Below is a Nason switch, something I pioneered into an aircraft safety system nearly 20 years ago. It is a very high quality pressure switch, from a company with makes very rugged electrical sensors. Its job is to cut off the primary electrical fuel pump automatically, in case the pilot has a mishap and forgets to or is unable to shut off the critical master putting power to the primary fuel pump.
Picture a 650 just touched down at a busy airport, and running into the wake of a jet that had landed previously. Aircraft gets blown off the runway and the pilot gets knocked out. In this instance, any airplane with a running electrical fuel pump, (not just a Corvair powered one) will continue to pump fuel. If a line is ruptured, it’s a very bad leak. Here is where the Nason switch works: It cuts off the power to the primary pump when the oil pressure drops below 5 psi. Engine stops, fuel flow by the primary pump stops.
I say primary, because on a corvair installation, the Nason switch is just on the primary pump, the back up pump has no switch, to make it as simple as possible. This also allows the engine to be primed and started on the back up pump, as it will flow fuel on command, even without the engine running.
Cars have similar systems, but they don’t have simplicity shown here. 15 years ago I had one builder who loved German things go out of his way to show that he could rig the ignition timing switch from a VW Jetta ( A car that makes the MGB appear to have a reliable electrical system) to do the same function with a bunch of relays and wire. Yes, the Nason switch isn’t the only way to do this task, but when people put their ego aside, its probably the best way.
Again, these just apply to Corvair Powered planes running two electrical fuel pumps, like a 601/650. They don’t apply to gravity feed planes.
3 Replies to “Nason Switch”
Wow. That’s quite an insult for the VW Jetta, considering that the MG-B used an electrical system, produced by the ‘Prince of Darkness’, Lucas Electrics.
Question. Why do Lucas Electrics wires have insulation?
Answer. To hold the smoke in.
I’ve been looking to do something similar to implement a fuel cutoff on a gravity-fed carburetor. There are a number of reliable solenoid valves out there that can shut off a fuel line based on an electrical system. In my day job, I just got done implementing something for somebody who needed to cut off a poisonous gas line if pressure dropped in a way that suggested Boron Triflouride or Arsine gas was being vented into a place where people might be. And in the back of my head, since I do have a gravity-fed fuel system, I’ve thought, “This could be handy.”
And having just bought a ’72 Norton Commando that I had to completely rewire, I’m quite familiar with Lucas Electrics.
Q: Why do the British drink their beer warm?
A: Because Lucas makes refrigerators.
There used to be a website called, “I love Lucas Electrics”. I emailed a friend about it, and he replied that he couldn’t open it. I told him the reason he couldn’t open it was that it was black.