NTSB Identification: CHI93FA182.
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Nonscheduled 14 CFR
Accident occurred Thursday, May 27, 1993 in CAMERON, MO
Probable Cause Approval Date: 06/24/1994
Aircraft: AEROSPATIALE AS-350B, registration: N782LF
Injuries: 2 Fatal,2 Serious.

NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.

LIFEFLIGHT HELICOPTER WAS EN ROUTE WITH PATIENT WHEN NURSE HEARD LOUD 'POP' FOLLOWED BY 'CLATTERING' & HORN ALARM. SOON THEREAFTER, HELICOPTER IMPACTED A FIELD ON SOUTHERLY HEADING ABOUT 70' SOUTH OF TALL TREES. GROUND SCARS & DEBRIS WERE LOCATED ALONG 150' BY 80' PATH. WITNESSES SAID WIND WAS STRONG/GUSTY FROM SOUTH. HELICOPTER WAS EQUIPPED WITH TURBOMECA ARRIEL 1B ENGINE WITH TU-76 MODIFICATION. IT HAD TOTAL FLIGHT TIME OF 2482 HOURS. RECOMMENDED TIME BETWEEN OVERHAUL WAS 2500 HOURS. INVESTIGATION REVEALED ENGINE LOST POWER DUE TO FAILURE OF LABYRINTH SEAL IN 2ND STAGE TURBINE NOZZLE GUIDE VANE. TUBOMECA PERSONNEL PROPOSED THAT UNDER THERMAL LOW CYCLE FATIGUE, A CRACK COULD INITIATE ON 2ND TURBINE NOZZLE GUIDE VANE HUB, WHICH COULD LEAD TO RUB BETWEEN INNER DIAMETER OF HUB & INNER TURBINE LABYRINTH LIPS. THERE WERE 7 ENGINE FAILURES OF SIMILAR NATURE, ALL RELATED TO TU-76 STANDARD NOZZLES. TURBOMECA SERVICE BULLETIN 292 72 0153 WAS ISSUED TO IMPROVE MECHANICAL STRENGTH OF INTERNAL ENVELOPE OF 2ND STAGE NOZZLE GUIDE VANE.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

THE LOSS OF ENGINE POWER DUE TO FAILURE OF THE SECOND STAGE TURBINE LABYRINTH SEAL. FACTORS RELATED TO THE ACCIDENT WERE: THE UNFAVORABLE WIND AND TALL TREES BORDERING THE EMERGENCY LANDING AREA.

Full narrative available

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