FAR 91.159 and the Cruising Altitude Rule: East Is Odd, West Is Even, and Why the Plus Five Hundred Matters

FAR 91.159 requires VFR aircraft in level cruising flight to fly at specific altitudes based on magnetic course. Flying eastbound (magnetic course 0–179°), you cruise at odd thousands plus 500 feet - 3,500, 5,500, 7,500, and so on. Flying westbound (magnetic course 180–359°), you fly at even thousands plus 500 feet - 4,500, 6,500, 8,500. The rule only applies when you’re more than 3,000 feet above the surface beneath you, and “surface” means terrain, not mean sea level.

What does FAR 91.159 actually say?

Three phrases in the regulation each do specific work. “Level cruising flight” means the rule governs cruise, not climbs, descents, or maneuvering. “More than three thousand feet above the surface” measures from the terrain directly below, not from sea level. “Magnetic course” means your intended track over the ground - not the heading your nose is pointed.

Each of these distinctions comes up on oral exams, and each is covered below.

East is odd, west is even - but what about the plus 500?

“East is odd, west is even” is correct but incomplete. The altitude is always the thousand-foot mark plus 500 feet - never the bare thousand.

If your magnetic course is 0–179°, you’re in the eastbound group. That covers every direction from due north, sweeping through northeast, due east, and southeast, all the way to just short of due south at 179°. Fly at odd thousands plus 500: 3,500 ft, 5,500 ft, 7,500 ft, 9,500 ft.

If your magnetic course is 180–359°, you’re in the westbound group. That covers due south at exactly 180°, through southwest, west, and northwest, up to but not including due north at 360°. Fly at even thousands plus 500: 4,500 ft, 6,500 ft, 8,500 ft, 10,500 ft.

A student heading due east at 6,500 feet is at a non-compliant altitude. That’s a westbound altitude. The correct eastbound choices are 5,500 or 7,500 feet.

Why does the 500-foot offset exist?

Under IFR, eastbound traffic flies at odd thousands - 5,000, 7,000, 9,000 feet. Westbound IFR traffic flies at even thousands - 4,000, 6,000, 8,000 feet. The VFR cruising altitudes are intentionally slotted between those instrument layers.

A VFR aircraft at 7,500 feet sits exactly halfway between IFR traffic at 7,000 and 8,000 feet. That 500-foot offset isn’t arbitrary - it’s separation built directly into the regulatory structure. It doesn’t replace see-and-avoid, but it makes the airspace more predictable for everyone using it.

When does FAR 91.159 not apply?

This is where oral exam candidates stumble most often. Two situations take the regulation off the table entirely.

Below 3,000 feet AGL. The rule only applies when you’re more than 3,000 feet above the surface beneath you. At or below that threshold, select any altitude that clears obstacles and meets cloud clearance requirements for your airspace. The east-west altitude structure doesn’t apply.

ATC altitude assignments. When operating in Class B, Class C, or Class D airspace with an ATC-assigned altitude, you follow the clearance. ATC has the full picture of the traffic environment, and their instruction takes precedence.

How does terrain affect the three-thousand-foot threshold?

The 3,000-foot measurement is above the surface beneath you - not above sea level. This distinction matters most during cross-country planning over high terrain.

Consider a flight over the Colorado Plateau, where terrain sits at roughly 6,000 feet MSL. At 9,000 feet MSL, you’re exactly 3,000 feet above the surface - right at the threshold, not above it. Climb to 9,100 feet MSL over that terrain and the rule applies. Stay at 9,000 and it doesn’t.

This means a flight at 12,000 feet MSL over ridgelines sitting at 10,000 feet is only 2,000 feet above the surface. FAR 91.159 wouldn’t apply. Altitude selection there depends on terrain clearance, cloud separation, and aircraft performance at that density altitude - not the odd-even structure.

Over flat terrain near sea level, 3,500 feet MSL already puts you more than 3,000 feet above the surface and the rule is fully in effect.

How do I select a cruising altitude for a cross-country flight?

Altitude selection integrates several factors. Work through them in this order:

1. Terrain clearance. Identify the highest obstacle along your route. Standard guidance is at least 1,000 feet of clearance, with considerably more in mountainous terrain. Check the sectional for what you’d overfly if you drifted a few miles off your planned course.

2. Weather and cloud clearance. In Class E airspace above 700 feet AGL, you need 500 feet below clouds, 1,000 feet above, and 2,000 feet horizontal separation. Build real margin - don’t plan to the legal minimum when a broken layer is nearby.

3. Winds aloft. The FAA Aviation Weather Center publishes wind forecasts at 3,000, 6,000, 9,000, 12,000, and other altitudes. Run the numbers for your candidate altitudes. A favorable wind makes a meaningful difference in fuel burn and time en route on a long cross-country.

4. Apply FAR 91.159. Confirm your magnetic course and select a compliant altitude. If the best altitude for terrain, weather, and winds doesn’t match the required family, move to the next compliant option.

A typical scenario: routing from western Kansas to central Missouri on a magnetic course of approximately 105° puts you solidly in the eastbound group. If winds aloft favor 7,500 feet, that altitude clears terrain with a comfortable margin, stays well below the 12,500 feet MSL threshold where supplemental oxygen is required on flights over 30 minutes, and it’s a valid odd-thousand-plus-500 eastbound altitude. Compute fuel burn at cruise for that altitude, confirm reserves meet minimums, and the altitude decision is complete.

On the oral exam, expect the examiner to walk through your planned altitude directly. The answer they’re looking for isn’t just the number - it’s the full reasoning chain: magnetic course bracket, why that altitude over nearby alternatives, terrain clearance margin, and how it fits with the winds aloft.

What’s the difference between course and heading under FAR 91.159?

Your course is your intended track over the ground. Your heading is the direction your nose is pointed, which includes any wind correction angle you’re holding to stay on course.

FAR 91.159 uses course, not heading. If you’re making good a course of 175° - firmly in the eastbound group - but holding a heading of 185° to compensate for a northerly wind, you fly the altitude that matches the 175° course. Your heading appears westbound. Your course does not. The rule cares where you’re going.

This distinction shows up on written exam questions designed to catch students who confuse the two. On most flights the difference is small, but a significant crosswind can push your heading across the 180° boundary while your course remains in the other group entirely.

What if my route changes direction?

Plan each leg separately. A flight that tracks eastbound for the first segment and then turns to a southwest heading after a fuel stop requires a new altitude calculation for the second leg. Southwest falls in the westbound group - even thousands plus 500. You may need to climb or descend to a legal westbound altitude after the stop.

Note the required altitude for each leg in your flight log before departure. Working it out airborne, tired from the first leg, is not the right time.

Key Takeaways

• FAR 91.159 applies to VFR aircraft in level cruising flight more than 3,000 feet above the surface - above terrain, not MSL.
• Eastbound (0–179° magnetic course): odd thousands plus 500 feet - 3,500, 5,500, 7,500.
• Westbound (180–359° magnetic course): even thousands plus 500 feet - 4,500, 6,500, 8,500.
• The 500-foot offset exists to slot VFR traffic between IFR altitude layers - it’s separation built into the regulatory structure, not an arbitrary buffer.
• Altitude selection combines terrain clearance, cloud separation, winds aloft, and FAR 91.159 compliance - document the full reasoning, not just the final number.