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 I’ve spent part of the last few weeks looking over the shoulders of some of the engineering disciplines here at Fort Worth as they deciphered data from many recorders scattered about the F-16 to try to quantify the attempt of one pilot who did his damnedest to see just how high the F-16 will fly. (The pilot and his unit will remain nameless.) This data reduction was not in the interest of documenting a record, other than one for a pilot not thinking through the consequences of his actions.
The high-altitude attempt, which I'll discuss in detail later, reminded me once more of how the fighter community may have little or no knowledge as to how various limits are established for their airplanes. Back in the good ol' days when I was still a young pup in the F-104 and F-4, even I had a hard time understanding the reasoning behind certain limits. On more than one occasion, I suspected that my airplane was physically capable of doing whatever I wanted it to do, regardless of what the Dash One or wing regulations mandated. But two stints at Edwards AFB and one at the Tactical Air Command Systems Office at Wright-Patterson AFB (looking over shoulders at the F-16 System Program Office, or SPO) gave me an appreciation as to how and why most of the limits are established for this or that airplane.
From the start, everyone should be clear on the following: This is a discussion of various limits that apply to the F-16 in particular and to all aircraft in general. It is neither an encouragement nor a license to exceed any operating limit. Once you read everything I'm going to say about some of the limits (especially the physical ones) on the F-16, you should better understand why they exist and, I hope, be much more inclined to abide by them.
Limits fall generally into three broad categories: economic, physical, and political (yes, political).
Situations in which the customer or the SPO feels that it is unnecessary to fund tests beyond a given point produce economic limits. You will frequently encounter these limits with munitions (weapons, las armas, les armes, Waffe, and so on). Have you ever considered it odd that nearly all the air-to-ground weapons have a Mach 0.95 carriage limit? You won't find some little-known Newton's Fourth Law that states that the upper limit for air-to-ground weapons is Mach 0.95. This limit usually exists because the money necessary to fly the sorties and reduce the data to extend the limit was not available.
Be advised that a Mach 0.95 limit does not necessarily imply an economic limit. For example, this limit is realistic in a physical sense when it comes to some types of fuses, though internal electric fuses go a long way to improve this situation. Also, some weapon configurations may have a flutter problem or at least a limit cycle oscillation problem beyond Mach 0.95.
You may have noticed that the limits on some weapon configurations have been extended to allow supersonic carriage. Most of these are parent carriage on the MAU-12. However, the majority of configurations still restrict you to Mach 0.95 employment because the load was not tested (because of money) for release at higher airspeeds.
Though economic in nature, these limits often have as much of an impact operationally as do the physical limits. For example, some have complained that the tack-driving accuracy that the F-16 is known for was not replicated in the Iraqi fracas. Videos revealed, however, that the tactics used produced a large number of releases in the vicinity of Mach 1.05 to mach 1.1. The fact that none of the weapons hit the airplane is good testimony that such releases work. But why no good hits? At the risk of sounding like a broken record, economic reasons.
Since the tests did not call for supersonic releases, supersonic tests were never performed. Further, the near-flow-field effects change drastically when the airplane is transonic. The information in the fire control computer is in error at these speeds because its information cuts off at Mach 0.95. If the separation effects were quantified in the Mach 0.95 to 1.2 range, the accuracy would return to the level normally expected of the F-16. Though this may sound a little simple, quantifying these effects would require one thing – money. Still no free lunch.
Let's look at political limits before getting physical. I can recall two good illustrations. One involves the flight control system of the F-16. Like most of you, the first time I was exposed to the F-16 flight control description, I was not sure that I liked what I read. In particular, the system had no physical connection between the stick and the flight controls, only electrical signals. Having flown with the system all these years, I now have the utmost confidence in it. Having said that, however, I would still like it to be investigated a little further.
One of the possible failure modes with the F-16 flight control system involves one or more of the flight control surfaces being voted out of the flight control computer loop, in which case the surface would be instructed to lock in the faired position. I'd like to know just how well the airplane would fly in this configuration. I suspect that it will still fly fairly well as long as the pilot doesn't ask for some maximum input in the affected channel. But we don't know for sure.
