The development of the Pratt & Whitney F100 turbofan began in August of 1968 when the USAF awarded contracts to both P & W and General Electric for the development of engines to be used in the projected F-X fighter, which was later to emerge as the F-15 Eagle. 
In 1970, Pratt and Whitney was declared the winner of the competition and was awarded the contract for the engine for the F-15. The engine was to be designated F100. Two versions of the engine were planned, the F100 for the USAF and the F401 for the Navy. The latter engine was intended for later models of the F-14 Tomcat, but was cancelled when the size of the planned Tomcat fleet was cut back in an economy move. 

The F100 is an axial-flow turbofan with a bypass ratio of 0.7:1. There are two shafts, one shaft carrying a three-stage fan driven by a two-stage turbine, the other shaft carrying the 10-stage main compressor and its two-stage turbine. For the F100-PW-200 version, normal dry thrust is 12,420 pounds, rising to a maximum thrust of 14,670 pounds at full military power. Maximum afterburning thrust is 23,830 pounds.

The F100 engine was first tried in service with the F-15 Eagle. The Air Force had hoped that the F100 engine would be a mature and reliable powerplant by the time that the F-16 was ready to enter service. However, there were a protracted series of teething troubles with the F100 powerplants of the F-15, compounded by labor problems at two of the major subcontractors. Initially, the Air Force had grossly underestimated the number of engine powercycles per sortie, since they had not realized how much the F-15 Eagle's maneuvering capabilities would result in abrupt changes in throttle setting. This caused unexpectedly high wear and tear on the engine, resulting in frequent failures of key engine components such as first-stage turbine blades. Most of these problems could be corrected by more careful maintenance and closer attention to quality control during manufacturing of engine components. Nevertheless, by the end of 1979, the Air Force was being forced to accept engineless F-15 airframes until the problems could be cleared up.

However, the most serious problem with the F100 in the F-15 was with stagnation stalling. Since the compressor blades of a jet engine are airfoil sections, they can stall if the angle at which the airflow strikes them exceeds a critical value, cutting off airflow into the combustion chamber which results in a sudden loss of thrust. Such an event is called a stagnation stall. Stagnation stalls most often occurred during high angle-of-attack maneuvers, and they usually resulted in abrupt interruptions of the flow of air through the compressor. This caused the engine core to lose speed, and the turbine to overheat. If this condition was not quickly corrected, damage to the turbine could take place or a fire could occur.

Some stagnation stalls were caused by "hard" afterburner starts, which were mini-explosions that took place inside the afterburner when it was lit up. These could be caused either by the afterburner failing to light up when commanded to do so by the pilot or by the afterburner actually going out. In either case, large amounts of unburnt fuel got sprayed into the aft end of the jetpipe, which were explosively ignited by the hot gases coming from the engine core. The pressure wave from the explosion then propagated forward through the duct to the fan, causing the fan to stall and sometimes even causing the forward compressor stage to stall as well. These types of stagnation stalls usually occurred at high altitudes and at high Mach numbers.

Normal recovery technique from stagnation stalls was for the pilot to shut the engine down and allow it to spool down. A restart attempt could be made as soon as the turbine temperature dropped to an acceptable level.

When it first flew, the YF-16 seemed to be almost free of the stagnation stall problems which had bedeviled the F-15. However, while flying with an early model of the F100 engine, one of the YF-16s did experience a stagnation stall, although it occurred outside the normal performance envelope of the aircraft. Three other incidents later occurred, all of them at high angles of attack during low speed flights at high altitude. The first such incident in a production F-16 occurred with a Belgian aircraft flying near the limits of its performance envelope. Fortunately, the pilot was able to get his engine restarted and land safely. The F-16 was fitted with a jet-fuel starter, and from a height of 35,000 feet the pilot would have enought time to attempt at least three unassisted starts using ram air.

When the F100 engine control system was originally designed, Pratt & Whitney engineers had allowed for the possibility that the ingestion of missile exhaust might stall the engine. A "rocket-fire" facility was designed into the controls to prevent this from happening. When missiles were fired, an electronic signal was sent to the unified fuel control system which supplied fuel to the engine core and to the afterburner. This signal commanded the angle of the variable stator blades in the engine to be altered to avoid a stall, while the fuel flow to the engine was momentarily reduced and the afterburner exhaust was increased in area to reduce the magnitude of any pressure pulse in the afterburner. Tests had shown that this "rocket-fire" facility was not needed for its primary purpose of preventing missile exhaust stalls, but it turned out to be handy in preventing stagnation stalls. Engine shaft speed, turbine temperature, and the angle of the compressor stator blades are continuously monitored by a digital electronic engine control unit which fine-tunes the engine throughout flight to ensure optimal performance. By monitoring and comparing spool speeds and fan exhaust temperature, the unit is able to sense that a stagnation stall is about to occur and send a dummy "rocket-fire" signal to the fuel control system to initiate the anti-stall measures described above. At the same time, the fuel control system reduces the afterburner setting to help reduce the pressure within the jetpipe.

