ATA 24

Aircraft Grounding Points
Several instances have been reported of failing electrical grounding points. These connections have developed significant arcing, enough so that the cables have eroded a hole completely through the bulkhead frames. Compliance with the Service Bulletins 1124-24-120 will help minimize this. The areas to pay special attention to are the grounding cable GND 87, 88, 89 and 90, these are the attached at frame station 383 on the ceiling. Also the external power connection to the airframe, this is located at frame station 402.810. Holes in the frame may not be visible until the cables are removed.

ATA 24

Several instances have been reported of electrical grounding points failing. These connections have developed significant arcing, enough so that the cables have eroded a hole completely through the bulkhead frames. Compliance with the Service Bulletins 1124-24-120 will help minimize this. Pay special attention to the grounding cable GND 87, 88, 89 and 90. These are the attached at frame station 383 on the ceiling. Also, pay attention to the external power connection to the airframe. This is located at frame station 402.810. Holes in the frame may not be visible until the cables are removed.

ATA 24

Reverse Current Relay (RCR)

Have you experienced trouble getting the left or right generator to come on line after an engine start? Mechanics have experienced this many times and have fixed this problem by simply swapping the two generator Reverse Current Relays (RCRs). The RCRs have an internal case ground, and once the RCR is installed in the DC Contactor Box, it is then grounded to the airframe. The wiring prints do not show this case ground anywhere except in 24-30-00 (the large generic schematic). When I talked to the repair shop that overhauls the RCRs, I was told that the internal case ground is a common problem with those particular units due to corrosion and fatigue issues. So, it's not by magic that the RCRs tend to fix themselves by swapping them. In actuality, removing and installing the RCR from the DC Contactor Box more than likely cleans the corrosion from the grounding points at the mounting bolts. The first step in troubleshooting the RCR is to find out if it has power going to it. I use a simple test light made for a 28 VDC circuit with two insulated alligator clips. Clip one end of the test light to the SW terminal of the RCR and the other end of the test light to a known good airframe ground. After engine start, see if the test light is illuminated; if so, power is good, so you then have a "ground" issue with the RCR. After engine shutdown, you can check the case ground using an ohmmeter. Disconnect the SW lead from the RCR and check for continuity between the SW terminal on the RCR and a known good airframe ground. Remember that you will get resistance having to go through the coil inside the RCR, but you should have some continuity and not an open line. An open line more than likely means that the internal case ground is no longer good and the unit will need to be replaced or sent out for repairs. Compare this reading with the opposite "good side." If your resistance reading is higher on the "bad side" than the "good side," then you probably just need to clean the mounting bolts and pads by removing the RCR and cleaning all points of contact between the DC Contactor Box and the RCR, and then re-installing and re-checking. This should fix your problem, but if not, you will need to replace or repair the RCR.

ATA 24

Electrical Performance Deterioration
Electrical gremlins have been teasing us since Benjamin Franklin hung that key on his kite string so long ago. With that in mind, I thought a quick refresher on the importance of proper grounding was in order. How many times has troubleshooting led us down the wrong path because of either a poor or non-existent grounding point? It is easy to forget the simple things when we are looking for power supply or component integrity, a blown fuse, or a popped circuit breaker. A simple ground connection overlooked can be a missed opportunity to look like a troubleshooting genius. Our venerable fleet of Westwind Aircraft has been in service now for quite sometime with the newest aircraft having more than 20 years of duty. With time, corrosion, dirt, vibration, and/or combinations thereof can play havoc on the overall performance of an electrical system. Anything that will increase the ground path resistance is a gremlin’s paint brush slowly eliminating the required bonding. It is important that electrical bonds are properly maintained in order to minimize radio interference due to precipitation of static electricity and to provide effective electrical grounding of the aircraft. Static discharge build-up, St. Elmo’s fire, and lightning strike damage potential are all affected by poor electrical bonding. Good electrical continuity is required between all wing panels, fuselage panels, doors, stabilizers, and flight controls. When inspecting the aircraft for continuity, the use of stainless steel test probes is recommended to ensure penetration of protective metal coatings and accurate resistance readings. It is also important to use calibrated testing equipment that is in good repair. Resistance of test leads and jumper cables must be subtracted from readings obtained to get the actual bonding resistances. For surfaces fitted with bonding straps, measure resistance across structures adjacent to the bond strap attachment points. Do not measure at either end of the strap at fasteners or terminal lugs. Visually check bonding jumpers to assure that they are not broken, frayed, or missing. If you’re new to the aircraft, you may not catch a missing jumper, but if the area looks to you like it should probably have one, check the manuals. The Aircraft Maintenance Manual (AMM) has the specific details for the correct resistance between each measurement point and the zero reference point; it will vary between bonded and un-bonded components and surfaces. Never leave it to memory. Always use the latest revision of the appropriate manual when working on an aircraft, as we continue to update and improve the manuals on a regular basis.