Why planes fly at around 10,000 meters: cruising altitude and flight levels

You’re standing at the fence, the landings are done, and a white line cuts across the blue above you. On the flight tracker, next to it, it says “FL370”. But why is the jet flying up there in particular – and not two kilometers lower or higher? The cruising altitude of airliners is no coincidence and no whim of the crew. It comes from physics, weather, weight, wind and a set of rules that neatly stacks thousands of aircraft on top of each other. This guide explains what is behind those roughly 10,000 meters, what a flight level is and how to read altitude readings on a flight tracker like a spotter.

Cruising altitude: the short answer

Airliners mostly fly between about 31,000 and 38,000 feet, roughly 9.5 to 11.5 kilometers, because three things come together there: The air is thin enough to cut drag sharply. The engines work very efficiently at typical cruise speed. And the aircraft flies above most of the weather. Exactly how high a given flight cruises then depends on weight, wind, route length, direction and traffic.

This altitude is given not in meters but as a flight level (FL for short). FL370 means roughly 37,000 feet – measured not above sea level, but as a pressure altitude with a worldwide uniform reference pressure. More on that in a moment.

Reason 1: thin air, less drag

Air gets thinner and thinner with altitude. In the ICAO standard atmosphere, which aviation and aircraft design use for their calculations, sea level has 1013.25 hPa at 15 °C, and the air density is about 1.225 kg/m³. At 11 kilometers, the pressure has dropped to about 226 hPa, the temperature to minus 56.5 degrees, and the density is only about 0.36 kg/m³ – roughly 30 percent of the ground value.

For an aircraft, that means: at the same true airspeed, it meets far less air mass. Drag falls accordingly. To still generate enough lift, the jet has to fly faster than at low altitude – and that is exactly what you want. A modern short- and medium-haul jet like an Airbus A320neo or a Boeing 737-800 typically cruises at around Mach 0.78, which is about 830 km/h. Widebodies such as the Airbus A350-900 or the Boeing 787-9 fly closer to Mach 0.85, or around 900 km/h. Near the ground, such speeds would be unthinkable for reasons of structure, noise and fuel burn.

Why the Mach number matters so much

Up in the cold, the speed of sound is also lower than at ground level. At minus 56.5 degrees it is about 1,060 km/h instead of roughly 1,225 km/h at 15 degrees. Airliners are optimized for a particular Mach number, that is, a fraction of the speed of sound. If they go faster, shock waves form on the upper surface of the wing, and drag rises abruptly. Cruise speed is therefore capped – and at high altitude a jet reaches this economical Mach number with comparatively little thrust.

Reason 2: efficient engines

Turbofan engines deliver less thrust at high altitude, because they take in less air. At the same time, though, the aircraft needs less thrust there to hold its speed. All in all, a jet in high-altitude cruise burns significantly less fuel per kilometer flown than at low altitude. That is especially true on long routes, where cruise makes up most of the flight time.

For propeller aircraft, the optimum is lower. Turboprops like the ATR 72-600 are designed for shorter routes and lower speeds and typically fly between FL150 and FL270 – how to tell the common models apart is covered in How to tell turboprops apart. For a one-hour route, the long climb to eleven kilometers isn’t worth it anyway – the jet would barely arrive at the top before it has to descend again.

Reason 3: above the weather

Almost everything we experience as weather happens in the troposphere, the lowest layer of the atmosphere. Its upper limit is called the tropopause. It sits at very different heights depending on latitude and season: around 16 kilometers over the equator, often only 6 to 9 kilometers over the polar regions. At our latitudes it is usually somewhere in between. Above it, in the stratosphere, the air is very dry and largely cloud-free.

At mid-latitudes, airliners often fly near the tropopause or just above it. That leaves many clouds, areas of precipitation and icing conditions beneath them. It isn’t completely weather-free up there, though: thunderstorm towers can reach the tropopause, cirrus clouds often mark its position, and near bands of strong wind there is clear-air turbulence. How it forms and why it is usually harmless is explained in the article Turbulence explained.

