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Physics of flight: questions and answers
Physics of flight: the most searched Aviation questions, with a direct answer, an explanation and sources.
Physics of flight
Why does the speed of sound vary with temperature?
Sound is a pressure wave passed along by molecular collisions, so it travels faster when molecules move faster, and molecular speed is set by temperature. In… more…
Physics of flight
How does a plane fly?
A plane flies by using engine thrust to move its wings through the air fast enough to generate lift greater than or equal to its weight. The wing's shape and… more…
Physics of flight
Why does a plane keep circling?
A circling aircraft is usually holding: air traffic control has asked it to fly a racetrack pattern while a runway, a weather cell or a traffic gap clears… more…
Physics of flight
Why do contrails form?
Contrails form when hot, humid exhaust from jet engines meets air that is cold enough, typically below about −40 °C at cruise altitude, for the water vapour… more…
Physics of flight
How high planes fly?
Commercial jets typically cruise between 9,000 and 12,500 m (30,000–41,000 ft), where thin air cuts drag and fuel burn while the engines still get enough… more…
Physics of flight
What happens when a plane goes supersonic?
When an aircraft passes Mach 1 the air can no longer move out of its way in advance, so pressure disturbances pile up into shock waves at the nose, wings and… more…
Physics of flight
How do supersonic planes work?
A supersonic aircraft is built to live with shock waves. Thin, sharply swept or delta wings and a slender fuselage keep wave drag low; intakes with movable… more…
Physics of flight
Why do planes board front to back?
Most airlines do not board front to back for speed: the premium cabin at the front boards first as a commercial perk, and economy then boards by zones, often… more…
Physics of flight
Why did supersonic planes stop?
Concorde stopped flying in 2003 because it no longer paid its way: it burned far more fuel per seat than subsonic jets, carried only around 100 passengers… more…
Physics of flight
How does the sound barrier work?
The sound barrier is the sharp rise in drag and control difficulty an aircraft meets as it nears Mach 1, about 1,062 km/h (573 kn) at cruise altitude. Air… more…
Physics of flight
Why do planes require airplane mode?
Airplane mode is required because a phone hunting for a distant ground tower transmits at full power, and regulators like the FCC and EASA want to rule out… more…
Physics of flight
Why does an airplane stay in the air?
An airplane stays airborne because its wings produce lift that matches its weight. Moving through the air, a wing turns the flow downward and creates lower… more…
Physics of flight
How far is the sound barrier?
The sound barrier is not a distance but a speed: Mach 1, the local speed of sound, about 1,225 km/h (661 kn) at sea level and 1,062 km/h at 11 km. Airliners… more…
Physics of flight
How does the sound barrier break?
The sound barrier is not broken so much as crossed: an aircraft with enough thrust and a shape that tolerates shock waves accelerates through Mach 1. Chuck… more…
Physics of flight
Why does the sound barrier exist?
The sound barrier exists because pressure disturbances travel through air at the speed of sound. Below Mach 1 the air ahead of an aircraft receives warning… more…
Physics of flight
What planes can go supersonic?
Only aircraft designed for it: military fighters and interceptors (F-15, F-16, F-22, F-35, Rafale, Eurofighter Typhoon, Su-57), the B-1B bomber and a handful… more…
Physics of flight
Why do supersonic planes have swept wings?
Sweep delays and softens the drag rise around the speed of sound. Only the airflow component perpendicular to the leading edge counts for compressibility, so… more…
Physics of flight
Why do planes zig zag?
Airliners rarely fly perfectly straight because they follow published airways and waypoints, deviate around thunderstorms and turbulence, and are vectored by… more…
Physics of flight
Why do planes use knots?
Aviation uses knots because a knot is one nautical mile per hour, and a nautical mile equals one minute of latitude on a chart. That link between speed… more…
Physics of flight
Why does the speed of sound change with altitude?
The speed of sound changes with altitude because it depends on air temperature, and temperature falls as you climb through the troposphere. Colder molecules… more…
Physics of flight
Why is the speed of sound so slow?
Sound is slow because it is a push, not a signal: air molecules must bump into their neighbours to pass the pressure wave along, and they do so at roughly… more…
Physics of flight
How much the speed of sound?
In air at 20 °C the speed of sound is about 343 m/s, or 1,235 km/h (767 mph, 667 kn). It depends on temperature, not pressure: in the standard atmosphere it… more…
Physics of flight
Why do planes dump fuel?
Planes dump fuel only when they must land while still heavier than their maximum landing weight, usually after an engine problem or a medical emergency… more…
Physics of flight
How much is the sound barrier?
The sound barrier is not a price or a quantity: it is the speed of sound, which an aircraft meets at Mach 1. That speed is about 343 m/s (1,235 km/h, 667 kn)… more…
Physics of flight
Why do planes vanish in the bermuda triangle?
They do not, at least not more than anywhere else. The US Coast Guard, NOAA and insurers find no higher rate of aircraft or ship losses in the Bermuda… more…
Physics of flight
Why do planes vibrate on takeoff?
