Summit height

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The height of the summit, or more precisely: the maximum height of the summit indicates the maximum height that aircraft can reach. It depends on the maximum total weight and the flight performance . In contrast to the service peak, the rate of climb at the peak is theoretically zero.

Physical background

With increasing altitude, for example, with the turbojet engines commonly used in commercial aircraft , less atmospheric oxygen enters the engines , since the air pressure and thus the density of the atmosphere decrease with increasing altitude . The lower air density also reduces the lift of the wings and thus limits the flight altitude .

Limit height

Furthermore, there is a physically determined limit altitude for aircraft that are not designed for supersonic flight . This is given by the relative course of the parameters Mach number (true flight speed) and minimum speed depending on the flight altitude.

The maximum speed is basically limited by the approximation to the maximum Mach number of the aircraft, with the air flow already reaching the speed of sound at some points before the aircraft itself reaches this. At the same time, the air density, which decreases with altitude, limits the minimum speed of the aircraft: slower flying would result in a stall and the aircraft would no longer be able to fly.

Mach number and minimum speed approach each other with increasing flight altitude, since the Mach number decreases with decreasing temperature (i.e. with increasing altitude) and the true minimum speed increases with increasing altitude due to the decreasing air density. The limit height is reached when both sizes meet; see Coffin Corner .

Even if an aircraft could continue to climb due to its engine power, the maximum theoretical altitude would be reached here, which corresponds to a certain airspeed. For a commercial aircraft, this limit altitude is usually just above the cruising altitude, or the wing and engine power are designed so that the desired cruising altitude is just reached.

special cases

Increase the true minimum speed

In this case, the speedometer of the aircraft shows the same minimum speed as near the ground. Due to the thinner air, it shows a value (IAS) that is significantly lower than the true speed (TAS) of the aircraft. The increase in error is approximately 2% per 1,000 feet .

  • Example 1:
    An aircraft is at flight level 300 (30,000 feet) and is flying at an indicated speed of 200 knots , its true airspeed is approximately 320 knots.
  • Example 2:
    Requires approx. 240 knots as a minimum speed near the ground in order to fly "clean", i.e. with retracted buoyancy aids. The airspeed indicator shows about 240 knots there (if IAS = CAS). To fly safely, the airspeed indicator must also read 240 knots at 30,000 feet, but in this case the aircraft is actually 384 knots (an increase of 2% per 1000 feet, an increase of 60% at 30,000 feet).

Usable speeds

An aircraft has a maximum Mach number of approx. 0.8. This corresponds to a true speed of approx. 528 knots near the ground (at 15 degrees Celsius). At an altitude of 30,000 feet (about −45 degrees Celsius), this corresponds to a speed of 470 knots.

This results in the usable speed range:

  • near the ground: 240-528 knots: 288 knots usable area
  • at 30,000 feet: 384-470 knots: 86 knots of usable area

See also: service ceiling