Cracked turbine blades on a Honeywell LTS101-600A turboshaft engine preceded the May 2008 fatal crash of a single-engined Eurocopter AS350 on Santa Catalina Island, according to a just-released final report by the US National Transportation Safety Board.

The air taxi flight, operated by Island Express Helicopters and en route from Long Beach, California to the island, collided with the ground after a "loud pop" was heard and flames were emitted from the single-spool 615shp (459KW) engine.

Investigators determined that the pilot had lost power and attempted to autorotate to a landing from an altitude of between 200ft (61m) and 400ft on his arrival. Three of the six on board, including the pilot, were killed in the crash and post-crash fire.

 AS350 crash 1
 Crash site on Santa Catalina Island
©NTSB
"Because of the relatively low altitude at which the loss of power occurred, it is likely that the accident pilot had to trade rotor rpm to maintain the altitude needed to clear the obstacles to reach the open field," the NTSB says, adding that the pilot had cleared "numerous obstacles, including buildings and power lines" to the reach an open location that was short of the intended landing area.

Post crash investigation revealed localised damage to four consecutive power turbine blades, two of which were broken at mid-span and which were later found to have fatigue cracking features despite the engine being operated and maintained properly.

 AS350 crash 2
 View of power turbine looking inward from rear of engine
 ©NTSB

The LTS101-600A consists of single-stage axial and a single-stage centrifugal compressor, followed by a reverse-flow annular combustor, single-stage turbine and lastly, a power turbine rotor that drives the helicopter's three-blade main and two-blade tail rotors.

The NTSB notes that Honeywell issued two service bulletins after the accident, requiring the removal and inspection of the turbine assemblies to check for cracking at the mid-span point "that can lead to a blade separation and subsequent inability to maintain powered flight resulting in potential injuries and damage to the aircraft".

A US Federal Aviation Administration proposed airworthiness directive on the blades, issued in December 2009 and not yet finalised, notes that analysis of the two fractured and "field returned blades identified a manufacturing processing issue, with some blades, which may result in crack development between the platinum core pins and the blade material".

Honeywell, while supporting the AD that includes its two service bulletins (LT 101-71-00-0252 and LTS101-71-00-0253), says it "does not fully agree with the NTSB’s probable cause finding... and is evaluating its options."

Source: Flight International

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