Why Planes Don't Fly Higher Than 40,000 Feet?

Ajouté : 2026-06-02

[00:00:00]Picture this, you're on a long haul flight somewhere over the Atlantic and you look out the window. Nothing but darkness and a thin blue line where the atmosphere ends. You're already 35,000 ft up, nearly 7 mi above the ground.

[00:00:15]And a thought crosses your mind.

[00:00:17]Why don't planes just go higher?

[00:00:20]If the air up here is thinner and there's less drag, wouldn't it be even better to climb a little more? Maybe 50,000 ft?

[00:00:29]60,000?

[00:00:30]It sounds logical and it is up to a point.

[00:00:34]But somewhere above where you're sitting right now, the physics of flying starts to collapse [music] in on itself. And the answer to why planes can't simply climb out of it involves not one problem, but four completely separate ones. Each one compounding the last.

[00:00:50]By the end of this video, you'll understand exactly why 35,000 ft isn't a compromise. It's a balancing act on a razor's edge.

[00:01:00]Let's start with what's actually true.

[00:01:03]Flying high is efficient.

[00:01:05]The basic equation for drag is this.

[00:01:08]Drag equals a drag coefficient times half the air density times velocity squared times wing area.

[00:01:16]What that tells you is simple. Less air density means less drag.

[00:01:21]And as you climb, density drops fast. By the time you're at 35,000 ft, the air is roughly a third as dense as it is at sea level. That means the engines don't have [music] to push as hard against the air to maintain speed. The aircraft burns less fuel [music] for every mile it travels.

[00:01:39]Airlines call this specific air range, distance covered per unit of fuel, >> [music] >> and it peaks at altitude.

[00:01:47]That's why every commercial jet climbs as quickly as possible after takeoff.

[00:01:52]There's another benefit, too. At very high altitudes, the air is almost always calm. You're well above most weather systems, above storm clouds, above the choppy air that makes your drink slosh around.

[00:02:05]Smoother air means a more stable flight and less structural stress on the airframe over time.

[00:02:12]So, given all that, why not go to 50,000 ft or 70,000?

[00:02:18]Here's where it gets complicated. The same thin air that reduces drag also reduces lift.

[00:02:25]Lift uses the same variables as drag. It depends on air density, too.

[00:02:30]When density drops, the wing generates less lift [music] for the same speed.

[00:02:35]To compensate, the aircraft has to fly faster.

[00:02:39]Specifically, as you go higher, true airspeed has to increase roughly in proportion to the square root of the drop [music] in density just to hold the same lift and keep the plane level.

[00:02:51]This is where the Mach number becomes critical.

[00:02:54]Mach is simply your speed as a fraction of the local speed of sound.

[00:02:58]>> [music] >> And the speed of sound isn't constant.

[00:03:01]It drops as the air gets colder.

[00:03:04]In the upper atmosphere, where temperatures can sit around minus 57° C, the speed of sound is noticeably lower than at sea level.

[00:03:13]So, here's the trap. As you climb, you need to fly faster in terms of true airspeed, but the speed of sound is simultaneously dropping.

[00:03:24]You're eating into your Mach margin from both ends.

[00:03:28]Most commercial [music] jets cruise around Mach 0.78 to 0.85, and there's a hard ceiling on that.

[00:03:35]Push past it and things get dangerous very quickly.

[00:03:40]When an aircraft's wings approach the critical Mach number, which is lower than the actual speed [music] of sound, airflow over certain parts of the wing locally goes supersonic.

[00:03:51]This creates shock waves.

[00:03:53]Those shock waves aren't just a bump.

[00:03:56]They cause what's called shock-induced boundary layer separation. Essentially, the smooth airflow over the wing suddenly detaches.

[00:04:06]The result is high-speed buffet, violent vibration that can be felt throughout the aircraft.

[00:04:13]In severe cases, it can cause a phenomenon called Mach tuck, where the center of lift shifts rearward and the nose pitches down sharply, accelerating the aircraft even further toward overspeed.

[00:04:28]This is why there's a maximum operating Mach number, MMO, on every commercial aircraft.

[00:04:34]For most wide-body [music] jets, it sits around Mach 0.86 to 0.90.

[00:04:41]The aircraft will not let you exceed it in normal operation.

[00:04:45]On modern fly-by-wire jets, the flight computers actively prevent it.

[00:04:50]>> Now, here's where it gets truly uncomfortable. At low altitudes, there's a wide gap between the speed you must fly to avoid stalling, the stall speed, and the maximum Mach you must stay under to avoid Mach buffet.

[00:05:03]That gap is your safety margin. It's the room the pilots in the aircraft have to maneuver, handle turbulence, make corrections.

