While in cruise flight between Bundaberg and Brisbane, the crew of the Shorts 360 aircraft heard a loud noise followed by the automatic feathering of the right propeller. The crew shut down the right engine, declared a PAN and continued to Brisbane.
The most likely source of the electrical discharge damage was the starter-generator unit, which couples directly to the starter-generator gear. This was consistent with the failure pattern in previous number 1 bearing failures in PT6A engines. Tests carried out on a starter-generator of the same model as that fitted to the failed engine, indicated that the problem is exacerbated by the accumulation of brush dust. Armature leakage electric current from the unit would be conducted via the dampener assembly, through the starter-generator gear and to the number 1 bearing via the coupling gear and shaft assembly. Intermittent pitting on the starter-generator gear and continuous pitting on the coupling gear indicated that the most likely source of electric potential was the starter-generator assembly. The effect of utilising FAA PMA components rather than original manufacturer parts in the overhaul of the starter-generators is undetermined.
- The failure of the left engine was a result of the failure of the number 1 bearing.
- The failure of the number 1 bearing in the left engine was a result of electrical arcing damage to the bearing inner race.
- The number 1 bearing inner race electrical arcing damage was most likely a result of electrical discharge from the starter-generator assembly.
While in cruise flight between Bundaberg and Brisbane, the crew of the Shorts 360 aircraft heard a loud noise followed by the automatic feathering of the right propeller. The crew shut down the right engine, declared a PAN and continued to Brisbane.
Initial investigation by the operator found that the engine would not rotate. Subsequent specialist examination revealed that the number 1 bearing had failed. The surfaces of the bearing showed signs of discolouration and blackening associated with extreme overheating. The bearing balls and inner race showed heavy localised wear and metal flow associated with the sliding contact of the balls against the inner race. There was also evidence of electrical arcing damage on the inner race. That was traced back through the accessory gearbox components to the starter-generator input shaft. Four equally spaced groups of two or three teeth on the starter-generator drive gear were pitted. The electric current required to cause that pitting was an alternating or pulsed frequency, equal to four times the rotational speed of the starter gear. The coupling gear that mated with the starter-generator drive gear showed continuous pitting over the whole contact surface.
Tests carried out by the operator on a starter-generator of the same model as that fitted to the failed engine, showed pulsed electric current discharges from the starter-generator output shaft. When an accumulation of armature brush dust was blown from the housing of the starter-generator, the measured voltage of the pulsed discharge decreased. The investigation noted that during overhaul, the starter-generators may be fitted with Federal Aviation Administration (FAA) Parts Manufacturer Approval (PMA) components, such as rotor electrical brushes.
The operator had previously experienced four engine failures in Shorts 360 aircraft. Those occurred in November 1995, June 1999, October 1999 and April 2000. All involved failure of the number 1 bearing. In three cases, there was evidence to suggest that an electric current from the starter-generator gear shaft, passed through the accessory gearbox gear train and the compressor hub splined coupling. The electric current initiated spalling damage to the bearing (the cracking and flaking of particles out of a surface). The reason for the electric discharge was not determined. In one case, the reason could not be determined because of the severity of secondary damage to the bearing. The four inflight engine failures in Australia, attributed to electrical discharge damage to the number 1 bearing, occurred between 60 and 640 hours after the starter-generator became unserviceable and a replacement unit was fitted. There are indications that in some occurrences a previously installed starter-generator may have initiated damage to the number 1 bearing.
The engine manufacturer has since reported that seventeen engine failures attributed to electrical discharge damage of the number 1 bearing have occurred in the PT6A worldwide fleet. The aircraft types involved included the Shorts 360, Beech 1900 and Beech Kingair 350 aircraft. To date, the engine and starter-generator combination has been limited to the PT6A-60A, 65B, 65R, 67D, 67R series engines equipped with Lucas Aerospace (TRW Aeronautical Systems) 23078 and 23085 model starter-generators.
A diagram showing the general construction of the PT6A engine and the relationship between the starter-generator and the number 1 bearing is included in this report available on the ATSB web site at Technical Analysis Report 200003399.
The ATSB Technical Analysis Report 200003399 (BE/200000014) details the examination of the failed number 1 bearing and is also available on the ATSB web site at Technical Analysis Report 200003399 or from the Bureau on request.
Local safety action
As a result of this occurrence, the Australian Civil Aviation Safety Authority (CASA) have taken the following actions:
- Issued a direction on 5 July 2001, under the provision of Civil Aviation Regulation 38, to all Australian Certificate of Registration holders of Pratt and Whitney Canada PT6A turboprop powered aircraft, conducting fare-paying passenger operations. That direction required the operators to include in their System of Maintenance the following:
a. 'Periodic in service S/G [starter-generator] field cleaning and resistance checks to be performed in accordance with the procedures detailed in TRW Lucas Maintenance Manual Number 23700, Revision 9 at intervals not to exceed 300 hours S/G time in service; and
b. Oil system monitoring of engines in service from which a S/G was removed to rectify a reported engine starting or electrical generation defect that was confirmed to be caused by the S/G.'
- Sent an advisory letter on 5 July 2001, to all Certificate of Registration holders of Pratt and Whitney Canada PT6A turboprop powered aircraft. The letter advised the following:
'CASA investigation into reports of 17 in-flight shut downs (IFSD) of PWC PT6A-60 series engines in the world fleet, 5 in Australia, has determined those defects resulted from electrical discharge damage (EDD) to the engine number 1 bearing. The electrical current source is the engine starter-generator (S/G) TRW Lucas models 23078 and 23085.
