As VFR pilots, we should first consider the terrain elevation and then the wind when we select an altitude to fly, but then there is that confusing Even/Odd plus 500 question. It may be more complicated — and more important — than you think…
The Rules: Federal Aviation Regulations require that airplanes flying cross country VFR between 3,000 AGL and 18,000 MSL use the following altitudes: For Magnetic Course (MC) of 360 degrees to 179 degrees use odd-numbered altitudes plus 500 (3,500, 5,500, 7,500 etc.) and for Magnetic Courses of 180 degrees to 359 degrees, use even-numbered altitudes plus 500 (4,500, 6,500, 8,500, etc.). Important: These altitudes are based on Magnetic Course (MC).
Problem: We never calculate MC when we complete our navigation planning.
The common method is to take the TC, add or subtract a wind correction angle and get a True Heading (TH). Then Magnetic Variation is considered, which yields a Magnetic Heading (MH). Consequence: A pilot flying a near-North or a near-South path could improperly choose TH or MH instead of MC — which could cause the pilot to select the wrong altitude.
Example: In 1985 this confusion apparently led to a fatal mid-air collision between a Piper and a Mooney. The wind was from the West and the Piper had a “crab” angle, which gave it a TH of about 185 degrees, BUT an MC of 173 degrees. Therefore, the Piper should have flown the trip at an odd thousand plus 500-foot level. The Mooney had an MC of 347 degrees and was correctly flying an even thousand plus 500-foot level. The collision occurred at 4,500 feet.
SOLUTION: Plan and fly your altitude based on your MC. The MC is found by taking the airplane’s True Course (TC) and either adding or subtracting the Magnetic Variation for the area the flight will be flown. Hint: You may be able to display TC as an option on your GPS. Be sure to have it shown when you fly VFR — and don’t forget to add the magnetic variation to get MC!