Practical Use of Aeronautical Charts

Practical air navigation consists of transferring real-world information to a two-dimensional plane: the aeronautical chart. To do this successfully, it is necessary to master the use of measurement tools, understand the properties of the Earth, and know how to interpret terrain symbols and radio aids. Below is a detailed step-by-step explanation of how to apply these concepts.

Tools and Distance Measurement

To navigate on paper, we use tools such as the plotter (for measuring angles and drawing lines), the ruler, and the compass (for drawing distance arcs).

A fundamental concept on any chart is distance measurement based on latitude. Since the Earth is almost a perfect sphere, one degree (1º) of latitude always equals 60 Nautical Miles (NM). Therefore, one minute (1') of latitude equals 1 NM.

• How to measure: If you don't have a graphic scale at hand, measure the length of your route and compare it with the degrees of latitude on a nearby meridian (vertical line).
• Scales: Charts have specific scales, such as 1:500,000 or practical conversions like "1 inch = 15 NM".

Headings and Magnetic Variation

When drawing a line between two points (for example, two aerodromes), we first measure its angle with respect to True North (the chart's meridians). This gives us the True Track.

However, aircraft compasses point to Magnetic North. The difference between the two norths is called Magnetic Variation and is indicated on the chart by dashed lines (isogonals). To convert the true heading to the magnetic heading that the pilot will fly, we use essential mnemonic rules:

• Variation West, Magnetic Best: If the variation is West (W), it is added to the true heading.
• Variation East, Magnetic Least: If the variation is East (E), it is subtracted from the true heading.

Positioning with Radio Aids (VOR and DME)

To know "where we are" without GPS, we use ground stations.

• VOR Radials: A radial is a magnetic heading that goes from the station. To draw it, align your plotter with the VOR and trace the line in the indicated direction. Remember that radials are already magnetic, so you don't always need to apply variation if you use the station's own compass rose.
• DME Arcs: The DME equipment measures the slant range to the station. On the chart, this translates to a circle or arc drawn with a compass around the station.
• Fixing Position: The point where two radials cross, or a radial and a DME arc, marks your exact position (Fix).

Navigation with NDB and the Convergence Concept

Navigating with NDB stations is a bit more complex because it requires additional calculations.

1. Relative Bearing (RBI): It is the angle that the ADF needle shows relative to the aircraft's nose.
2. True Bearing to Station (QUJ): It is calculated by adding the aircraft's heading to the relative bearing: $\text{QUJ} = \text{Heading} + \text{RBI}$
3. True Bearing from Station (QTE): It is the reciprocal of the previous one (±180º).

The Convergence Problem: On charts such as the Lambert Conformal Conic, meridians are not parallel; they converge toward the poles. This means that north at the aircraft's position is not parallel to north at the NDB station position. To draw a precise line from a distant NDB toward the aircraft, we must apply the Chart Convergence.

• Formula: $\text{Convergence} = \text{Change in Longitude} \times \sin(\text{Mean Latitude})$
• If we don't apply this angular correction, the line drawn on the map will not pass through the aircraft's actual position.

Projections and Route Types

The way the Earth is "flattened" affects navigation:

• Great Circle (Orthodromic): It is the shortest distance. On Lambert charts, it appears as a straight line.
• Rhumb Line (Loxodromic): It is a line of constant heading. On Mercator charts, it appears as a straight line, but it is not the shortest distance.
• Conversion Angle: It is the angular difference between the Great Circle route and the Loxodromic. It equals half of the convergence.

Terrain Interpretation and Safety

Finally, a chart must warn us about terrain hazards.

• Contour Lines: Lines that join points of equal elevation. If they are very close together, they indicate steep terrain (mountains, cliffs). If they are separated, the terrain is gentle.
• Layer Tinting: Use of colors to indicate elevation (greens for low areas, browns for high areas).
• Spot Heights: Black dots with a number that indicate the maximum elevation of a specific peak or obstacle.
• Relief: The variation between the highest and lowest point of an area.

Understanding relief and maximum elevations is crucial to maintain adequate vertical separation and avoid terrain impacts, especially in low visibility conditions.