Squawk Box

December 2017 Issue




Master Switches

The following information is derived from the FAA’s Service Difficulty Reports and Aviation Maintenance Alerts.

Cessna Model 172S Skyhawk SP
Arcing Avionics Master

Pilot reported burning smell and failure of avionics #2 buss during run-up. Troubleshooting revealed the right side of the avionics master switch (#2 buss) had been arcing internally. Replaced switch; ops check okay.
Part total time: 1018.0 hours

Cessna Model 172S Skyhawk SP
Arcing Avionics Master II

ATC stated transponder wasn’t showing altitude. Went through five transponders before manufacturer figured out the avionics master switch (#2 buss) controls the transponder and was spiking the circuit, frying a component. The switch had been arcing and its contacts were deformed. System has been working flawlessly since. Avionics master switch was original to the airplane.
Part total time: 1720.2 hours

Cessna Model T182T Turbocharged Skylane
Intermittent Master Switch

Pilot reported if the master switch was cycled several times, it would eventually turn on aircraft power. New master switch installed; operation normal.
Part total time: 881.0 hours

Cessna Model 425 Conquest I
Defective Avionics Master

Avionics master switch was found inoperative upon functional check. Part was replaced. Old part was discovered to have a manufacturing defect.
Part total time: Unknown

Learjet Model 55
Inoperable Steering

Crew reported ground steering problem. Examination revealed a fully operational system except for the co-pilot’s control yoke steering master switch, which was found to be sticking from FOD. Removed, cleaned and reinstalled co-pilot’s control wheel steering switch.
Part total time: Unknown

Piper PA-31-350 Navajo Chieftain
Defective Avionics Master

Shortly after takeoff, aircraft lost all avionics. Crew returned to departure airport without incident. Removed and replaced defective avionics master switch.
Part total time: Unknown

Piper PA-34-200T Seneca II/III
Corroded Master Switch

Master switch failed closed, energizing electrical system when switch was in off position. Investigation revealed switch very difficult to move to on or off position. Replaced switch with new and opened up the old one. Discovered switch internally wet and corroded; suspect condensation as there is no evidence of leaks and the aircraft was hangared.
Part total time: 203.0 hours