Definition

A Magnetic Fuel Level Indicator (MFLI), also sometimes called a Mechanical Fuel Level Indicator, is a calibrated measuring rod built into aircraft fuel tanks that provides a backup method of measuring fuel quantity.

Description

An MFLI allows direct reading of fuel quantity when the aircraft is on the ground. Typically, they are installed in multiple locations in aircraft fuel tanks, and personnel access the indicators from underneath the wings. For example, on the Embraer 170, there are three MFLIs under each wing. On the Airbus A318, A319, and A320, there are four MFLIs under each wing and one per center tank. 

The calibrated indicator rods are usually released by means of a screwdriver. The rods then extend downward to permit reading of fuel level. With most installations, the mechanic will lower the rod fully, then press it upward until the magnet engages. The value shown on the rod where it is flush with the aircraft skin shows the fuel level, usually by reference to a table in the aircraft maintenance manual (AMM). For precise readings, the calculations may take into account precise aircraft attitude as obtained from the Flight Management System (FMS) and the measured fuel density.

In most cases, MFLIs are used according to guidance from the aircraft's Minimum Equipment List (MEL). If an aircraft's cockpit fuel level indication system becomes inoperative, the MEL may allow flight under limited circumstances until the indicating system is repaired. This requires maintenance personnel to confirm adequate fuel quantity with the MFLIs prior to each flight. Most MELs allow operators to fly the aircraft in this condition only a set number of hours.