While at 8,000 ft on descent for a landing at Melbourne Airport, the crew of the Fairchild Industries Inc SA227-AC, Metro aircraft heard a loud bang from the left engine together with associated aircraft yaw. The pilot carried out the initial engine failure actions, noting that the left engine torque had dropped to 15%. The flight continued to Melbourne with the engine still operating. The pilot then carried out an uneventful approach and landing, shutting down the engine at the end of the landing roll.
An examination of the left engine, commissioned by the operator, found evidence of heat and burn damage in the turbine assembly, with one of the first-stage turbine nozzle vanes almost completely burnt away. A first-stage turbine blade had also detached from the first-stage turbine rotor assembly. That blade had then passed through the subsequent stages of the turbine section, extensively damaging the turbine components. The engine failure was fully contained.
The first-stage turbine blades and first-stage turbine nozzle assembly were forwarded to the ATSB's Technical Analysis section laboratory for further investigation. That investigation (see ATSB Technical Analysis Report, BE/200200024) found that many of the nozzle vanes had developed thermal fatigue cracking in the leading edges during operation. That cracking had led to the perforation of the internal cooling air galleries in one vane, resulting in the partial interruption to the flow of cooling air through it. The interrupted flow of air through the vane had resulted in its overheating and complete burn through. The burnt-through nozzle vane had subsequently disturbed the downstream flow of the combustion gas through the turbine and excited a once-per-revolution vibration of the first-stage turbine blades. The investigation concluded that the vibration contributed to development of fatigue cracking within the root of a single blade from the stage-one turbine, and the subsequent liberation of the cracked blade from the turbine rotor.
The engine was last overhauled on 20 January 1997 and had completed 2,327.9 engine hours and 2,473 cycles since that time. The operator's system of maintenance required that the turbine section be inspected at 3,500 hr intervals. To accomplish that inspection, the turbine section had to be removed from the engine. The forward end of the turbine was not normally examined outside of those inspection intervals.
During the 20 January 1997 overhaul, all 36 first-stage turbine blades were replaced with new items of part number 3108125-1. The 12 first-stage turbine nozzle segments were also replaced.
The 12 first-stage turbine nozzle segments were divided into two part number groups. Eight were of part number 3103820-4. Two of the ` - 4' segments were new, with the six remaining segments having been overhauled in January 1997. The remaining four nozzle segments were of part number 3103820-2 and included the failed segment. All of the ` - 2' segments had been overhauled in December 1996.
During the overhaul process the manufacturer required that the turbine nozzle segments be inspected for missing sections of vane material.
A 'Caution' note in the engine manufacturer's maintenance manual stated:
'CAUTION: MISSING MATERIAL (BURN THROUGH) AT VANE LEADING OR
EDGE RESULTS IN IMPULSE (CYCLIC) LOADING OF BLADES.'
The engine manufacturer indicated that similar turbine blade failures had been observed in engines with clogged or streaking fuel burner nozzles.
The manufacturer further indicated that there was a `Hot Gas Path Inspection' borescope inspection procedure that could be carried out to determine the in-service wear of the first-stage turbine nozzle and first-stage turbine rotor. That inspection could be accomplished through the fuel nozzle body ports in the engine's combustion case, and was recommended if `fuel nozzle induced distress is suspected'. Fuel nozzle distress could be indicated to a pilot by an engine running hotter than normal or had reduced performance. The fuel nozzles were required to be serviced every 450 engine hours as part of the operator's system of maintenance. That maintenance had been carried out 197 hours prior to the incident.
Post-incident examination of the nozzles in the engine, carried out on behalf of the operator, did not reveal any nozzle clogging.
The operator carried out in-flight engine trend monitoring. That monitoring did not reveal any ongoing problem with the operation of the engine.
The engine failure appears to have been initiated by the complete burn through of a vane in a turbine nozzle guide vane segment. That resulted in the disturbance of the airflow into the first stage of the turbine assembly and the ensuing failure of one of the first-stage turbine blades at the blade root. The reason for the nozzle guide vane segment burn through could not be fully determined. There was evidence of heat damage to many of the nozzle vanes. That damage can arise from in-service incidents such as partially blocked burner nozzles, however, there was no evidence to indicate that this occurred.
Cracking and erosion of the guide vane segments may have been detected by an in-service borescope inspection before the component failed. That inspection, however, would have required the operator to suspect that there was `fuel nozzle induced distress'. The evidence available from the operator's trend monitoring figures did not indicate any ongoing problem.
The ATSB has distributed this report to all known Australian operators of Metro aircraft.
Technical Analysis Report
Fairchild Industries Inc. SA227-AC, VH-VEH
1 FACTUAL INFORMATION
While on descent for a landing at Melbourne airport, the crew heard a loud 'bang' from the left engine and noted the aircraft yaw to the left. Observing that the left engine output torque had dropped to 15 percent and all other indications were normal, the pilot elected to leave the engine operating at low power and continued the flight to Melbourne.
Initial investigation by maintenance staff found many damaged and missing blades within the turbine assembly of the left engine. To conduct an investigation into the engine failure, the ATSB subsequently obtained the first stage turbine rotor disk and all segments from the first stage inlet guide vane assembly.
Fig. 1. Forward face of the stage-one turbine wheel - missing blade indicated.
|Date:||11 October 2001||Investigation status:||Completed|
|Time:||2300 hours EST|
|Location:||46 km ENE Melbourne, Aero.|
|Release date:||19 December 2002|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||Fairchild Industries Inc|
|Type of operation||Charter|
|Damage to aircraft||Nil|
|Departure point||Canberra, ACT|
|Departure time||2200 hours EST|
|Role||Class of licence||Hours on type||Hours total|