What a flight level is

An altimeter in an aircraft doesn’t measure distance to the ground but air pressure. From that, it calculates an altitude. Because air pressure constantly fluctuates, every pilot would need to know the current pressure – and on a cruise over hundreds of kilometers with changing pressure systems, that would be impractical and error-prone.

The solution is the flight level: above a certain altitude, all crews set their altimeters to the standard pressure of 1013.25 hPa. The displayed altitude in hundreds of feet is then the flight level. If the altimeter reads 35,000 feet, the aircraft is flying at FL350 – roughly 10,700 meters on paper. The actual height above sea level can differ from that when air pressure and temperature don’t match the standard. For safety this doesn’t matter: all aircraft in the same airspace measure against the same reference, so the spacing between them is correct.

Transition altitude and transition level

Below the transition altitude, aircraft fly using the local air pressure (QNH) and altitudes in feet above sea level – important for clearance from terrain and obstacles. In Germany, the transition altitude is generally 5,000 feet above sea level, but at least 2,000 feet above ground. On descent, the transition level applies, which lies between FL60 and FL80 depending on air pressure. In the US and Canada, the transition altitude is much higher at 18,000 feet. If you’re interested in how airspace is divided up, there is more in Airspace classes A to G.

Term	What it means	Example
QNH	local air pressure converted to sea level	Altitude “5,000 ft” on approach
Standard pressure	uniform reference of 1013.25 hPa	Basis of all flight levels
Transition altitude	switch to standard pressure on the climb at this point	Germany: 5,000 ft
Transition level	switch to QNH on the descent at this point	FL60 to FL80, depending on air pressure
Flight level	pressure altitude in hundreds of feet	FL350 ≈ 35,000 ft

Eastbound odd, westbound even

To keep opposing traffic from flying at the same altitude, the semicircular rule applies. What counts is the course over the ground:

• 000 to 179 degrees (roughly eastbound): odd flight levels, for example FL330, FL350, FL370.
• 180 to 359 degrees (roughly westbound): even flight levels, for example FL340, FL360, FL380.

A flight from Frankfurt to Eastern Europe will therefore typically end up on an odd flight level, and the return flight on an even one. Individual countries have different rules. In Europe’s upper airspace, the rule is also supplemented by route structure and air traffic control instructions – who gets which altitude when is decided by controllers at places like DFS Deutsche Flugsicherung, coordinated through the network management of Eurocontrol.

RVSM: twice as many altitudes in the main band

It used to be that above FL290 there had to be at least 2,000 feet of vertical separation between two aircraft. With RVSM (Reduced Vertical Separation Minima), that was halved to 1,000 feet between FL290 and FL410. In Europe this has applied since January 2002. Instead of seven, this band has offered 13 flight levels ever since – exactly where most jets like to fly. That increases capacity and lets more flights fly close to their optimum altitude, which saves fuel.

For that, aircraft have to meet special requirements: two independent altitude measuring systems, a warning for deviations from the selected altitude and an autopilot that holds the flight level in straight flight to within a few dozen feet. Above FL410, larger separations apply again.

Weight, wind and distance: why every flight cruises at a different height

The perfect cruising altitude doesn’t exist – it depends on the individual flight.

• Weight: A fully fueled long-haul jet is so heavy at the start that its optimum altitude is relatively low. Every hour it burns tons of kerosene, gets lighter, and the optimum altitude rises. That is why crews on long flights request a step climb: they climb stepwise by one or two flight levels, for example from FL340 to FL360 and later to FL380.
• Wind: Around the tropopause lie the jet streams, bands of strong wind with core speeds of often 200 to 500 km/h. With a tailwind, crews look for the altitude with the strongest push from behind. With a headwind, they sometimes divert lower or onto a different route. Especially over the North Atlantic, routes are therefore adjusted daily to the position of the jet stream – more on route planning and curves is in Why planes fly curved routes.
• Route length: On short routes, the climb to the highest flight levels isn’t worth it. Short domestic flights therefore often stay lower than a long-haul flight that spends hours in cruise.
• Traffic and airspace: If the preferred altitude is occupied or a sector is overloaded, air traffic control assigns a different flight level.
• Weather: With reported turbulence or thunderstorms, crews move to a smoother altitude.

