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You're planning your cross country, when the all-important question comes up: what altitude should I file? Here are seven things you should be thinking about before you pick your altitude.
It's usually the first thing that comes to mind: what are the winds doing? After all, the last thing you want to do is buck a 40 knot headwind. That's where tools like ForeFlight's winds aloft overlay come in. By dragging up and down on the altitude selector, you can get a quick look at how strong the winds are aloft.
Next up, you need to make sure you're flying at the right altitude for your direction of flight. According to FAR 91.159 if you're more than 3,000 AGL, you need to be flying an odd-thousand MSL altitude +500 feet on a magnetic course of 0-179. And if you're flying a magnetic course of 180-359, you should fly an even-thousand altitude +500 feet. An easy way to remember this is the phrase "East is odd, West is even odder."
There's nothing that will ruin your day like hitting terrain or an obstacle. So how do you make sure you're clear on your route? If you're flying VFR, one of the easiest ways is to open your sectional chart and look at the MEF (Maximum Elevation Figure) altitudes for your route.
The MEF is the bold blue altitude, in hundreds of feet MSL, listed in the middle of each quadrant of your sectional. That altitude guarantees you at least 100 feet (up to 300 feet, in some cases) of clearance from all terrain and obstacles in the quadrant.
As long as you pick an altitude above the MEF, you can rest easy in knowing that you're not going to hit something sticking out of the ground.
You need to be practical with your altitude choice. If you're flying a short distance, it doesn't make a lot of sense to spend the majority of your flight in a climb.
That's where your aircraft's Fuel, Time and Distance to climb chart comes into play. For most aircraft, your time-to-climb is pretty linear, but if you're flying a normally aspirated airplane above 10,000 feet MSL, your climb rate can start tailing off significantly. On top of that, you're burning extra fuel in a climb, and flying a slower indicated airspeed as well.
But the opposite is true when it comes to true airspeed. The higher you go, the higher your true airspeed. The rule-of-thumb is that you gain 2% of true airspeed for every 1,000 feet you climb, and that can make a big difference. Consider this: if you're flying at 140 knots indicated at 5,500' MSL, your true airspeed will be roughly 154 knots. But if you fly the same indicated speed at 11,500', your true airspeed shoots up to 170 knots. That's a gain of 16 knots, which is big difference, especially on long flights.
There's controlled airspace, special use airspace, and just about every kind of airspace you can think of listed on sectional charts.
Fortunately, tools like ForeFlight can help you navigate around tower controlled airports and special use airspace along your route. But there's another way to make life easy on yourself: simply climb above it.
If you climb above 10,000 feet MSL, you've all but guaranteed yourself clearance above tower controlled airspace, even Class B. There are of course a few exceptions, like the Denver Class B that extends up to 12,000' MSL, but those are few and far between.
Unfortunately, the same can't be said for restricted areas and other special use airspace, but a quick check on your sectional chart can clear up any questions about that.
On most flights, you need to contend with the weather. And mother nature isn't always cooperative when it comes to flying.
That's where your METARs, TAFs and PIREPs come into play. When you're checking the clouds, think about coverage and altitude. If you're looking at few or scattered clouds, climbing above them might be an option, but if there's a broken layer along your route, it's probably best to stay below it.
Also, remember that METARs and TAFs list cloud bases in AGL, not MSL. You'll need to do some math to figure out where the bases will be to maintain your VFR cloud-clearance requirements.
There's one final consideration, and it's quite possibly the most important thing: what are your passengers going to think on your flight?
If you're getting bounced around because of turbulence, your passengers might not be very impressed. One place you're almost guaranteed to find turbulence is around shear layers in the winds aloft.
While you obviously want to consider your headwind or tailwind along your route, you also want to make sure you're keeping yourself clear of any significant shear layers aloft.
On this route from KGCY-KEHO, there's a 24 knot wind velocity difference between 3,000' and 6,000', with a nearly 50 degree wind direction direction difference. And if you're thinking things would be bumpy in that area, you're right.
Taking a look at the PIREPs confirms what you'd expect: a Cherokee pilot reported continuous moderate turbulence below 4,500' MSL.
Unless you want to pack extra sick sacks for your passengers, it's a good idea to be on the lookout for the "smooth ride" altitudes, along with the favorable winds aloft.
There's a lot to consider when you're picking your cruise altitude. But if you're thinking about obstacles, your plane's performance, and the weather and winds along your route, you'll have a smooth flight, and hopefully some happy passengers as well.
Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at firstname.lastname@example.org.