# NTSB SEA95LA051: Sikorsky S-58JT, N1099T, Marblemount, WA, February 13, 1995 - Type: NTSB accident investigation - Classification: Accident · Serious injuries · Limited investigation · Final report - Date: February 13, 1995, 10:20 local (18:20 UTC) - Place: Marblemount, WA - Coordinates: 48.440578, -121.269271 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=48.4406&lon=-121.2693&mi=10 - Phase of flight: [Maneuvering](https://openairdocs.org/ntsb-phases/450) (OpenAirDocs' grouping) - Highest injury: Serious injuries - Injuries: 1 serious - Status: Completed, report July 24, 1995 - Report: Approved by Office Director - NTSB case key (mKey): 42110 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=SEA95LA051 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/42110/pdf - Page: https://openairdocs.org/ntsb/SEA95LA051 ## Probable cause FATIGUE FAILURE OF THE MAIN ROTOR DRIVESHAFT. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Airframe/component/system failure/malfunction, maneuvering - rotor drive system,main rotor mast(drive shaft) - fatigue (cause) - Forced landing, emergency descent/landing - autorotation - attempted, pilot in command - In flight collision with terrain/water, emergency descent/landing - terrain condition - mountainous/hilly ## Analysis THE MAIN ROTOR DRIVESHAFT FAILED DUE TO FATIGUE FAILURE WHILE THE PILOT WAS CONDUCTING HELICOPTER LOGGING OPERATIONS. THE MAIN ROTOR DRIVESHAFT HAD 952 HOURS SINCE NEW, AND HAD BEEN INSTALLED NEW AT THE MOST RECENT GEARBOX OVERHAUL. OVERHAUL PERIOD AND MAIN ROTOR DRIVESHAFT RETIREMENT TIME ARE 2500 HOURS. THE PILOT WAS NOT AT AN ALTITUDE OR AIRSPEED WHERE A SUCCESSFUL AUTOROTATION COULD BE ACCOMPLISHED. ## Factual narrative On February 13, 1995, approximately 1020 Pacific standard time, a Sikorsky S-58JT, N1099T, sustained substantial damage during a forced landing after a main rotor mast failure while conducting helicopter logging operations. The commercial pilot, who was the sole occupant of the aircraft, was seriously injured. There was no fire, and no flight plan was filed for the flight, which was conducted near Marblemount, Washington, and had departed approximately 1010. The ELT actuated, but did not aid in locating the accident site. A witness stated that the pilot had just hooked a load, then released it and descended rapidly to impact. He described hearing an engine sound increasing in rpm before the crash. The pilot stated that he had a 150 foot long-line attached, and about ten minutes into his cycle, heard a loud bang while picking up a load. He said he tried to move the load downhill, away from the ground crew person, released the load and long-line, and turned the aircraft to head down slope. The aircraft impacted the terrain. Post accident investigation revealed that the main rotor shaft, P/N S1635-20059-2, S/N HR243, which had been installed new on August 16, 1993, during the last main gearbox overhaul, had separated through the shaft-to-flange radius. The part had 952 hours total time since new. The normal transmission overhaul period and main rotor shaft retirement time are both 2500 hours. ## Aircraft - Registration: [N1099T](https://openairdocs.org/deregistered/N1099T-2013-09-25), registration cancelled 2013-09-25, S/N 581646, built 1963 (OpenAirDocs) - Reported make and model: SIKORSKY S-58JT - Registry model: [Sikorsky S-58JT](https://openairdocs.org/model/sikorsky-s-58jt) (registry code 8141807) (OpenAirDocs) - Registry engine: [Pratt & Whitney Canada PT6A-60A](https://openairdocs.org/engine/pratt-whitney-canada-pt6a-60a) (registry code 52043, turbo-prop) (OpenAirDocs) - Category: Helicopter - Serial number: 58-1646 - Airframe hours: 9776 - Engines: P&W PT6-T3 1800 horsepower turbo shaft - Damage: Substantial damage - Flight: Other work use, Part 133 - Registered owner: Glacier Helicopters - Airworthiness certificate: Standard, Transport - Last inspection: Continuous airworthiness, February 9, 1995, 14 hours before ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 54; commercial; airplane ratings: single-engine land; instrument ratings: airplane; medical: class 2, valid medical--w/ waivers/lim; seat: left; flight hours: 11,389 total, 3,139 in this make and model, 11,389 as pilot in command, 95 in the last 90 days, 53 in the last 30 days, 2 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 45° at 5 knots, gusting 8 - Visibility: 75 statute miles - Lowest ceiling: none - Temperature: 28 °F - Altimeter: 29.00 inHg ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Accident 19950213001699A](https://openairdocs.org/incident/19950213001699A), Feb 13, 1995: Skrsky S58JT, Marblemount, WA, Substantial damage. Loud bang. Dropped load. Impacted terrain. Main rotor shaft separated. Shaft had 952 hrs. Normal overhaul 2500 hrs. From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.