Seven miles above your head, the outside air is thinner than the summit of Everest and colder than an Antarctic winter, around minus 55 degrees Celsius, and roughly a quarter of sea-level pressure. It would render an unprotected person unconscious in under a minute. Airliners choose to spend their working lives there, hour after hour, because that hostile band of atmosphere between about 31,000 and 42,000 feet happens to be the most economical place on Earth to move people quickly. The story of why is a neat bundle of physics, engineering and air traffic bureaucracy, and once you know it, the altitude readouts on a tracking map begin to tell stories of their own.
The short answer, with numbers
Commercial jets typically cruise between 31,000 and 42,000 feet. Most long-haul traffic settles between 35,000 and 41,000 feet, climbing higher as fuel burns off and the aircraft lightens, a technique called step climbing. Narrowbodies on shorter hops often cruise lower, in the low-to-mid 30s, because a 45-minute sector does not justify the climb to 39,000 feet. Turboprops like the ATR 72 work far below the jets, around 18,000 to 25,000 feet. Business jets go highest of all: types like the Gulfstream G650 are certified to 51,000 feet, cruising above the airline traffic in nearly empty sky. The certified ceiling for most airliners sits around 41,000 to 43,100 feet; the Airbus A380 and Boeing 787 both top out at 43,100 feet.
| Aircraft category | Typical cruise | Certified ceiling (approx.) |
|---|---|---|
| Regional turboprop (ATR 72) | 18,000-25,000 ft | 25,000 ft |
| Narrowbody jet (737, A320) | 33,000-39,000 ft | 41,000 ft |
| Widebody jet (787, A350, A380) | 35,000-41,000 ft | 43,100 ft |
| Business jet (G650, Global 7500) | 41,000-51,000 ft | 51,000 ft |
| Concorde (retired) | 50,000-60,000 ft | 60,000 ft |
Why thin air is cheap air
Drag, the force the engines must constantly overcome, is proportional to air density. At 35,000 feet the air is roughly a quarter as dense as at sea level, so the airframe slips through with far less resistance, and the jet engine, which thrives on cold intake air, runs at its most efficient. The result is that an airliner at altitude covers each mile on dramatically less fuel than it would lower down. Climb costs fuel up front, which is why very short flights stay low, but on anything over an hour the altitude investment repays itself handsomely.
There is a ceiling on the bargain. Climb too high and the thin air can no longer hold the aircraft up at its current weight without flying close to both its low-speed stall limit and its high-speed Mach limit, a narrowing margin pilots informally call coffin corner. This is why heavy jets cannot reach their optimum altitude right after takeoff with full tanks: a fully loaded 777 leaving for a 14-hour sector may initially cruise at 31,000 or 33,000 feet, then step up to 37,000 and finally 39,000 or 41,000 feet as hundreds of thousands of pounds of fuel burn away. Watch a long-haul flight for a few hours on the live flight map and you can see the staircase drawn in its altitude trace.
Above the weather, inside the wind
Altitude also buys smoothness. Most convective weather lives below 30,000 feet, and cruising jets overfly the bulk of it, though the tallest thunderstorms can tower past 50,000 feet and must be steered around. The other atmospheric player is the jet stream, the fast river of wind meandering between roughly 30,000 and 39,000 feet, described in detail by NOAA. Dispatchers plan altitudes to ride it eastbound and dodge it westbound, and its shear zones generate the clear-air turbulence that arrives without warning, a phenomenon our travel-section article on the science of turbulence covers from the passenger's seat.
An airliner's cruising altitude is a negotiation: physics sets the menu, weight sets the limit, and air traffic control sets the table.
Flight levels: Why pilots say three-five-zero
Above 18,000 feet in the United States, altitude stops being measured against local sea level and becomes a flight level. Every aircraft sets its altimeter to the same standard pressure of 29.92 inches of mercury, so all altimeters agree with each other even though the reading no longer matches true height. Flight level 350 means 35,000 feet on that standard setting. The system exists so that two jets converging from different weather systems are measuring altitude with the same yardstick; the rules are laid out in the FAA's Aeronautical Information Manual.
