how does the fuel control unit (FCU) meters fuel to the engine in response to different ambient condition?

I saw it in DC-3 Carburetors and some Garret APU-FCU like a vent float valve but call it an evacuated capsule since FCU have holes for atmosphere this evacuated capsule sense air/temperature becoming less or higher and the air inside the capsule either exits or accepts air from atmosphere. These causes the capsule end part which is the vent float valve to either goes up or down thereby allowing or restricting fuel for combustion. Very similar with cars carburetors. This is now the metering of fuel in response to ambient atmosphere.

I hope i explained it nearly to what I have learned eons of years ago.

it's even in car engines.

i believe there is a unit called something like "air weight", truly measuring pressure and temperature. out of these, the maximum fuel allowed for burning is estimated, based on oxygen available (from a standard atmosphere).

using various physical processes / mechanical, or electric operated valves, the FCU regulates the maximum fuel flow allowed. additional governors regulate that flow downwards towards desired power output or towards limits regulated.

the FCU we are using, plainly governs N1, N2 TOT and TQ of the engine. whichever reaches maximum, it triggers GOV MAX signal, that blocks any further flow increase requests.similarly, there is a minimum flow bypass, that provides minimum flow for a sustained engine operation at idle.

moreover, the unit compares valuesto values of the other engine, and throttles the the engine towards ballanced N2 output. in case of one engine failure, the other peaks up to keep up the N2 at reasonable values.

regarding the ambient conditions, really the only value regulated is maximum allowed fuel for optimised burning (pouring more fuel than what is possible to burn cools the engine, increases fuel consumption and decreases performance. it's somewhat similar to injecting too much fuel into your motorbike piston at startup or upon ignition failure. there is too much fuel in the piston then, and cannot burn properly, and the engine misfires.

btw, it's what you see when trucks start acceleration - too heavy feet on throttle result in nonperfect burning and lots of sooth.

another example may be throttling up, using too heavy grade in manual shift cars. beyond certain point pouring more fuel only decreases the performance, up to stalling of the engine.