Recently, I went on a road trip to troubleshoot a flap problem on a Cessna Citation 650. The flaps would begin to move downward and after a one second run, they would stop and restart. Sometimes they would come to a complete stop and trigger a flap fail light. After running the troubleshooting program and reading the maintenance data provided, a power relay was identified as the most likely culprit. Several hundred dollars later, the problem still existed. After speaking with a tech rep. as well as an avionics lab technician, it was decided to pull the flap relay junction box as well as the flap control handle and send them in for testing. Minor problems were found, and several thousand dollars later, the problem remained. An inspection of the flap angle gearboxes as well as the flex cables and actuators revealed no problems. After several more hours of troubleshooting and using basic electrical knowledge, I was finally able to track the problem down to the High Speed Relay not getting enough power to hold it open. An inspection of the wiring revealed no chafes, and a steady source of 28V dc. Load testing the wiring from the Flap Control Unit to the High Speed Relay revealed no problems. This left one component as the culprit. The Flap Control Unit was not sending the proper signal (voltage) to the High Speed Relay, which was causing the system to malfunction. All in all, this task turned out well, but left me wondering what I could have done differently to avoid the costs of parts not needed.
Troubleshooting guides are just what they say they are. They are simple guides that are designed to point you in the right direction. Sometimes the language of a trouble shooting guide can be misleading, so it's very important that if it directs you to a component that failed, then check everthing that leads up to that component before you go ordering parts. The manufacture is in business to make money, so they will be glad to sell you all the parts you want.....even if you don't need them. Always dig deeper to find the total problem, and you may find the problem was caused by something as simple as a wire chafe.