There I was, high over the Texas prairie, enjoying the view as Junior?s autopilot drove us home after a long (but successful) day of flying the RV0-1 in Fort Worth. Suddenly, without warning ? the oil pressure annunciator light comes on! Now THAT got my attention, so I looked for a confirming ?Master Alarm? light from the EFIS?.and none came. I looked at the oil pressure on the panel ? nice and steady at 78 psi. Well, it varied just a little ? enough to tell me that it was, in fact, reading correctly, and still showing plenty of pressure. The annunciator is driven by a simple oil pressure switch screwed into the transducer manifold, and obviously, something was wrong with the circuit. I put it out of my mind for the rest of the trip, and when I got home, I pulled the cowl to check it out.
The pressure switch that I use is a regular automotive switch, used almost universally in cars, trucks, generators, pumps ? any place where you want a switch to complete a circuit when you have less than 5 (or so) psi of oil pressure. I?ve never had one go bad in decades of using them in an airplane ? but this one really had an interesting failure mode ? not only didn?t it work electrically (I checked the circuit quickly by pulling the wire off - the light went out- and grounding it to the airframe -the light came on), this one also failed with a LEAK! The case of the switch was wet with oil, and a drizzle of engine oil ran down the firewall. I check the connections, and everything was tight ? the case of the switch itself was leaking. There was no general spray of oil, so the leak was really tiny ? but it clearly had failed in a bad way, and I quickly replaced it with a spare.
This is one of the reasons for the restrictor orifice in the oil pressure fitting on the engine ? in case of a gauge leak, we won?t lose much oil. But it is something to check, as I know that many, many people use these automotive switches for this job. I know that some will say that you shouldn?t use an automotive witch in this application, but I have looked at the ?aircraft? ones sold by the popular supply houses ? they appear to come from a very similar source. This certainly isn?t a catastrophic failure (so long as you have the orifice fitting), but it is something to note the next time you have your cowl off.
Paul
The pressure switch that I use is a regular automotive switch, used almost universally in cars, trucks, generators, pumps ? any place where you want a switch to complete a circuit when you have less than 5 (or so) psi of oil pressure. I?ve never had one go bad in decades of using them in an airplane ? but this one really had an interesting failure mode ? not only didn?t it work electrically (I checked the circuit quickly by pulling the wire off - the light went out- and grounding it to the airframe -the light came on), this one also failed with a LEAK! The case of the switch was wet with oil, and a drizzle of engine oil ran down the firewall. I check the connections, and everything was tight ? the case of the switch itself was leaking. There was no general spray of oil, so the leak was really tiny ? but it clearly had failed in a bad way, and I quickly replaced it with a spare.
This is one of the reasons for the restrictor orifice in the oil pressure fitting on the engine ? in case of a gauge leak, we won?t lose much oil. But it is something to check, as I know that many, many people use these automotive switches for this job. I know that some will say that you shouldn?t use an automotive witch in this application, but I have looked at the ?aircraft? ones sold by the popular supply houses ? they appear to come from a very similar source. This certainly isn?t a catastrophic failure (so long as you have the orifice fitting), but it is something to note the next time you have your cowl off.
Paul
