“a Cessna drops out of the sky through metal fatigue of the propeller and there’s no enquiry?”
Technically not the propeller.
There are two types of turbines in the PT-6A engine a compressor turbine (CT) and a power turbine (PT). The power turbine turns the shaft on which the propeller is bolted.
Page 15 of this guide to the PT-6A has a cut away view of the engine.
http://www.pwc.ca/files/en/Know_your_PT6A.pdf
You might be interested in this Australian government bulletin
Apparently they sometimes use after market turbines when they overhaul these engines.
Not a good idea.
Abstract On 31 December 2009, a Cessna Aircraft Company model 208, registered VH-UMV, was engaged in parachuting operations from Cairns Airport, Queensland. While climbing through 12,500 ft in preparation for a parachute drop, the engine failed. The parachutists exited the aircraft and the pilot completed a glide approach and uneventful landing at Cairns Airport.
The failure of the Pratt and Whitney PT6A-114 engine was probably precipitated by fracture of the compressor turbine blades. Federal Aviation Administration (FAA) parts manufacturing approval information indicated that part number T-102401-01 compressor turbine blades that had been installed in the engine during the most recent overhaul were not approved for the PT6A-114 model. As a result of this occurrence, the Civil Aviation Safety Authority (CASA) released Airworthiness Bulletin AWB 72-005, alerting all operators and maintainers of PT6A engines, of the potential for installation of these compressor turbine blades in unapproved PT6A engine variants, and to raise awareness of the restrictions placed on the use of approved after-market blades.
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Excellent contribution to the thread. Butter, you need to see this. Images in report clearly show damage to Compressor turbine blades...