Builders,
Here is a look at a simple, but important part of the Corvair installations which require electric fuel pumps. Please note: While this part looks identical to the switch we used from 2003-2005, it has a critically different pressure rating, and no Corvair powered plane with electric fuel pumps should be flying with the earlier number. Nason’s part number for the correct unit is SM-2C-5F. I mention this because just this year I found an aircraft in the Corvair fleet still flying the wrong part number.
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The purpose of the switch is simple: If the plane has an accident, and the engine stops but the primary fuel pump is left on, The switch will detect the oil pressure dropping, and automatically cut the primary electric fuel pump off, without the pilot having to act. Note that the system is not used on the back up electric fuel pump, for reasons of having the simplest back up possible. Our 601XL, N-1777W, may not have been the first experimental aircraft to use such a system, but we were the first people to widely popularize the need for it in all planes with primary electric pumps. It was nominated for an EAA award for safety design of the year, but nothing came of this and the idea was not published beyond our personal efforts.
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There are alternatives to this derived from a Bosch system extracted from German cars which cut off the pump if it detects the coil is no longer firing, but no one should ever connect any device to the Corvairs’s ignition system that it does not need. Here is an example of that mistake: MGL vs Corvair ignition issue. No one should connect a tach, sensor or any other device to the ignition system, it is a failure point. I have been writing that for 20 years, but people still do it, and it has caused issues, but thankfully no one has been seriously hurt…yet. Don’t be the first.
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Above is a 2008 picture from our website, with 13 Nason switches, part No. SM-2C-5F. We were reminding builders then to switch to use. This switch automatically cuts off the fuel pump when the oil pressure drops below 5 psi. The original switch was the same function, but the switched closed at 20 psi.
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We started with the 20 psi switch because we originally used the Corvair’s mechanical fuel pump (We stopped this in 2004 when we conclusively demonstrated that all modern replacement Corvair mechanical pumps were prone to leaking), as the primary. The electric back up fuel pump was automatically activated when the mechanical dropped below 4 psi fuel pressure, and was automatically stopped when the oil pressure was below 20 psi. This prototype mechanical/electric system was replaced by modern system we have today in the summer of 2004. We originaly kept the 20 psi switch.
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However, with many builders retrofitting 5th bearings, some engines would have a hot idling oil pressure below 20 psi, and this could potentially lead to a builder gliding in on final with a hot engine and the low oil pressure cutting off the primary fuel pump. Switching to a 5 psi Nason prevents this from potentially happening, We have promoted this almost 9 years, but some builders with 5th bearings missed this important change.
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We now have a large number of SM-2C-5F’s in stock and will be glad to supply them at $43 including S&H in the U.S. It is part number #3410 at this link: http://www.flycorvair.com/products.html
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Above is the ignition wiring diagram for a a Corvair system. The Nason switch is on the upper left. Note that it is only wired into the primary fuel pump, not the back up .-ww.
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Thanks much for the timely diagram, I will integrate this into circuitry I am currently wiring!
I see I have 3410 on order and pleased to find that it wires to the hour meter as well.
Philosophical question: How many people will go back to the original (protected) ignition/pump switch position if they experience an engine problem that is NOT remedied by the alternate position? Personally, if my engine starts running rough, carb heat would be the first thing tried. Second would be an alternate ignition source followed by alternate fuel source (if installed) and full rich mixture. If none of that works, best glide speed and tighten the harness. Do I go back to the normal ignition position? Maybe not since the focus has changed to off-airport landing survival, not trouble-shooting.
I’m going to suggest alternatives that are NOT ideal in that they add one switch to the system. Suggestion 1 (and the one I prefer). Wire both pumps through the Nason switch and have another switch labeled “Emergency Fuel Pump Override” or similar that would bypass the Nason switch and possibly provide power to both pumps simultaneously. This provides for continued operation of the engine in a low oil pressure condition using either ignition/pump switch position. Suggestion 2. Rather than have the Nason switch in just the primary system, wire it to a separate switch labeled “Emergency Fuel Cutoff” or similar. It would deactivate BOTH pumps in the event the oil pressure went away completely. As long as it was not activated, either switch position would operate the engine should the oil pressure drop below the cutoff threshold. This would require positive pilot action to add the protection which makes it my less preferred choice.
There is no perfect solution for positive pressure fuel systems and both suggestions require a different level of thought during emergency situations. Some are not comfortable with switches labeled “Emergency Anything” and others are. Just stirring the pot…
Dave
Dave,
Several things you probably missed because you are a Pietenpol builder, and have not studied installations with fuel pumps. 1) I don’t want anyone to be switching back and forth when the have a rough engine on the primary, I want them to switch it, and land at the next airport. 2) you can’t run both pumps at one time, because if you do, they will over pressure and flood the carb. 3) we specifically selected the DPDT switch, and we teach builders to just move the switch to the other position, as a one step, first item on the rough engine operation check list. They can apply carb heat/full rich afterward, but the whole point of having the DPDT switch is to preclude low time pilots from having to do any diagnosis at a stressful moment. Your B-52/ATP background is very different from most builders, and they are at lower risk with the simplest of systems. You can make it as complex as you like, but the way we have it serves the average builder better.
If, as you recommend, pilots move the switch to the alternate position and leave it there, they lose the Nason switch benefit. That was the point of my suggestion. If having the switch protection is important, then it should be available in both primary and alternate switch positions. Having the Nason switch in both circuits is ONLY a problem if you’re trying to limp a short distance with oil pressure below the switch cutoff. The override in my option 1 is ONLY used if the oil pressure is low OR if the Nason switch malfunctions, as indicated by normal oil pressure on the gauge and the Low OP light on. Mea culpa on the pumps in series.
As I said, it’s a philosophy question. Add a switch (and complexity) to get increased protection or KISS.
I DO agree with the one DPDT switch choice versus the separate fuel pump and ignition switches for the reasons you have outlined in past postings. If ANY of the four components (two ignitions and two pumps) malfunction, as evidenced by the switch “fixing” the problem, then the “Direct Nearest Airport” button on the GPS is the one and only intelligent choice. That’s all the analysis you need.
If I understand the functioning of the Nason switch correctly, then wiring both fuel pumps thru the switch is clearly safer, but counter-productive since you can’t start the engine; i.e. you need one of the fuel pumps to run to establish fuel pressure to start the engine and keep it running long enough to establish and maintain sufficient oil pressure to close the Nason switch so that the fuel pumps can run. On the other hand, the purpose of the Nason switch is to make sure that fuel is not being pumped out thru broken fuel lines to start/feed a fire in the event of a crash, and if you are playing with the switches to possibly bypass that function, then conditions are such that the probablity of a crash in the very near future have just risen dramatically. Either way, you think thru your options, you make your choices, and take your chances. I should caveat my opinion by stating that I have yet to finish my airframe or engine. The airframe is proceeding slowly, but surely, and the engine is dead in the water, waiting on the nest parts to be installed.