How high should we fly today? Choosing an altitude for a flight may seem like a simple decision at first, but there are actually a lot of variables to consider. What is the purpose of the flight? How is the weather? Are there any airspace restrictions? What is the performance of the airplane? This may sound like a lot to consider, but with some help from your mobile EFB app and some practice, this process will become second nature to you.
Flight Training Maneuvers
The airplanes you’ll be flying in the early stages of your training can be used for a variety of missions, including flight training, sightseeing, or cross-country transportation. Let’s start with flight training since this is the stage where you will be first tasked with picking an altitude for your lesson. Early on your instructor will guide you to the practice area, to focus on learning the four fundamentals, aerodynamic stalls, slow flight and ground reference maneuvers.
When considering an altitude to practice these maneuvers, you have to think in terms of minimum and maximum altitudes, and then you’ll find the optimum altitude is somewhere in the middle. When practicing stalls or slow flight, the minimum altitude is fairly cut and dry – the Airman Certification Standards (ACS) for Private Pilot says the task must be completed no lower than 1,500 feet above ground level (AGL). That’s the absolute minimum, so a good rule of thumb is to tack on at least another 1,000 feet above that and use 2,500′ AGL as your minimum starting altitude.
That’s the height above ground level, so you’ll then need to add the elevation of the ground above sea level to determine the actual altitude to fly using the altimeter in your airplane. Say you’re flying in the midwest where the elevation is typically 800′ mean sea level (MSL), you’d want to climb to at least 3,300′ MSL before starting the maneuvers.
So if a little altitude buffer above the minimum is good, a large altitude buffer is even better, right? The answer is maybe, but like with everything in life, this isn’t always practical. Yes, you could climb up to 10,000 MSL to practice stalls, but you’d waste a lot of time and fuel during the climb to get there, especially since training airplanes start to climb quite sluggishly above 8,000 or 9,000′ MSL.
There are of course exceptions and times you may want to climb higher to practice your flight training maneuvers. In the summer it can be quite warm still in the 3,000 – 4,000′ MSL range, so climbing up to to the cooler air around 6,000′ might provide some relief. Along those lines during windy and turbulent days, you may also want to climb up higher to get above the rough air, where you’ll find more comfortable conditions.
Just when you think you have it all figured out and have found the optimum altitude for practicing slow flight or stalls for the afternoon, you check out the METAR for the airport and see there is an overcast cloud layer 3,500′ AGL (METARs always display cloud heights as above ground level). Doing the math, given that the reporting airport’s height elevation is 800′ MSL, this puts the bases of the clouds at 4,300′ MSL – a new limiting factor. Assuming your flight school’s practice area is in Class E airspace (the most likely case), you must remain 500 feet below the clouds at all times. This puts your maximum altitude for the day at 3,800′ MSL.
When it’s time to leave the local area and head to a new airport on a cross-country flight there are additional variables to consider. First, start by considering the performance of the airplane. You’ll soon learn that most airplanes have a “sweet spot”, which is the altitude that you can climb to efficiently, maintain a fast cruise speed while not burning excessive fuel.
For the Cessna 172R with a normally aspirated engine, this is typically around 8,000′ MSL, where you’ll achieve a 120-knot true airspeed at 9 gallons per hour. Go a little lower and you won’t be able to go as fast. Go higher and you’ll add more time to the climb, and again you’ll lose a few knots of true airspeed. There is one advantage though to going higher, which is a reduced fuel burn rate due to the decreasing air density. It only takes a minute or two to study the Cruise Performance table for your airplane to find this optimum altitude.
Next, you’ll want to consider winds aloft. When flying near strong weather systems the wind speed can vary by 10 or 20 knots over just a few thousand feet of altitude, which could quickly change from a tailwind to a headwind. So while 8,000′ MSL might be ideal for the C172 in a no-wind situation, you may find it better to go a little higher or lower based on the wind forecast. A helpful tool here is ForeFlight‘s altitude advisor – enter a departure and destination airport in the flight plan, select the aircraft type (ForeFlight includes performance data for common training airplanes), and tap the altitude button at the lower left corner of the route advisor. This will display a listing of estimated time en route and fuel burn at each altitude, based on the forecast winds.
So let’s say now that the optimum altitude for a longer flight is 13,000′ MSL due to a kicking tailwind. This might sound good, but if you look at the C172 maximum rate of climb table you’ll see that it’s only going to give you 145 feet per minute climb at 12,000′, so it’d be a struggle to get up to 13,000′. You’ll also want to know the certified service ceiling for the airplane, which is a limitation. For the C172R, it’s 13,500′, if you were able to make it up that high.
Climb into a Cessna 182 with additional horsepower, and you’ll have the ability to climb up 14,000 or 15,000′ MSL. At that altitude, though there is something new to consider – supplemental oxygen. The FARs state that the pilot you must use supplemental oxygen when flying for longer than 30 minutes between 12,500′ MSL and 14,000′ MSL, and then always use oxygen above 14,000′ MSL. If flying above 15,000′ MSL, you must supply all the occupants of the airplane with oxygen.
Ok, back to planning a cross-country trip in the Cessna 172R. We’ve established the limits on the high altitude side of things, but what if you’re flying over a scenic part of the country and would rather sacrifice some speed and fuel efficiency to fly at a lower altitude for a better view? You first have to consider the legal aspect, where the FARs specify minimum safe altitudes for flight.
For uncongested areas, which will likely be the environment for most of the flight, you must remain 500 feet above the surface. While this may be legal, it’s not exactly practical or safe without lots of planning and scouting for towers or other obstacles along the route. And the FARs also specify that you must fly at an altitude of at least 1,000 feet above the highest obstacle within a 2,000′ radius, requiring you to climb higher above or fly around towns if you started out at the minimum altitude of 500′ AGL.
More importantly, though, flying at minimum altitudes doesn’t give you much time to prepare for an emergency landing should you experience engine issues. For this reason (and for more practical obstacle avoidance), building in some additional margin and maintaining at least 2,000′ above the ground makes a lot more sense.
The next consideration is looking at the airspace that your flight will traverse. If the en route phase of the flight comes across Class C airspace around a busy airport, you can typically fly over the top of the upper limit, since Class C typically goes up to 4,000 above the primary airport surface.
Class B is a different story though, which usually tops out at 10,000′ MSL. If you don’t have the performance (or patience) to climb above this upper limit, check out the VFR fly-way charts available in most mobile apps. This will depict preferred routes and altitudes to stay clear of the Class B airspace, without having to go around it altogether.
Then consider special-use airspace, like MOAs, restricted and prohibited areas. You’re required to stay clear of restricted and prohibited areas, and it’s good practice to stay clear of MOAs when they’re active. More often than not you will typically need to adjust your flight routing, instead of adjusting altitude since these extend up to higher altitudes.
This is another example where a mobile app like ForeFlight can help you plan ahead for potential airspace conflicts. After entering the departure and destination airport, press the Profile button in the lower right corner of the route editor, and you’ll see a profile view of your flight and any airspace along the way. Airspace is color-coded and you can tap on the shape to determine type and altitude. You may find you need to go just a little higher to stay clear or modify the route using a VOR or GPS waypoint to steer clear of it altogether.
As you can see there are a lot of variables to picking an altitude for a flight, but after logging some time in the practice area with your instructor and practicing several cross-countries, you’ll soon develop your own mental checklist and a greater understanding of what matters most in making this decision.