How High

There are a lot of factors in play when it comes to choosing our altitude. I often get asked how high we’ll be flying, and I always have to hold back the "well it depends on..." and launch into all the different factors that come into play when choosing how high we're going to fly. I don't think my passengers usually want the answer in the amount of detail I'd like to give. That's what a blog is for. So here we go.

Every airplane has a favourite altitude – the altitude where it performs most efficiently. The higher we get the thinner the air gets, which carries with it both a pro and a con. The pro is that since there’s less air, there’s also less resistance, meaning it takes less power to move through the air then it would at a lower altitude. Less air also unfortunately means the engines can’t produce as much power, which is where the trade-off is. An airplane's most efficient altitude is where we burn the least amount of fuel and go the fastest. This is accomplished by finding that trade-off of drag vs power produced. We can go even further to say that this depends on the power setting we want to cruise at. If we want to fly for maximum range, then we'll have to cruise at a lower power setting. In that case since we’re not asking a whole lot of the engine, it makes sense to climb up high to the thin air where both power and drag are reduced. The higher power setting we want to cruise at, generally the lower we have to fly. Lets say we want to fly at max cruise, which is usually 75% of the maximum rated horsepower. Our most efficient altitude will be somewhere near to the height at which we have the throttles fire-walled with the engine only producing that 75% power of its full power horsepower at sea level. Usually for most normally aspirated (non-turbocharged) airplanes that’s around 6000-8000 ft. In the Twin Comanche (which I happen to be flying in while I write this) it is 8000 ft. Any higher than that and the engine power stands a good chance of falling off below 75%. Any lower however and the thicker air will be holding us back in the form of extra drag. So engine power vs drag is our first consideration when choosing the best cruising altitude.

For this reason turbocharged airplanes have the ability to fly both faster and much higher. A turbocharger compresses the air before it enters the engine, which eliminates (to a point) the disadvantage of a power decrease at higher altitudes. This allows an aircraft to fly in the higher altitudes where the air is thinner, resulting in a higher cruise speed while still be able to produce the same level of power as it would much lower. How high exactly depends on how much boost the turbocharger has, or how much it is able to compress the air. For a turbocharged airplane the most efficient altitude occurs at the highest altitude where the turbocharger is able to produce the desired power setting. Below that altitude and the turbocharged engine can produce more power then is required, but above that the turbocharger’s ability starts to decline and engine power decreases.

Our second biggest consideration is what are the winds doing? Wind strength usually increases with altitude. So if we’re fighting a headwind, we have to decide if the stronger winds aloft are going to cancel out any advantage we’re gaining by flying high. If we’re in a tailwind, the winds aloft may very well give us a bigger push then we’d achieve by flying at our airplanes most efficient altitude. Considering that it’s usually a good practice to fly higher in a tailwind, and lower in a headwind.

Weather is also a factor, especially if we’re flying VFR. VFR airplanes aren’t supposed to fly through clouds, some sometimes that can limit us to flying lower underneath, or in some cases overtop. Weather considerations for IFR airplanes can include icing, which may mean that airplane has to stay lower in the warmer air where airframe icing is not occurring.

Other factors include obeying the Cruising Altitude Order, which is a rule that governs certain altitudes airplanes flying in certain directions cruise at. VFR aircraft flying westbound should always cruise at even thousands plus five hundred feet. For example 4500, 6500, 8500, etc. VFR aircraft flying eastbound should cruise at odd thousands plus five hundred feet – 5500, 7500, 9500, etc. For IFR traffic it’s the same except without the “plus 500 ft”. This in theory is supposed to reduce the risk of a mid-air collision. I think it works fairly well.

Turbulence also comes into play sometimes as well, especially when carrying passengers. Usually convective turbulence caused by the sun heating the earth on sunny days is stronger at the lower altitudes. So sometimes for your passengers’ sakes, it may be worth it to cruise a little higher in the smooth air, even if it means catching a stronger headwind or giving up some engine power. A smooth flight that’s slightly longer is usually more enjoyable then a short flight bouncing around. Especially if it means you don’t have to clean up puke afterwards.

How far the destination is can also be a factor, especially when flying at aircraft with marginal performance. Sometimes staying lower then the altitude where you’ll fly the fastest can be better in the long run if its just a short hop. Spending 20-30 minutes climbing at a slow airspeed and burning obscene amounts of fuel may not make much sense when it’s only a 40-minute flight. That being said, during the descent it is possible to convert much of that altitude back into airspeed, but its never possible to gain back all of the spent time and energy.

Finally, and this generally only applies to single engine airplanes, safety in the form of gliding distance should also be considered. If we’re flying over bodies of water or unfriendly terrain its wise to consider choosing an altitude high enough to leave us with options should an engine fail and we become a glider.

This all being said, all this may be great theory, and though it may be possible to sit down, check the weather reports and aircraft performance charts and compute the absolute best altitude to fly at, it rarely makes sense or is practical to spend the time doing that. Usually after having flown an airplane for a reasonable amount of time, one gets to know the airplane enough to form a pretty good idea of its most efficient altitude and time it will take to climb to it. The difference on all but the longest flight may mean only +/- 5%. Winds can make a big difference, but if you don’t have the chance to check the upper winds, going on the rule of thumb of flying high in a tailwind and low in a headwind usually works reasonably well. Everything else can pretty well be figured out on-the-fly (pun not intended).

Well, been flying for an hour and forty minutes so far, one hour fifty to go… lousy headwind…