CASA has determined electrical current can flow to the engine from the S/G armature output shaft through armature leakage or an electrical short of the armature. The most probable cause of armature leakage is an accumulation of brush dust. The TRW S/G Maintenance Manual (M/M) 23700 includes periodic field cleaning and resistance checks that will prevent brush dust build up and detect a decrease in armature leakage resistance and a hard short.
Enclosed is further background information relating to this subject.
Unfortunately, the aircraft manufacturer's maintenance schedules for aircraft types known to have experienced PT6A EDD do not reflect that detailed in the TRW Lucas M/M 23700. As such, the aircraft manufacturer's instructions for continuing airworthiness for those particular aircraft are deficient in respect of S/G maintenance.
Whilst there is no evidence of EDD in PT6A models other than PT6A-60 series engines incorporating a TRW Lucas S/G model 23078 or 23085, CASA recommends all PT6A operators carry out a review of their elected aircraft maintenance schedule in consideration of information learnt from the CASA investigation of the PT6A-60 series IFSD events.
As of this date, CASA has no evidence to support mandatory action for PT6A powered aircraft operators other than those conducting fare paying passenger operations in PT6A-60 powered aircraft. CASA recommends that you review the enclosed information and initiate any changes to your aircraft maintenance schedule as you believe necessary to ensure the continued airworthiness of your aircraft.'
CASA has also assisted the operator to introduce the following local safety actions:
- In conjunction with the aircraft manufacturer, carried out a bonding check to ensure that an appropriate electrical discharge path was available from the starter-generator.
- Assisted the manufacturer in performing tests on starter-generators removed from service after 600 hours to determine any source of electrical leakage.
- Installed a supplemental chip detector system on each engine accessory gearbox. Prior to and at the completion of each flight, the flight crew is required, to test the chip detector to determine that no metal has bridged the chip detector probes. Every 120 hours the accessory gearbox chip detector is to be removed and inspected. The chip detector probes are to be wiped and the results from the wipe are to be sent to a power plant repair facility for analysis.
- Reduced the starter-generator overhaul period from 1,500 hours to 1,000 hours. At each 250 and 750 hour inspection the starter-generator is to be cleaned (brush dust removed) and the brushes inspected. The brushes are to be replaced at the 500 hour inspection, the starter-generator cleaned and an armature resistance check is to be carried out (to identify any path that may allow voltage to leak from the starter-generator). The allowable resistance values set by the operator are more restrictive than that recommended by the manufacturer of the starter-generator.
In addition, the aircraft manufacturer (Bombardier Aerospace, Shorts Brothers plc) has issued the following service documentation to aircraft operators:
- SB360-24-24, dated December 2000, detailing the installation of a new earthing point between the engine firewall assembly and the starter-generator, to provide supplemental bonding for the starter-generator.
- SIL SD360-IL-207, dated August 2000, detailing starter-generator removal and installation instructions and advising that operators ensure the integrity of the engine starter-generator electrical bonding.
- SB360-72-01, dated December 2000, recommending that aircraft operators carry out Pratt & Whitney Canada Service Bulletin PT6A-72-13348 and PT6A- 72-14304 within 25 flight hours of a starter-generator failure or an unscheduled starter-generator removal during the last 1,000 flight hours.
The aircraft manufacturer has also advised that they have agreed with proposed TRW Lucas modifications to electrically isolate the starter-generator output shaft from the engine starter gear.
The engine manufacturer (Pratt and Whitney Canada) issued SB's PT6A-72-14304, PT6A-72-13348 and PT6A-72-14318 on 15 Dec 2000 recommending engine oil filter patch inspections within 25 flight hours. Those inspections may detect debris in the oil system, originating from the number one bearing area.
Safety Recommendations
As a result of this occurrence, the Australian Transport Safety Bureau issues the following safety recommendations.
R20020120
The Australian Transport Safety Bureau recommends that the Civil Aviation Safety Authority continue to examine the circumstances of electrical discharge damage to the number 1 bearing of the Pratt and Whitney (Canada) PT6A engine models equipped with TRW Lucas starter-generators and develop an appropriate safety assurance strategy to ensure the continuing airworthiness of Australian registered aircraft fitted with similar engine and starter-generator combinations.
R20020121
The Australian Transport Safety Bureau recommends that the United States Federal Aviation Administration examine the circumstances of electrical discharge damage to the number 1 bearing of the Pratt and Whitney (Canada) PT6A engine models equipped with TRW Lucas starter-generators and develop an appropriate safety assurance strategy.
SAN 20020122
The Australian Transport Safety Bureau suggests that Transport Canada should note the deficiencies identified relating to electrical discharge damage to the number 1 bearing of the Pratt and Whitney (Canada) PT6A engine models equipped with TRW Lucas, model 23078 and 23085, starter-generators.
SAN 20020123
The Australian Transport Safety Bureau suggests that Pratt and Whitney Canada should note the deficiencies identified relating to electrical discharge damage to the number 1 bearing of the Pratt and Whitney (Canada) PT6A engine models equipped with TRW Lucas, model 23078 and 23085, starter-generators.
SAN 20020124
The Australian Transport Safety Bureau suggests that the United Kingdom Civil Aviation Authority should note the deficiencies relating to electrical discharge damage to the number 1 bearing of the Pratt and Whitney (Canada) PT6A engine models equipped with TRW Lucas, model 23078 and 23085, starter-generators.