Engine General Description

    The ATF3 engine is a 4,000 to 6,000 pound thrust class turbofan engine. It has 3-spools, with a moderate-bypass-ratio, high-pressure-ratio, and an aft-mounted accessory gearbox/oil-tank. All engine accessories are mounted on the aft end of the engine under an engine tail-cone. This configuration yields a very slender Turbofan engine for its thrust class, fitting completely inside a 36-inch diameter nacelle. The ATF3's small diameter reduces aircraft frontal area induced drag, and engine installation losses. The ATF3 is readily recognizable with its unique exhaust where the engine core-exhaust exits from cascades mixing with fan discharge air in the bypass duct, and then exits rearward out an annular opening in the aircraft nacelle.

    The engine has a 2.88:1 bypass ratio at Sea Level (2.52. at 40,000 feet, MACH 0.8), and a 21:1 pressure ratio. FAN spool-speed maximum limits are 10,400 rpm for ATF3-6 engines and 10,700 rpm for ATF3-6A engines. LP (low-pressure) spool speed limit is 17,200 rpm for all ATF3 engines, and the HP (high-pressure) spool speed limit is 36,900 rpm for all ATF3 engines. The ITT (inter-turbine temperature) maximum limit is 1850 degrees Fahrenheit (1010 degrees Celsius) for all ATF3 engines. Transient limits for the ATF3 engine are 102 percent N2, 103 percent N3, and 1868 degrees Fahrenheit (1020 degrees Celsius) ITT. Transient overspeed for the FAN spool is not allowed.

Engine Spools and Main-shaft Bearings

    The three engine spools contain a total of seven compressor stages (six axial and one radial) and six turbine stages (all axial). Two main-shaft bearings, one ball-bearing to react rotor thrust loads and one roller-bearing support each spool. The six main-shaft bearings are housed in four bearing-sumps, sealed by carbon-face-seals that are pressurized with “buffer-air” from the outside by secondary (backup) labyrinth seals.  The #4-5 Bearings are sealed by carbon ring seals. The bearings, and in some cases carbon seal rotors, are oil-pressure lubricated and cooled with oil.

Fan Spool

    The Fan Spool has three versions of a single-stage 30-inch diameter mid-spanned axial-flow fan (the pre certification “C Fan”, and the post certification “CD Fan” with three strengthening ribs outboard of the mid-span and the blade leading edges “clipped” at the blade tip, and the “E6 Fan”).  The Compass Cope and Tacit Blue aircraft were the only aircraft to use ATF3-6 engines with the 36-Blade 1.5 pressure ratio “C Fan.”  The USCG ATF3-6 engines have a 36-blade 1.5:1 pressure ratio “CD Fan”, and the Commercial and French Navy ATF3-6A engines have a 30-blade 1.6:1 pressure ratio “E6 fan.” The fan spool incorporates an ice-shedding conical-spinner that does not require performance-robbing anti-ice heating.  All ATF3 engines use a three-stage tip-spanned axial-flow turbine to drive the Fans.  The three turbine stages have tip-shrouded blades and are identical for all engine models, except for part numbers and cycle life limits required by the higher operating speeds of the “E6 Fans.” The 2nd, 3rd, and 4th stages of the fan turbine are cooled with low-pressure compressor discharge air, except for the aft side of the 2nd stage that is cooled with high-pressure compressor discharge air.

Low Pressure Spool

    The low-pressure spool has a 5.5:1 pressure-ratio, five-stage axial-flow LPC (low-pressure compressor) with variable geometry IGV's (inlet-guide vanes) in front of the 1st stage rotor, and a two-stage tip-spanned axial-flow LPT (low-pressure turbine). The LPC and LPT blade-tips and knife-seals have abradable tip shrouds and seal rings for minimum running clearances and maximum efficiency. The LPT is cooled with LPC discharge air. The IGV angle is positioned by the EEC (electronic engine control) allowing the LPC to operate surge free at maximum efficiency and pressure ratios throughout the entire engine-operating envelope.

