You may have noticed that some of the newer F- l6s entering the field are equipped with the F110-GE-100 engine and a few have the F100-PW-220. Both of these new engines are a tremendous improvement over the present Fl00-PW-200 (which itself is no slouch). This installment in the Semper Viper series is about the F-16’s powerplant. Since more than 1,500 F-16s out there are powered by the older Pratt & Whitney, I’ll limit my discussion to that engine.
During the time that the prototypes and Full Scale Development F-16s were being designed, the Fl00-PW-200 engine was the best one available that provided the required performance. The F100 is different from any other engine you’ve experienced in previous fighters, so a little discussion is worthwhile. A few courses of action worked out ahead of time (You do this for all situations you might run across in flight, don’t you?) will keep you on the good side of everybody involved.
How about the backup controller, or BUC? I’ve really gotten upset with all the misinformation that’s going around about the BUC. First of all, it’s a simple backup system for the main fuel control, really not much more complicated than a kitchen faucet. Because it is simple, you (the pilot) must make allowances for its simplicity. It’s important to remember this. If you treat the BUC the way it’s supposed to be treated, it’s nearly foolproof. But if you insist on being Joe Cool and not paying attention to just how the engine control system works (EEC, UFC, or BUC), it’ll jump up and bite you so fast your head will swim. Read the books, ask questions, and when it becomes obvious that the main fuel control is going south, don’t be afraid to switch to BUC. It’s up to you to know - and know cold - the BUC operating envelope, the rate you can move the throttle, and exactly where the BUC start and BUC idle positions are on the throttle quadrant. If you know this and can make allowances for the fact that it is such a simple system, then it works. And it works well.
Another major place for getting into trouble is in the area of stagnations – one of the unfortunate side effects of pushing the state of the art with afterburning turbofans.
Excuse me. That should read augmented turbofan. Every new design engineer wants to become famous, so he or she changes the name of everything just because it happens to be slightly modified. (Pilots aren’t exempt from this disease, either.) As a result, you see power lever instead of throttle, augmentor instead of afterburner, intermediate instead of MIL, crew station instead of cockpit, and so on. Sometimes, it’s really necessary to read between the lines.
Where was I? Oh, yeah. Afterburning turbofans. Since the fan duct now allows almost direct access from the afterburner to the compressor, these new engines are much more likely to stall when selecting afterburner than are the turbojets you’re used to operating. Also, it’s possible for these stalls to progress to an even less desirable event called a stagnation. (Don’t get the impression that you can expect one of these on every flight, but – once again – forewarned is forearmed.) The stall can continue, driving the basic engine down to a revolution per minute from which it will not self-recover (all the while putting out no useful thrust). This is bad with the F100 because, if you don’t recognize the condition and do something about it, you’ll be left with a melted ingot in the engine bay.
So how do you recognize one of these hummers? It’s real easy, but more on that in a minute. First, the engine must stall in order to stagnate. Did I mention cues? With the exception of an off-idle stall, there’s nearly always some duct rumble that precedes a stall. With an off-idle stall, the energy available to make noise is not there and you might not hear any rumble. It’s also possible you might not even hear the stall. This is certainly not the case with the engine in any other condition. There’s almost always a lot of duct rumble and then a very definite "bang" or "pop" when the engine stalls. The engine control design incorporates an automatic throttle retard if the engine stalls, but I’ve yet to see a fighter pilot who couldn’t beat the automatic system, hands down. If the engine stalls, you’re not getting much thrust, so it makes little sense to leave the throttle in the afterburner range. Retard the throttle to MIL, allow time for the system to reset, and try it again – cautiously – if you’re sure you’re not in region three. But don’t get carried away. If you know you were at 400 knots, one g, and 20,000 feet, you are obviously well into region one. An engine stall here indicates a sick engine. Bring it home without further attempts.
Now about those stagnations. If you allow the engine to continue stalling, it’s very likely you’ll get a stagnation. How do you recognize one? Very simple. The book says that if rpm is low, fan turbine inlet temperature, or FTIT, is high, and the engine does not respond to the throttle, then the engine is stagnated. Actually, however, recognizing a stagnation is far simpler than that. Even with the wildest temperature fluctuations (weather-wise), you’ll never see ground idle rpm lower than sixty-one to sixty-two percent, and this idle rpm increases with altitude. So it’s incredibly simple: If the throttle is in the operating range and the rpm is below sixty percent (especially if you have heard and/or felt the engine stall), chances are extremely good that you have a stagnation on your hands. There is never an excuse to let the FTIT get anywhere near the peg.
You should have enough situational awareness to know the stagnation exists almost as soon as it starts. Don’t be spring-loaded to the stagnation position – there’s always the possibility that you could have a problem with the electronic engine control, or EEC, or even the upfront control, or UFC. But the chances are very good that, if you’ve felt or heard the engine staff and the rpm is below sixty percent (especially if the throttle is above idle), you’ve very likely inherited a stagnation. Check the FTIT. If it’s increasing, you can be sure you have a stagnation. Don’t wait. It’s not going to get any better. Shut the engine down and prepare for an air start. If you’ve caught the stagnation early enough, the FTIT will be below 700 degrees almost immediately and all you’re doing now is waiting for the rpm to get within the twenty-five- to forty-percent range specified in the Dash One. Bring the throttle around the horn and you should be well on your way to an air start.
Remember that the F100 can take what seems like an eternity to complete a start (sometimes over a minute), so don’t abandon a successful start because of impatience. If you didn’t catch the stagnation early and the FTIT is on the peg, you’re in for a long wait until it falls below 700 degrees. Not catching the stagnation early also means the rpm is lower once you decide to shut the engine down. Lower rpm and higher FTIT make it much more difficult to arrive at the below-700 degrees FTIT and twenty-five- to forty-percent rpm that you need. You might even have to move the throttle out of cut-off early in order to keep the engine rpm from going below twenty-five percent. (You should be aware that the rpm will decay very rapidly, especially at the lower altitudes, and you don’t get any ignition if the rpm is below fifteen percent.) As a result, the first start attempt will very likely go hot. You’re now worse off than when you started. So it behooves you to recognize a stagnation ASAP. Know what the inflight limits are for the jet fuel starter, the JFS. The JFS can help a lot to preserve what rpm you have left. It also helps unload the rotor at the lower rpm, thus improving the stall margin.
If you think it necessary to attempt an air start using BUC (a non afterburner-related stall/stagnation, for example) make sure you know – cold – the position on the throttle quadrant of both BUC start and BUC idle. Although your body clock will probably be running about the same rate I mentioned while we were discussing deep stalls, make sure you don’t rush the BUC start attempt. It’s very easy to get ahead of the engine and wind up with a hot start or worse. If you can afford to spend the altitude, it’s better to make sure the engine has lit off and is beginning to accelerate before you start moving the throttle. If you’ve started to move the throttle because the time has elapsed and there is no indication of light-off, do so at a very slow rate until you do get a light-off. Even then, let the engine accelerate on its own after light-off before you start moving the throttle forward again.
The only place where the Dash One is overly pessimistic concerns maximum FTIT when attempting a BUC start at low altitude. If the start is going well but altitude is becoming critical, you might want to go above the recommended FTIT limit. But do so in a smooth, controlled manner. Tests have shown that the FTIT limit is not so critical at the lower altitudes. So it’s worth the gamble to push the limit a little instead of going directly to the ejection seat. This is not to suggest a cavalier attitude toward your emergency. It’s just one more tidbit of information to file away.
