Occurrence
A Delta Helicopters Ltd. Bell 204B helicopter (registration C-GTNP) was conducting slinging operations in the Wadlin Lake, Alberta, area. While descending to position a load of seedlings at approximately 1420 mountain daylight time, the pilot heard a scraping sound from the engine. The engine then lost power, and the helicopter descended rapidly, resulting in a hard landing and substantial damage. The helicopter remained upright with the load attached to the longline. Another company pilot located the wreckage at about 1915 and found the pilot with serious but non-life-threatening injuries. The pilot was extracted from the site at 2145.
Engine Examination
Disassembly of the Lycoming T53-11B engine (serial number LE08253) revealed that two blades had separated from the power turbine disc, causing extensive damage to other blades, shrouds, and the power turbine nozzle. Blade No. 1 fractured at the disc platform from a pre-existing fatigue crack extending approximately 50% of the blade chord back from the leading edge. Blade No. 52 fractured about ¼ inch above the platform from a pre-existing fatigue crack extending about 35% of the blade chord forward from the trailing edge. It could not be determined which blade failed first, but the second blade failed from debris impact. Scanning electron microscope examination did not reveal the cause of the cracks, as no mechanical damage, discontinuities, inclusions, or corrosion were evident.
All remaining blades on the turbine disc were examined, and no further cracks were found. Two blades were sectioned to investigate possible overheating, but results were inconclusive due to lack of comparison data from the manufacturer. Other hot-section components were undamaged. The turbine disc (part number 140-250-6, serial number 4454) was new with an overhaul life of 2000 hours when installed on 14 June 2000; total time since overhaul was 1284 hours. The turbine blades were maintained "on condition" with no timed replacement life. The last major inspection, an engine mid-life hot-section inspection, was completed about 191 hours prior to the occurrence on 13 July 2002 at 1092.7 hours since overhaul.
Load and Weight Information
The load consisted of 130 boxes of seedlings; the average weight of each box (from a 10-box sample) was about 28.5 pounds. With a weight reduction of one pound per box for water loss, the load weight was calculated at approximately 3575 lb. The carousel weighed 80 lb, and nets and longline were estimated at 120 lb, for a total load weight of about 3775 lb. The helicopter's gross weight was calculated at approximately 9680 lb at takeoff and 9480 lb at the crash site. C-GTNP had been modified per Bell Service Instruction 204-3 to increase the maximum allowable gross weight with external load from 8500 lb to 9500 lb. The calculated out-of-ground-effect hover weight for the conditions was 8600 lb.
Findings
The engine failure was caused by damage to the power turbine and nozzle following the fracture of two turbine blades. The blades failed due to overstress extension of pre-existing fatigue cracks. The initiation of the cracks was not determined. The loss of engine power occurred at low altitude and airspeed, and a hard landing ensued. The helicopter was being operated at a weight exceeding the maximum out-of-ground-effect hover weight but within the increased gross weight limit with an external load.
Safety Actions
The company initiated changes to improve flight crew awareness, including training on performance charts, more detailed examination questions, and an in-house pilot decision-making course. Management supervision was enhanced with greater field oversight and reviews of work zone plans, load control, and communications.