I can, however, point to a previous experience with the F-16XL. During a test point looking at the roll response of the airplane at very high Q (a fancy scientific term for high calibrated airspeed), one of the test pilots (nice guy by the name of Jim McKinney) had the misfortune of making a maximum roll input to the left at very high airspeed coincident with hitting a pretty good patch of turbulence. The sharp additional load from the turbulence physically broke the horn on the bell crank connecting the right elevon to the hydraulic actuator. Jim said that it sounded like a cannon going off in the cockpit. (The elevon, the inboard control surface on the XL wing, is primarily for pitch control. But the control surfaces help each other depending on the rate requested and whether one is being tasked in its primary channel.) The air loads caused the right elevon to snap back over center, which violently checked the roll. So violently that it snapped off the vertical fin cap, the right dummy AIM-9, and the front and rear third of the right missile launcher.
The elevon was now free floating and damped to the still free-floating, but generally streamlined, position. Jim prudently elected to discontinue the flight and returned to land. Obviously he did not make any gross control inputs, but he had no indication that he had lost the use of one of the major flight control surfaces until after the landing. Based on this episode, I feel the little airplane will also fly okay with a locked flight control surface. But I don't know for sure and won't until someone goes up on a briefed flight with an instrumented, cine-tracked, debriefed, and data-reduced F-16.
When such a test was requested, one of the past center commanders refused to allow it. He said, "Let the operators find out about it." So for political reasons, the flight control system of the F-16 was not tested completely. As I said, I have great faith in the F-16 flight control system. But given enough time, it is surely going to happen at about two o'clock in the morning during a deployment that Blue 4 announces, "Hey, lead, my right stabilator just locked." Now what do you do? Do you let him refuel? Do you send him to Keflavik that's reporting 200 and one-half? It would be nice to know how the airplane will respond ahead of time.
Another good political limit with the F-16 is the crosswind landing limit. During the test program, I landed the F-16 with a direct crosswind of thirty-eight knots. I must admit that the airplane was very unhappy and my gains were a little higher than normal. As a result, we recommended a direct crosswind landing limit of thirty-five knots. The four-star test pilot, however, said twenty-five knots.
"But, General, we've landed in excess of thirty-five knots."
"Twenty-five knots!"
"Okay, twenty-five knots."
That's one limit not tested because of politics and one established because of politics.
Now for the promised physical limits. Two examples are worthy of discussion. The first involves the calibrated airspeed limit on the F-16. The Dash One sets the knots calibrated airspeed, or KCAS, limit at 800 knots. Interestingly, the airspeed limit is actually based on the engine operation. With the original -200 version of the F100 engine, you almost had to dive into the point to exceed 800 knots. But just such a dive could surpass the physical limits of the engine because the -200 engine control system essentially runs open loop. That is, the control system would allow the compressor discharge pressure to increase beyond the physical limits of the engine. Therefore, the limit was established at 800 knots to ensure that the -200 engine remained within its envelope.
A tragic accident exemplifies the frivolity (read stupidity) of exceeding this limit. One of the pilots in the test squadron at Eglin (someone who certainly should have known better) took it upon himself to see just how fast the F-16 would go. On the way back from a rather mundane test mission, he climbed to 16,000 feet, turned the wick full up, and nosed over into a dive. In reconstructing the flight, we feel that he got well on the high side of 850 knots. Since the compressor discharge pressure was uncontrolled, the pressure became high enough to distort the engine case. The turbine rubbed the engine case at a ferocious rate, went through the turbine tip seals, and began eating into the engine case with equal fury.
The engine casing in this area is constructed mostly of titanium. Those of you familiar with the periodic table of elements may remember that titanium is in the same family of elements as sodium, lithium, and magnesium. The temperature to get titanium burning is very high (that's one reason it is used in the engine). Once any of these metallic elements start burning, though, you have one hell of a fire. With a fire of this intensity in this part of the airplane, you have effectively wrapped prima cord between the trailing edge of the wing and the leading edge of the horizontal tail and lit that sucker off.
The intense fire quickly burns around the entire fuselage and through both hydraulic control systems in spite of the best efforts of design engineers to isolate the two systems from collateral damage resulting from multiple combat hits. Under such conditions, all bets concerning the control of any aircraft are off. Even the most thoughtful attempt to design redundant safety systems can be sidestepped by stupid actions of a pilot. With no hydraulic pressure, you are nothing more than a passenger in an aircraft going down at almost 1,000 miles per hour. The airplane continues to descend and you don't even have a vote. That's not a feeling I want to experience.