High Pressure Spool

    The high-pressure spool has a 2.6:1 pressure-ratio, single-stage radial-flow HPC (high-pressure compressor), and a single-stage axial-flow HPT (high-pressure turbine), both with abradeable shrouds. The HPT is cooled with HPC discharge air. The relatively small 9-inch diameter of the HP spool compared to the much larger diameter FAN and LP spools results in unique engine acceleration and deceleration characteristics. These will be discussed later under "ATF3 Operational Characteristics.”

Combustion Section

    The ATF3 engine has an annular combustor with eight dual-tipped atomizers. Each atomizer has two fuel-nozzle tips, each with a primary orifice, and a secondary orifice. A fuel-flow-divider in each atomizer body blocks the secondary fuel-passage routing all fuel through the primary fuel-passage to the smaller primary orifices for good fuel-atomization during engine starts. As the engine approaches idle-speed the flow-divider opens supplying fuel to both passages and the primary and larger secondary atomizers reducing fuel pump discharge pressures at higher engine powers. Each atomizer has a check valve that closes at engine shutdown (about 5 psid) preventing fuel from dripping onto and coking the atomizer tips, or out of the combustor drain onto the flight-line ramp. The combustor also has two electronic igniters that are controlled by the EEC and power lever position during engine starts. These igniters are also turned on manually by the pilots during takeoffs, landings, and when operating in in-climate weather to protect against any potential engine flameout.

Accessory Gearbox Section

    ATF3 engine accessory gearboxes are identical with the exception of the material used for the gearbox housings.  All military ATF3-6-2C/4C engines and the French Navy Gardian ATF3-6A-3C use Aluminum for its light weight and superior corrosion resistance.  Commercial ATF3-6A-4C engines use Magnesium for weight considerations as these engines do not normally operate in the highly corrosive low altitudes over the Earth’s Oceans.  The only difference in the accessories used on military aircraft engines is a step-down gearbox and second hydraulic pump mounted on the aft surface of the Permanent Magnet Generator (PMG) to drive an additional AC generator located in the aircraft tailcone.  All other accessories are identical for all ATF3 engine models.

Lubrication System

    The ATF3 engine has a regulated oil pressure system. The oil tank is integral with the engine cast aluminum (for military engines) or Magnesium (for commercial engines) accessory-gearbox located on the aft end of the engine and driven by a high speed quill shaft splined into the high-pressure spool.

    A BPV (breather pressurizing valve) maintains gearbox/oil-tank and engine sump pressurizes at or above 4.7 psia above 24,000 feet, to maintain scavenge pump efficiency and prevent oil frothing. 

    The MOP (main oil pump) is mounted on the accessory-gearbox aft surface at the 7 o’Clock position and has one oil pressure element (2 g-rotors) and four oil scavenge elements (6 g-rotors).

    An Oil pressure regulator is located near the center of the gearbox/oil tank aft surface. The regulator is adjustable and normally set to 72 ±2 psig (cruise oil pressure limits are 65 to 82 psig). The engine oil pressure (cockpit gage) fitting is also at this location. 

    The MOP (Main Oil Pump) delivers hot oil from the engine oil-tank to the FOH (fuel/oil heater), used to prevent fuel filter icing. From the FOH the oil travels to an oil-temperature-control-valve used for rapid oil warm-up, and clogged oil cooler bypass. Hot oil is routed from the temperature-control-valve to the surface AOC's (air-oil-coolers) located in the fan bypass duct, to the FOC (fuel-oil-cooler) located in the oil tank, then the oil filter (located in the bottom of the gearbox/oil tank). From the oil filter cooled oil is routed to the oil-pressure-regulator, then to the oil jets, engine bearings, and seals. Only 20 percent of the cooled oil is required for lubrication, the other 80 percent being used to cool the bearings and seals. Hot oil from the bearing-sumps is returned by the MOP scavenge elements past an oil-temperature-pickup and chip-detector back to the oil tank.

