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By August 18, 2019Simon's facts about


What happens if the fuel quantity indication delivers no more data for the right wing tank?

– What happens if the fuel quantity indication from the right wing does not work?
– Will the aircraft still be allowed to fly if the engineer cannot fix the problem?

The fuel quantity indication system is a computerized system consisting of fuel level, fuel densitiy and temperature measurements.
A maintenance procedure is necessary to make the aircraft airworthy again if the engineer cannot fix the problem. This enables to calculate the tank content even if the fuel gauge is faulty. However, to release the aircraft back to service a functioning “fuel used indication” is necessary to enable the calculation of the remaining fuel in the tank in every flight phase.

Many of you may know an oil dipstick to check the engine oil level in your car. With a car engine you only have to immerse the dipstick, pull it out and read the oil level. The system to check the fuel level of an aircraft fuel tank is almost identical and is called Magnetic Level Indicatior (MLI). In each fuel tank there are several MLI to be able to read the fuel level. Exactly this system is then used to determine the filling level.

On the picture you can see a float bobbing on the surface of the fuel and magnetic towards the pipe. In the pipe there is a rod with a magnet at the upper end. If you pull the rod downwards, a scale becomes visible on it. The instruction is to pull the rod down to the end. Then carefully lift the MLI until you feel the magnets engage. Now you can read the unit mark nearest to the bottom-skin of the wing and write down the number. Please note the title image.
Next we have to have a look at the table for the respective Magnetic Level Indicator (MLI=3) with an aircraft attitude of pitch=0 (table column =4) and roll=0 to determine the level in litres.

Units of MLI: 17

Litres in fuel tank: Middle between 16-18 = 5850-5950 l = 5900 litres
Density from fuel truck or aircrafts densitometer: 0,796 kg/l

In order to determine the fuel weight now, we must calculate the following:

Litre x fuel density = fuel weight

5900 l x 0,796 kg/l = 4696,4 kg

This method achieves an accuracy of +/- 5%. The deviation corresponds to +/- 235 kg for a fuel quantity of 4700 kg.

I don’t want to withhold a technical fact from you. There are airplanes that have so-called “dripsticks”. This means that you have to lower the tube with the units on it until fuel just starts to drip to get a depth reading. And I promise this is literally a mess for an engineer…

For the readers, who would like to know the maintenance procedure in more detail, it goes on here……

Step 1: Use the MCDU to get access to the FQIS (Fuel Quantity Indication System) to read out the aircraft’s PITCH and ROLL data and note the average.

Step 2
Use the PITCH data to find the equivalent number and use the ROLL data to find the equivalent letter in the table below.
Put together the number and the letter to identify the square to use the correct MLI table.

Use the table D for MLI No. 3 and the column No. 4 for the pitch of 0.

To make the weight calculation, use the density 0.796 kg/l of the FQIS Input parameters printout.

Thank’s for reading and don’t forget – A good technician is always learning 😉

August 18, 2019