Trouble With the Air Force's Eagle

TWICE pages IN have RECENT carried MONTHS, discussions THESE of an Air Force fighter plane known as the “Eagle,” or F-15. Last May, in “America’s High-Tech Weaponry,” the F-15 was used to illustrate the costs and benefits of rolling back the frontiers of technology when creating new military machines. “I Fly With the Eagles,” published in November, emphasized the skills that pilots must possess to handle these aircraft—and the perils awaiting the layman who is lured into their realm.

There is a third chapter in the Eagle’s saga, which connects the themes of the previous two and which figures prominently among the Air Force’s current concerns. This story, which is still unfolding, begins with decisions about the performance of machines, but it leads to questions pertaining to human beings.

The machine is the F-100 engine, two of which propel every F-15. (The engine should not be confused with the F-100 fighter plane.) Another Air Force fighter, the F-16, uses a single F-100 engine. The engine is built in Connecticut by Pratt & Whitney.

The F-100 was one of several completely new systems that were incorporated in the design of the F-15 in the early 1970s, and the properties of the new engine were crucial to the hopes for the Eagle’s performance. The engine’s virtue was its extremely high thrust-toweight ratio; while one engine added 3,000 pounds to the weight of the plane, each F-100 was (according to initial plans) supposed to generate 23,800 pounds of thrust. When two such engines were mounted on an aircraft whose total weight was about 40,000 pounds, the result was a plane whose overall thrust-to-wTeight ratio was greater than one, which was supposed to make the Eagle more maneuverable than any other plane and faster than most. Its proponents said that the F-15 would be America’s answer to a new generation of Soviet planes.

But to wring such high propulsion from so light an engine, the F-lOO’s designers were knocking at the limits of materials and of engineering almost everywhere they turned. A modern jet “turbofan” engine, such as the F-100, takes in air through inlets at the front and then compresses it to very high pressures through a series of “compressor stages,” which are essentially fans all pointed in the same direction. In the F100’s case, the air is compressed to pressures about twenty-three times greater than when it enters the engine. Then it is combined with jet fuel and explosively ignited. The resulting jet of exhaust is directed out the rear of the engine, where it provides the thrust for the plane. On its way out, it passes through a series of turbine blades and turns them, as wind turns a windmill. The energy consumed in turning the blades is subtracted from the thrust that goes out the rear of the engine, but the energy of the turbines’ rotation is conveyed to the front of the engine, where it drives the compressor stages that keep the engine running.

In designing such an engine, engineers must make trade-offs among a number of complicated and sometimes opposing values, including the compression ratio and the operating temperature, in the hope of producing the optimum balance between fuel consumption and thrust.“The result,” according to two Air Force officers, Major Charles L. Fox and Lt. Colonel Dino A. Lorenzini, who WTOte about the F-100 in the Naval War College Review two years ago, “was a design that indeed gave the pilot every possible ounce of performance.” They continued, “However, the last few ounces were purchased at the price of persistent stagnation stall problems and engine operating temperatures so high as to degrade considerably the F-lOO’s durability.”

The “stagnation” and “stall” problems the officers referred to arose from a disturbance in the flow of air through the engine. The compressor fans at the front of the F-100 may be thought of as a number of small airfoils—like little airplane wings or propeller blades—that “fly” in a circular path around the fan’s hub. Instead of providing “lift” for the entire aircraft, as wings do, these airfoils “lift” wind backward into the engine. All airfoils derive their power from the difference in curvature between top and bottom surfaces, which forces the air to flow at different rates past those surfaces. Up to a point, the airfoils provide increasing lift as the air flows past them at a more extreme angle. But when the “angle of attack” becomes too great, the flow of air is disturbed and the airfoil’s lift disappears. If this happens to the wings, the plane stops flying and starts falling out of the sky. If it happens to the blades of the compressor fan, the engine suffers a stagnation or stall. The F-100 proved to be unusually susceptible to these mishaps w’hen turbulent air flowed into the engine, as was the case during certain aerial maneuvers.

