Nautical Air Miles

Nautical Air Miles

Many of the graphs for the Single-engine Piston (SEP), Multi-engine Piston (MEP) and, later, the Medium Range Jet Transport (MRJT) aircraft, refer to nautical air miles (NAM). They are a measure of the air distance flown by an aircraft, i.e. the distance flown at the True Airspeed (TAS). In still air (or when there is no wind component along the aircraft’s heading vector) the NAM flown are equal to the Nautical Ground Distance (NGM) flown. NGM is the distance flown by the aircraft over the ground, as may be measured on a chart.

Usually the air is moving and an aircraft flying through this moving air will fly a different distance over the ground. If the air is moving in the opposite direction to the aircraft (a headwind, or minus wind component) then the aircraft will fly more NAM than NGM. (See Figure 3.1.)

Figure 3.1 NAM greater than NGM

Similarly, if the wind is blowing in the direction that the aircraft is flying, (a tailwind or plus wind component ) the NAM will be less than NGM. (See Figure 3.2.)

Figure 3.2 NAM less than NGM

The relationship between NAM, NGM, TAS, GS (ground speed ) and wind component (WC) is:

NAM = TAS

NGM GS

Nautical Air Miles 3

Miles Air Nautical 3

Example

An aircraft flies at TAS 142 kt for 63 NAM. If the WC is -20 kt, how many NGM does it fly?

x = 122/142 × 63 = 54

OR on your Navigation Computer

Red cursor: 142 on the inner scale is set against 122 on the outer. Blue cursor: 63 is read on the inner scale against 54 on the outer.

Figure 3.3 NAM to NGM using navigation computer

If the TAS (or GS) are not known, for example in a climb or descent, the conversion can still be carried out.

Consider an aircraft with a TAS of 100 kt:

In one hour it would fly 100 NAM. But in that hour the air has been moved by the wind component. If the WC is -25 (25 head), then the air has moved 25 NM in the direction from which the aircraft came. So the ground distance flown is:

100 - 25 = 75 NGM

Similarly, if the WC is +25 (25 tail) the ground distance is:

100 + 25 = 125 NGM

We can see that the difference between air and ground distance is the +/- WC per minutes flown, or:

Example

An aircraft climbs to cruising level in 11.5 minutes, covering 23.5 NAM. If the wind component is -30 kt, how many NGM are flown in the climb?

NGM = 23.5 - ( 3060 × 11.5 )

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