MC-130 Zero Offset Testing

MC-130 Combat Talon IIby L. D. Alford
Photo Courtesy of the U.S. Air Force

The MC-130H, Combat Talon II is a sneaky secret infiltration/exfiltration aircraft designed to take special operations forces and cargo deep behind enemy lines. It is an overt aircraft for a wartime mission, but can be used in a covert role under certain circumstances.

The aircraft is a normal C-130H with the addition of a high power terrain following radar, night vision electronic displays, new autopilot, Electromagnetic Countermeasures systems, Special Operations radios, and a beefed up rear cargo door for high speed airdrop. The radar gives the Combat Talon II its distinctive bulbous nose. Everyone who flew it at Edwards lovingly called it Opus after the penguin in Bloom County, but the brass didn’t like the name, and the SOF guys and SOF brass didn’t like the image. We tried a couple of times to get an Opus painted on the nose, but that only lasted a week or two before we were ordered to paint it over.

The Combat Talon II was the first Air Force aircraft to have primary electronic cockpit displays. The displays were all green on black CRTs for night vision compatibility. These displays were the first of their kind in any aircraft and gave us incredible development problems. The display rate was one critical problem. In time, the displays got better, but they were digital tapes like many current electronic display aircraft. Digital tapes are much harder and less intuitive than round dials. They are always much harder to read and interpret. That is why the Air Force standards I helped develop call for round dials in all electronic displays from heads down to heads up. Displays are important, but the critical part of the testing of the Combat Talon II was the radar.

The radar on the Combat Talon II had to look out 20 miles to determine climb angles and turning angles for the aircraft. The truth about terrain following radars is the slower you fly, the lower you go, and the greater your weight, the further you have to be able to look out. The reason for this is geometry. Speed, altitude, and weight directly relate to energy. Slow, low, and heavy means low energy. You have to be able to decide whether to climb over or turn away from terrain. If the terrain is too high and close to climb over you must turn away. If the terrain is low and far enough way to climb over, you need to determine when to start a climb. You also have to consider power available, for example, what happens if you lose an engine.

The MC-130H was tested from top speed (318 knots) to 1.3 times Vstall (stall speed); that’s really slow for an unconfigured C-130 (about 160 knots) with four and three engines. The top weight was 155,000 pounds; that’s really heavy. The set clearance plane (minimum altitude above the terrain was 250 feet); that’s really low. You gotta see out 20 miles to keep a big aircraft like this out of the dirt.

The Combat Talon II was fantastically missionized for this purpose. The flight director was a dot in a circle. The pilot flew the aircraft to kept the dot in the circle—no autopilot available here for terrain following. As long as the dot stayed in the circle, the aircraft was clear of terrain in front and on either side. The navigator controlled the radar and the flight direction. The copilot and the navigator monitored the radar system outputs. The aircraft could turn using the terrain following radar system alone. The pilot would start a turn keeping the aircraft within the radar scanning profile (this was dynamically shown on the pilot’s display). As the radar gained more information about the new flight path, the flight display would give the pilot a greater cut at the turn until a full IFR turn (about 20 degrees) was available. This system allowed the crew to make unplanned turns in very high terrain without fear of hitting anything. The system is basically foolproof and pilotproof as long as the pilot followed the flight director.

Of course the major part of flight testing is to ensure that the system does what it is supposed to under all conceivable conditions (and some not so conceivable). The radar and flight director system was supposed to keep the aircraft clear of terrain at a set clearance plane of 250 feet and 90 feet from the wing tips. In other words, it wasn’t supposed to get closer than 250 feet to the ground (or anything sticking up from the ground) and it wasn’t supposed to get closer than 90 feet to either wingtip. The SOF guys constantly complained about the Combat Talon II’s capability compared to the Combat Talon I (MC-130E). The Combat Talon I was an analogue computational aircraft with a similar terrain following radar mission. The Combat Talon II had a digital computational radar. The Combat Talon II was designed never to break the clearance plane 250 feet from the ground and 90 feet from the wingtip. The Combat Talon I couldn’t help breaking the clearance plane in every direction. It was designed for a similar set clearance plane, and any variation was due to the analogue characteristics of the system and not the intentional design of the system. So a flight development flaw became a basis point for comparison of the two aircraft. Additionally, the SOF guys insisted on a hard ride for the terrain following on the Combat Talon II. Everyone knew that a soft ride gave a lower average set clearance plane, but the engineers could never get this through the SOF mindset. All of this has direct application to zero offset testing on the Combat Talon II.