Why not even higher?

If thin air has so many advantages – why doesn’t an airliner fly at 15 kilometers? Because the window closes toward the top. The thinner the air, the faster the aircraft has to fly to generate enough lift. At the same time, it must not exceed a certain Mach number. At some point, minimum and maximum speed are so close together that hardly any margin is left. Pilots like to call this zone the “coffin corner”. In addition, available thrust drops, and the pressurized cabin has to withstand an ever greater pressure difference.

The cabin doesn’t hold ground-level pressure but a pressure equivalent to a lower altitude. US certification rules for airliners limit this cabin altitude to 8,000 feet at most in normal operation. Airbus advertises a cabin altitude of 6,000 feet for the A350. Why your ears still hurt is explained in the article Cabin pressure and pressure on your ears.

Aircraft class	Typical cruise range	Example
Turboprops	about FL150 to FL270	ATR 72-600
Airliners	about FL300 to FL410	Airbus A320neo, Boeing 787-9
Large business jets	up to 51,000 ft service ceiling	Gulfstream G650, Bombardier Global 7500
Supersonic (historic)	above about 15,000 m	Concorde

Business jets often fly above scheduled traffic, because they are lighter and find free airspace there – tips on how to get them in your viewfinder anyway are in How to spot business jets. Concorde, which flew in scheduled service until 2003, cruised higher still thanks to its supersonic speed – its story is told in the article Concorde history.

Reading flight levels on a flight tracker

For spotters, the altitude reading on a flight tracker is a handy tool. The altitude comes from the aircraft’s transponder – how that works technically is covered in Transponder Modes A, C and S. It is transmitted as pressure altitude, so it matches the flight levels. Here is how to use it:

• Constant flight level: If the value stays the same for a long time, say at FL360, the jet is in cruise. An overflight, not an approach to your airport.
• Falling values: Airliners often begin their descent more than a hundred kilometers before the destination. If you want to know when a plane will arrive at your spot, this is an early hint. More on that in How accurate are arrival times on flight trackers?
• A jump of usually 2,000 feet in cruise: This is often a step climb, but can also be an air traffic control instruction.
• Even or odd: If the flight level fits the direction of flight, you’re seeing the semicircular rule live.

What the other symbols, colors and fields on the map mean is explained in the guide Flight tracker symbols and colors. And if you want to capture the jets up there in a photo: focal length, exposure and the right weather for overflights are described in Photographing aircraft at cruising altitude.

What to keep in mind

• A flight level is not meters above ground. It is a pressure altitude. Over high terrain or at extreme air pressure, the true height differs significantly.
• Feet remain the standard. In almost all countries, altitude is given in feet. A few states historically use metric flight levels, which hardly matters for everyday spotting in Europe.
• Long-haul flights climb as they go. A jet that is still at FL320 shortly after takeoff can be at FL380 or higher hours later.
• Cruising altitude has limits for photography. At more than ten kilometers up, and often at an even greater slant distance, even a widebody remains a small dot.

Conclusion

Those roughly 10,000 meters are a compromise that has settled in over decades: thin air lowers drag, the engines work efficiently at the typical Mach number, and most of the weather stays beneath the aircraft. How high an individual flight cruises is determined by weight, wind, route and air traffic control. Altitude is measured in flight levels – pressure altitudes with a uniform reference that keep opposing traffic neatly apart via the semicircular rule and RVSM. For you as a spotter, that makes every altitude reading on a flight tracker readable.