ATF3 Performance Recovery/Restoration

By: John C. Evans

This page is dedicated to the Garrett Performance Restoration Program addressing the high ATF3 Turbofan Engine performance rejection rates during engine Repair & Overhaul, and to the individuals instrumental in restoring performance margins.

Garrett R&O performance rejects reach unacceptable levels.

     In 1984 the Garrett Repair and Overhaul shop was having trouble making performance and shipping ATF3 engines.  Many engines had three or more performance rejects before shipping with barely acceptable performance margins. 

Author of this article transfers from Site B Torrance CA to Phoenix AZ

    On April 1, 1984 (April Fools Day) John C. Evans transfered from Site B, Torrance CA to Phoenix AZ Project Engineering.  Shortly thereafter I was invited to a meeting at Garrett R&O in the 601 building.  The people present at this meeting were Bob May (R&O Engineering Manager), Dale Rogers and Scott Seale (both TFE731 & ATF3 Overhaul Engineers) and myself.  Early on this meeting took on the flavor of a job offer.  I had worked on the ATF3 since December 2, 1968 (for over 13-years) as laboratory Technician, created the first ATF3 measurements forms and assembly instructions, and worked in Tool Design and Engineering supervising ATF3 engine testing.  Phoenix AZ Repair & Overhaul had a long working relationship with Site B Torrance CA and recognized the talent and expertise of the Site B staff.  I had fallen into their lap, and they wanted to solicit my aid in a performance recovery program.  As I was new to the Project Engineering group I declined their offer, only to find myself on semi-permanent loan to them shortly thereafter.

ATF3 Turbofan engine Performance Restoration Team formed.

    Garrett R&O forms a multi-disciplined performance recovery team to quell the abnormally high performance rejection rate of Overhauled or Repaired ATF3 Turbofan Engines.  This team was made up of seven people, Bob May (Manager), the author John C. Evans (ATF3 Project Engineering), Dale Rogers & Scott Seale (R&O Engineers), Zee Zubia (Material Expediter), Joe Kavanaugh (Manufacturing Engineer at the hardware repair shop) and Rick Burkmier & Bob Rittenhouse (both R&O Mechanics).  This team was to interface closely with Don Tyler and Fred Fuller (ATF3 Project Engineering).

    In addition the Maintenance Service Plan (MSP) administrator Mary Fredette worked closely with the performance recovery team from the outset to financially cover the performance improvements.  It was recognized early on that all of the changes in the world would not make a difference until they were incorporated into the fleet.  It was costing the Garrett MSP plan $5-million a month to support the ATF3 fleet, and it was cheaper to give the fleet the fixes than continue to bleed money. 

    The first task of John C. Evans was to identify performance critical hardware, with the aid of Dale Rogers and Scott Seale.  One of the first things we discovered was that no two engines were alike.  They all had different speed matches, and different performance margins (the margin between where the engine operates and its maximum limits).  We quickly identified the problems as compressor efficiencies, compressor discharge leakage (internally and overboard), and the engines thrust margins and spool speed matches.  It would not only be necessary to rework some hardware dimensions back to new production limit, but it would also be necessary to mechanically rework the A2 (2nd stage turbine nozzles) and A5 (5th stage turbine nozzles) to nominal area’s determined by the resulting engine speed matches and performance margins. 

    The area’s of performance loss were quickly identified as compressor efficiency loss from worn knife seals and blade tips and shrouds, poor spool speed matches (from non uniform turbine nozzle areas), and loss of compressor discharge air through turbine seals and secondary cooling passages.  In less than nine months the team resolved the ATF3 performance issues and passed ten engines in a row with no performance rejects.  The ATF3 engine ITT ship limits were increased to assure that the performance increases and resulting longer engine life would remain.

    Once the critical hardware was identified Dale Rogers created a “Green Memo” (later a Blue Memo) identifying the critical components and areas that required reworks back to “new production limits.”  Team members would then have the identified rework routings modified to revise the identified critical dimensions.  Manufacturing Engineer Joe Kavanaugh would follow the critical hardware through the repair and inspection process to assure compliance with the routings.  Zee Zubia would interface with all parties to assure adequate flow of reworked hardware to meet overhaul and test requirements.

    Within nine-months an ATF3 Overhaul performance rejection average of three-rejects per engine had been resolved with ten consecutive ATF3 engines passing performance without any rejections.  These engines not only passed but passed with greater margins than when they shipped as new engines.  To assure that the performance margins did not decrease with time the Test Instruction ITT (T8 temperature) margin was increased to three-times the new production engine limits and the thrust margin limits were tightened to be between 50 and 100 LbFn (pounds thrust) greater than minimum takeoff requirements.  the ATF3 was unusual as when inservice engine ITT margins were decreasing the engine thrust margins were actually increasing (the engine was becoming more powerful).

    The Lessons Learned by the performance restoration team are clear.  1) Performance is recovered by improving the efficiency of airflow through the engine, especially the engine core.  2) Any core airflow lost through overboard leakage or excessive internal leakage in the secondary cooling passages of the engine core will result in engine inefficiencies and lost performance.  3) Any increase in compressor or turbine blade tip or knife seal clearances reduces efficiencies.  4) Any compressor discharge air that leaks into the engine turbine section through secondary cooling passage reduces cooling flow in the combustor and ITT Probes resulting in higher inter-turbine temperatures (ITT) and performance rejects.  5) Properly matched engine spool speeds with selected turbine nozzle areas are critical for maximum engine efficiency.  6) The dimensional quality of the hardware used to build or repair the engines will determine if they will have good performance, or will fail to pass the test cell run.  7)  Proper rigging of the engine Inlet Guide Vanes (IGV’s) is critical to the Fan (N1)  vs. Low Pressure Spool (N2) speed match and engine ITT margin.  8) The key to good engine performance is the control of engine hardware quality, PERIOD!

    To my total amazement I received an individual Engineering Award for what was a team effort (passing ten consecutive ATF3 engines with no test cell performance rejects).  I could not have accomplished the ATF3 engine performance restoration without the support and dedication of every person on the team, and they all deserved to be recognized for their contributions to the success the team achieved.

ATF3-6-2C Turbofan engine corrosion and performance problems.

    Shortly after the first 100-hours of engine operation in the U.S. Coast Guard HU-25 aircraft the ATF3 engines started to have a high premature removal rate due to loss if ITT margins (loss of performance).  Tear-down inspections of USCG engines removed and returned to Garrett Repair & Overhaul exhibited heavy corrosion and loss of Low Pressure Compressor (LPC) shroud (Silicone/Aluminum) abradeable material resulting in loss if low pressure compressor (LPC) efficiency and performance.  The cause was determined to be salt laden ocean air corrosion.  The short term fix was for the USCG to water wash the engine core with a soap solution after after they washed the airframe, to be accomplished after low over the water operation.  This procedure doubled the engine life to about 200-plus hours while Garrett worked on a more corrosion resistant bronze-polyester compressor shroud material. 


Created on: 8/21/2008, Revised 6/9/2009

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