Somehow our intrepid (but not too forward-thinking) aviator got out of the ruined airplane. But he broke both arms in the process. He ended up drowning in the Gulf of Mexico. A very bad scene from any perspective.
Those of you who may have been getting in the books or listening to the engine awareness briefings of both Pratt & Whitney and General Electric will probably now want to point out that the later -220 and -229 versions of the F100 and all models of the GE F110 engine have positive, closed-loop control systems. I'm glad you've been paying attention. The -220 engine has more thrust than the -200, once you get it moving. But it is seldom more than ten percent better. So you'll still usually have to dive the airplane to exceed 800 knots.
But the -229 and all versions of the F110 have enough power to go right through 800 knots like it isn't even there, straight and level, sometimes as high as 15,000 feet. So why not exceed the limit?
Pay attention. The newer engines control the compressor discharge pressure for the most part by rolling back fan revolutions per minute, which cascades through the engine and, thus, effectively limits the N2 (the high-pressure compressor) discharge pressure. They are not taking thrust away, but they are preventing thrust from building at the rate it would like to because of the increased ram effects. However, improvements throughout the rest of these engines result in a lot more thrust in this part of the envelope. Furthermore, this extra thrust comes without the risk of hurting the engine. So if the engine is not the limiting factor, what is?
Well, it is not the F-16's flutter limit, which is theoretically on the plus side of 900 knots. That is an impressive number, n'est-ce pas? Under test conditions, I've been as fast as 845 knots with the early GE engine. Further, it was readily apparent that the airplane was nowhere near ready to quit. It is really a ride to feel those levels of acceleration for that length of time and still know in your heart that there is a lot left. We could have easily taken that airplane (one of the old beat-up, full-scale development airplanes, F-16A No. 1) and set a low-altitude speed record with no preparation other than getting the Federation Internationale de Aeronautic to Edwards to certify the timing. But once again the political climate was not right to grant the F-16 any favorable notoriety. Plus, we were told in no uncertain terms that we were not even to talk about such an attempt. Too Bad. It would have been fun to see just how fast we could have gone. It most certainly would have been a big number.
I almost choke, however, when I hear that some pilots have had the airplane as fast as 870 knots. Even considering that the speed may inflate a little every time the story is told, I really don't like to hear about anyone exceeding 800 knots for no apparent reason.
Why? Anybody have a clue?
The answer concerns the canopy. It has never been qualified at the kind of airloads and temperatures involved with flight in excess of 800 knots. If you don't think the airspeed effects on the canopy are real, the next time you have the opportunity to fly for any period of time with the clock reading more than 500 knots, take your glove off and feel the inside of the canopy. It gets damned warm. Further, the effects are exponential. And going from 500 to 800 knots is a hell of a lot more than the sixty-percent increase that simple, linear arithmetic would lead you to believe.
Don't get me wrong. In combat (the operative word), knowing what I know about the F-16 engine combinations available, if I had a MIG-29 just out of range at twelve o'clock (or worse yet, just in range at six o'clock), I wouldn't hesitate to go over 800 knots. But doing it as a matter of routine in peacetime because it's fun is plain dangerous. No one really knows just how many pressure and temperature cycles involved with these airspeeds the canopy can take before it starts to get tired. And I can guarantee that you will not want to be the name behind the data point associated with a face full of canopy somewhere in excess of 700 knots just because you and your squadron bubbas have ignored the airspeed limit one time too many. This is a very real physical limit. I strongly encourage you to respect it.
This brings me back to the high-altitude attempt mentioned in the beginning. From what we can determine, our stalwart aviator decided to see just how high he could fly. He started out right in that he climbed to a fairly high altitude, pushed over into a very good Ratowski path, and adroitly accelerated to maximum Mach at the tropopause. He was now out at the Es lines you have seen only on some of the classified charts in Intel. No problem, so far.
But from this point, he became dumber than dirt. He pulled the airplane into almost a vertical attitude and peaked out at a very high altitude. There is no reason to give any credibility or acclaim as to the exact height reached, but trust me – it was way up there. If he had flown the profile correctly, he probably could have gone even higher. But this is not the first thing he did wrong.
