Nearly everyone has been on an airline flight at some point in their lives and seen the word from a 30,000 foot (often more) foot view. It’s amazing up there and one of the reasons you go that high is so that you can go fast, unobstructed by the world below or by the winds and weather that are near the surface. But a common questions is, do airplanes fly faster at higher altitudes?
Technically, no. As altitude increases, air density decreases, which decreases engine performance but gives it better fuel efficiency. However, less air density with high altitude does mean that the airplane has less wind resistance and drag, which partially offsets the decrease in engine performance. Each airplane has an optimal cruising altitude range that is the best tradeoff of speed and fuel efficiency.
All things equal, if you took an airplane and flew at 30,000 feet and measured its speed, then took the exact same airplane and went to 40,000 feet and measured its speed, it would almost always be universally slower at 40,000 feet. Why is that? And if that’s the case, why do airplanes fly so high in the first place? Would it be better and faster to fly low to the ground? Well, let’s find out.
How Do Airplanes Fly Faster?
Let’s set one thing straight – when we’re referencing “speed” in the context of this article, we’re referring to true airspeed. This is the purest form of speed measurement because it filters out the wind direction and speed, non-standard temperature and atmospheric pressure conditions, and calibration errors of the instrumentation in the airplane.
In other words, true airspeed is how fast the airplane is truly flying. Ground speed is how fast you are moving across the ground and is used in the calculation of time to your destination, but this is influenced greatly by the wind and so that’s why we aren’t referring to ground speeds in this article.
To go “faster”, airplanes have two options: increase thrust or decrease drag. Trust is created by the engine and drag is created by anything that produces wind resistance. Airplanes get the greatest amount of thrust actually at lower altitudes, because the air is more dense, meaning there is more oxygen to mix with fuel and combust, leading to more thrust from the engine. On the other hand, lower altitudes have much greater wind resistance because the air is more dense, which slows the airplane down. Therefore, there is an optimal altitude range that balances the airplane’s thrust, drag, and fuel efficiency.
Is Higher Faster?
If you take an airplane that is going at full speed, and measured its speed at two different altitudes, where would it be going faster? It would actually go faster at the lower altitude, all things equal. If you look at the operating handbook for a Cessna Skyhawk or a Phenom jet, you’ll notice that the true airspeed decreases as altitude increases. This is because you have worse engine performance at high altitudes because of the much thinner air. Think of it like your lungs at sea level versus your lungs up in the mountains. You have to work harder to breathe, don’t you? An engine feels the same way when it is starved of air. It performs worse but it will use less fuel.
Is Higher Better?
If airplanes produce more thrust at lower altitudes and can fly faster, then wouldn’t you just want to fly as low as possible to fly as fast as possible? Well, no. And for a few reasons.
First off, there is a speed limit below 10,000 feet above sea level of 250 knots. This is for noise abatement and air traffic control’s spacing of aircraft. So, jets have their airspeed really pulled back until climbing through 10,000 feet to abide by this rule.
Secondly, most weather phenomena happen below 30,000-40,000 feet (except for some of the super cell thunderstorms that can tower up to 60,000 feet or more), and so traveling high keeps you out of this for the most part. Turbulence can happen at any altitude but it is generally more prevalent closer to the ground because of convection (rising heat) from the surface and some obstruction turbulence where the wind more or less bounces off of hills, buildings, and the like. Staying high helps you avoid this as well.
Lastly, flying higher leads to better fuel efficiency. Engines burn a combination of fuel and air, and that ratio is determined by the weight of the air and the weight of the fuel. When you fly higher, the air is less dense and therefore weighs less, meaning that you have to mix less fuel with it in order for it to combust. Said another way, you use less fuel.
Why fly different altitudes?
Altitudes are largely selected based on the best combination of fuel efficiency and wind direction/speed, and weather (thunderstorms, icing, turbulence, etc.). In my Cessna 182 (a normally aspirated, or non-turbocharged engine) I usually fly somewhere between 5,000 and 10,000 feet, but my performance starts to slip after about 5,500 feet because the engine is producing less power. If there are more favorable winds higher up to where my ground speed would increase, then I will usually do that.
Jets traveling high are in a similar situation, where they are generally flying between 35,000 and 45,000 feet on any given day. It largely depends on the wind direction and speed at various altitudes as well as the weather in the route of flight.
How fast can you go in a commercial airliner?
If there is a speed limit down low (below 10,000 feet, and in certain airspaces), then a natural question is to ask is there a speed limit at high altitude? For civil aircraft there is an airspeed limit of Mach 1 which is 667 knots or around 767 miles per hour. Military aircraft often travel much faster than this but there is a regulation on civil aircraft staying under this limit.
Airliners are still usually traveling a decent margin underneath this airspeed. Why would that be? Well, without getting too technical it’s because near Mach 1 some of the air going over the wings actually starts to travel faster than Mach 1 which starts to create some control issues for the airplane. It’s called Mach Tuck and in order to avoid this, each airplane has what’s called a Critical Mach number or the highest airspeed the wing can reach before Mach Tuck sets in. Airliners stay below this threshold which is why they aren’t traveling at or very near the speed of sound.
Conclusion
By the book, airplanes won’t fly faster at higher altitudes. But there are a lot of practical reasons why it’s still more beneficial to fly at higher altitudes, whether that’s getting above speed limiting altitudes (10,000 feet), getting above troublesome weather, or gaining greater fuel efficiency. You won’t fly significantly slower at higher altitudes and so these tradeoffs are often beneficial at the end of the day to sacrifice a little bit of speed.