Pratt & Whitney Update
Recently there have been some concerns at Pratt & Whitney regarding information getting circulated throughout the industry. While every effort is made to distribute all current information to operators and maintainers, sometimes information may be missed or not utilized correctly. Service Bulletins (SB) and Service Information Letters (SIL) are the primary means of conveying information by manufacturers. Pratt and Whitney have allowed access to their SB and SIL listing through their web site (requires a logon). (www.pwc.ca/en/service-support/technical-publications)
Two items that were requested to be brought to the attention of affected operators are:
SIL # 200-050 (Automatic Low Cycle Fatigue (LCF) Counting Display). This SIL affects engine models PW206B and PW206B2 installed in some Eurocopter helicopters.
- SIL # GEN-113 pertains to PT6 1st stage carrier bolt fracture. This is an ongoing concern that was addressed previously by overhaul manual updates in 2008 and 2010 as well as service difficulty advisory 10 July 2008. All PT6 overhaul facilities will need to be aware of the procedures outlined therein.
On a final (and more generic) note we are reminded to ensure the fuel we are using is contamination free. SIL GEN-091 and SIL GEN-108 provide useful information regarding fatty acid methyl esters. Specific limits are set regarding concentration levels and procedures if levels are exceeded.
When Transport Canada needs to convey non-mandatory safety information, a Civil Aviation Safety Alert (CASA) is issued. These CASAs may be accessed at Transport Canada’s web site: (http://www.tc.gc.ca/civil-aviation-safety-alert). These are formerly service difficulty advisories and service difficulty alerts.
Dual Hydraulic System Failure
During flight, the aeroplane experienced a complete loss of hydraulic fluid quantity and pressure from the right-hand engine driven pump (EDP). Shortly after the flight crews’ acknowledgement of the lost of hydraulic pressure, it was noted that the opposite left-hand EDP hydraulic system pressure was well below its operational limit, also disabling its function.
The crew managed to land the aeroplane despite the complete hydraulic failure. Upon maintenance investigation, it was determined that a failed non-return valve (NRV) allowed the remaining operational left-hand hydraulic pump pressure to “bleed-off” into the inoperative right-hand pump system, draining the complete system pressure.
The type design of BAE 3112 hydraulic system is such that both EDPs feed into a single hydraulic system. Therefore the two EDPs share the hydraulic demand of the complete system and provide the sufficient fluid flow and pressure to support the required operations of the aeroplane.
With this type of design, there comes the requirement to isolate the two EDPs from each other in case of a single pump failure. This is done through the installation of two NRVs downstream the pressure output of each EDP.
The NRVs are a fully mechanical check-valve style unit which simply allows hydraulic flow in a single direction. With each valve being positioned down-stream and in series with their respective EDP pressure output hydraulic line, the possibility of a reverse-flow condition that can “bleed-off” and disable an operative EDP is removed.
With the event described above, the right-hand EDP pressure output supply line ruptured, draining its fluid and pressure overboard. Coupled with the dormant failure of the associated NRV, the left-hand < abbr title="engine driven pump">EDP hydraulic pressure was also lost.
Through the co-operation with BAE engineering, scheduled maintenance task TR 29/5 and TR 29/6 have been issued to check the function of the NRV downstream of each EDP.
The maintenance schedule is being amended to call up the new task at a D-check interval for low utilization aircraft, and at a 4000 flight hour interval for normal utilization aircraft.
Transport Canada Civile Aviation is advising all BAE 3112 operators, owners and maintainers of this possible dormant NRV failure condition and the newly released maintenance task TRss 29/5 and 29/6.
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