Engine Life Limit Exceedance Caused Logging Helicopter Fatal Accident (UH-1B N64RA)
On 8 March 2019 the pilot of restricted category Richards Heavylift Helo Inc (Bell) UH-1B N64RA, registered to Iron Eagle Helicopters, died while conducting Part 133 Helicopter External Sling Load Operations (HESLO) at a logging site near Forks, Washington. After responding to a mayday call of “I’m going down, I’m going down I’m going d…” fellow workers found the helicopter inverted amongst trees 1/4 mile way.
Wreckage of Iron Eagle Bell UH-1B Logging Helicopter N64RA (Credit: NTSB) Note: A blue long line and two black hydraulic hoses can be seen
running uphill to the grapple hook.
The Safety Investigation
The US National Transportation Safety Board (NTSB) safety investigation report was published on the 5 May 2021. The pilot was the owner of Iron Eagle and had 6610 flying hours in total, 729 on type. He was reportedly averaging 30 lifts per flying hour while logging, using a grapple logging system that only requires one person on the ground (vs four for choker logging).
The Grapple of Iron Eagle Bell UH-1B Logging Helicopter N64RA (Credit: NTSB)
The investigators report that:
Examination of the engine revealed that the gas generator first stage [static] sealing disk had fractured and separated into three major pieces.
Gas Generator Turbine Static Sealing Disc from Honeywell T53 of Iron Eagle Bell UH-1B Logging Helicopter N64RA (Credit: NTSB)
Multiple internal components of the engine were subsequently damaged as a result, including all four turbine rotors and nozzles.
Honeywell T53 Gas Generator Components of Iron Eagle Bell UH-1B Logging Helicopter N64RA (Credit: NTSB)
Subsequently, the Honeywell T53-L13B turboshaft engine…
…lost all power, necessitating a forced landing via autorotation. Due to the low altitude and densely tree covered terrain, it is unlikely that the pilot had reaction time to release the load which became entangled in the trees…
The NTSB explain that:
An examination of the first stage sealing disk revealed fracture features consistent with low cycle fatigue.
Cracks in Gas Generator Turbine Static Sealing Disc from Honeywell T53 of Iron Eagle Bell UH-1B Logging Helicopter N64RA (Credit: NTSB)
Fracture Surface Low Cycle Fatigue (LCF) Beach Marks on Gas Generator Turbine Static Disc from Honeywell T53 of Iron Eagle Bell UH-1B Logging Helicopter N64RA (Credit: NTSB)
The overhauled engine was installed into the accident helicopter about 8 months before the accident [2 July 2018]. The sealing disk was installed into the engine at the time of overhaul and had previously accrued 1,067.3 hours and 2,134.6 cycles. Documentation of engine cycles was not accomplished daily. Engine cycles were documented irregularly from July 2, 2018, to September 17, 2018, after which there was no documentation of engine cycles. [however], the helicopter was equipped with an AKV N1/N2 cycle counter.
According to Honeywell SB T53-L-13B-0020 the hour/cycle life limit of the first stage sealing disk is 25,000 hours or 6,900 cycles.
A review of maintenance records and the helicopter’s electronic cycle counter revealed that the sealing disk had exceeded the published life limit of 6,900 cycles. The last documented cycle value was 9,023.13. The cycle counter had recorded 12,023.19 cycles.
NTSB Probable Cause
A total loss of engine power due to the failure of the gas generator sealing disk as a result of the operator’s exceedance of the sealing disk life limits at too low an altitude for the pilot to accomplish a successful autorotation which resulted in the collision with terrain.
Safety Resources
The European Safety Promotion Network Rotorcraft (ESPN-R) has a helicopter safety discussion group on LinkedIn. You may also find these Aerossurance articles of interest:
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