Cracking Open and Controlling a 747 Fuel Gauge | Hacker News Reader

Cracking Open and Controlling a 747 Fuel Gauge

bikerglen.com

71 pointszdw15 comments

From the drawing in the article, the fuel capacity is 47,210 gallons. Also from the article the weight of fuel is 6.7 - 6.8 pounds per gallon.

Thus, a fully loaded 747 has 316,000 pounds of fuel.

To put this in context, the maximum takeoff weight of a 737-700 is 154,000 pounds.

A fully fueled 747 has 2x the maximum weight of a 737 just in fuel!

The 747 horizontal stabilizer has an area of 136.6 m², while a 737 main wing has an area of only 102m² to 127m².

The engine nacelles on a 777-300 are the same diameter as the entire fuselage of a 737. The big planes are very, very big.

I really like this picture that demonstrates this size difference: https://www.flickr.com/photos/ramonkok/27081197680

If you want to see a bunch of these classic steam gauges in action this is a great video (starts right before spool up):

https://youtu.be/JSKYoqVPtWw?t=86

Here's a video of a 747-100 from 2001 that has run through of the flight engineers console but low-def:

https://www.youtube.com/watch?v=x00ta8xC4oc

Then there is this amazing video that captures the sound of the 747 classic "buzzsaw" sound better than any other (due to microphone placement and the 747SP having engines closer to the cockpit):

https://youtu.be/3lhHwKK-6ms?t=313

I guess I've never heard the "classic" 747 engines. But reminds me of the GE TF39 used on the c5 galaxy.

Much more of this https://m.youtube.com/channel/UC4Kz0sEXSqNkfi7W1PBKyMg

Man, I need to think of any reason I can spec a 0.06% linearity potentiometer without having the BOM guy put a hit out on me.

The centre tanks are so much larger than the outboard wing tanks. I would be very interested to know if the rate at which the fuel can be pumped up from the centre tank to the wing tanks exceeds the rate at which the engines consume it. If so, is fuel transferred on multiple separate occasions during the flight, or is it instead pumped continuously throughout the flight at a rate approximately matching the rate of consumption by the engines?

The fuel pump rates are very extreme. A plane can take off heavier than it can land. So if it is fully fueled and shortly after take off it needs to land due to emergency, it will need to dump thousands of pounds of fuel in minutes. The typical design and workflow of a larger jet like this is the engines are feed by the fuel tank nearest them. The flight engineer is on constant duty to continuously pump fuel from the center tank and/or other fuselage tanks to the tanks feeding the engines.

The rate at which fuel can be pumped from the center tank to the wing tanks does exceed the rate at which the engines consume fuel, so fuel transfer can occur continuously throughout the flight.

The actual rate of fuel transfer and the frequency of transfers will vary depending on the specific flight conditions, such as the weight of the airplane, the distance of the flight, and the amount of fuel remaining in the tanks. The fuel management system on the 747 is designed to optimize fuel usage and ensure that the airplane remains balanced throughout the flight.

There are two possible interpretations of GP's question. I took the opposite one of the one you did. "Can the fuel be transferred more quickly than engines consume it, such that there is a risk of the wing tanks becoming overfilled from the center tank?" is the one I took. You took "Can the fuel be transferred fast enough such that there is no risk of engine fuel starvation?" which is also valid.

Fuel tanks have vents to prevent overfilling from damaging them. From one of the Boeing documents I found, the 767-400ER will vent tanks into the fuel dumping system, passively pushing the excess overboard.

That fuel was presumably loaded for a reason. Pushing it overboard still represents a risk to the flight.

The alternative would be a burst fuel tank.

I think I prefer the “dump fuel” failure mode.

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