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Maintenance Minute - EC135 Blue Light Special

Posted 1 years 249 days ago ago by Admin

Troubleshooting an intermittent yaw caution light with the autopilot and FCDS installed can be a pain sometimes. Since both the yaw SAS SEMA and the yaw autopilot SEMA are mounted on the same control tube at the Fenestron, the yaw SEMA 1 can influence yaw SEMA 2, or vice versa. The trick is to separate the systems by turning off the autopilot and pulling into hover; if the yaw light indication returns, it should be the yaw SEMA 2. If not, it should be yaw SEMA 1. 

Much to a mechanic’s displeasure, the vent grill openings/ducts in the fuselage side panels for the aft avionics deck have been the root of many electrical issues. The problem starts when water is applied with force (e.g. water hose while washing aircraft, etc.) at either vent grill, allowing the water to inundate the components on the avionics deck. One fix adopted by numerous operators is to fabricate and install a simple “hat” or deflector to the top section of the interior fiberglass vent tubes. The deflector should be mounted above the vent tube edge on tabs, and designed to channel/deflect any water to the interior surface of the fuselage side panel. This will still allow hot air to vent out while directing the water toward the lower floor tunnel drains via the side panels.  It’s not 100% corrective, but coupled with additional training to avoid these vent areas during aircraft washing, it keeps these issues to a minimum.

And speaking of water infiltration...sometimes water mysteriously finds its way into the aircraft master electrical boxes. Water can come from either the vent tubes mentioned above, unsealed FADEC harness panels on the engine deck, condensation issues, or simply magic! But not to fear. For those who remember the infamous AS355 Twin Stars and their dual electrical boxes, the EC135 system is just the latest and greatest design, right down to the “Z” cards. Thankfully these boxes are now robust metal boxes instead of the old “Tupperware” containers with lids that didn’t fit. But I digress. Any time unusual electrical problems pop up that normal troubleshooting methods can’t isolate, check both master electrical boxes for signs of moisture. Be sure to look at both boxes since they contain microchips that monitor each other. With aircraft power OFF, disconnect the aircraft battery and remove the cover panels on the electrical box. There will be five “Z” cards inside, including one that is a double-stacker. Each one slides out with a little persuasion. Check for signs of moisture or traces of corrosion. If excessive water is noted, the interior of the boxes can be dried with a fan or blower. Use a pencil eraser to clean the card connection bar if needed. Once dry, reinstall cards, reconnect the aircraft battery, and perform your standard smoke check. 

About the author: After 32 years maintaining helicopters in various capacities, Scott concluded a full-time career with a major operator in 2014. When not pursuing future writing projects, he can still be seen around the flight line tinkering on aircraft for beer money. He can be contacted at [email protected]