ATF3 Silver Bullet Modifications

By: John C. Evans


The “Silver Bullet Modifications” page was created not because it was the only or most productive ATF3-6 Turbofan improvements, but because it is one of the most recognizable by the end users.

This page is dedicated to all of the programs the Garrett Corporation initiated to improve ATF3 Turbofan Engine reliability, durability, customer acceptance, and to the individuals instrumental in making them happen.  The “Silver Bullet Modifications” was only one aspect of the ATF3 engine improvement programs.  The bulk of the engine modifications, improvements, and revised training programs that I will address on this page ran in the background, and were only noticeable to the end users when presented in Technical Training, the “Advisory Board Meetings” or as improved engine durability, reliability, and as Technical and Program Support.


Garrett Management and Sales Engineers Identify Problems and Corrective Actions.

    ATF3 Program Manager Earl Cummings and Sales Engineer George Purpura made the decision to take on and correct AMD Dassault and Flacon 200 operator complaints about ATF3 engine durability and cost of ownership issues.  Earl and George decided what was needed was a fast action team of ATF3 engine experts to immediately dispatch to any facility with an ATF3 engine problem.  Several names for the team were considered before settling on Falcon 200 Aircraft Support Team as the name with the most acceptable acronym “FAST.”  It was decided that the primary team would be made up of five Garrett Personnel, Field Service Engineering Manager John Elam, two Engineers from Project Engineering, and two Field Service Engineers.  John C. Evans and Gary Saarup were picked from Project, and Jerry Palyash and Homer Shroma were picked from Field Service Engineering as initial team members.  Many more talented personnel from the Field Service Engineering, Overhaul & Repair, Technical Training, Manuals & Bulletins, and Site B would ultimately be contributors to the success of the FAST program.


Garrett Performance Restoration & FAST (Falcon Aircraft Support Team).

    The Performance Restoration program and FAST team were both highly successful programs for the Garrett AlliedSignal Corporation.  They addressed and and corrected almost all on the customer and AMD Dassault, Falcon Jet Corporation, and Operators concerns about the ATF3 engine and its durability.  These programs didn’t have a positive beginning or end but just flowed from one problem to another taking them in stride and resolving them to the satisfaction of all concerned.  In fact many of the same persons worked on both programs.


    The FAST team had addressed and correct a number of the issues brought up by the ATF3 Advisory Board including;

  1.     Heavy salt air corrosion of the ATF3 inlet and compressor section on USCG HU-25 aircraft.

  2.     Unavailability for aircraft dispatch due to engine operational problems and unscheduled engine removals.

  3.     Most problems were not repairable at Hangar Facilities requiring unscheduled engine removals and bank engine rentals to continue to operate their aircraft.

  4.     High oil consumption and main shaft carbon seal leaks.

  5.     Aircraft Cabin noise from Fan unbalance and vibration.

  6.     Low frequency beat noise in the aircraft cabin from miss-matched N2’s (low pressure spool speeds) at matched N1’s (fan spool speeds).  Cabin beat noise could not be eliminated with either N1 or N2 synchronizing.

  7.     Wet socks smell in the cabin when flying in rain or through clouds.


    Garrett Project Engineering and Repair & Overhaul personnel realized that the ATF3 still had hardware durability issues to resolve, and they had the personnel from the afore mentioned programs in place to resolve these new issues.  Some of the largest were turbine section hardware temperatures in excess of 1300-degrees Fahrenheit (hot enough to melt steel), and combustion and oil residue recirculation into the engine gas path upstream of the customer bleeds.  These and many other issues had been gathered into one group of engineering changes that would be referred to as “The Silver Bullet Modifications.”  the intent of the Silver Bullet Modifications was to address and eliminate all remaining ATF3 problem areas.


    Assistant Project Engineer A. W. (Fred) Fuller took charge of the ATF3 modification “Silver Bullet” program in Project Engineering.  with the two previous programs under his belt, Fred was on a tear to make the ATF3 engine into the best engine that was possible.


Garrett ATF3 Silver Bullet Modification program.

    By late 1986 Fred Fuller had been compiling a list of all of the known problem areas in the ATF3 engine, and picking the brains of everyone he could think of including Site B Engineers Jerry Steele, John Huber, and Bob Lawrence, and Facility Managers Bob Kimes & Bob Scurfield.  In 1987 a new University of Florida graduate Kurt Lammon came on board to round out the ATF3 project “Silver Bullet” team to four members, ATF3 Engineering Manager Fred Fuller, ATF3 Engineer Greg Hansen, ATF3 Jack of all Trades John C. Evans, and ATF3 Design and Test Engineer Kurt Lammon.  These four engineers (with inputs from Site B engineers John Huber, Jerry Steele, & Bob Lawrence) were the core team members that that created the silver bullet modifications.