In the extreme version, a stagnation, the entire engine can run down to a halt. Stalls are momentary coughs or hesitations in the engine’s performance, as one portion of a fan is affected. In both cases, the flow of air into the engine is choked off. The flow of fuel continues; the reduced air supply is still enough to support combustion but not to cool the engine. As a result, the engine’s temperature soars. This sudden “spike” of heat, which can take turbine blades to the temperatures at which they weaken or melt, is thought to be one of the main reasons the F-lOO’s turbine blades wear out so fast.

THE DELICATE NATURE OF THE F100 was evident from the start. In 1973, the F-lOOs that Pratt & Whitney submitted to the Air Force failed to meet a standard durability requirement. The officer who was then managing the project, General Ben Beilis, attracted publicity by deciding to waive the requirement and accept the engines. Pratt & Whitney had proposed that they take more time to test the engine’s durability, by mounting an F-100 beneath a test plane and flying it for extended periods. General Beilis had rejected that proposal as too costly and time-consuming. In the years since then, the focus of practical concern about the F-100 has been its appetite for turbine blades. The blades, which range from three to ten inches long, now cost an average of more than $500 apiece. (There are 268 blades in every engine.) Apart from the expense of replacing them, the Air Force found that it was burning up the blades faster than they could be made. By the late 1970s, turbine-blade producers were straining to meet commercial and military demands. As one response to the turbineblade problems and the rapid flow of F100s into the repair shop because of stalls and overheating, the Air Force ordered a change in its maintenance procedures that led to the engines being “detuned.”

Detuning, also known as “trimming down,” or “turning down the wick,” means adjusting an engine so that it burns at a lower temperature. This eases the wear and tear on the turbine blades, but it also reduces the thrust that the engine had originally been designed to maximize. The detuning took place in two stages, one in 1976 and another in 1979. The first step reduced the engine thrust by about 5 percent. According to the Air Force, the second step was taken principally to save time during the tuning procedure. The Air Force’s official claim, considered optimistic by several pilots and aircraft designers with whom I spoke, is that the second detuning lowered the thrust of the average engine by another 2 percent, and by 5 percent in some cases.

Because of the formula used to determine an airplane’s ability to accelerate (acceleration equals the thrust of the engine minus the drag of the plane, divided by its mass), small differences in thrust can be magnified into large differences in acceleration when the aircraft in question is one like the F-15, whose drag at high speeds is very great. Beyond that, there were indications that the F-100’s performance had fallen off by more than was officially claimed. F-15 pilots routinely fly against “aggressor” pilots in F-5s, much smaller and lighter planes that simulate the role of Russian fighters. The F-5 costs about a fourth as much as the F-15 and requires less maintenance; its top speed is supposed to be 1.6 times the speed of sound, as opposed to 2.5 times for the F-15. Yet in a number of exercises, pilots noticed that at certain altitudes and airspeeds the F-15s were no longer able to out-accelerate the F-5s.

The most noticeable rumblings of protest came from Nellis Air Force Base, in Nevada, where some of the Air Force’s best pilots train at the Fighter Weapons School. In drag races last year at Nellis, the F-15 was eventually overtaken by the older F-4 Phantom jet, which the Eagle theoretically should have outclassed. In one case, according to reports from Nellis, when an F-100 engine was placed on the test stand, its thrust proved to be 17,000 pounds—or 29 percent lower than the original standards. There was a wide variation in performance from plane to plane and engine to engine. There was another engine problem even more worrisome to pilots: the F-100, designed to burn smoke-free, was beginning to emit the smoke trails that pilots regard as a grave handicap in aerial combat.

In July, Major General Jack Gregory, the commander of the test range at Nellis, sent a letter reflecting his pilots’ complaints to the Air Force’s Tactical Air Command. By that time, Pratt & Whitney and McDonnell Douglas, which is the principal contractor for the F-15, were well aware that they had a problem; along with the Tactical Air Command and the Air Force Systems Command, they were investigating its origins. They did little, however, to publicize the inquiry, even within the military, until Bruce Ingersoll, of the Chicago Sun-Times, and John Fialka and Walter Mossberg, of the Wall Street Journal, broke the story of the pilots’ complaints early in October.

