Navigation Fundamentals for the CPL Examination
Core Concepts
A commercial pilot must move fluently between three ways of describing position and direction: true, magnetic and compass. The chain is true → (apply variation) → magnetic → (apply deviation) → compass, remembered as Variation West, Magnetic Best; Variation East, Magnetic Least. Always work from the known quantity toward the required one, and check the sign with a sketch of the isogonal lines.
Understand the triangle of velocities: heading and true airspeed form the heading vector, wind direction and speed the wind vector, and the resultant is track and groundspeed. Solve it by the wind-triangle method or the mentally quick 1-in-60 rule: a track error of 1 nm in 60 nm equates to 1°. The same rule converts crosswind into drift: drift angle ≈ (crosswind component ÷ TAS) × 60.
Know the Earth's geometry: great circles are the shortest path, rhumb lines cut all meridians at a constant angle, and the difference matters most at high latitude and on long legs. Convergence of meridians is approximately longitude difference × sin(latitude), which explains why grid and polar navigation need special treatment.
Traps in Questions
- Confusing track error (angular difference between present track made good and required track) with closing angle (angular difference between present position and destination).
- Forgetting that wind velocity is given as the direction from which it blows.
- Mixing up heading (where the nose points) with track (the path over the ground).
- Applying variation to a compass heading as though it were a true heading.
Worked Example
TAS 120 kt, heading 090°(T), wind 180°(T)/20 kt. The wind is a pure crosswind, so drift ≈ (20 ÷ 120) × 60 = 10° to the right; the resulting track is roughly 100°(T) and groundspeed ≈ √(120² − 20²) ≈ 118 kt. If variation is 5°E, the magnetic heading to hold that track is about 085°(M).