Concorde VMax

Well, I will try to explain it very simple. 400 kts is maximum speed which is not changing with altitude. For example when you are in 10 000 or 50 000ft it is still 400kts as it is designed for this speed. But Mach number is increasing with altitude so, for example, imagine you fly at 5000ft at 400kts, your Mach number is much lower, than it will be at 60 000ft. So that's why, very simple explanation. Hope you understood.

Remember that air density reduces to eventual near vacuum with increasing height. No WW2 era aircraft could do anything remotely near 400 knots at sea level, all their top speeds were achieved in the semi vacuum at extreme altitudes.

At 30.000 feet the mean sea level pressure of 1013 hectopascals/millibars has fallen to a deadly to human life low of 300 mb/hpa. At the Concorde cruise at twice that height the pressure is an intimidating 71 hpa/mb. At only a slightly higher altitude your blood, saliva and tears would boil in the event of a cockpit depressurisation, for Concorde to fly higher the passengers and crew would have needed to wear U2 style space suits. Concorde did Mach 2.0 in this hellish environment.

The Indicated Air Speed (IAS) is what the plane feels in the current air stream, a true air speed of 1,300 mph in an extreme high altitude near-vacuum would only register as several hundred knots on the Air Speed Indicator.

A straighforward Google search brings up this Concorde acceleration and climb profile.

"concorde acceleration and climb profile

Concorde's flight profile was

a precision-engineered sequence designed to transition from a loud, fuel-heavy takeoff to a highly efficient Mach 2.0 cruise at the edge of space.

1. Subsonic Departure & Initial Climb

• Takeoff: Concorde used all four engines with afterburners (reheat) engaged, rotating at approximately 200–214 knots.
• Initial Climb: Immediately after takeoff, it accelerated to 250 knots. At light weights, it could achieve climb rates of 5,000–6,000 feet per minute (fpm).
• Noise Abatement: About 60–80 seconds after takeoff, pilots performed a "noise cutback," reducing power and turning off afterburners to limit noise over populated areas.
• Transition to Mach 0.95: After clearing initial restrictions, it accelerated to 400 knots (Mach 0.95) and climbed toward FL280 (28,000 ft).

2. Transonic Acceleration & Supersonic Climb

• Acceleration Point: Usually over the ocean to avoid sonic booms on land, pilots re-engaged afterburners at roughly Mach 0.95 and FL280.
• Pushing through Mach 1.0: The aircraft accelerated through the high-drag transonic region (Mach 0.93 to 1.4). During this phase, the Flight Engineer began pumping fuel aft to shift the Centre of Gravity (CG) for supersonic stability.
• Supercruise Transition: Afterburners were typically switched off at Mach 1.7. From this point, the engines were efficient enough to continue accelerating to Mach 2.0 using dry power alone.

3. The "Cruise Climb" (FL450 – FL600)

Unlike subsonic airliners that fly at fixed "steps," Concorde used a continuous cruise climb.

• Dynamic Altitude: As the aircraft burned fuel and became lighter, it naturally drifted higher to stay in the most efficient air.
• Cruise Block: It typically operated in a block between FL450 and FL600 (45,000 to 60,000 ft).
• Final Altitude: By the end of a transatlantic flight, Concorde would often be cruising at nearly 60,000 feet.

4. Descent Profile

• Subsonic Transition: Descent began approximately 200–250 miles from the destination. The aircraft decelerated through Mach 1.0, and the Flight Engineer moved fuel forward to shift the CG back to subsonic limits.
• Rapid Descent: At subsonic speeds, it descended rapidly at 350 knots with rates often exceeding 3,000 fpm.
• Approach: Final approach was flown at a high nose-up attitude (approx. 10–12 degrees) due to the delta wing design, with landing speeds between 150 and 162 knots."