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Above the flight engineer's panel in a 747-100 there is an instrument that looks like a curved spirit level.

What was it used for?

747 engineers panel

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It's for measuring the airplane's inclination on the ground for the longitudinal axis. There will be another one on a lateral bulkhead for measuring lateral inclination.

When doing fuel quantity calculations from the manual dipsticks, you have to take into account the effects of the plane's tilt on the fuel depth measurements, so you will read the values on the inclinometers and consult some kind of documentation that applies corrections for off-level conditions.

There are also other situations where they come in handy, like when you want to park in a level spot, or to keep the plane level during jacking.

Just about all Transport Category airplanes have inclinometers like that somewhere in the cockpit.

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  • $\begingroup$ Would such an instrument offer more precision than other banking indications, or is the purpose of having the instruments their ability to operate even when a plane is powered down? Would the instruments be used to conform calibration of inertial instruments? $\endgroup$
    – supercat
    Feb 20, 2023 at 19:01
  • $\begingroup$ In flight it will show deck angle if you're flying steady state, but only steady state and would be totally useless for showing pitch attitude. The lateral one will show slip/skid (not roll angle) like a skid ball on the instrument panel, but can't show bank angle. As far as using them to try the fly the plane, they are fairly useless. They're only utility is showing the long and lat axis levelness on the ground. They aren't a requirement for initialization of Inertial Reference Systems. $\endgroup$
    – John K
    Feb 20, 2023 at 20:21
  • $\begingroup$ I wasn't thinking that they would be useful in flight, but that they might be useful when the plane is stationary on the ground as part of a pre-flight check (if the artificial horizon shows a half-degree bank to the left, the inclinometer should do likewise). Otherwise, would the advantage over the artificial horizon be the fact that it's always ready for use, or would it have other advantages as well? $\endgroup$
    – supercat
    Feb 20, 2023 at 20:29
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    $\begingroup$ The gyro instruments (the actual gyros are remote in black boxes under the floor somewhere), erect themselves and don't need any help from the pilot. As far as cross checking, modern airliner has 3 separate attitude gyros, and if there is a disagreement between them, the warning systems tell you about the miscompare. When I was flying RJs we never consulted those clinos, because the only time the flight crew would ever use them is measuring fuel quantity with the magna sticks b/c the fuel quantity system is US and I never had that problem. $\endgroup$
    – John K
    Feb 20, 2023 at 20:38

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