Direction of flight organizes the layers. Broadly, eastbound traffic flies odd flight levels (FL330, FL350, FL370) and westbound traffic flies even ones (FL340, FL360, FL380). Since 2005, reduced vertical separation minimum, or RVSM, has allowed properly equipped aircraft to be stacked just 1,000 feet apart between FL290 and FL410, doubling the usable cruise levels and adding enormous capacity to the system. That is why a passing jet 1,000 feet above you at cruise can look startlingly close from your window: it is supposed to be there, precisely there, and both autopilots are holding altitude to within a few tens of feet. Air traffic controllers assign and monitor every one of these levels for every flight in their sector, from initial climb clearance to the final descent handoff.
Reading altitude on a flight tracker
Altitude is one of the most informative fields in tracking data. A target at 36,000 feet is in cruise. One descending through 11,000 feet within 40 miles of a city is arriving; below 10,000 feet, it is also limited to 250 knots indicated airspeed, which is why traces visibly slow there. A jet circling at 8,000 feet is probably holding. And a reading of 2,000 feet climbing steeply means you caught a departure. Trackers report barometric altitude broadcast by the aircraft's own transponder, so during unusual pressure weather the number can differ slightly from true height; the differences are small at cruise and irrelevant for hobby purposes. If you want the deeper plumbing of where those numbers come from, our technology-section article on how ADS-B works follows the data from the aircraft to your screen.
Did you know?
Concorde cruised as high as 60,000 feet, and its passengers could see the curvature of the Earth and a sky turning deep indigo at midday. It flew above nearly all weather and all other traffic, climbing gently as fuel burned in a continuous cruise-climb that controllers permitted because nothing else was up there.
The edges of the envelope
A few aircraft live outside the airline band. Military and research aircraft such as NASA's ER-2, a descendant of the U-2, work above 60,000 feet in pressure suits. Gliders have soared past 76,000 feet in mountain wave experiments. At the other extreme, sightseeing flights, survey aircraft and helicopters spend their careers below 10,000 feet where the scenery is. General aviation pilots without pressurized cabins must use supplemental oxygen for extended flight above 12,500 feet under FAA rules, a regulation that quietly defines the ceiling of weekend flying. Every one of these layers is visible on a busy day over any major metro area, stacked like geological strata, and learning to read them is one of the small pleasures of the hobby, right alongside identifying the aircraft themselves or spotting the difference between the trails they leave, the subject of our companion piece on why some planes make contrails.
Key takeaways
- Airliners cruise between roughly 31,000 and 42,000 feet because thin, cold air minimizes drag and fuel burn.
- Heavy jets step-climb through the flight, rising as fuel burns off and the optimum altitude climbs with them.
- Above 18,000 feet, altitude becomes standardized flight levels; RVSM stacks traffic just 1,000 feet apart.
- Eastbound flights generally use odd flight levels, westbound flights even ones.
- Business jets cruise above airliners, up to 51,000 feet; turboprops work far below, around 20,000 feet.
- Altitude on a tracker reveals flight phase instantly: cruise, descent, holding or departure.
Frequently asked questions
What is the highest a commercial plane can fly?
Most airliners are certified to between 41,000 and 43,100 feet. Business jets like the Gulfstream G650 reach 51,000 feet, and Concorde was approved to 60,000 feet. Certification ceilings reflect pressurization limits and aerodynamic margins, not engine failure points.
Why do planes fly at 36,000 feet instead of higher?
Because optimum altitude depends on weight. A heavily loaded jet cannot efficiently or safely cruise at its ceiling early in the flight, and air traffic control must also fit each aircraft into available levels. As fuel burns off, crews request higher levels in steps.
Do planes fly lower in bad weather?
Sometimes they fly around weather rather than under it, since thunderstorms can exceed cruise altitudes. Turbulence reports do drive altitude changes: crews routinely request a different flight level to find smoother air, which you can watch happening in real time on a tracker.
Why does my ears pop if the cabin is pressurized?
Cabins are pressurized to an equivalent altitude of about 6,000 to 8,000 feet, not sea level, so pressure still changes noticeably during climb and descent. Newer composite aircraft like the 787 hold a lower cabin altitude of about 6,000 feet, which many passengers find noticeably more comfortable.
The next time you watch a contrail crawl across a summer sky, put a number on it: seven miles, minus 55 degrees, a quarter of an atmosphere, and a few hundred people watching movies in shirtsleeves. Altitude is the quietest miracle in aviation, purchased with physics and maintained by an invisible bureaucracy of perfectly spaced layers.