[00:05:12]As altitude increases, something alarming happens to those two lines.

[00:05:16]The stall speed rises because the wings need faster true airspeed to generate lift in thinner air.

[00:05:23]And the Mach [music] limit, expressed in true airspeed, falls because the speed of sound itself falls with temperature.

[00:05:31]The two [music] lines move toward each other.

[00:05:33]At some point, they meet.

[00:05:36]The speed at which you'd stall and the speed at which you trigger Mach buffet become identical.

[00:05:42]There is no safe speed.

[00:05:45]This is called coffin corner, And the name is not an accident.

[00:05:51]Commercial jets never fly anywhere near actual coffin corner. Regulations require a meaningful margin between the aircraft's cruise conditions and both of these limits. Typically, at least 0.3 G of maneuvering room before entering either low-speed or high-speed buffet.

[00:06:08]In practice, that margin defines the absolute ceiling. [music] But even approaching coffin corner is dangerous. Turbulence, which increases load factor on the wing even briefly, can eliminate that margin [music] in seconds.

[00:06:22]Near the ceiling, a jolt from clear air turbulence can [music] simultaneously stall the wings and over-speed the aircraft with no safe recovery.

[00:06:32]There's another limit that has nothing to do with aerodynamics. It's about the structure of the aircraft itself.

[00:06:38]At 35,000 ft, the air pressure outside is roughly 26 kilopascals, about a quarter of sea level.

[00:06:46]No human can survive in that environment for long.

[00:06:49]The aircraft cabin is pressurized, typically to the equivalent of about 6,000 to 8,000 ft altitude.

[00:06:56]That pressure difference, roughly 8 to 9 lb per square inch acting outward on every square inch of fuselage, is enormous when you multiply it over the entire aircraft body.

[00:07:08]The fuselage is essentially an inflated tube under constant outward pressure, thousands of times per flight.

[00:07:16]>> [music] >> Every time the aircraft takes off and climbs, the fuselage expands slightly.

[00:07:21]Every time it descends and lands, it contracts.

[00:07:25]These cycles cause [music] tiny cracks to grow in the aluminum skin, frames, and window corners, a process called metal fatigue.

[00:07:34]The design of every airliner must account for this. Certification standards require that the aircraft withstand thousands of these pressurization cycles without failure.

[00:07:45]Critical structural areas are inspected at set intervals and the entire aircraft has a certified limit of validity. A maximum number of pressurization cycles it can accumulate before mandatory retirement or extensive structural rework.

[00:08:01]If you flew much higher, the pressure differential would increase >> [music] >> and those stresses would accelerate dramatically.

[00:08:07]The structure simply isn't built for it.

[00:08:10]So, we have four problems.

[00:08:12]Lift falls off. The Mach limit closes in.

[00:08:15]Coffin corner [music] arrives and the fuselage pushes back.

[00:08:20]But, here's what's interesting.

[00:08:22]All of these problems [music] interact with one more variable that pilots and dispatchers think about constantly.

[00:08:28]Weight.

[00:08:30]At the start of a long flight, the aircraft is heaviest, full of fuel.

[00:08:34]A heavier aircraft needs more lift, which means a higher minimum speed to avoid stall.

[00:08:41]That pushes the aircraft's safe altitude ceiling down because a higher minimum speed eats into the margin to Mach buffet faster.

[00:08:50]As the flight progresses and fuel burns off, the aircraft gets lighter.

[00:08:55]The stall speed drops. The gap between stall [music] and Mach buffet widens.

[00:09:00]The aircraft can safely fly higher.

[00:09:03]This is why long-haul flights use step climbs. An aircraft that departs at flight level 330, [music] that's 33,000 ft, might request flight level 350 [music] several hours into the cruise. Then, perhaps 370 as the flight approaches its destination.

[00:09:21]Each step moves into thinner air, where fuel burn per mile is lower, saving thousands of pounds of fuel on an intercontinental route.

[00:09:31]Flight management computers calculate the step climb points continuously based on the aircraft's current weight and the buffet margins available.

[00:09:40]The crew doesn't have to guess. The system knows when the aircraft is light enough to go higher safely.

[00:09:46]The engines face their own altitude problem. Thrust depends on how much air the engine can swallow. At altitude, the air is thin, so less mass flow through the engine per second.

[00:09:59]The result is lower thrust, a phenomenon called thrust lapse.

[00:10:04]Up to a certain altitude, typically the sweet spot for commercial jets, around [music] 31,000 to 38,000 ft, the engine efficiency actually improves. [music] The cold air allows better combustion conditions.

[00:10:19]Fuel consumption per unit of thrust [music] gets better.