The afterburner-induced stalls were addressed by a different mechanism. In an attempt to prevent pulses from coming forward through the fan duct, a "proximate splitter" was developed. This is a forward extension of the internal casing which splits the incoming air from the compressor fan and passes some of this air into the core and diverts the rest down the fan duct and into the afterburner. By closing the the gap between the front end of this casing and the rear of the fan to just under half an inch, the designers reduced the size of the path by which high-pressure pulses from the burner had been reaching the core. Engines fitted with the proximate splitter were tested in the F-15, but this feature was not introduced on the F-15 production line, since the loss of a single engine was less hazardous in a twin-engined aircraft like the Eagle. However, this feature was adopted for the single-engined F-16.

These engine fixes produced a dramatic improvement in reliability. Engines fitted to the F-16 fleet (and incorporating the proximate splitter) had only 0.15 stagnation stalls per 1000 hours of flying time, much better than the F-15 fleet.

In recent years, the USAF became interested in acquiring an alternative engine for the F-16, partly in a desire to set up a competitive process between rival manufacturers in an attempt to keep costs down, as well as to develop a second source of engines in case one of the suppliers ran into problems. In search of a source for an alternate engine for the F-16 and for the Navy's F-14 Tomcat, in 1984 the Department of Defense awarded General Electric a contract to build a small number of F101 Derivative Fighter Engines (DFE) for flight test. The DFE was based on the F101 used in the B-1 but incorporated components derived from the F404 engine used in the F/A-18. The Navy decided to adopt the DFE as a replacement for the Tomcat's TF30 turbofan, but the USAF announced that they were going to split future engine purchases between Pratt & Whitney and General Electric. GE was given a contract for full-scale development of its new engine, which was to be designated F110.

The General Electric F110 is similar in size to the Pratt & Whitney F100. The F110 has a three-stage fan leading to a nine-stage compressor, the first three stages of which are variable. The bypass ratio is 0.87 to 1. The annular combustion chamber is designed for smokeless operation, and has 20 dual-cone fuel injectors and swirling-cup vaporizers. The single-stage HP turbine is designed to cope with inlet temperatures as high as 2500 degrees F (1370 C). Blades are individually replaceable without rotor disassembly. An uncooled two-stage LP turbine leads to a fully-modulated afterburner. When afterburning is demanded, fuel is injected into both the fan and core flows, which mix prior to combustion.

All F110s ordered by the USAF were for the F-16 fleet, with the F-15 retaining the F100. The choice of engines for the Fighting Falcon began with the Fiscal Year 1985 Block 30 F-16C/Ds. About 75 percent of the F-16s purchased from that time on by the USAF were powered by the GE engine, with the remainder being powered by the P & W engine. However, it is not intended that individual units operate with F-16s powered by two different engine types, since that would create a spare parts and logistics nightmare. The choice of engines for the F-16 is made at the Wing level.
In an attempt to address some of the reliability problems of its engine, Pratt & Whitney developed the -220 model of its F100 turbofan. It has the same thrust as the -200, but is much more reliable, having improvements which radically lowered the number of. unscheduled engine shutdowns. Many older -200 engines were rebuilt to the -220E standard, becoming directly interchangeable with new-build -220 engines.

In an attempt to make the F100 more competitive with the General Electric F110, Pratt & Whitney introduced the more powerful F100-PW-229 version in the early 1990s. This engine is rated at 29,100 pounds of thrust with full afterburner. It has a higher fan airflow and pressure ratio, a higher-airflow compressor with an extra stage, a new float-wall combustor, higher turbine temperatures, and a redesigned afterburner. It has about 22 percent more thrust than previous F100 models. The first F-16s powered by the -229 engines began to be delivered in 1992. However, the degree of mechanical changes introduced in the -229 make it impractical to rebuild -200 or -220E engines to -229 standards.

On the export market, the higher thrust of the F110 made it the engine of choice through the mid to late 1980s. The more powerful F100-PW-229 finally gave P&W the chance of re-entering the export market. In 1991, South Korea chose the F100-PW-229 for its license-built F-16s, maintaining engine commonality with F-16Cs and Ds that were purchased earlier from the USA.

The F100-PW-200+ is intended for foreign air forces which operate significant numbers of F-16s that are powered by -200 and -220E engines, but which are denied access to the more powerful -229. It combines the core of the -220 with the fan, nozzle, and digital control system of the -229. It develops around 27,000 pounds of thrust with afterburning.

Cockpit of F16C

The AN/APG-66 is a pulse-doppler radar designed specifically for the F-16 Fighting Falcon fighter aircraft. It was developed from Westinghouse's WX-200 radar and is designed for operation with the Sparrow and AMRAAM medium-range and the Sidewinder short- range missiles. APG-66 uses a slotted planar-array antenna located in the aircraft's nose and has four operating frequencies within the I/J band. The modular system is configured to six Line-Replaceable Units (LRUs), each with its own power supply. The LRUs consist of the antenna, transmitter, low-power Radio Frequency (RF) unit, digital signal processor, computer, and control panel.

