" "for centrifugal impeller(s) 1-100-078-07/-08" (refer to ATTACHMENT SB-I). This Service Bulletin addressed only cycles and not engine or component operating hours. An FAA Airworthiness Directive, 86-09-05, Amendment 39-5293, effective May 6, 1986, references the T53-13B engine "to prevent centrifugal compressor impeller rupture" mandated the application of Textron/Lycoming Service Bulletin 52 but only for impeller Part Numbers 1-100-078-08 (refer to ATTACHMENT AD-I). The -07 impellers were not addressed, although the -07 and -08 impellers are virtually identical. TESTS AND RESEARCH At the request of the Safety Board, the engine was shipped to the facilities of Allied Signal Aerospace, Phoenix, Arizona, where it was partially disassembled on November 20, 1995. The two centrifugal compressor impeller halves, which were shipped as a separate package along with the engine, were briefly examined at the same time and then shipped to the Safety Board's Material Laboratory for further examination. Metallurgical examination of the centrifugal impeller sections revealed a fracture separation originating from one of two drain (weep) holes in the impeller body. Examination of the fracture surfaces revealed features typical of fatigue cracking emanating from multiple circumferential machining gouge marks on diametrically opposite sides of the drain hole. The gouge marks appeared most severe in the fatigue origin areas and extended through the entire length of the hole. The opposing weep hole was found to have fatigue cracking similar to that found in the opposing hole as well as the same machining gouge marks (refer to attached metallurgist's factual report). ADDITIONAL INFORMATION Subsequent to its disassembly the engine, exclusive of the impeller sections, was released to AIG Aviation Insurance Services through Allied Signal on 11/21/95. The impeller sections, at the conclusion of the Board's metallurgical examination, were shipped per the instructions of AIG Aviation to Air Transport, in care of Mr. Jarman, on 05/31/96 (refer to attached NTSB Form 6120.15).
the fatigue failure of the centrifugal impeller and the lack of suitable terrain for a forced landing. Factors contributing to the accident were: the lack of required compliance with the FAA Airworthiness Directive by the operator, the insufficiently defined FAA procedure within the AD (failure to include the -07 impeller and establish a cycle count program), and inadequate rotor RPM.
Source: NTSB Aviation Accident Database
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