We had pressure suits in the prototypes and flew the F-16 at sustained altitudes considerably higher than 50,000 feet. The zoom potential is even higher. But the production F-16s do not incorporate a pressure suit, and it is senseless to go above 50,000 feet without one. You have probably seen the demonstrations in the altitude chamber in which someone wearing a pressure suit and holding a beaker of water is explosively decompressed to some altitude well above 50,000 feet.
Your ninth grade general science teacher spoke with straight tongue when he or she taught you that, as the pressure is decreased, so does the boiling temperature. Sure enough, the water at room temperature violently (almost explosively) boils off in a cloud of vapor. While this demonstration supposedly shows you what will happen to your blood in the same situation, it is a little over dramatic in that it doesn't consider the pressure retention of your skin and clothing.
Ergo, you are not going to boil off at a similar rate. However, it is correct in that your time of useful consciousness is effectively nil under the same conditions. If you don't get down tout de suite, you are history without a pressure suit. Our pilot was above 50,000 feet without a pressure suit. Strike one.
Our high-flying pilot also failed to consider that the engine does not enjoy these altitudes any better than humans do. That is why you can't get them restarted at any ol' place in the envelope. You can tune a jet engine to run at these altitudes, but it is difficult. (This is why we have rockets.) So during the climb, the engine quits. Strike two.
At extremely high altitudes, the control surfaces have little or no effect. In case you haven't been paying attention, ask Ensign Wesley Crusher just what the calibrated airspeed is at Warp Eight. It is zero. And unlike the warp-drive equipped Enterprise, the F-16 and all other airplanes respond to calibrated airspeed to control where they go. (We don't even have impulse engines.) So our pilot rapidly approached very low calibrated airspeeds in his climb even though he may still have been supersonic. He was more appropriately classified as ballistic. Strike three.
The pilot tried to roll left, but the inertia effect of the engine took the aircraft to the right. (He will later complain about the flight control system for rolling the wrong way.) Fortunately, the bleed down rate of the cabin pressure when the engine quits is slow enough to allow him to subsist until the airplane fell to a lower altitude, where he has some hope of surviving and getting the engine restarted.
Back at the base, he has to explain why the emergency power unit fired. But this will be the least of his worries. He is lucky to be alive. All in all, a poorly thought through stunt that very nearly cost us a valuable airplane and an expensive pilot.
In case you think you are smart enough to get away with such a stunt, read carefully. In the old days with the F-100 and F-104, you could just happen to drop the film canister on the ramp if you had the camera running during some event you didn't want the world to know about. In the F-4, you had to be sure that the radar camera wasn't running and that your GIB, or the guy in back, wasn't suffering from a guilty conscience.
But the F-16 has any number of hidden places where interested parties can retrieve data and reconstruct an entire flight if necessary. For example, the altitude in the crash-survivable memory is limited to 50,000 feet mean sea level, or MSL. When the data in the crash-survivable flight data recorder from the flight I described above was downloaded, the readout indicated a long (very long) string of special events data had been recorded at altitudes in excess of 50,000 feet.
The engine monitoring system computer revealed that the engine problem occurred at an altitude greater than the engine recorder limit of 70,000 feet MSL. If you look still further, the altitude data stored in the electronic control assembly will record altitudes as high as 100,000 feet MSL, which is nearly 20,000 feet higher than the altitude limits of the central air data computer stored in the flight loads recorder and, if selected, on the video tape.
All of these sources include airspeed and altitude information and many other interesting flight profile and aircraft systems parameters as well. There are also other locations where data lurks that I didn't mention. Good luck dropping all of these on the ramp. The moral: 1984 has indeed long since come and gone in the F-16. All it takes is for someone to suspect that something is awry. The airplane can tell us very nearly exactly what you did from the time you started until the time you shut down.
If you don't like the economic limits, try to convince the chain of command to spend the money to get them changed. If the political limits bother you, use the proper channels and attempt to get them changed. But for Pete's sake, realize just what the physical limits truly are and don't exceed them. If you do, at best, you're going to get caught. And you could very easily lose your life. The airplane has some fantastic flying characteristics. Learn to use them to the fullest and surely you can have fun and still remain within the airplane's physical limits. Step outside them, even briefly, and either way, you're toast.
Check Six.
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