The Maximum Elevation Figure, the Two Little Numbers in Every Grid Square on the Sectional, and How to Build Terrain Clearance Into a Cross-Country Before You Ever Leave the Ground

The Maximum Elevation Figure (MEF) is the pair of stacked blue numbers printed inside each grid square on a sectional chart, and it tells you the height above sea level of the tallest obstacle in that box, plus a safety buffer, rounded up to the next hundred feet. It is not a safe altitude to fly - it is the top of the terrain and obstacles. To build a safe cross-country, you find the highest MEF along your route, add 1,000 feet of clearance over gentle terrain (or 2,000 feet over mountains), then round up to the correct hemispheric cruising altitude for your direction of flight.

What Are the Two Numbers in Each Grid Square on a Sectional?

Look at the empty space on any sectional chart - the areas between the airports and airways. In the middle of every grid box, printed in blue, sit two numbers stacked on top of each other: a large number over a small one. Most pilots have looked past them thousands of times without ever using them.

These numbers are the Maximum Elevation Figure. The FAA prints them inside every grid formed by the lines of latitude and longitude. In the lower 48 states, that grid is roughly 30 minutes on a side - a box about 30 nautical miles wide.

A big 2 over a small 3 reads as 2,300 feet. It means that somewhere in that box, the tallest object reaching up toward you tops out below 2,300 feet mean sea level (MSL).

What Does the Maximum Elevation Figure Actually Mean?

The MEF gives you the height of the tallest thing inside the box, above sea level, with a margin already built in. Chartmakers find the highest obstacle in the box - a mountain peak, a radio tower on a hill - then they:

• Add the height of possible unmarked objects on natural terrain features
• Add a margin for the survey tolerance in measured elevations
• Round up to the next 100 feet

The result is a number you can trust as the ceiling of terrain and obstacles in that grid.

Is the MEF a Safe Altitude to Fly? (No - Here’s Why)

This is the trap that catches pilots: the MEF is not your cruising altitude. If you fly at exactly the MEF, you are flying at the exact height of the tallest tower in the box. That is not clearance - that is a collision altitude.

Think of the MEF as the floor of your thinking, not the answer. It tells you the lowest altitude you could ever consider, and then you build up from there.

How Do I Turn the MEF Into a Safe Cruising Altitude?

Draw your course line on the chart, then examine every grid box the line passes through - and glance at the boxes just to either side, since you may drift or deviate around weather. Find the single highest MEF along the whole route. That worst-case box sets your minimum.

Say the highest MEF you find is 9,500 feet. Here’s how to build a real altitude from it:

1. Add your terrain clearance. The FAA’s guidance - reflected in the Aeronautical Information Manual and in how search-and-rescue thinks - is at least 1,000 feet above the highest obstacle near your course over anything but flat, open country. In the mountains, many experienced pilots push that to 2,000 feet, because lee-side downdrafts on a ridge can exceed a light airplane’s ability to climb.
2. Do the math. 9,500 + 1,000 = 10,500 feet. For real mountains, 9,500 + 2,000 = 11,500 feet.
3. Round to a legal hemispheric altitude for your direction of flight (see below).

Do the terrain math first. Winds aloft and fuel burn tell you how to choose between safe altitudes - terrain tells you which altitudes are even on the table.

How Do Hemispheric Cruising Altitudes Fit In?

Those clearance numbers - 10,500 and 11,500 - aren’t random. They’re hemispheric cruising altitudes under FAR 91.159. When you’re more than 3,000 feet above the surface flying VFR:

• Magnetic course 0° through 179° (eastbound): odd thousands plus 500 (e.g., 9,500 / 11,500)
• Magnetic course 180° through 359° (westbound): even thousands plus 500 (e.g., 10,500 / 12,500)

Your terrain math and the regulation are trying to shake hands. Suppose you’re headed westbound and terrain says you need at least 10,500 feet - but 10,500 is an eastbound altitude. You step up to 12,500 feet, the correct westbound altitude, which also gives you even more margin. Terrain sets the floor; the regulation snaps you to a legal, sensible level.

Does My Terrain Altitude Trigger the Oxygen Rules?

When terrain pushes you up near 12,500 feet, another rule enters the picture: FAR 91.211, the oxygen requirement.

• Above 12,500 feet cabin pressure altitude: if the flight lasts longer than 30 minutes at that altitude, the required crew must use supplemental oxygen.
• Above 14,000 feet: the required crew must use supplemental oxygen the entire time.

A cross-country plan is a web. Pull the terrain string, and the oxygen string moves. The terrain forced you up to 12,500 for clearance, and that same altitude just triggered an oxygen requirement on a longer leg. This is exactly why you plan on the ground, with the whole chart in front of you, instead of sorting it out at altitude with your hands full.

Should I Fly Straight Over the Mountains or Follow the Valleys?

Consider a scenario. You’re a fresh private pilot with around 200 hours, flying from the valley to visit family on the other side of a mountain range on a severe-clear day. The direct line runs over the high country, and the worst MEF along it is 11,200 feet. Your normally aspirated trainer will climb to 12,500 - but slowly, and it’s wheezing up there loaded with people and bags on a warm day.

You have a decision, and it’s exactly the kind the Airman Certification Standards (ACS) wants you to make out loud. The examiner is looking for you to show you considered terrain, selected an altitude with a reason, and thought about aircraft performance at that altitude on that day.

Your options:

• Climb over the top at 12,500 feet, accepting about 1,300 feet of margin - knowing your climb rate up there is poor and your options in a downdraft are thin.
• Follow the valleys and passes. You give up the straight line and the route gets longer, but your worst-case MEF drops dramatically because you’re over the low ground between the peaks, not the peaks themselves. Now maybe your highest MEF is 7,800 feet, and a comfortable 9,500 feet clears it with room to spare, keeps you below the oxygen numbers, and leaves your airplane still climbing like it means it.

The straight line is rarely the smart line in the mountains. The Maximum Elevation Figures let you discover that at the kitchen table instead of over the ridge.

What Should I Check Before Locking In My Altitude?

Once you’ve picked an altitude, sanity-check it against your airplane. Pull the performance charts: what’s your climb rate at that altitude at the temperature you’ll actually see that day? Density altitude on a hot afternoon can turn a book number into fantasy. If the chart shows you climbing at 200 feet per minute at your planned altitude, that’s not an airplane clearing a ridge with authority - that’s an airplane you should route around the terrain, not over it.

Then confirm whether your terrain-driven altitude bumps you into the oxygen rules, toward airspace ceilings, or toward the flight levels. Terrain doesn’t care about regulations - you have to hold all of it at once.

Key Takeaways

• The MEF is the height of the tallest obstacle in a ~30 NM grid box, above sea level, buffered and rounded up to the next 100 feet - it marks terrain, not a safe altitude.
• Find the single highest MEF on your route, then add 1,000 feet (gentle terrain) or 2,000 feet (mountains) of clearance.
• Round the result up to the correct hemispheric cruising altitude under FAR 91.159 for your direction of flight.
• Watch for FAR 91.211: above 12,500 feet for more than 30 minutes, and above 14,000 feet at all times, the required crew needs supplemental oxygen.
• Always verify your planned altitude against aircraft performance and density altitude - and remember the valley route is often safer than flying over the peaks.