Light Aircraft Fuel Timer

Device to assist with monitoring fuel in a light aircraft's multiple tanks.

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Flying a light aircraft requires keeping track of the fuel onboard at all times. This is easy enough to do with just a wrist watch, pen and paper, but it's nice to have a countdown timer running to reference at a glance and as a backup.

Some aircraft owners keep a digital countdown timer stuck on the panel somewhere, and some panels have a digital countdown timer built in. Full "glass" panel aircraft (eg. with a Garmin G1000) appear to have fuel/time management features in their software, however none of the aircraft I fly regularly have (full) glass cockpits.

This project aims to build a small device that helps manage fuel time available in multiple tanks - essentially a glorified countdown timer with multiple timers. It originally started as an exercise in how much UI could be packed into a microcontroller, but it's definitely a tool I want in the cockpit now.


(This list is mirrored from in the GitHub repository)

Core functionality

  • Display time remaining in each tank
  • Swtich active tank (including crossfeed)
  • Pre-flight programming state allowing at least configuration of time in each tank
  • Start flight action
  • End flight action (to prevent warning features tripping)
  • Keep accurate time and state across loss of main power and reset.

Nice to haves

  • Display elapsed flight time
  • Display elapsed time on current tank
  • Show elapsed time vs. reserve time vs. total remaining time
  • Show use of reserve time differently visually (more urgent)
  • Display flight start time
  • Protect against accidental changes to tank quantity
  • Automatic fuel balancing - alter user configured fuel change time in flight to balance fuel in tanks.
  • Sleep between updates to save power? We only need to be awake to update the screen every few seconds during a flight.
  • Alert sound loud enough to be heard in the cockpit:
    • short chirp to indicate it's time to switch tanks, or time is overdue
    • a more worrying alert to notify entering reserve time, plus a repeated alert every five or ten minutes to remind the pilot of the status.
    • persistent alarm should sound when running dangerously low on tank time or on total remaining flight time. For tank time where there is adequete fuel remaining in the other tank, the alarm may not be dismissable. For other cases, the persistent alarm may be silenced to a chirp every minute.
  • Configure fuel change time
  • Configure date and time
  • Display time in UTC
  • SAR time monitoring with audible warnings to update/terminate (active even outside of flight mode)

Other (probably feature creep)

  • Multiple count up/down timers for navigation
  • Scroll through log of fuel changes this flight
  • Log previous flights (duration), aircraft registration / "totals flight log"
  • Allow entering fuel quantities and have the program work out the time per tank. At minimum, this could be a single screen where instead of configuring time for two tanks, you config quantity in each tank, plus burn per hour and unusable fuel. The burn per hour and unusable could be recalled from EEPROM.
  • Low power mode where the user has to wake with an interupt to see the display for five seconds at a time. could show time remaining in current tank/flight, or some configurable value.

  • 1 × 2.42" 128x64 OLED screen (SSD1306) Any size of 128x64 OLED screen will work - SPI or i2C
  • 1 × Rotary encoder with push button

  • Doing the math

    Stephen Holdaway03/05/2019 at 09:20 0 comments

    From the original list of features I sketched out a year or two ago, I primarily want address the following:

    • As a pilot I want to know how much fuel is onboard, expressed as time.

    • As a pilot I want to be alerted when I am approaching my reserve fuel time.

    • As a pilot I want to know when the active fuel tank needs to be changed so I can keep the tanks balanced.

    • As a pilot I want to know when my search and rescue deadline (SARTIME) is approaching so I can update or terminate it.

    • As a pilot I want this device to keep state accurately across loss of power or reset so I do not lose information.

    Common to all of these are two calculations:

    • Difference between two date/times
    • Time remaining given a quantity and rate of consumption

    These calculations have to be reliable, but not necessarily fast. As long as the system stays responsive on a human scale (eg. a few hundred milliseconds when a button is pushed). This makes the main constraints program size and memory.

    Read more »

  • One year of tinkering update

    Stephen Holdaway12/04/2018 at 11:32 3 comments

    Monochrome 128x64 reflective LCD driven by Atmel AVR ATMEGA328P
    Root menu on a ST7565 based reflective monochrome LCD.

    This project has existed in breadboard-form on my desk for 18 months. Though this is the first update in a while, it's a project I tinker on a lot: there's a menu system that's easy to work with, views can be pushed and popped, user input is transferred around automatically, and the event-loop is an interesting way to organise an embedded project. Problem is, none of these are core features. These things are polished not because they need to be, but because I've been putting off the harder task of making decisions about the in-flight views of the device.

    Earlier in the project I'd sketched out a bunch of screens to see what might fit into in the limited screen resolution:

    Various view sketches. The top, second from the left screen is the original way I considered displaying in-flight information.

    Read more »

  • Fuel management in light aircraft: why use time?

    Stephen Holdaway02/17/2017 at 18:50 1 comment

    Is this really state of the art, measuring used fuel just by time? Are there no fuel level sensors available just like in any car, or at least flow meters which measure fuel usage?

    If my life depends on not running out of fuel it seems quite disturbing to just rely on absolutely constant fuel usage over time during the flight.


    Great questions. Using time is definitely not state of the art technology, and you're quite right that running out of fuel in an aircraft means you're gonna have a bad time.

    From my flight training, experience and some research, I would say that the use of time comes down to operational aspects more than a lack of technology. Specifically, managing the phase of flight where accurate fuel information is the most critical: pre-flight planning.

    Read more »

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asarangan wrote 10/09/2018 at 14:57 point

I ran across this site by accident. I have been trying to build such a device for a long time. Given that fuel exhaustion is the leading cause of engine stoppage, it is surprising we don't have something in the market. The timers that Sporty's and ASA sell are don't do the job because they are not LED (hard to see), and they don't sound any alarms. What I would like to see is a simple but bright countdown timer (minutes, not seconds), and would starting making a lot of noise and flash lights when you get below a certain threshold. 

  Are you sure? yes | no

Martin wrote 02/17/2017 at 15:31 point

Is this really state of the art, measuring used fuel just by time? Are there no fuel level sensors available just like in any car, or at least flow meters which measure fuel usage? 

If my life depends on not running out of fuel it seems quite disturbing to just rely on absolutely constant fuel usage over time during the flight.

  Are you sure? yes | no

Stephen Holdaway wrote 02/17/2017 at 22:25 point

Good questions. There's too much to explain in a comment, so I've written a post in answer:

  Are you sure? yes | no

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