Refer to figure.

According to the first table "Ground to Air Miles Conversion" for a 20 kt headwind:

• If the ground distance = 50 NM, the air distance = 55 NM;
• If the ground distance = 100 NM, the air distance = 108 NM.

To find the air distance corresponding to 80 NM ground distance we must interpolate:

• From the ground distance of 50 NM to 100 NM, the air distance increases by 53 NM (108 - 55);

• Therefore, when the ground distance increases by 30 NM (50 NM to 80 NM), the air distance will increase by:
  
  NAM = (30 x 53) / 50 NM = 32 NM

An Ground Distance of 80 NM corresponds to an Air Distance of: 55 NM + 32 NM = 87 NM

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According to the second table "Trip Fuel and Time Required":

• If the air distance = 50 NM and landing weight = 50 000 kg => fuel required equals 600 kg;
• If the air distance = 100 NM, and landing weight = 50 000 kg => fuel required equals 900 kg;

To find the fuel required corresponding to 87 NM air distance we must interpolate:

• From the air distance of 50 NM to 100 NM, the fuel required increases by 300 kg;
• Therefore, when the air distance increases by 37 NM (50 NM to 87 NM), the fuel required will increase by:
  
  Fuel required = (300 x 37) / 50 = 222 kg

The Fuel required to the alternate (air distance of 87 NM) equals = 600 kg + 222 kg = 822 kg