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Safety Action

Summary

Local safety action

As a result of this occurrence, the Australian Civil Aviation Safety Authority (CASA) have taken the following actions:

 

  1. 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 resitance 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.'

  2. 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:

 

  1. 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.
  2. Assisted the manufacturer in performing tests on starter-generators removed from service after 600 hours to determine any source of electrical leakage.
  3. 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.
  4. 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:

 

  1. 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.
  2. 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.
  3. 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.

Technical Analysis Report 200003399

 
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