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Ceengland
Joined: 11 Oct 2020
Posts: 398
Location: MS
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 Posted: Tue Mar 10, 2026 8:54 am Post subject: Re: Fuel System Manager |
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Planes that run Continental fuel injection systems with fuel regulator 'bypass' flow going to one particular tank in a multitank airplane have crashed due to the pilot not noticing that he's pumping fuel overboard.
I never understood why the FAA certified the design in the 1st place; there are several very effective cures for the problem.
Just sayin'...
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Eric Page
Joined: 15 Feb 2017
Posts: 275
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| Posted: Thu Mar 26, 2026 3:04 pm Post subject: Re: Fuel System Manager |
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Project update:
Circuit boards and components for Pete's low fuel warning module arrived and I built up a prototype. It behaved strangely when powered with 14V and didn't work at all when powered with 28V. Not very helpful in a 28V aircraft!
I discovered a couple of design errors:
1) The pads for the 12V voltage regulator had the input and output pins swapped, resulting in ~26V where there should have been 12V. This exceeded the 18V max for the SA555P timer IC, causing it to behave erratically. Miraculously, it survived the abuse.
2) The unused input pins for the second comparator in the LM2903P were tied directly to ground. This was changed to 10k and 10M pull-down resistors, per Texas Instruments Application Report SNOAA35B, page 13: https://tinyurl.com/bdfvewje+
With the first problem fixed by rotating the voltage regulator 180 degrees, the module works as expected: when the LO_SNSR input has 28V removed so it's open circuit, there's a delay of 20 seconds, after which the EFIS_ALM output pulls low and the LED+ output begins to oscillate. Re-applying 28V to LO_SNSR resets the module.
I've updated the schematic (attached to this post) and PCB layout. New boards and components are on the way. I'll report again when I have the new board assembled and tested.
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Low Fuel Warning Module (Rev C).pdf |
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Eric Page
Joined: 15 Feb 2017
Posts: 275
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| Posted: Tue May 12, 2026 9:10 am Post subject: Re: Fuel System Manager |
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Now that the forum is back online, I can report that the second assembly of this project was completed and successfully tested a couple of weeks ago.
Its "+24V_A" and "LO_SNSR" pins were connected to +28VDC. When the "LO_SNSR" pin was made open circuit there was a 21 second delay then the module began flashing its "LED+" output at ~1.5Hz, and its "EFIS_ALM" output pulled low. The "LED+" output sourced 20mA, even into a short circuit. Reapplying +28VDC to the "LO_SNSR" input caused the "LED+" output to turn off and the "EFIS_ALM" output to return to a high impedance state. Behavior is identical when powered via the "+24V_B" pin.
See the attached photos showing the bare board as tested, and the finished module as shipped to Pete (minus adhesive-lined shrink wrap over the board).
For anyone wanting to build this in the future, use the following links. The last one goes to a very long DropBox URL so I used a URL shortening service to make it manageable in this post; the first link (with the "+" at the end) goes to a preview page and the second will download the .zip file.
Circuit boards are available here:
https://oshpark.com/shared_projects/98df16xF
Electronic components are available here:
https://www.digikey.com/en/mylists/list/CFEKJK5A1K
Options for instrument panel warning LED and connectors are available here:
https://www.digikey.com/en/mylists/list/2RBW4Q2Y1I
Design files (DipTrace v5.2.0.4.) and RS-274X Gerber files are available here:
Link Preview: https://rb.gy/453ftf+
Download Link: https://rb.gy/453ftf
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