In the Air Force, Functional Check Flights (FCFs) are flown when an airplane comes out of ‘phase’ maintenance, or when it has been repaired following damage. An FCF profile ‘wrings out’ the airplane to make certain it is ready to ‘fly and fight’.
When an engine was worked on or changed, FCFs involved inflight engine shutdowns to make certain they would restart. They also involved some distinct high and low-speed maneuvers including supersonic flight (not in the OV-10!), and pulling lots of G’s (nine of them in F-16s!).
When I was flying F-16s at MacDill AFB (MCF) in Tampa, FL, you knew you were watching an FCF by how the takeoff was flown. Always flown in a ‘clean’ airplane, (no external stores like fuel tanks, bombs, or missiles), FCFs are also single-ship, meaning no wingman. After taking off and raising the gear, the pilot would level off at 100-150 feet and continue accelerating to the end of MCF’s 12,000’ runway in full afterburner. At the departure end, they would do an Immelmann maneuver to arrive at 14,000’ AGL directly over the field. This wasn’t done as a ‘shine your @$$’ maneuver; rather, it put the pilot in the position to do an engine shut-down and restart. If the engine didn’t restart, then the airplane was at ‘high-key’ to make an immediate flameout landing. If the engine restarted, the pilot would fly to a nearby Military Operating Area to continue the FCF profile.
I only flew FCFs when I was a Forward Air Controller (FAC) flying the OV-10 Bronco in Southeast Asia (SEA) – callsign Nail 49. Join me now as I take you through some exciting times as an FCF pilot.
During my checkout, I took my first ride in the back seat to observe an FCF first-hand. We had a regular crew chief and two fire guards launching us that day. The #1 engine start was normal, but when #2 was cranked, a ‘blob’ of fuel dropped out of the exhaust stack and fell to the ground, landing next to our right main gear. Did I mention that it was lit? The crew chief signaled us to shut down, and the PIC pulled both engine condition levers into the ‘Fuel Shut-off’ position. However, he continued to crank the #2 engine as residual (and lit!) fuel continued to drip from the exhaust stack! Meanwhile, the two fire guards sprang into action, which quickly became a Keystone Cop routine.
While moving the fire bottle closer to the fire, one of the guards pulled out the hose readying it to extinguish the fire. However, the other one pulled the discharge trigger prematurely and doused his partner in fire retardant. The poor guy staggered off wiping his face. The remaining guard quickly smothered the fire. With the fire out, we shut everything down – the FCF was done before it got started! The fire guard who got hosed down was none the worse for wear (or extinguisher fluid!).
Although FCFs were flown with a clean airplane, the ground crew closed the hooks that secured our combat load-out (three rocket pods and our 230 gallon centerline fuel tank) in their place. This was done so we could verify they would open when needed, such as when an engine failed on takeoff and you needed to jettison as much excess weight and drag as possible.
On FCF takeoffs, we would go through the Critical Action Procedures (CAPs) for an Engine Failure on Takeoff: 1) Gear – UP; 2) Stores – JETTISON; 3) Condition Lever – FEATHER. Step 1 reduced drag; step 2 reduced weight and drag; step 3 also reduced drag by keeping the dead engine’s propeller from becoming a speedbrake. We did this so the muscle memory would be there if an engine quit on any takeoff.
Of course, we actually did step one, and, although we didn’t have any stores to jettison, we also mashed the jettison button, opening the hooks closed earlier. After landing, we checked to make certain they had all opened. Finally, we only touched the condition lever; but didn’t move it out of the ‘Takeoff & Land’ position.
With the Engine Failure on Takeoff CAPs complete, we continued down the runway and activated a spring-loaded switch mounted on the gunsight in front of us. The OV-10 has a reservoir in the left main gear-well containing hydraulic fluid or oil. Holding that switch open pumped fluid from the reservoir into the exhaust of the #1 engine creating a smoke trail. That smoke helped fighters spot us FACs over the jungles of SEA. There was apparently a ‘new guy’ in the tower one day because, when I hit the smoke switch, he broadcast, “NAIL 49, YOU’RE ON FIRE!” I thanked him and assured him I was fine. I’m sure the ‘old-head’ tower operators had a laugh at his expense.
