A Peek Inside A 747 Fuel Gauge

It isn’t that often that we civilians get the chance to closely examine the fantastic internals that make up the modern marvels of avionic engineering. Luckily for us, [Glen] got his hands on a 747 fuel gauge and tore it down for our benefit. Not only does he tear it down, but he also builds a controller to display values.

Unlike your typical automotive fuel gauge that reports the distance from the top of the tank to the fuel level, this gauge reports the number of pounds of fuel. The fact that the indicator pictured above can go all the way to 95,000 pounds of fuel hits home the sheer scale of the fuel tanks on a 747 compared to your Volvo. Of course, where this gets interesting is the teardown with the metal sleeve removed. A 400 HZ AC servo motor moves the pointer and counter through the gearing with the help of a feedback potentiometer. The resistance tolerance is only 3%, as there are adjustment knobs on the back. But the linearity spec is only 0.06%, putting this part in a different grade from most pots.

One of the indicators was in worse shape than the others, so [Glen] got to work tapping into the internals of the gauge to drive the motor directly. A custom AC power supply repurposed from another project provided power, and a Raspberry Pi Pico was the PID controller. For [Glen], it isn’t all roses. Unfortunately, a noisy spot around 22,500 prevents accurate placement around there.

The code is up on GitHub, and we love having a gauge on the desk to show whatever value we like. If you are curious about more 747 instruments, this retro control unit might interest you.

28 thoughts on “A Peek Inside A 747 Fuel Gauge

        1. Amateur…. Kamala is a pro at grabbing the verbal salad tongs and tossing a word salad served up with croutons!
          Could you imagine a KamalaChatgpt ?

          “We’ve got to take this stuff seriously, as seriously as you are because you have been forced to take this seriously.”

      1. I once read a book from a decently large publishing company where a lot of the story revolved around a mountain, where Peak was part of the mountain’s name. But 100% of the uses of the mountain’s name in the book was Peek. The reply to my email to the publisher was properly embarrassed about how they managed to let that big a screwup through. The only place in the whole e-book that had it correct was the cover art image.

        They couldn’t simply do a find-replace of Peek to Peak (with and without capital P) because ISTR some instances of proper use of the work peek in the book which F&R would screw up. Some poor sod would have to go through the whole book to search for every use of Peek and peek and manually correct each wrong one.

        1. In Western MA, there’s a stream with a waterfall named Peck’s Falls (there’s been a sign for probably at least 50 years) and a trail, Peck’s Falls Trail. Google had it listed as “Peaks Falls” for many years and refused to correct it, even when shown pictures (Google is notoriously stubborn, sometimes, in refusing to correct incorrect names in Google Maps.)

        2. There are programs that do find-ask, requiring a single keystroke or click for each potential replacement. This should take about an hour for most books, an insignificant effort in the general scheme of things.

  1. Dammit hackaday for your effective moderation! I spent a long time replying to the casual racist comment above with a properly sourced and well thought out argument, and I hit reply and the comment was already gone.

    1. I think it’s because Reddit is leaking everywhere since some of their subs have gone private to protest Reddit’s pricey plan. A lot of subs were planning to be shut down June 12-14 so that means a few thousand racist people would end up forced to troll somewhere else for a while.

      Being a moderator shouldn’t be a thankless job as they have to deal with nasty stuff before most of us would ever see it.

  2. Reminds me of the time we had to leave a plane after we had boarded due to the fuel gauge not working properly. As I was leaving the plane the pilot was there and I asked him if he tried tapping on the glass!

    1. This gauge doesn’t have it but some of these have a little orange flag that drops into view upon loss of power or if the input signal differs too greatly from the displayed value.

    2. This indicator doesn’t have one but some of these have a little orange flag that drops into view upon loss of power or if the displayed value differs too greatly from the input signal.

    1. That’s ultra-modern considering the same company made the workhorse strategic bomber that is still flying today and will likely be until past its 100th birthday: Boeing B-52.

      Seriously though, the “modern” age of aviation started during WW2 with large all-metal construction aircraft and the jet engine. IMHO we are still in that age. The fundamentals haven’t changed at all since then.

  3. You can go to the kitchen and *make* the fuel gauge for various Piper Cub models (provided you save a few wine corks, that is). Simple is good, though the thought of that fuel tank sitting just above your lap has always alarmed me just a little.

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