The system has ten operating modes, which are divided into air-to-air, air-to-surface display, and sub-modes. The air-to- air modes are search and engagement. There are six air-to-surface display modes (real beam ground map, expanded real beam ground map, doppler beam- sharpening, beacon, and sea). APG-66 also has two sub-modes, which are engagement and freeze.

In the search mode APG-66 performs uplook and downlook scanning. The uplook mode uses a low Pulse Repetition Frequency (PRF) for medium- and high-altitude target detection in low clutter. Downlook uses medium PRF for target detection in heavy clutter environments. The search mode also performs search altitude display, which displays the relative altitude of targets specified by the pilot.

Once a target is located via the search mode, the engagement sub-mode can be used. Engagement allows the system to use the AMRAAM , Sidewinder , and Sparrow missiles. When engaging the Sidewinder , APG-66 sends slaving commands that slaves the missile's seeker head to the radar's line-of-sight for increased accuracy and missile lock-on speed. An Operational Capability Upgrade (OCU) was developed to modify the APG-66 to use the AMRAAM missile. The OCU is designed to provide the radar with the necessary data link to perform mid-course updates of the missile. The Sparrow 's semi-active homing seeker is facilitated in the engagement mode by a Continuous Wave Illuminator (CWI). The CWI also permits APG-66 to be compatible with Skyflash and other missiles with similar semi-active homing seekers.

Target acquisition can be manual or automatic in the track mode. There are two main manual acquisition modes, single-target track and situation awareness. The situation awareness mode performs Track-While-Scan (TWS), allowing the pilot to continue observing search targets while tracking a specific target. While in this mode, the search area does not need to include the tracked target's sector.

Four Air Combat Maneuvering (ACM) modes are available for automatic target acquisition and tracking. In the first ACM mode, a 20 x 20-deg Field Of View (FOV) is scanned. This FOV is equal to that of the Head Up Display (HUD). Once a target is detected, the radar performs automatic lock-on. The second ACM mode's FOV is 10- x 40-deg, offering a tall window that is perpendicular to the aircraft's longitudinal axis; this proves especially useful in high-G maneuvering situations. A boresight ACM mode is used for multiple aircraft engagement situations. The boresight uses a pencil beam positioned at 0-deg azimuth and minus 3-deg elevation to "spotlight" a target for acquisition. This is especially useful in preventing engagement of friendly aircraft. A slewable ACM mode allows the pilot to rotate the 60- x 20-deg FOV. The automatic scan pattern gives the pilot up to 4 sec of time. This mode is designed for use when the aircraft is operating in the vertical plane or during stern direction conversion.

The slant range measurement to a designated surface location is generated by the Air-to-Ground Ranging (AGR) mode. This real-time mode acts with the fire-control system to guide missiles in air-to-ground combat. AGR is automatically selected when the pilot selects the appropriate weapons deployment mode.

Terrain in the aircraft's heading is displayed via the real beam ground map mode. The radar provides the stabilized image mainly as a navigational aid in ground target detection and location. An extension of this mode is the expanded real beam ground map. The expanded real beam ground map provides a 4:1 map expansion of the range around a point designated by the pilot via the display screen's cursor.

Doppler Beam Sharpening (DBS) is available to further enhance the higher resolution of the expanded real beam ground map. This mode, which enhances the range and azimuth resolution by 8:1, is only available from the expanded real beam ground map mode.

In the Beacon mode the system performs navigational fixing. It also delivers weapons relative to ground beacons and can be used to locate friendly aircraft that are using air-to-air beacons.

The high-clutter environment of the ocean surface is countered in the sea mode. There are two sub modes in the sea mode. The first sub-mode, Sea-1 is frequency-agile and non- coherent to locate small targets in low sea states. The second sub-mode, Sea-2, is fully coherent, with doppler discrimination for the detection of moving surface crafts in high sea states.

The freeze sub-mode can only be accessed through the air- to-ground display modes. It pauses the display and halts all radar emissions as soon as the freeze command is received via the controls. The aircraft's current position continues to be shown on the frozen display. This mode is useful during penetration operations against stationary surface targets when the aircraft needs to prevent detection of its signals, yet continue to close in on the target.

The system's displays include the control panel, HUD, radar display, with all combat-critical controls integrated into the throttle grip and side stick controller.

The modularity of the LRUs allow for shortened Mean Time To Repair (MTTR) since they can simply be replaced, involving no special tools or equipment. The MTTR has been demonstrated to be 5 minutes, with 30 minutes for replacement of the antenna unit. APG-66 has also demonstrated a Mean Time Between Failure (MTBF) of 97 hours in service, but the manufacturers contend that it has achieved 115 hours. A cockpit continuous self-test system monitors for malfunctions. The manufacturers claim that the system's Built-In-Test (BIT) routine can isolate up to 98% of the faults to a particular LRU in the event of a malfunction.

A new version of the AN/APG-66, designated the AN/APG-66(V)2 is being installed in F-16A/B aircraft as they are modernized in the Midlife Update program. The equipment is lighter and provides greater detection range and reliability for the modernized F-16s.