F-15A Engines

Last revised February 20, 2000

The F-15A is powered by two Pratt & Whitney F100-PW-100 afterburning turbofans. The powerplant is designated F100-PW-100 by the company and JTF22A-25A by the USAF. It is an axial-flow turbofan with a bypass ratio of 0.7:1. There are two shafts, one carrying a three-stage fan driven by a two-stage turbine, the other carrying the 10-stage main compressor and its two-stage turbine. 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. Left and right engines are interchangeable with each other. Unlike in the F-14 Tomcat, the engines in the F-15A are mounted close together in order to minimize asymmetric handling problems when one of them is out.

The F100 dates back to August 1968 when the USAF awarded development contracts to Pratt & Whitney and General Electric for a next generation fighter engine, with the Pratt and Whitney engine being selected in 1970 by the USAF for further development. A parallel version, the F401, had been proposed for the later models of the Navy's F-14 Tomcat. However, the F401 was cancelled when the size of the Tomcat fleet was cut back in an economy move.

The engines are fed by a pair of laterally-mounted straight, two dimensional external compression air intakes. The intakes are swept forward from bottom to top, in order to ensure that an adequate amount of air is admitted to the engines at high angles of attack. The intakes are pivoted at their lower edges and can be adjusted in flight to angles of as much as 4 degrees above or 11 degrees below the horizontal. The air intakes "nod" up or down under the control of an air data computer to keep the aperture facing directly into the airstream in order to maintain a smooth flow of air into the engines. The angle of the intakes can also be adjusted to prevent more air than necessary from being taken in to the engines. The intake surfaces have an additional function in providing extra maneuvering control, in a manner similar to the function of the canard foreplanes fitted to aircraft such as the SAAB JAS-39 Gripen. At supersonic speeds, the effectiveness of the "nodding" intakes is almost a third of that of the horizontal stabilators. The intakes stand away from the fuselage sides to prevent boundary layer air from entering the engines, making complex diffuser plates unnecessary. Downstream of the intake are moveable ramps which control the amount of air admitted to the engines. The exhausts of the F100 engine have fully-dilating nozzles to control the mass flow of air from the exhaust. The dihlating nozzles were initially fitted with "turkey-feather" vanes, but these were later removed on most aircraft.

The F100 engine had numerous teething troubles, which might have been expected for such a new and advanced aircraft engine. Initially, the Air Force had grossly underestimated the number of engine powercycles per sortie, since they had not realized how much the Eagle's maneuvering capabilities would result in abrupt changes in throttle setting. This caused unexpectedly high wear and tear on key engine components, 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. However, the most serious problem 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. 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. This was especially dangerous in a twin-engined aircraft like the F-15, since the pilot might not notice right away that one of his engines had failed. To correct for this, an audible warning system was attached to the turbine temperature reading.

Some stagnation stalls were caused by a "hard" afterburner start, which was a sort of mini-explosion that took place inside the afterburner when it was lit up. "Hard" afterburner starts could be caused either by the afterburner failing to light 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. Of course, if this happened during the stress of combat, the pilot would be dead meat.

There were frequent groundings and delays in engine deliveries while an attempt was made to fix these problem. Strikes at two major subcontractors delayed the delivery of engines. By the end of 1979, the USAF was forced to accept engineless F-15 airframes and place them in storage until sufficient numbers of engines could be delivered. A massive effort by Pratt & Whitney helped to alleviate this problem, but the F-15 suffered from an engine shortage for a long time.

The installation of a quartz window in the side of the afterburner assembly to enable a flame sensor to monitor the pilot flame of the augmentor helped to cure the problem with "hard" afterburner starts. Modifications to the fuel control system also helped to lower the frequency of stagnation stalls. In 1976 the F-15 fleet had suffered 11-12 stagnation stalls per 1000 flying hours. By the end of 1981, this rate was down to 1.5. However, the F100 even today still has a reputation of being a temperamental engine under certain conditions.


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