    The oil-filter location just prior to the engine oil jets protects them from clogging by any engine-generated or foreign debris. Locating the chip-detector and oil-temperature-probe in the oil scavenge return line will indicate any potential bearing or seal failure by a chip indication and/or rapidly climbing oil temperatures.

Engine Controls

    The ATF3 engine uses an airframe mounted EEC (electronic-engine control) and an engine mounted hydro-mechanical FCU (fuel control unit) attached to a positive displacement MFP (main fuel pump) mounted on the lower right hand aft surface of the gearbox assembly. The EEC has a PMG (permanent-magnet-generator) dedicated power source for normal operation, and uses 28-vdc aircraft buss voltage in the event of a PMG failure. The MFP has a fuel filter, a filter bypass-valve, a maximum fuel pressure regulator, and a temperature control valve. The temperature-control-valve prevents fuel-filter icing by routing cold fuel to the FOH (discussed earlier in the lubrication system section). The MFP also provides 400-psid motive-force fuel-pressure to the IGV actuator mounted, on the aft surface of the gearbox. IGV position is controlled by the EEC in normal mode, and by power lever position in manual mode.

    The hydro-mechanical FCU fuel-delivery is controlled by the EEC and Pcd (pressure, compressor discharge) in normal mode, and has a backup mechanical-governor for overspeed protection and engine control in case of EEC faults or malfunctions.
The EEC monitors pilot inputted power demands and all engine-operating parameters. For engine steady state operation, the EEC controls the engine N1C2 (fan spool speed corrected for non-standard-temperature) and non-standard-pressure in response to pilot inputted power-lever-position. The EEC controls FCU fuel-delivery, IGV position, and SBV (surge-bleed-valve) position to maintain all engine parameters within acceptable operating limits. The EEC also has "fault-monitoring-circuitry" to sense malfunctions (out-of-limit-conditions), and if detected removes power from the EEC, reverting to the FCU mechanical-governor for continued engine control.

    In aircraft climb and cruise operation the EEC automatically adjusts N1C2 (fan spool speed corrected for non-standard temperatures) to hold a constant engine ITT providing "locked throttle" climb and cruise. This feature prevents undetected engine over-temperature greatly reducing pilot workload required to hold engine ITT while climbing or flying through changing air temperatures. The EEC also monitors all engine spool-speeds, spool-speed-matches, and engine ITT preventing limit exceedance. The EEC has "fault-monitoring-circuitry" to monitor all input and output devices, and if a fault is detected it will cut 28 vdc electrical power to the EEC and revert to FCU manual (backup-mode) control.

    During engine transient operation, the engine accelerates and decelerates on N2C2 (low-pressure-spool-speed) and N3C2 (high-pressure-spool-speed) speed match. During engine acceleration the EEC controls IGV position to prevent LPC surges and fuel delivery to prevent HPC surges. During engine deceleration the EEC controls IGV position, opens the engine surge valves, and controls fuel delivery to improve engine deceleration and prevent LPC surges.

    An altitude "N1 rated acceleration" of 3%/Sec was added to EEC PN's 2101484-13 & -52 and subsequent to slow engine acceleration above 18,000 feet improving engine controllability for the smaller power adjustments required in-flight.

ATF3 Operational Characteristics

    With 1966 technology, the ATF3 engine required some unique tradeoffs to obtain the desired power and efficiency in an acceptable engine size and weight package. To keep engine diameter, length and weight down a radial-flow high-pressure compressor was selected. As a concentric three-spool engine with a radial-flow compressor would be unacceptably large in diameter, a concentric Fan and HP spool with an aft mounted HP spool design was selected.  Counter rotating spools were used to reduce engine acceleration and deceleration torque effects. The tradeoff's resulted in an acceptable engine package, but not without some consequences. The relatively small HP spool accelerated and decelerated more quickly than the larger diameter Fan and LP spools initially resulting in speed match and engine surge problems. As the spools are aerodynamically matched, this required precise turbine nozzle area’s and a more complex control system.

