Private Pilot Flight Training and Instruction Flight Training Article Library | Back To 4VFR.COM 521 Maintenance During WWII one of the very first airborne radar sets from England was code-named the 521. Aircraft using this set were equipped with a set of Yagi transmitting and receiving antennae projecting forward on the wings. The Yagi was named after its Japanese inventor and would be easily recognizable even today in those areas that use housetop antenna instead of cable for TV reception. This set consisted of a set of components consisting of a combination transmitter/receiver, a box that contained the circuits that made the pulse waves to be transmitted and the timing circuits used to present the signals on the cathode ray florescent screen. The radar operator had to use a scale on the screen to determine the distance from transmitter to target by halving the time of the transmitted wave and reception of the echo. By watching the movement of the target 'blip' on the screen it was barely possible to determine the size and speed of movement of the target relative to the transmitting aircraft. I had occasion to take a training flight out of Boca Raton, FA using such a set in a Lockheed Hudson bomber that had been returned from Britain as war weary. It still had the bullet holes to show why it was put out to training pasture. It had the 521 installed and was being flown for training purposes. While I was being trained as the operator, the pilot asked if I had a target and if I could provide any identification for him. Fact is I had seen the target, I had no idea as to what it was but was able to give distance as approximately thirty miles. It was a large target but too slow for an airplane and too fast for a ship. I was invited into the cockpit to see a blimp on anti-submarine patrol out by the Bahamas. As I returned to my position I found that the set had failed. It was then that I learned about 521 maintenance. The various components of the 521 were interconnected with cables and 'cannon-plugs' that consisted of male and female components that went into each other and were tightened into position by screwing a threaded cover over the connection. The back of each box could have any where from three to six cables installed so as to feed the required power and signals to each component. The cannon-plugs were a frequent cause of electronic failure due to corrosion and aircraft vibration that affected the connections. It did not take experienced operators long to uncover the practical solution to cannon-plug failures. All it took was for the operator to lean back in his seat and lift his feet in such a way as to allow you to give a component a good kick with the bottom of your heavy G.I. shoes. This corrective action came to be known as 521 maintenance among airborne radar mechanics and operators. On occasion, I used 521 maintenance on later model airborne radar sets with the B-29s in India. With the miniaturization of electronics political correctness made such kicking inappropriate. Use your hand. Written by Gene Whitt Flight Training Article Library | Back To 4VFR.COM