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Feature Article: 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.
Last Modified January 27, ©2026 TAGE.COM
