#3410-Nason switch-(For planes with electric fuel pumps)

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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