[00:10:22]But beyond that band, the thrust loss outpaces the efficiency gain.

[00:10:27]The engines have to work harder to maintain cruise speed. Exhaust gas temperatures climb toward their limit, and eventually, there simply isn't enough thrust to sustain level flight at the desired Mach number.

[00:10:41]There's also the engine out scenario.

[00:10:44]Every long-haul aircraft must be capable of maintaining safe flight on one engine in the event of a failure.

[00:10:51]The altitude [music] at which a single engine can sustain level flight, the OEI ceiling, is significantly lower than the normal cruise ceiling.

[00:11:01]Long-range flight planning always accounts for this. If the planned cruise [music] altitude is above the OEI ceiling, crews must plan a drift down route, a controlled [music] descent to a lower altitude where the remaining engine can hold level flight. And that route must stay clear of terrain [music] all the way to a diversion airport.

[00:11:21]Given all of these converging limits, it might seem astonishing that commercial aviation operates as reliably as it does.

[00:11:29]Part of the answer is the automation that manages the envelope in real time.

[00:11:34]On fly-by-wire aircraft, computers monitor angle of attack, airspeed, Mach number, and load factor continuously.

[00:11:42]If a pilot's input would push the aircraft outside safe boundaries, the flight control laws simply don't allow it.

[00:11:49]Stall protection kicks in before the wing actually stalls.

[00:11:53]Overspeed protection prevents exceeding MMO.

[00:11:57]In some aircraft, the computers will even apply full thrust automatically [music] if angle of attack climbs too high.

[00:12:04]The speed tape on a modern cockpit display shows this visually.

[00:12:08]There's a colored band at the bottom showing minimum safe speed, and a barber pole at the top showing the Mach and [music] overspeed limit.

[00:12:16]At normal cruise altitude, these bands are well apart.

[00:12:20]As the aircraft approaches its ceiling, the bands creep together.

[00:12:24]Crews are trained to read that convergence as a warning.

[00:12:28]Near coffin corner, the priority is always to back off, descend, reduce load, restore margin.

[00:12:36]Trying to stretch the envelope for a marginal fuel saving is not a trade-off any trained crew would make.

[00:12:43]The atmosphere adds one more layer of variability.

[00:12:47]Jet streams, bands of fast-moving air at roughly 30,000 to 40,000 ft, [music] can either be a major tailwind or a significant headwind, depending on the route and direction.

[00:12:59]Sometimes, a lower [music] cruise altitude that catches a favorable jet stream burns less fuel overall than climbing [music] to the theoretical optimum.

[00:13:08]Clear air turbulence is closely associated with jet stream boundaries, [music] where there are strong vertical wind shear gradients.

[00:13:16]It's invisible. Radar can't detect it, and it can be severe.

[00:13:21]Recent research suggests that clear air turbulence at high altitudes has increased over the last few decades, likely tied to changes in atmospheric circulation patterns.

[00:13:32]This turbulence risk is another reason aircraft don't push to the absolute aerodynamic ceiling.

[00:13:39]At maximum certified altitude with minimal buffet margin, an unexpected turbulence encounter could simultaneously violate both the low speed and the high speed limits.

[00:13:50]Staying a few thousand feet below the ceiling provides real protection.

[00:13:56]So, what determines [music] where a commercial jet actually flies?

[00:14:00]It's not one number.

[00:14:02]It's the intersection of at least six independent constraints, all of them active at once.

[00:14:08]The aerodynamic envelope sets hard limits, stall speed at the bottom, Mach buffet at the top, with coffin corner marking where those limits collide.

[00:14:18]The aircraft's weight moves those limits throughout each flight, which is why step climbs make sense.

[00:14:25]Engine thrust lapse sets a practical ceiling on how high the aircraft can actually reach.

[00:14:31]The pressurization system and fatigue life of the fuselage set limits on how much differential pressure is acceptable over tens of thousands of flight cycles.

[00:14:41]Weather and jet streams influence which altitude is actually most economical on any given day.

[00:14:48]And engine out contingency planning puts a cap on cruise altitude to ensure [music] a safe drift down path in the event of failure.

[00:14:57]The number that comes out of that, usually somewhere between 33,000 [music] and 41,000 feet for most commercial jets, isn't arbitrary.

[00:15:06]It's where the physics, the engineering, [music] the regulations, and the economics all agree.

[00:15:13]So, the next time you're on a flight and you notice the aircraft leveling off at 37,000 feet, you'll know it didn't stop there because of some conservative default.

[00:15:23]It stopped there because that is for that aircraft on that day at that weight in those atmospheric conditions, the best place it can possibly be.