Zero offset testing is the end point for all terrain following flight testing. In terrain following flight test, you start with flat terrain and check out the system there. In each defined terrain, you test the different clearance planes, the different terrain following modes, and terrain following turning. You accomplish this first at the median speed about 250 knots then at max speed 318 knots and finally at 1.3 Vstall. Then you repeat it on three engines. If the system doesn’t show good characteristics or it breaks an obvious design point, you have to go back home and let the engineers and software guys figure out what’s wrong and fix it.

If the system passes all the terrain testing points, you move the testing to zero offset. In normal terrain following testing you are basically doing everything similar to the way a SOF pilot would fly the aircraft—you are a little more anally retentive: you hold airspeed as constant as possible and you keep the flight director right in the design parameters. In zero offset testing you do something that no pilot in their right mind would try. You aim intentionally at terrain you know the aircraft can’t climb over and you direct the flight path of the aircraft intentionally into the terrain. If it works, the terrain following system will safely turn you away from the terrain, and you will clear everything by 250 feet elevation and 90 from either wingtip. If it doesn’t work, you theoretically are supposed to be able to physically keep the aircraft from hitting the terrain when the system directs you to closer than 250 feet and 90 foot wingtip clearance—that’s what you get the big test pilot bucks for (editor’s note: test pilots are not paid any more than regular pilots in the Air Force). It all sounds simple—doesn’t it? It would be simple if anyone flying in a C-130 could tell when you are at 249 feet above the ground instead of 250 feet and when the wingtip is at 89 feet instead of 90 feet. I have about 2000 hours in almost every make of C-130 and I’m telling you, it can’t be done.

When we would start doing offset points the crew would always ask me if I wanted to fly or act as the safety observer. I would always say “fly.” From the pilot or copilot’s seat in the cockpit, the wingtip is about 50 feet behind you and 50 to the side. If my life depended on it (and it did), I could never tell if the wingtip was going to clear or hit the ground. You would have to be prescient, and this has never been one of my life skills. If we were going to be directed into the ground by the terrain following computer, I always wanted to be the guy at the controls because, if I survived, I could say with confidence: I did what the test plan and the plane directed. I certainly didn’t want to be the guy who came back and said, I thought we were clear.

This isn’t the worst of offset testing. You always started with an easy computer solution: the terrain right ahead. This is easy because the decision making is simple for a computer. If the system passes, you don’t hit the ground—that’s easy. Incrementally, however, you move to more and more difficult solutions. The most difficult is the zero offset parallel. In this test, you set zero offset parallel to a cliff side. Remember, the navigation system is directing the aircraft to hit the cliff. The terrain following system is supposed to cozy you up to the cliff with 90 feet to spare. I was never so sure.

One day we were flying a zero offset test on the flat. This is the easiest of all the zero offset tests. I was in the pilot’s seat, and the other test pilot’s (we always fly elevated risk experimental flight test with two test pilots) name will go unmentioned. At a 250 foot set clearance plane, I pulled the aircraft around the initial point (a small hill) at 250 knots and started into the cliff side. As we made the turn, the other pilot turned to me and said, “Whoo wee that was pretty close.”

“What do you mean by close?

“Oh I suspect we cleared the wingtip by about 20 feet.”

