When You’re Hot, You’re Hot – When You’re Not, You’re Not
But you still may spark! Part 2
By Fred Workley
To be a good trouble-shooter on aircraft, you sometimes need to be like Sherlock Holmes. Not that you have to wear a hat like his character or smoke a pipe, but you need to be a good investigator. We found out last issue that Mel and Ray had a lot of information about their airplane. What would you expect them to tell you if they brought you their Turbo Arrow III for repair? You have to search out the clues and solve the riddle. "When You’re Hot You’re Hot, When You’re Not Your Not; But You Still May Spark!"
Look for the clues
How much do you know about the aircraft that you are about to trouble-shoot? Do you have the maintenance manual available and do you have time to study it? I owned a Turbo Arrow III for 20 years, so I got to know the maintenance manual quite well. Not everything is in the maintenance manuals. Your logical sense of reasoning has to come into play. We have certain known information about the airplane like the hours on the airplane, who owns it, a history of the in-service use and damage history.
Often, the clues we need to repair the airplane come from the pilot. They get to know the individual airplanes that they fly. The pilot reports symptoms of a fault or failure. In Mel’s case he is flying solo and he is busy. Pilots usually develop a routine. This routine has them constantly checking and cross-checking everything. If anything is "not normal," it quickly gets the pilot’s attention. Pilots also perform checklists to determine if everything is normal. Subtle changes in noise, vibration and gauge readings are indicators to the pilot. Mel realized that during his flight, there were several things that were not normal.
We found out that the Number Two VOR navigation radio was passing the noise and interference generated somewhere in the aircraft into the navigation radio. Even though this situation is highly improbable, Mel could see that the navigation signal was unreliable. The circuits that supply power to the radio are filtered so that noise (static) from the aircraft should not pass into the radio.
Finding: The Number Two VOR navigation radio is connected through the coaxial cable to a dipole "Cat’s Whiskers" antenna. Upon inspection, it was found that the anchor screw in the micarta had broken allowing the right pole of the antenna to move freely in a seven-inch arc. This broke the wire at the wire crimp on the screw where it was mounted on the antenna. The signal was most probably somewhat less with only one pole of the dipole antenna connected.
This antenna coax had been routed through a diplexer coupling system that was also connected to a Number Two glide slope receiver for the HSI (Horizontal Situation Indicator). This is not a good practice because the Glide Slope receiver is UHF (Ultra High Frequency) with its signal being received from a VHF (Very High Frequency) antenna. Note: The duplexer is a component of the radar system that permits it to transmit and receive simultaneously. This broken wire would not have affected the Number One VOR navigation receiver. Sometimes, air turbulence will finally break a wire that has been breaking, strand by strand, for some time. Poor connections in wire crimps or wire with long unsupported runs between clamps sometimes break or wires pull out of crimped connections due to the constant vibrations in the aircraft.
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Training and troubleshooting as a basis for industry knowledge