My most memorable FCF was when I had an airplane whose #1 engine had been worked on. I had to shut it down and then do an air-start in order to verify that the procedures worked. During my checkout, I picked up the technique of raising the guard on the start switch for the engine I was shutting down. That was so I would engage the correct start switch when restarting the engine. It became apparent to me that day that it also served another purpose.
To begin, I moved the #1 condition lever from ‘Normal Flight’ to the ‘Fuel Shut-Off’ position and the engine started winding down. As the propeller slowed to one to two revolutions per second, I moved the condition lever to the ‘Feather’ position and #1 came to a full stop. And that’s when it happened – the #2 engine started winding down as well! I now saw the advantage to raising the guard on that start switch!
I quickly shoved the #1 condition lever back into ‘Normal Flight’ and engaged the #1 start switch. The engine started winding up immediately and, thankfully, #2 came back to life with no input from me – other than some strong, non-aviation related language! I logged about 0.2 seconds of glider time and, deciding it was time to get this Bronco back to the stables, I declared an emergency and promptly got the airplane safely back on the ground.
We discovered the problem was the engine-driven, fuel boost pumps. Both engines had their own boost pump and one of them would provide adequate fuel flow to both engines throughout the flight envelope. But, in this instance, only the #1 boost pump was working. After I started #1 on the ground, #2 had fuel provided to it by #1’s pump, so it too started normally and I was able to taxi, takeoff, and begin the FCF. However, when I shut down #1 in-flight, there was no fuel being fed to either engine, so #2 also quit.
As a result of this ‘incident’, the procedure for engine start prior to takeoff became: start #1 and then #2; with #2 running, shut down #1; if #2 continued running, restart #1 and press.
I mentioned earlier there were some distinct maneuvers done on FCFs. To check for two possible yaw problems on the OV-10, we did what we called “The Dive For Death!” (DFD). The Bronco was a great airplane from which to observe things, which was our job as FACs. The large expanse of plexiglass comprising our canopy helped us do our job. However, a drawback to all that plexiglass was the airplane’s limiting airspeed of 350 knots; exceeding that speed could cause canopy failure! But there were yaw checks to be made, thus the need to fly DFDs!
The first yaw check was accomplished in that dive. It looked for any induced yaw caused by misalignments in the twin booms/rudders extending from the Bronco’s wings. After pulling out of the dive, the second check looked for adverse yaw. We executed an aileron roll in each direction before the airspeed dropped below 250 knots. That checked that the spoilers on top of the wings were operating. Then stick left, check left aileron up, right aileron down, and left spoilers up to counter the adverse yaw – repeat the procedure to the right.
Needless to say, DFDs were an exhilarating ride neither Disney nor Six-Flags can duplicate! DFDs also prepared me for my FCF ‘closing act’. Prior to launching, I would tell the ground crew that, if I ‘released’ the airplane (meaning it passed the FCF), I would signal that on my return by doing what we called a Nail-Break.
We Nail FACs often did Nail Breaks upon returning from combat missions, but we especially did one on our ‘fini-flight’ – our last mission. You may ask, “What’s a Nail Break?” I’ll explain.
A Nail-Break is similar to what one hears when a powerboat makes a hard turn. The laminar flow of the water across the boat’s propeller is no longer smooth and streamlined, it becomes irregular and chaotic – and it gets noisy as the prop cavitates. Similarly, a high-speed, hard turn in a Bronco caused our propellers to cavitate and create a lot of noise.
We executed Nail Breaks by entering the traffic pattern on a right or left downwind at 1000’ AGL. However, we initiated that entry the same way we did a DFD. We’d inform the tower we were two miles east (or west) for a downwind entry to the pattern. The ‘old-head’ tower operators knew we were actually two miles overhead and slightly offset from the downwind. Meanwhile, the ‘new guys’ would be looking for an OV-10 around 1000’ AGL and two miles east (or west) of the downwind. Once cleared to enter the downwind, we would execute a DFD pulling out in a steeply banked, 5-6 G turn to arrive at 1,000’ on downwind. The induced drag from both the G-loading and the cavitating propellers quickly slowed us belowlanding gear lowering speed. The resulting roar created by our cavitating propellers was heard across the base and let the maintainers know they had a Bronco ready to ‘fly and fight’!