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Fuel System Manager
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Eric Page

Joined: 15 Feb 2017
Posts: 268

Posted: Sat Feb 21, 2026 7:49 am Post subject: Re: Fuel System Manager

Here's a quick conceptual doodle showing how the switch wiring might be done to handle OFF-MANUAL-AUTO switching, using a 2TL1-1 (or, for a much cheaper alternative, the B&C S700-2-1).



Switched LPFS ground connection to be shared with HPFS using diode steering.



AFC logic remains the hurdle...

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911pete

Joined: 09 Aug 2018
Posts: 15

Posted: Sat Feb 21, 2026 8:33 am Post subject: Re: Fuel System Manager

Not being an electronics guy my question is would latching relays utilizing the fuel sensors be a path to control the logic?

user9253

Joined: 28 Mar 2008
Posts: 1958
Location: Riley TWP Michigan

Posted: Sat Feb 21, 2026 8:40 am Post subject: Re: Fuel System Manager

Pete, you wrote in a previous post, "The goal is to always keep the main tank 

full until the wings are empty".   That seems reasonable.  It is hard for me to 

understand how the fuel computer knows when to open and close the wing tank

valves when there are only two level sensors in the main tank, high and low.  It 

would not be desirable to let the main tank fuel level go down to the low level 

sensor before opening the wing tank valves.

  Suppose the computer closes the wing tank valves when the main tank is full.  

As fuel is used, the high level sensor tells the computer that the main tank is no 

longer full.  The computer could then wait for a period of time, say 5 minutes,  

before opening the wing tank valves again.  That time delay period needs to be 

short enough to keep the main tank relatively full.  The time delay also needs to 

be long enough so as not to wear out the solenoid valves due to short cycling.

_________________
Joe Gores

911pete

Joined: 09 Aug 2018
Posts: 15

Posted: Sat Feb 21, 2026 9:18 am Post subject: Re: Fuel System Manager

I think you are mistaking where the low level sensor is. The main tank is approximately 75 gallons. The low level sensor is set at around the 60 gallon level. The plane burns 1.5 to 2 gallons per minute at non takeoff settings, so the valves would typically cycle every 7 to 10 minutes. If the fuel gets below the low level sensor for longer than 20 seconds the low level light would come on giving you around 30 minutes to land.

user9253

Joined: 28 Mar 2008
Posts: 1958
Location: Riley TWP Michigan

Posted: Sat Feb 21, 2026 11:28 am Post subject: Re: Fuel System Manager

OK, I modified my drawing in post number 5 by moving the lower fuel level 

sensor from the bottom of the tank to a higher position.

_________________
Joe Gores

Eric Page

Joined: 15 Feb 2017
Posts: 268

Posted: Sun Feb 22, 2026 6:37 am Post subject: Re: Fuel System Manager

Pete, do you have a datasheet link for your LPFS and HPFS?  I'd like to see their electrical requirements and what position feedback, if any, they offer.



Yes, latching relays could definitely be part of the solution, but they can't be driven directly by the fuel level sensors.  This type of relay requires a brief pulse (a few tens of milliseconds) to set or reset its coil(s) (some have two), not a continuous current like a normal relay.  This would require some electronics to turn the long-duration on/off cycles of the level sensors into these short pulses.



Also, we want the absence of fuel at the Low Level Sensor to open the solenoids, but we don't want them to close when that sensor sees fuel again; they have to wait until the High Level Sensor also sees fuel.  That's going to require some electronics too.

user9253

Joined: 28 Mar 2008
Posts: 1958
Location: Riley TWP Michigan

Posted: Sun Feb 22, 2026 9:07 am Post subject: Re: Fuel System Manager

Eric,

What do you think of the attached circuit?  It uses discrete components.  No computer necessary.  Integrate it with your switch circuit.

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Joe Gores

user9253

Joined: 28 Mar 2008
Posts: 1958
Location: Riley TWP Michigan

Posted: Sun Feb 22, 2026 9:29 am Post subject: Re: Fuel System Manager

After thinking about it, why use an AND Gate?

The upper liquid level switch could energize the relay.

The relay can hold through the lower liquid level switch.

_________________
Joe Gores

Last edited by user9253 on Sun Feb 22, 2026 10:04 am; edited 1 time in total

user9253

Joined: 28 Mar 2008
Posts: 1958
Location: Riley TWP Michigan

Posted: Sun Feb 22, 2026 9:48 am Post subject: Re: Fuel System Manager

Simple relay circuit attached.

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_________________
Joe Gores

911pete

Joined: 09 Aug 2018
Posts: 15

Posted: Sun Feb 22, 2026 5:07 pm Post subject: Re: Fuel System Manager

Eric



Here is a copy of the specifications for what I will use for the low pressure solenoid. The original spec’d solenoid is not available. The high pressure one is a Duke’s aerospace part, they are the original supplier of the low pressure one as well, but they are no longer in business as well. All the solenoids just have 2 wires so I assume they are simple on open or closed.



As far as the logic I started investigating Arduino controllers and that might be the path of least resistance for this whole project.



Pete

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Eric Page

Joined: 15 Feb 2017
Posts: 268

Posted: Mon Feb 23, 2026 6:30 pm Post subject: Re: Fuel System Manager

Joe:  Yep, I was over-thinking it.  Well done.  Your second diagram does exactly what's needed with nothing but one relay per side.  You have to wonder why they used a PLC in the first place.



Pete:  An Arduino isn't necessary.  In fact, the original PLC wasn't necessary.  The diagram in Joe's last post does exactly what you want with just a relay.



I've started work on fleshing out Joe's design to a full schematic with specific part numbers.  I have some other stuff going on so it may take a few days...

911pete

Joined: 09 Aug 2018
Posts: 15

Posted: Mon Feb 23, 2026 6:58 pm Post subject: Re: Fuel System Manager

Great news. No rush.

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