The International Cessna^® 170 Association

I am interested in a ground power switch. One that would let you operate the radios and a GPS and the cigar liter, I can see an electric clock in the future also. I would like not to reinvent the wheel, any do or don’ts, good tested logic would be appreciated.
Thanks in advance
Dave

davevramp wrote:I am interested in a ground power switch. One that would let you operate the radios and a GPS and the cigar liter, I can see an electric clock in the future also. I would like not to reinvent the wheel, any do or don’ts, good tested logic would be appreciated.
Thanks in advance
Dave

In my own airplane, I think the only unswitched things drawing current when the master is on are the electric turn coordinator, the alternator field, and the master relay. If I added a switch for the turn coordinator and pulled the alternator field breaker, I think I could just use the master switch for what you're talking about. Am I missing something?

In my aircraft everything is switched, including the alternator and electric turn coordinator. The GPS, 2nd nav/com and transponder are switchd by the avionics master. I can turn the master on without unintentionally powering anything. This setup has saved me more than once from a mechanic forgetting to turn off the master.

Robert Eilers wrote:In my aircraft everything is switched, including the alternator and electric turn coordinator. The GPS, 2nd nav/com and transponder are switchd by the avionics master. I can turn the master on without unintentionally powering anything. This setup has saved me more than once from a mechanic forgetting to turn off the master.

That's what I was getting at. If all the equipment is switched off, the master solenoid is the only thing drawing current when the master is on.

davevramp wrote:I am interested in a ground power switch. One that would let you operate the radios and a GPS and the cigar liter, I can see an electric clock in the future also. I would like not to reinvent the wheel, any do or don’ts, good tested logic would be appreciated.
Thanks in advance
Dave

Dave, if you have C.B.'s then this is a simple matter.* Pull the T&B/STALL warning CB and then turn on the MASTER. Everything else should be individually turned on, unless you have a radio Master, in which case you will need to either turn it on, or pull the Radio CB.

In the best/most sophisticated Radio-Master installations, a CB protects the radio Master and pulling the CB disables the radio-Master switch, thereby powering the radios directly from the battery. This is not likely the case in most 170s. It is the way most large and corporate aircraft are wired, however.

As for your suggestion.... if you anticipate installing an electric clock, it's likely you'll want it to keep ordinary time even when the aircraft is hagared, in which case the clock will be "hot-wired" anyway and the "ground-power" switch you contemplate will not affect the clock.

By the way... the term "ground-power-switch" is probably not the best terminology to use for this purpose. Industry-wide, "ground power" refers to the use of an external power-source for use on the ground by maintenance personell and occasional flight-crew use. A ground-power-switch would "switch" or connect that external source to the aircraft electrical system.

What you are envisioning is an "emergency avionics" switch, or master... which would serve double-duty in the event of an in-flight failure of your regular aircraft Master and/or it's supply. Such an "Emer. Avionics" switch would by-pass the regular aircraft Master and allow the radios (and any other connected items) to receive power directly from the battery.

Be certain to provide a suitable fuse or C.B. if you do this. I recommend you do NOT include the cigar-ligher receptacle in the set-up, unless you have re-designated it to a power-supply-receptacle only and reduced it's fuse/CB value accordingly. Otherwise excessively large-gauge wire will be required for the new circuit and an increased hazard of quickly draining your battery would exist.

*- If you have a fuse system instead of C.B.'s, the same things apply, but fuses are clearly not as convenient to utilize as a "switch".