Caption: Fragmented KAflex from the accident helicopter, source ATSB
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The Australian Transport Safety Bureau has issued a Safety Advisory Notice to operators of the UH-1H helicopter, recommending that they inspect the helicopter's 'KAflex' driveshaft.
The Safety Advisory Notice stems from the ATSB's on-going investigation into a fatal accident involving a UH-1H that was conducting firebombing operations at Labrina, near Launceston, Tasmania on 14 February 2022.
The ATSB previously released a preliminary report from the investigation on 28 April, however, ongoing examination of the wreckage has determined that the helicopter's main driveshaft had failed, with evidence of severe frictional and wear damage to one portion of the shaft.
"It is important to note that this investigation is on-going and the ATSB is yet to make formal findings as to the contributing factors to this accident, and technical examination of the KAflex shaft is continuing," said ATSB Director Transport Safety Stuart Macleod.
"However, the driveshaft's manufacturer has advised the ATSB that the presence of frictional damage is evidence that the shaft had entered fail-safe mode during operation."
The KAflex driveshaft was manufactured by Kamatics Corporation in the early 1980s as part of a US Army UH-1H driveshaft retrofit program. It uses flexible plates to accommodate relative movement between the engine and gearbox, and has a fail-safe mode intended to allow for uninterrupted drive for up to 30 minutes of helicopter operation.
However, the manufacturer has advised the ATSB that if a flex-frame attachment bolt were to release, the time before complete shaft failure may be significantly reduced.
Additionally, the Safety Advisory Notice notes that operators should be aware of the manufacturer's concern of a certain serial number range of shafts for the UH‑1H that may be fitted with legacy flex-frame attachment hardware.
A number of previous KAflex driveshaft failures had already prompted the US Federal Aviation Administration to issue an airworthiness directive to UH-1H operators, which with effect from 25 February 2022 requires them to replace the KAflex before further flight.
"While the specific circumstances of this accident are still under investigation, the ATSB advises UH-1H operators to note the preliminary details of this accident, the FAA's airworthiness directive, and to look for the presence of corrosion, fretting, frame cracking, and missing or damaged flex-frame attaching hardware during all inspections of the KAflex driveshaft," Mr Macleod said.
The ATSB also advises UH-1H operators to familiarise themselves with Civil Aviation Safety Authority (CASA) Airworthiness Bulletin 63-004, which recommends closer inspection of the KAflex driveshaft.
The CASA AWB was re-issued in June 2021 in response to an ATSB investigation into a separation UH-1H accident where the helicopter conducted a forced landing after a driveshaft failure. The AWB was first issued in 2007.
"Any identified defects should be notified to CASA and the ATSB," Mr Macleod said.
The UH-1H Iroquois (or 'Huey') was originally manufactured by Bell Helicopter for service with the United States Army and other militaries. A number of companies, including Garlick Helicopters, held supplement type certificates to convert the helicopter for civil operation and registration in the limited/restricted category.
You can find here the safety advisory notice: SAN AO-2022-006-SAN-001 UH‑1H helicopter main drive shaft failure
Last update 15 June 2022
