Starting with an AUX power source

[MoonFlyer68]

Fellow Mooney people

I had my A&P install the AUX power port in my 68F.  While I have never needed the AUX power port before, I did come very close once trying to start a hot engine at a much higher elevation than I was used to.  Now that I have this installed, I have been looking for any specific procedures I should be using for an AUX power start.  I looked at an M20K model POH and all it said was there are no special starting procedures required.  Is this correct?  Is there anything that I should be mindful of (12 vs 24 Volts of course) if using AUX power to start?

Thanks

[Slick Nick]

I don’t think there’s any special procedures. It’s been ages since I’ve used mine, but I think once power is connected it automatically closes the master relay? I think I turned on my battery first, then connected the power supply. Leave the alternator master off until the power supply is disconnected after start. This is out of the service manual for my ‘91 J model 28V. 
 

Edited March 5, 2025 by Slick Nick

[Fly Boomer]

8 hours ago, Slick Nick said:

It’s been ages since I’ve used mine, but I think once power is connected it automatically closes the master relay?

On the K, the aux power relay is energized by the small, short pin so that the longer, larger power pins are already engaged in the receptacle before power comes on.  No effect on the master contactor.  Probably the same on your J.  For the OP, if you are going to start using aux power, you need to have a lot of current available -- a battery charger isn't going to cut it.

[A64Pilot]

On 3/5/2025 at 7:31 AM, Fly Boomer said:

On the K, the aux power relay is energized by the small, short pin so that the longer, larger power pins are already engaged in the receptacle before power comes on.  No effect on the master contactor.  Probably the same on your J.  For the OP, if you are going to start using aux power, you need to have a lot of current available -- a battery charger isn't going to cut it.

That is the way it is most often done and is the safest as the two pins aren’t “hot” until the external power is plugged in.

However for the Thrush as an example the plug is just wired into the battery direct. Probably because it’s easy to keep a charger on the battery thru the Aux power plug, so you may want to check just to see. Easy check, just take a multimeter and see if the pins are hot or not.

However I agree for starting you need a big battery to connect, a Golf cart is an excellent start cart, just connect to the correct voltage battery on the cart because they are often between 36V and 48V, Golf cart also makes an excellent tug.

As I assume your 12V your car or truck is just fine, I travel with jumper cables that had the clamps removed from one end and the aircraft plug installed so any car or truck could jump me.

Edited March 7, 2025 by A64Pilot

[MoonFlyer68]

I purchased the adapter from aircraft spruce that has the standard aux plug with lugs to hook up your jumper cables from a car.

[1980Mooney]

On 3/4/2025 at 8:52 PM, MoonFlyer68 said:

Fellow Mooney people

I had my A&P install the AUX power port in my 68F.  While I have never needed the AUX power port before, I did come very close once trying to start a hot engine at a much higher elevation than I was used to.  Now that I have this installed, I have been looking for any specific procedures I should be using for an AUX power start.  I looked at an M20K model POH and all it said was there are no special starting procedures required.  Is this correct?  Is there anything that I should be mindful of (12 vs 24 Volts of course) if using AUX power to start?

Thanks

The design is foolproof.  If you reverse polarity on your auxiliary power source by accident, then the solenoid will not energize the relay and there will be no connection with your airplane’s battery. It’s just like jumpstarting your car – the auxiliary power will first flow to the battery adding whatever charge is left there and then onto your starter.

I’ve used it once that way and it worked just fine.  Many years ago while I was heading for an engine conversion. My Lycoming was on its last legs and on my final refueling stop the engine would not restart.  The only way to get it started was by hooking the cables up to an FBO truck while it was revving its engine. 

Since then, I’ve only used the auxiliary port  to charge my battery from time to time. 

Edited March 8, 2025 by 1980Mooney

[DC_Brasil]

I recently had a problem with my battery while away from home. It showed charge but wouldn't start the engine. The FBO had a GPU and I followed normal start procedure as if I did not had the groud power. My M20J also doesn't say anything else in the POH about using external power.

When I replaced my battery, the technician told me to use the GPU whenever available/practical for starting the engine. He said it greatly increases battery life as the big startup current is what stresses the battery the most. 

I don't know if that's true or not, however, since the place I hangar my plane has a GPU available, I have been using it whenever it's possible (e.g. other planes aren't using it).

[wombat]

I built my own ground power unit like this guy did.   It works great and I don't mind leaving the power on for a few hours while I mess with avionics.