    Kurt would later say that his assignment to the ATF3 Silver Bullet team was the best thing that could have happened to him.  Engineering Graduates fresh out of college are normally spend several years doing small tasks for more senior and experienced engineers, but this was not to be for Kurt.  Kurt was immediately given the task of designing and redesigning development hardware for the ATF3 engine.  Kurt had oversight from other team members to assure his success, but the designs were largely Kurt’s.  Kurt’s responsibility on the Silver Bullet Team did not end with the design of development hardware.  Kurt was given a development ATF3 engine to install and test his hardware at Site B CA.  Kurt also participated in every aspect of the program including the analysis of the test data, and any required redesign and additional testing.  In Kurt’s four years on the ATF3 program he got to do it all.  Most engineers will spend an entire career gathering the experiences that was exposed to is his short career with Garrett/AlliedSignal.  After four years kurt decided to leave Garrett and return to college to get his Masters Degree.   Kurt was gone before his contributions to the Silver Bullet Modifications were introduced into the ATF3 fleet.


    To say that Fred Fuller got his moneys worth out of Kurt Lammon would be a gross understatement.  Very few new hire engineers receive the opportunities that Kurt had on the ATF3 Silver Bullet team.  Even fewer have the success that Kurt had.  Although kurt’s contributions to the success of the ATF3 Silver Bullet Modifications were substantial it was the entire teams efforts and contributions that resulted in the success of the program.


ATF3 Silver Bullet Modifications

     The Silver Bullet modifications were primarily directed at lowering turbine section metal temperatures, improving ATF3 engine performance and durability, resolving main shaft carbon seal leakages, fixing gearbox lip seals, and improving the cooling efficiency of the turbine section cooling passages, many other fixes were also incorporated.  the creative use of Ceramics helped reduce metal temperatures as much as 300-degrees Fahrenheit.  Some of the Silver Bullet changes were;

  1.     The common secondary cooling passages were split into two paths, one for the cold section (compressors) and one for the hot section (turbines).  This change resulted in cooler “buffer air” directed to the main-shaft carbon seals, and prevented combustion products from entering the engine core gas path upstream of the customer bleeds.

  2.     Repositioning the secondary cooling air intakes to the leading edge of the crossover duct struts just downstream of the LPC discharge resulted in higher total air pressure into the cold section secondary cooling passages.

  3.     The high pressure turbine blade tip shroud was changed from a two piece single piston ring design into a three piece two piston ring design.  Removing the piston ring(s) from the blade tip shroud eliminated uneven cooling of the shroud and a severe coning that had caused substantial blade tip wear.  The blade tip shroud was also ceramic coated on the inner diameter resulting in a more uniform blade tip clearance and improved performance.

  4.     The 2nd, 3rd, and 4th stage fan turbine nozzle tip shrouds were also ceramic coated on the inner diameter resulting in cooler metal temperatures and a better control of the turbine rotor tip seal clearances.  The addition of a Ceramic coating to the blade tip shrouds required reduction of the blade tip knife seal diameter to maintain proper seal clearance.

  5.     A cooling shroud was added to the fan turbine stators to keep the cooling air close to the stators improving cooling efficiency.  This fan turbine cooling shroud directed air past the fan turbine nozzles into the 5th stage low pressure turbine nozzle hollow air cooled stator vanes.

  6.     The four step 4th turbine knife seal was replaced with a two step (still four paired-knives) eliminated the locked fan problem just after shutdown.  The new design allowed substantially increased “thermal travel” of the knife seals across the much wider honeycomb shrouds.  The new design allowed the Knife seal clearance on the 12-inch diameter seal to be reduced from 0.018-inches to 0.008-inches.  This one change reduced leakage of this seal and reduced the engines ITT by 18-degrees Fahrenheit (10-degrees Celsius).  An increased engine ITT margin of 10-degrees Celsius from one knife seal redesign is huge!

  7.     The high pressure compressor shroud was re-contoured to better match the high pressure compressor blade tips when the engine was operating and high speeds.

  8.     Dual lipped main gearbox Teflon lip seals were incorporated into the design.  The added outer lip kept dirt and debris from damaging the sealing lips.

  9.     Secondary seals inside the carbon seals were changes to low-swell materials to prevent them from binding, resulting of more effective pressure balancing and sealing of the carbon face seals.

  10.     Numerous other small improvements were incorporated into the Silver Bullet Modifications, and the ATF3 engine is continuing to be monitored for improvements to this day.


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Created on: 8/15/2008, Last updated: 12/28/2008


Please email me with any suggestions or corrections to this page.