Most takeoff vibration comes from the runway, not the aircraft: tyres rolling over expansion joints and grooved pavement at 250 km/h (135 kn) shake the whole… more…
Physics of flight
Why do jet engines have a spiral?
The spiral painted on a jet engine's spinner is a safety marking. When the fan is turning, the spiral blurs into a visible, pulsing pattern that warns ground… more…
Physics of flight
Why does an airplane fly?
An airplane flies because its wings turn the oncoming airflow downward, and the air pushes back with an upward force called lift. The wing's shape and angle… more…
Physics of flight
Why are supersonic planes banned?
Supersonic aircraft are not banned; what is prohibited is civil supersonic flight over land in the United States (since 1973) and many other countries… more…
Physics of flight
How high do supersonic planes fly?
Supersonic aircraft cruise far higher than airliners. Concorde flew at 15,000 to 18,300 m (50,000 to 60,000 ft) and the SR-71 operated above 24,000 m (80,000… more…
Physics of flight
How high are contrails?
Contrails usually form between about 8 and 12 km (26,000–40,000 ft), the cruise band of jet airliners, where the air is colder than roughly −40 °C. Below that… more…
Physics of flight
Why does the sound barrier break?
The sound barrier is not a physical wall: it is the sharp rise in drag and loss of control that pilots met near the speed of sound (about 1,225 km/h, 661 kn… more…
Physics of flight
Why are supersonic planes coming back?
Supersonic airliners are returning because three obstacles are easing: the overland boom ban, which the United States moved in 2025 to replace with a noise… more…
Physics of flight
How does the speed of sound change with altitude?
The speed of sound depends only on air temperature, not on pressure or density, so it falls as the troposphere cools: about 340 m/s (661 kn) at sea level in… more…
Physics of flight
Are supersonic planes coming back?
Yes, but not yet for passengers. Concorde retired in 2003; since then Boom's XB-1 went supersonic in January 2025, NASA's X-59 flew in October 2025 to test… more…
Physics of flight
Why do jet engines not melt?
Turbine inlet gas can exceed 1,600 °C, above the melting point of the nickel superalloys in the blades, yet they survive because the metal never touches that… more…
Physics of flight
Why do jet engines use kerosene?
Jet engines burn kerosene because it packs about 43 MJ/kg into a dense, cheap, easy-to-store liquid that stays fluid down to −47 °C (Jet A-1) and does not… more…
Physics of flight
How long do contrails last?
Contrails last anywhere from a few seconds to several hours. In dry air at cruise altitude the ice crystals sublimate almost immediately; when the surrounding… more…
Physics of flight
Why do planes need airplane mode?
Airplane mode is required mainly by regulation and caution, not by a proven history of accidents. Regulators such as the FAA and EASA require devices to stop… more…
Physics of flight
How far is the speed of sound?
The speed of sound is a speed, not a distance: in air at 20 °C and sea level it is about 343 m/s (1,235 km/h, 667 kn), so sound covers one kilometre in… more…
Physics of flight
How does a plane stay in the air?
A plane stays in the air as long as four forces stay in balance: lift from the wings opposes weight, and thrust from the engines opposes drag. In level… more…
Physics of flight
Why is the sound barrier?
There is no physical wall in the air: the sound barrier is the sharp rise in drag and loss of control that early aircraft met near Mach 1, when shock waves… more…
Physics of flight
Why do planes need air start?
Most jet engines have no electric starter: the compressor must be spun to around 20-25% of rated speed before fuel is lit and the turbine can sustain itself… more…
Physics of flight
What are supersonic planes?
Supersonic planes are aircraft that fly faster than the speed of sound, above Mach 1, about 1,060 km/h (573 kn) at cruise altitude. Most are military… more…
Physics of flight
Why do planes vanish?
Planes vanish from tracking when their transponder and ACARS stop transmitting, through failure, deliberate switch-off or loss of power, while beyond ground… more…
Physics of flight
Why do airplanes fly so high?
Airliners cruise at 9–12 km (30,000–40,000 ft) mainly to save fuel: the air up there is a third to a quarter as dense as at sea level, so drag falls and jet… more…
Physics of flight
How does the speed of sound vary with temperature?
The speed of sound in air rises with the square root of absolute temperature: about 340 m/s (661 kn) at 15 °C at sea level, falling to about 295 m/s (573 kn)… more…
Physics of flight
Why does the sound barrier boom?
The boom is not the aircraft breaking anything: it is the shock waves that any supersonic aircraft drags behind it, heard as a sudden pressure jump when the… more…
Physics of flight
How far can the b 21 fly?
The B-21 Raider's range is classified: neither the US Air Force nor Northrop Grumman has published a figure (data of September 2026). The Air Force describes… more…
Physics of flight
What are contrails?
Contrails are condensation trails: lines of ice crystals formed when the hot, humid exhaust of a jet engine mixes with air colder than −40 °C. Water vapour… more…
Physics of flight
Why do planes just sit in the sky?
Planes never sit still; a cruising airliner covers about 900 km/h (490 kn). It looks motionless because it is far away: at 11 km (36,000 ft) altitude its… more…