This month’s Ask a Tech feature focuses on airframe and avionics issues. While not overly complex, the wrong troubleshooting effort could derail the ultimate fix. IA Mike Berry and avionics Repairman Larry Anglisano weigh in. First, to the front end of a Piper single.
The nose strut on a Piper PA-32-260 keeps going flat. There are no visible fluid leaks and the aircraft’s owner reports that the strut’s Schrader valve was replaced several years ago when he had this same symptom. Now, it was suggested that the strut needs to be rebuilt, and the owner assumes that means replacing the O-rings. But he asks that if the O-rings were the problem, shouldn’t there be evidence of fluid on the strut? Further, can the strut develop a small crack that would allow the nitrogen to escape without leaving a fluid stain?
Nitrogen can leak without leaving a trail of fluid in the process. Air doesn’t have the same “clotting” effect that oil and fluid have. The suggestion is to replace the Shrader valve housing the O-ring, the valve itself and all seals in the strut, which when done properly should likely eliminate the problem.
Cracked struts usually mean both the air and the fluid are gone. This Piper doesn’t seem to have that problem. When a strut overhaul is needed, ensure that both the strut and O-rings and associated hardware are brought back to correct specs. Plus, struts can be re-chromed by approved repair stations. Trouble is, we see far too many parked aircraft with signs of strut neglect and it’s certainly a maintenance issue you want to stay on top of. Look carefully at the strut during the preflight walk- around and also after flying the aircraft.
Over to the Cessna—a 172N with a King KX155 navcomm and a TKM MX300 navcomm, which is an old slide-in replacement for the original ARC Cessna radio. There’s also a vintage KMA20 audio panel. The owner has been battling a problem where both VHF radios develop severe noise (static) after a certain amount of time in flight, essentially making them useless for receiving.
Before tearing into the wiring and even bench testing both radios, it’s worth a walk around the aircraft to locate each of the VHF comm antennas. No, two radios seldom (if ever) share one antenna, so there will be two. On most single-engine Cessnas of this vintage, you’ll likely find two fiberglass whip antennas on the top of the fuselage, inboard of each wing root. If you only see one antenna, look on the belly for a bent-whip antenna. Inspect the base of each antenna for signs of cracking and make sure there aren’t major gaps between the base and skin. Water intrusion between the base and skin can cause severe corrosion and bonding issues, which can kill performance. Fiberglass comm antennas generally have an anti-static coating to fight P-stat buildup. If you see cracks in the antenna and worn-out fiberglass, it’s long since time for replacement. In flight, turn off the alternator (pull the field breaker) to see if the noise goes away.
Last, the age of the old KMA20 audio panel in this plane immediately piqued our suspicions. We’ve seen small-component failures that induce noise into the headphone audio. Find another KMA20 (a dime a dozen these days) and simply swap it out. With this one, our instinct was spot on and it was a faulty audio panel. Got a tech question? Send it in and we’ll help noodle it.
