Which unit provides an accurate fuel reading when the aircraft is not in level flight?

The inertial reference system (IRS) is designed to provide precise information about the aircraft's attitude, including its orientation in space relative to the Earth's surface. This capability is crucial for calculating various flight parameters, including fuel readings. When an aircraft is not in level flight, the IRS can determine the angle of bank and pitch, allowing for adjustments in the reported fuel quantities that take these dynamics into account.

In contrast, other systems like the fuel quantity indicator primarily measure the direct quantity of fuel in the tanks without accounting for the aircraft’s orientation. The fuel management system manages fuel distribution but also does not inherently adjust for the aircraft's flight dynamics when delivering readings, and altitude sensors focus on vertical position rather than fuel metrics. Therefore, the IRS is the most reliable unit for obtaining accurate fuel readings when the aircraft's position is not level.