FAR Ninety-One Point One Fifty-Nine, the Altitude Rule That Puts VFR Traffic Between the IFR Layers, and the Magnetic Course Confusion That Catches Students Every Time

Federal Aviation Regulation 14 CFR 91.159 establishes which altitudes VFR pilots must fly during en route cruise. Eastbound flights use odd thousands plus 500 feet; westbound flights use even thousands plus 500 feet. The altitude selection is based on magnetic course - not magnetic heading - a distinction that catches pilots off guard more often than any other part of this rule.

When Does FAR 91.159 Apply?

Three conditions must all be true simultaneously for the rule to apply.

First, you must be in level cruising flight. The rule does not apply while climbing or descending - only once you are established in level cruise. Second, you must be more than 3,000 feet above the surface. That is 3,000 feet above the terrain directly below you, not 3,000 feet mean sea level. Third, you must be below Flight Level 180 (18,000 feet MSL). At or above FL180 you are in Class A airspace, an instrument rating and ATC clearance are required, and this regulation does not apply.

The rule therefore governs the band from 3,000 feet AGL up to 17,500 feet MSL - the core en route cruise environment for general aviation.

Which Altitudes Do Eastbound and Westbound VFR Pilots Fly?

Direction of flight is determined by magnetic course. If your magnetic course falls between 0° and 179°, you are eastbound and must fly at odd thousands plus 500 feet: 3,500 / 5,500 / 7,500 / 9,500 / 11,500 / 13,500 / 15,500 / 17,500.

If your magnetic course falls between 180° and 359°, you are westbound and must fly at even thousands plus 500 feet: 4,500 / 6,500 / 8,500 / 10,500 / 12,500 / 14,500 / 16,500.

The boundary is exactly 180° magnetic. A course of 179° is eastbound; a course of exactly 180° is westbound. Zero degrees is eastbound. Three hundred fifty-nine degrees is westbound.

What Is the Difference Between Magnetic Course and Magnetic Heading?

This is the single most common source of errors on this rule. The regulation specifically says magnetic course, not magnetic heading, and on virtually every cross-country with any wind component they are different numbers.

Your magnetic course is the direction you are actually tracking over the ground - the straight line drawn on a sectional chart from your departure point to your destination, expressed as a magnetic direction. Your magnetic heading is the direction you are pointing the nose to compensate for wind drift and make good that course.

When a crosswind pushes you to one side, you angle into the wind with a wind correction angle. Your heading shifts; your course does not. The two numbers diverge the moment wind is a factor.

Example: Your magnetic course is 165° (eastbound, within 0°–179°). A strong southwest wind requires a 15° right wind correction angle, putting your magnetic heading at 180°. If you mistakenly use your heading instead of your course to determine altitude, you classify yourself as westbound and fly 4,500 feet. Your course says you belong at 3,500 or 5,500 feet. Same airplane, same flight, wrong altitude - because you used the wrong number.

How Do I Find My Magnetic Course Before a Flight?

If planning by hand with a sectional chart, measure your true course with a plotter, then apply the magnetic variation for your area. Sectional charts print isogonic lines - dashed lines labeled with local variation. In the eastern United States, variation is typically westerly, meaning you add degrees to convert true course to magnetic course. Variation in the western United States varies significantly by location; the chart has the values you need.

If you are using ForeFlight, Garmin Pilot, or similar software, magnetic course is calculated automatically for each route segment. Look at the course column in your flight plan - not at the heading indicator displayed for navigation. Those are different values.

If you divert mid-flight, your new magnetic course may be substantially different from your original routing. Recalculate before settling on an altitude. You cannot assume the cruise altitude that was correct for the first leg applies to the diversion leg.

Why Does the Rule Add 500 Feet Instead of Using Round Thousands?

The 500-foot offset is not arbitrary. It exists specifically to place VFR aircraft between IFR altitudes, not at them.

Pilots flying on IFR flight plans are assigned altitudes in round thousands. Eastbound IFR traffic: 3,000 / 5,000 / 7,000 / 9,000. Westbound IFR traffic: 4,000 / 6,000 / 8,000. Those round-number altitudes are built into the instrument en route system, and ATC expects instrument traffic there.

