[tlb]

Garmin G1000 units (popular in small planes) show this, as a circle around the current position, including fuel reserve. However, it depends on an accurate fuel level, which is mostly dead-reckoned from integrating fuel consumption. Measuring the actual fuel level in wing tanks is imprecise and the sensors sometimes get stuck and give bogus readings.

Perhaps the big innovation needed is accurate fuel quantity measurement.

[avereveard]

I think a plane can measure an approximation of weight loss from its own airspeed and angle of attack, won't give you exact weight measurement because weight at take off is an approximation, but might give a estimate of weight loss. Depends on the precision of the aoa sensor tho.

[cjrp]

Commercial airliners, and even some GA planes, have an accurate measure of fuel consumed using a fuel flow transducer.

[tlb]

Indeed, but that system will report nothing unusual when there's a leak directly from the tank, or in the pipe between tank and flow sensor (which I think is usually near the engine, downstream of a fuel pump) or if the tank wasn't filled to spec.

Any sensor has a failure rate. If the probability that a sensor has failed isn't dramatically lower than the probability of a leak, then the pilots will do just what they did in the incident and assume a bad fuel sensor reading rather than a leak.

Another failure mode of fuel tank sensors is short-circuiting and blowing up the plane: https://en.wikipedia.org/wiki/TWA_Flight_800