I thought I was going to have a heart attack right there. I couldn’t speak for a couple of moments. When I could finally express myself without opening the swing window and depositing the copilot onto the desert floor, I started a climb and called, “Knock it off and data off.”

The copilot looked at me as though I had ruined the party. Maybe he didn’t realize 20 feet of wingtip clearance is definitely outside the aircraft and my design parameters.

On another occasion, we were transiting between test points—zero offset test points. They say test point transitions are the most dangerous times for all flight test. I agree. During a test point, you are waiting for something to go wrong. You are primed and ready. During transitions, you let down your guard and you are just flying the aircraft. Check the records—more accidents in transitions than during any types of tests. Same copilot; I should have known better. We were crossing the big ridge in the Edwards test area to go from the medium zero offset tests to the mountainous zero offset tests. I started up a canyon with a ridge that the terrain following system said we should be able to easily top. About halfway up the terrain following still said we could make it. I wasn’t so sure. A quick look around the crew showed that many shared my apprehension. I said to the copilot, “I don’t think we are going to clear the ridge.”

“Sure we are. The radar says we will, and it looks good to me.”

Remember, this is the guy who thought 20 foot wingtip clearance was if not reasonable, then at least acceptable.

I said, “Eng compute 1.3 Vstall for me.”

I trolled up the ridge a little longer, right until we hit 1.3 Vstall. That was the lowest speed we were supposed to see during any test point or transition. That’s where I called knock it off. I made the prettiest half Chandelle turn you have ever seen. We saw stall speed and a lot of ground in the windshield. The moment, I stared the turn, the whole crew let out a collective sigh of relief. I reviewed my life right before my eyes. It was the most scared I have ever been in an aircraft. When we rolled out at the bottom gaining airspeed and clear of the terrain, the copilot said, “I still think we could have made it.” The best is I have the whole thing on tape.

Better to be old than bold.

Zero offset testing was the worst, but any terrain following test point could get you in trouble. We were completing a test sequence one day on the medium terrain runs. Now you might think rolling hills when I say medium terrain. There were rolling hills, but there was also a ridge right across the middle—a kind of a pre-mountainous test point. We came over the ridge that day on a slow point (1.3 Vstall), and I noticed that the airspeed increased 10 knots and we descended about 100 feet. That’s kind of significant when you are flying at 156 knots and stall speed is 120 and you are only 250 feet above the ground to begin with. Since we were coasting down hill, the change in airspeed and altitude was remarkable, but not ominous. I remarked to the other pilot that coming back the windshear over the ridge might cause us real problems. Unfortunately, you can’t call quits during fight test just because of a little windshear.

I was flying the point on the way back. I was right at 161 knots (Vstall plus 5, a little margin for mom and the kids) when we hit the windshear. This time we were climbing over the ridge and not coasting down the backside. When we hit it, we lost 100 feet and 10 knots, and the ridge swooped up to meet us. I did the only thing that was possible. I pushed all four engines all the way to the stops and pulled back to the tickle. Thank God, we kept climbing.

Ironically as the ground was coming up to meet us and the engines were putting out everything their little mechanical hearts could, the engineer said, “Pilot, the engines are overtorquing. Do you want me to pull them back?”

I replied with something about if you touch those engines we are going to die or something like that. The engineer didn’t touch the engines—good for him, he was a great flight engineer. I’m glad he asked. This is one of the examples I used to explain to new C-130 pilots why the navigators are officers and the engineers are enlisted. The engineer makes life and death decisions and calculations, but he always has to ask permission of the pilot to act on them. The navigator makes similar life and death decisions, but doesn’t have to ask permission. Good example, not the easiest way to add it to your bag of pilot tricks.

We did significantly overtorque the engines, but luckily there was no damage to them and we went out flight testing the next day.

The MC-130H was the best missionized aircraft in the US Air Force inventory for a while. It was really good because of the extensive terrain following testing accomplished—and especially the zero offset testing.

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