THE DIFFICULTIES AFFECTING THE F-100 obviously underscored the gap that separates the ideal from the real on the frontiers of military technology. Many modern military systems exact high prices, not only in dollars but in day-to-day reliability, for capabilities that are supposed to make a dramatic difference in military effectiveness. In practice, the real items sometimes fall short. No one claims to be exactly sure what has happened to the F-100; yet another factor seems to be that adjustable “vanes” in the rear compressor, which are supposed to direct the flow of air for maximum efficiency, have worked loose in their lightweight titanium bearings, further reducing the engine’s thrust. There is even some doubt about whether the problems are confined to the engines, or whether the airframe design may be involved in some way. All that is certain is that the $20-million-plus airplane in the fleet in 1982 is not the one advertised to Congress, the public, and the pilots ten years earlier.

Yet in some parts of the Air Force, a harsher lesson is drawn from this episode. It is based on the fact that when the Air Force leadership decided to detune the engines and degrade their performance, nobody told the pilots.

The vehicle for informing the pilots would have been the performance charts in the pilots’ flight handbooks that describe the airplane’s characteristics and capabilities at various altitudes and speeds and when conducting different maneuvers. From the charts the pilot learns the airplane’s range as well as the performance he can expect during combat maneuvers. Inaccurate data leads the pilot to think that he can do things he cannot do, and that he enjoys an advantage while making maneuvers that may mean his destruction. The F15 pilot would expect on the basis of his charts that in combat against certain fighters he could suddenly dash to overtake—or break away from—the other plane. But the detuned plane he was flying would not allow him to escape.

I spoke by phone with pilots at three of the bases where F-15s operate. None would be quoted by name, because of the sensitivity of the issue within the Air Force; most affirmed their overall enthusiasm for the Eagle as a “real fine plane.” But nearly all expressed sentiments like those of a pilot at Nellis: “If they’re going to make the changes, they damn well better tell me about it, because I’m going to rely on it in a fight.”

Of course, the most experienced pilots had already figured out that they could not rely on the handbooks. They learned from their own exercises what they could and could not do. Before a squadron of Eagles was sent to Germany for exercises in 1980, its members conducted cross-country flights to see what the plane’s range actually was and where they would need to refuel en route. In terms of peril to life and limb, the F-15’s smoky engine would probably be a more serious handicap in combat than the misleading charts. But it hardly increases the pilots’ faith in thenleadership to learn that they cannot trust the information officially provided.

Why weren’t the pilots told? Colonel Gerald Blake, who became head of the F-15 program just in time to answer questions about the detuning, told the Wall Street Journal that “a lot of pilots were simply not aware of that. We’ll take the hit in that area.” Lt. General Tom McMullen, the vice-commander of the Tactical Air Command, told me that because the detuning was such a “gradual” process, and “because we didn’t realize it had gone as far as it had,” the Air Force did not change the manuals. (A project to rewrite them is under way.)

There is an alternative hypothesis within the Air Force, which holds that political arguments had as much to do with the F-100 misadventure as did any technical considerations. The F-15 carries, in addition to missiles, a great deal of political freight. In the Air Force, enthusiasm for the plane is sometimes indistinguishable from loyalty to the nation. It is an inconvenient fact for the pro-Eagle group, which includes most of the service’s leadership, that a newer, lighter, and cheaper plane called the F16 has been far freer of engine problems, even though it also uses the F100. One possible explanation for the difference is that the F-16 is, and was designed to be, light enough to be quite speedy and maneuverable with just one engine—and its single engine’s inlet location, under the belly of the plane, is inherently less susceptible to turbulent air flow than are the inlets for the F-15’s twin engines, which are mounted on the sides. Some people in the Force consider it no accident that the Air Force issued charts that overstated the F-15’s capabilities. They point out that the F-16’s performance charts, some of them based on estimates, were adjusted to reflect the results of the first round of detuning in 1976, which applied to F-100 engines used in both planes. (The F-16 engines were not detuned the second time, because they were not creating the severe repair problems that the F-15s were.) The F-15’s charts, however, also based on estimates, were not changed to reflect the detuning.

But to accept this hypothesis would be to conclude that leaders placed loyalty to their machines above loyalty to their pilots. That is a conclusion so distasteful that most are compelled to reject it. □