When are the fuel quantity indicators considered unreliable?

Fuel quantity indicators can become unreliable primarily at high angles of attack and angles of side slip. This is due to the way fuel is stored and measured in aircraft fuel tanks. When an aircraft is in a high angle of attack or during significant side slip, the fuel may shift away from the fuel sensors, leading to inaccurate readings. The movement of fuel within the tank can cause fluctuations in measurements, resulting in potential misrepresentation of the actual fuel quantity.

This behavior reflects the dynamics of fluid mechanics within a tank that is not in a straight-and-level flight attitude, as the sensors may not be able to provide accurate readings when fuel is not evenly distributed. In contrast, other situations such as specific phases of flight, like takeoff or landing, do not inherently compromise the integrity of the fuel gauge readings under normal operating conditions. Thus, the focus on high angles of attack and side slip makes this condition particularly critical for pilots to monitor.