The +500 bump moves VFR aircraft to the midpoint between adjacent instrument altitudes. Picture the vertical layering: IFR eastbound at 5,000, VFR eastbound at 5,500, IFR westbound at 6,000, VFR westbound at 6,500. When everyone follows the system, opposite-direction traffic is always 500 feet away vertically - not at the same level. The offset is the mechanism that keeps VFR and IFR traffic from sharing altitudes in uncontrolled airspace.

Why Does the Rule Only Apply Above 3,000 Feet AGL?

Below 3,000 feet above the ground, pilots are typically operating in terminal areas, working with approach control, flying traffic patterns, or descending toward airports. ATC is actively providing structure, and separation is built into that environment. The cruising altitude rule is less critical there.

Above 3,000 AGL in en route cruise, VFR aircraft are largely self-managing. In most of the low-altitude en route environment below Class B and Class C shelves, there is no radar separation for VFR aircraft. No controller is placing you at a specific altitude. The VFR cruising altitude system is how pilots organize themselves into predictable, conflict-reducing flows without ATC direction.

How Does Mountainous Terrain Affect This Rule?

The regulation references “the surface” - meaning the actual terrain below the aircraft, not sea level. In flat terrain this is straightforward. In mountainous areas, it requires careful attention.

If you are flying over terrain sitting at 8,000 feet MSL and you are 3,000 feet above it, you are at 11,000 feet MSL. The rule still applies. Pilots flying cross-country over the Rockies, the Sierra Nevada, or the Colorado Plateau need to account for this: 3,000 AGL at one point on a mountain route might be 12,000 MSL or higher.

The regulation does not care about sea-level elevation. It cares about your height above the ground directly beneath you at any given moment during the cruise phase.

How Do These Scenarios Play Out in Practice?

Scenario 1 - Straightforward cross-country: Magnetic course of 045° (northeast). That falls within 0°–179°, so eastbound. Odd thousands plus 500. Terrain tops out around 2,000 feet MSL. A restricted area has a floor at 8,500 feet. 5,500 feet MSL is the right choice: odd, plus 500, eastbound, clear of both terrain and the restricted area.

Scenario 2 - Mid-flight diversion: Currently at 5,500 feet eastbound. Weather forces a divert. New magnetic course: 250° (westbound). Even thousands plus 500 required. Four thousand five hundred is too low given terrain. 6,500 feet MSL is the correct answer - climb to the appropriate westbound altitude, coordinating with ATC if in contact.

Scenario 3 - Classic checkride setup: The examiner states: magnetic heading is 095°, winds are 240° at 25 knots, true course is 080°. What is your VFR cruising altitude?

Work through it: True course is 080°. Variation in the area is 12° west - add 12° to convert true to magnetic. Magnetic course is approximately 092°. That is eastbound (0°–179°). Odd thousands plus 500. Options include 3,500 / 5,500 / 7,500 and up.

The examiner intentionally gave a heading and a true course - not a magnetic course - to test whether you know the difference, can apply variation correctly, and understand the altitude selection logic.

What Belongs on Your Nav Log?

Before every cross-country, write the magnetic course for each route segment on your nav log. Not the heading - the course. That is the number needed for altitude selection, for position reports, and for any PIREPs you call in.

The regulation also applies to any level flight, regardless of intended duration. If you level off momentarily at an intermediate altitude due to traffic, the FAA’s Aeronautical Information Manual is clear: the appropriate action is to level at a correct VFR cruising altitude, not at a random intermediate altitude. Choosing an incorrect altitude - even briefly - puts you out of compliance.

The rule does not require the lowest applicable altitude. Terrain, turbulence, airspace structure, icing, winds aloft, and aircraft performance can all justify a higher selection. 5,500 feet eastbound is just as compliant as 15,500 feet eastbound. Select the correct family of altitudes for your direction of flight, then choose the specific altitude that serves your flight safely.

Key Takeaways

• FAR 91.159 applies in level cruise, more than 3,000 feet AGL, below FL180.
• Eastbound (0°–179° magnetic course): odd thousands plus 500 feet. Westbound (180°–359°): even thousands plus 500 feet.
• Altitude selection is based on magnetic course, not magnetic heading - these differ whenever wind correction is applied.
• The +500-foot offset keeps VFR aircraft between IFR altitudes, providing built-in vertical separation.
• In mountainous terrain, 3,000 AGL can be well above 10,000 feet MSL - calculate for actual terrain, not flat-map assumptions.
• Write the magnetic course for each segment on your nav log before departure; recalculate if you divert.