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ATF3
Production Engine Performance Ratings and Aircraft Applications
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Engine Model |
Thrust Rating Cold Day |
Thrust Rating Hot Day |
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ATF3-All |
6000 Maximum |
1850 Deg F ITT (1010 Deg C) |
Maximum Certified Thrust and ITT for all ATF3 Engine Models |
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F104-GA100 |
4050 to 86 Deg F |
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Teledyne-Ryan Compass-Cope YQM-98A |
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ATF3-6-2C |
5440 @ 59 Deg F |
5050 @ 76 Deg F |
U. S. Coast Guard HU25-A (F20G) |
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ATF3-6-4C |
5440 @ 59 Deg F |
4747 @ 86 Deg F |
U. S. Coast Guard HU25-A (F20G) |
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ATF3-6A-3C |
5440 @ 59 Deg F |
5050 @ 86 Deg F |
French Navy Gardian (F20H) |
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ATF3-6A-4C |
5200 to 80 Deg F |
5050 @ 86 Deg F |
Falcon 200 (F20H) |
by John C. Evans
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 tailcone. 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 36,350 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 Mainshaft Bearings
The three engine spools contain a total of seven compressor stages (six axial and one radial) and six turbine stages (all axial). Two Mainshaft-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 from the outside by secondary (backup) labyrinth seals. The bearings, and in some cases carbon seal rotors, are oil-pressure lubricated and cooled with oil.
Fan Spool
The Fan Spool has a single-stage 30-inch diameter mid-spanned axial-flow fan and a three-stage tip-spanned axial-flow turbine. ATF3-6 engines have a 36-blade 1.5:1 pressure ratio fan, and ATF3-6A engines have a 30-blade 1.6:1 pressure ratio fan. The fan incorporates an ice-shedding conical-spinner that does not require performance-robbing anti-ice heating. The three turbine stages have tip-shrouded blades and are identical for all engine models, except for part numbers and cycle life limits. 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), 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 FAN and LP spool diameters result 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 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 inclimate weather to protect against engine flameouts.
Lubrication System
The ATF3 engine has a regulated oil pressure system. The oil
tank is integral with the engine cast aluminum (for ATF3-6 engines) or Magnesium
(for ATF3-6A engines) gearbox located on the aft end of the engine and driven
by a quill shaft splined into the high-pressure spool.
A BPV (breather pressurizing valve) maintains gearbox/oil-tank and engine
sump pressurizes to 4.7 psia above 24,000 feet, to maintain scavenge pump
efficiency and prevent oil frothing.
The MOP (main oil pump) with one pressure element (2 g-rotors) and four scavenge
elements (6 g-rotors) is mounted into the aft lower left side on the gearbox.
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 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 for cooling.
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 oil jets protects them from clogging
by any engine-generated-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/cruise operation the EEC adjusts N1C2 to hold a constant
engine ITT providing "locked throttle" climb and cruise. This feature
prevents undetected engine over-temperature greatly reducing pilot workload.
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 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 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. 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. This required a more complex control system.
To overcome acceleration 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 surge problems, two LP SBV's (surge bleed valves)
were added in the crossover duct just forward of the cascades. To prevent
engine surges during deceleration, the EEC 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) schedule and then the MIN (minimum)
schedule for rapid surge free engine deceleration.
This 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-slow
power lever movements, the EEC internal schedules control the engine rate-of-acceleration
and deceleration. The power lever position only determines the N1C2 speed
where the EEC will stop the acceleration or deceleration.
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.
Conclusion
Once flight crews have mastered the ATF3's 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 34-year history, the only serious injury was a fatal accident where a man was ingested by an engine during a high power ground run.
updated 3/29/2002