Turn on the UHF and the Plane Flips Over

September 09, 2015

Many years ago while I was a naval avionics technician new to the F-4 Phantom aircraft, a pilot wrote a gripe against one of the aircraft saying, "Every time I key the UHF the aircraft tries to invert." Obviously all of the tecnicians and most of the pilots got a good laugh. The pilot who wrote the gripe was not a newbie and knew no one would believe it, so he had arranged with the tower at Miramar for a no radio takeoff with the same aircraft and a fly over where he would transmit on the UHF as he passed overhead. We had a UHF radio in the shop so we could test the radios before we installed them, allowing us to listen to his transmission.

When he passed overhead and keyed the radio the aircraft did act like it was trying to invert, one wing lifted very quickly. The pilot unkeyed the radio and the aircraft uneventfully landed with no more transmissions. When the aircraft landed I met the pilot before he shut down and requested he only shut down the port (left) engine so that I could climb the ladder to the cockpit with power and hydraulics still running. Every time the UHF was keyed the Flaperons did a large flutter. This had never been seen before and baffled everyone. After much deliberation and looking at the schematics it was determined that the only electronic input to the flight controls was the Stabilization Augmentation System (StabAug System). This system is used in high speed jets to help smooth inputs to the flight controls. The StabAug System was then turned off, with no difference, the flaperons still fluttered when the UHF was keyed.

We went back to the schematics and discovered that when the StabAug was turned off, the only thing that really happened was the input to the sensor in the tail of the aircraft was grounded. These wires from the tail of the aircraft to the StabAug control box in the cockpit ran in the same wire bundle as the UHF antenna coax, which ran from the radio forward to an upper/lower antenna switch for the radio. We disconnected the output of the StabAug and the symptoms went away. Delving into the wire bundle, we found that the UHF coax was badly freyed, allowing the UHF to transmit into the poorly grounded wires going to the StabAug, which were now acting like an antenna. Replacing the coax from the radio to the antenna switch fixed the problem. The relay putting the ground on the input to the StabAug was also replaced since it has more resistance than it should when deactivated.

Four years later, I was the shop supervisor with orders to my new command in hand when a complaint came in saying, "Every time I key the UHF the plane goes squirrely." Many of the techs and pilots laughed. I was one of the few left in the squadron that had seen the first time it happened. I arranged for power and hydraulics carts to be connected to the aircraft and mustered the shop around the aircraft. The problem was demonstrated to all, including the shop chief and the maintenance officer. I then advised the junior crew what the problem was and how to fix it. Hopefully they will still be around the next time it happens.

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