Aircraft Manual Magnetic Fuel Level Indicator

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What is  Magnetic fuel level indicator (MLI)?


                                     
                                         Fig 01 Magnetic Fuel Level Indicator


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The magnetic fuel level indicators are a secondary direct reading system used to calculate the fuel quantity in LH wing, RH wing and Center tank when aircraft is on ground.

Fuel quantity indication system is a computerized system. With known parameter of fuel such as fuel level, fuel density and fuel temperature indication system determined the fuel quantity and displayed it to flight crew. During an aircraft operation there is a situation arise where fuel indication system is inoperative and such system put under MEL. In this case aircraft engineer has to calculate the fuel quantity with help Magnetic fuel level indicator. We can also determine the fuel quantity with help of MLI if no electrical power available in the aircraft exception to some cases where aircraft do not equip with aircraft attitude indicator.


Construction of MLI

MLIs are installed in aircraft fuel tank and consists of three parts,

Upper Outer part

Lower Outer part and

Inner assembly with graduation on it.

The outer assemblies made sealed tube which house the inner assembly.


Upper Outer part


Fig 02 Upper Outer Part


Upper outer part consists of tube, float assembly and mounting. Float assembly contain magnet and installed over the tube. Float assembly free to move along the tube and its motion stopped by collar at the one end and mounting at another end. The tube assembly attached to mounting assembly. Mounting assembly has opening in the center for inner assembly and is attached to fuel tank.


Lower Outer part


Fig 03 Lower Outer Part


Lower outer part is responsible for latching inner assembly. It consists of tube, mounting, collar and spring. The tube is installed in the mounting. The mounting contains collar and spring which help to latch and unlatch inner assembly. The mounting is attached to the aircraft fuel tank.


Inner Assembly i.e., MLI Rod



Fig 04 Inner Assembly i.e. MLI Rod


An inner assembly has graduated tube and graduations on tube show the contents of fuel in the tank. One end of inner assembly has magnet attached to it and this magnet attract to another magnet contain in float assembly when both assemblies brought together.  A latch is bonded on bottom end of inner assembly and has slot for insertion of screwdriver for latching and unlatching.


Operation of MLI

The float assembly on upper outer part is held by the fuel in the aircraft fuel tank and moves along the tube when the level of fuel changes.

The magnet on inner assembly connects to the magnet in the float assembly when inner assembly reaches near to float assembly fuel level. The number of graduations marks that can be seen changes when the level of fuel changes, and therefore show the amount of fuel in the tank. When MLI not in use MLI rod kept latch inside of MLI tube.

When there is fuel in tank, float will be at particular level in the tank. As the MLI rod retract for reading and when it reaches to float level there be magnetic attraction between MLI rod and Float and this particular position of MLI rod gives the fuel quantity reading.

When there is zero fuel in the tank, float will be at lower level. As MLI rod extend and retract for reading due to zero fuel in the tank and magnetic force of attraction between MLI rod and float cause rod and float move together along the MLI Tube. 



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