Lt.Col Tom Rodgers and what is left of his F-111 tail section. (Photo: Authors collection).
by Lt. Col. Tom Rodgers, USAF (Ret)
16 JAN 1979
A beautiful winter morning on the high plains of eastern New Mexico. There was not a cloud in the sky for hundreds of miles and temps were in the low 30’s. A great day to go fly.
I was an F-111D Instructor Pilot (IP) in the 481st Tactical Fighter Training Squadron based at Cannon AFB. My mission for the day (if I chose to accept it, as they said on Mission Impossible) was to take 1Lt Junior Hill (a new aircraft commander) on his “Dollar Ride” in the F-111D. This was mission TR-2 (transition) in the F-111D training syllabus and entailed showing the new guy the full envelope of the Aardvark; from fast to slow, high to low. The highlight of the ride was the Mach run where he got to see the plane literally go the “speed of heat”. The friction from the air at those speeds heated the surface of the airplane and the F-111 had an automatic countdown timer that told us when we had to slow and not exceed the skin heat limits.
We launched about mid-morning with Junior at the controls and headed northwest to the Las Vegas, NM TACAN. This was the start point for the supersonic corridor that ran south to the Salt Flat, TX TACAN, just east of El Paso. Climbing through 30,000 feet, Albuquerque Center cleared us to begin the high-speed run. Junior put both throttles into Zone 5 afterburner and pulled the wings all the way back to 72.5 degree sweep. As the indicated airspeed reached 400 knots, I told Junior to pull back on the stick and hold that airspeed as we climbed and the Mach number increased.
We accelerated smoothly, and there was no real sense of speed as we neared 50,000 feet. The only way we could tell how fast we were going was to look at the mach meter. The view was awesome. That high above the earth, we could see for hundreds of miles. The horizon was beginning to curve and the sky above was almost black. Though it was near noon, we could see stars above us in the sky. At about 2.3 mach, I began to feel a slight buzz or hum in the airframe. I thought we might be experiencing mach buffet and told Junior to back off the “G” loading a bit by pushing slightly forward on the control stick. He did as I asked but things got rapidly worse. We were still accelerating, and soon we were at 2.5 mach. The hum had increased quickly in intensity until it became a heavy, violent vibration that made reading the cockpit instruments nearly impossible.
At that time I had nearly 2000 hours flying various models of the F-111, and had never seen or heard of a situation like what was happening to us. We were in uncharted waters, as they say. Several things went through my mind in more or less random order. Did we still have control of the airplane, because it felt like it was coming apart? If we did have control, could we slow down before it came completely unglued? Lastly, if we didn’t have control, we would have to eject at some point, but I didn’t want Junior to panic and eject us at 2.5 Mach and 50,000 feet.
I told him that I had control of the aircraft and to “put his hands in his lap and don’t touch anything.” I pulled the throttles back to min A/B to slow (idle power at that speed would cause both engines to compressor stall which would make the aardvark a supersonic glider provided the engines didn’t blow up first). As I slowed, the intensity of the vibration decreased and once I was subsonic, it was barely noticeable. I declared an emergency and called back to the base to talk to the Supervisor of Flying (SOF). Lt. Col. Larry Colliflower was pulling that duty, and I asked if he had any ideas about what might have happened. I also requested that another airplane join up with us and look us over for any external damage. He said that no one else was airborne to help but that we might have damaged the over wing fairing. This was the hinged top of the airplane that sealed the fuselage to the wing as the wing swept fore and aft. We had damaged a few fairings at Red Flag on occasion, outrunning the Aggressors. We were pretty much on our own, so the best we could do now was fly slowly back to the base and do a “controllability check”. If we could get the airplane into a landing configuration of some sort (wheels down and some slats and flaps) and still have fairly good control, we could land. If not, then Junior could pull the ejection handle and give us the rocket ride of our lives.
As it turned out, the airplane flew well in the landing configuration and the approach and landing were nominal, as NASA would say. I even had a 10-knot direct crosswind from the right, but it posed no problem. As we rolled down the runway with fire trucks all around us, the control tower called and told us that most of our rudder was missing. I could have done a fly-by of the tower before landing to have them look us over but in retrospect I think I would have been more worried about the crosswind landing if I had known the extent of the damage. Ignorance was bliss in this situation.
The post-incident investigation found that the damage to the rudder began right after we took off. The rudder is made up of a high tech foam aluminum honeycomb to reduce weight. In this case, moisture found its way into the bottom of the rudder between layers of the honeycomb. The heat from the afterburners turned this moisture to steam, which expanded and caused the rudder to delaminate. Once we were supersonic, the shock wave moved aft on the fuselage and became unstable when it reached the delaminated portion of the rudder causing it to vibrate. The yaw flight control computer through the yaw damper tried to counter the vibration. Instead, it caused some sort of harmonic vibration that was out of phase with the actual damper commands. This in turn caused the rudder to flutter at increasingly higher rates. Thus, the rudder basically ate itself alive until we slowed subsonic and the shock wave moved forward away from the damaged portion of the rudder. The irony of the whole situation was that the original delamination was about the size of your small fingernail. A piece of aluminum that small almost caused the loss of a 100,000 pound airplane. As the old saying goes, “For the want of a nail, the battle was lost.”
About the Author:
Tom Rodgers is Daedalian named member #364, and currently flies for American Airlines. His wife’s recently published novel, The Final Salute, by Kathleen M. Rodgers, was reviewed in the Fall 2008 issue of Daedalus Flyer. For more information access the Amazon page at www.facebook.com/l/ad83c;www.thefinalsalute.com.