Flight Time by Cruise Speed

Cruise speed: t = d / v

Cruise speed is the velocity an aircraft settles into for the level part of a flight, the sweet spot between fuel burn, range and time. Modern commercial jets usually hold Mach 0.78 to 0.85, which comes out to roughly 830 to 900 km/h True Airspeed at FL350 (35,000 ft / 10,700 m). Up there TAS always outruns Indicated Airspeed (IAS), since the thinner air means the pitot tube reads less than the real speed. The retired Concorde cruised at Mach 2.02 (~2,180 km/h); a Cessna 172 plods along near 200 km/h. Eastbound, jet streams hand you 100-250 km/h of tailwind, which is why JFK→LHR takes about 6 h while LHR→JFK runs 7.5 h. Example: 3000 km at 850 km/h → 3.53 h block time.

Applications

Flight planning and ETA estimates, picking a route (great-circle versus wind-optimal tracks), squeezing fuel with the Mach Number Indicator and cost index, dispatch calls, ATC time-on-arrival negotiations, building airline schedules, and weighing turboprop against jet equipment on performance.

FAQ

Why fly at Mach number instead of km/h? Drag climbs fast as you near the speed of sound. Holding a fixed Mach keeps an airliner in the efficient band no matter the temperature, and temperature is exactly what shifts the local speed of sound around.

What is the difference between TAS and IAS? Indicated Airspeed comes off the pitot-static system and rides on air density. True Airspeed is your actual speed through the air mass. At FL350 TAS can run 1.6× IAS.

Do tailwinds really save that much time? They do. Ride a 200 km/h jet stream across a 6,000 km leg and you shave roughly 1.3 hours off the clock, plus a good chunk of fuel.