    To overcome LPC surge problems, the IGV's were not allowed to open past +34 degrees until the LP spool speed was greater than 81 percent N2C2 (approximately 50 percent fan spool speed). Acceleration fuel flow was also limited in this range to prevent a HPC surge. This is why the ATF3 engine accelerates more slowly than competitors below 50 percent N1 (fan speed). Once above 81 percent N2C2 (approximately 50 percent N1 speed) the IGV's begin to open rapidly increasing engine core-airflow and fuel-flow resulting in very rapid engine acceleration (usually less than one second from 50 percent N1 to maximum power).

    To overcome deceleration LPC (Low Pressure Compressor) surge (Stall) problems, two LP SBV's (surge bleed valves) were added in the crossover duct just aft of the LPC discharge and forward of the exhaust cascades. To prevent engine surges during deceleration, the EEC (Electronic Engine Control) rapidly drives the IGV's closed to the +40 degree position and opens the LP SBV's. The fuel command is then reduced to the EEC USG (under-speed governor) deceleration schedule and then the MIN (minimum) schedule for rapid surge free engine deceleration.

    EEC IGV positioning to maximize LPC efficiency across the entire engine operating envelope while protecting the LPC from surges is why the ATF3 engine is so responsive to power-lever movements above 50 percent N1, but requires a little more planning by the flight crew below 50 percent N1. Moving the power lever rapidly will not change the engine acceleration or deceleration rate below 50 percent N1. With the exception of very-very-slow power lever movements, the EEC internal schedules control the engine rate-of-acceleration and deceleration. The power lever position only determines the N1C2 (fan spool speed corrected for non standard temperatures) where the EEC will stop the acceleration or deceleration.

    On engine snap accelerations a N1 (Fan spool speed) split of less than 2-percent can result in one engine reaching N1 of the day (Takeoff Power) while the other engine is at less than 60-percent N1.  This will result in severe yawing of the aircraft requiring pilot inputs to maintain directional control.  The best method of reaching takeoff power a quickly as possible is to advance the power levers half way up the quadrant (above 40-percent N1) while lining up on the active runway, then as soon as both N1’s are above 50-percent move the power levers smoothly to maximum (about a one second push).    

    As stated in "Engine Controls" above, an altitude "N1 rated acceleration" of 3%/Sec was added to EEC PN's 2101484-13 & -52 and subsequent to slow engine acceleration above 18,000 feet improving engine controllability for the smaller power adjustments required in-flight.


    Once flight crews have mastered the ATF3 engines operating characteristics, the AMD Falcon Jets it powers can be an absolute delight to fly. The ATF3 is a fairly complex engine, and requires maintenance by qualified personnel trained either by the AlliedSignal Aerospace Academy (in Phoenix, Arizona) or U. S. Coast Guard / French Navy maintenance personnel. When properly maintained, it is a unique and very reliable engine. The U. S. Coast Guard regularly operates its HU25 aircraft in extremely hostile conditions with the confidence that when maintenance personnel say the "aircraft is ready to fly", it will complete its mission and bring its crews back safely. In the ATF3's operational history since introduction in service 1971, the only serious injury was a fatal accident where a man was ingested by an engine during a high power ground run.


updated 4/19/2010

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ATF3 Engine Overview

by: John C. Evans

Disclaimer:  The specifications contained in this article are intended to be representative of the engine design only, and ARE NOT TO BE USED TO MAINTAIN ANY GARRETT /  HONEYWELL ENGINES.  Refer to current engine and aircraft manufacturers maintenance and service manuals for all specifications and maintenance recommendations for the engines depicted in the ATF3 Online Museum.