Grid navigation

Grid navigation is somewhat obsolete because of GPS and Inertial navigation systems, but nevertheless is still part of your ATPL syllabus and you must be able to do it on paper.

1. The convergence of longitude meridians, which required constant true heading adjustments

2. Magnetic pole proximity, causing unreliable compass readings

3. Limited ground-based navigation aids in remote areas

The solution involved overlaying a grid on navigation charts, typically aligned with a reference meridian (most commonly the Prime Meridian at 0 degrees). This system became known as "Greenwich Grid" when using the Prime Meridian as reference.

• Grid Convergence: The angular difference between True Track and Grid Track, measured from Grid North to True North

• Datum Meridian: The longitude where True North equals Grid North

• Grivation: The sum of magnetic variation and convergence

• Variation: The angle between Magnetic and True North

• Magnetic/True: "Variation east, magnetic least"

• Grid/True: "Convergence east, true track least"

• Magnetic/Grid: "Grivation east, magnetic least"

1. Initial Setup

   • Calculate grid heading for the runway

   • Align aircraft with runway centerline

   • Switch from magnetic to directional gyro

   • Adjust heading reference to grid value

2. During Flight

   • Compensate for longitude changes

   • Use celestial navigation to verify position

   • Make periodic heading corrections

3. Approach and Landing

   • Conduct final heading check

   • Either maintain grid reference or switch to local true reference

• Determine true heading through celestial observation

• Apply appropriate convergence calculations

• Switch from magnetic to grid reference

• Verify true heading

• Calculate magnetic heading using local variation

• Transition from grid to magnetic reference

This system provided reliable navigation in challenging regions before modern electronic navigation systems became standard.

Let's work through a practical example of converting headings at Iqaluit Airport (CYFB):

• Airport location: 63°45'N 68°33'W

• Runway: 34 (Magnetic heading 345°)

• Magnetic variation: 29°W

• Using Greenwich meridian (0°) as Datum Meridian

1. Convert Magnetic to True heading:

   • Rule: "Variation west, magnetic best"

   • 345° - 29° = 316° True heading

2. Convert True to Grid heading:

   • Convergence = Local longitude = 68°33'E (rounded to 69°E)

   • Rule: "Convergence east, true track least"

   • 316° + 69° = 025° Grid heading

1. Calculate Grivation:

   • Grivation = Variation + Convergence

   • 29°W + 69°E = 40°E (algebraic sum of opposite directions)

2. Apply Grivation to get Grid heading:

   • Rule: "Grivation east, magnetic least"

   • 345° + 40° = 025° Grid heading

Both methods arrive at the same grid heading of 025°, which would be used to align the directional gyro before takeoff.