Descent · make the energy visible

Why does TAS change during a constant-Mach descent?

A normal high-altitude transport descent begins on the approved Mach schedule, transitions at crossover to IAS, then reduces speed for restrictions and configuration. At constant Mach in the isothermal lower stratosphere, TAS is approximately constant. In a standard tropospheric descent into warmer air, TAS increases as speed of sound rises.

Reverse the climb schedule

A normal high-altitude transport descent begins on the approved Mach schedule, transitions at crossover to IAS, then reduces speed for restrictions and configuration. At constant Mach in the isothermal lower stratosphere, TAS is approximately constant. In a standard tropospheric descent into warmer air, TAS increases as speed of sound rises.

IAS also increases during the constant-Mach descent. At crossover the IAS schedule takes over; continued descent at constant IAS generally decreases TAS as density increases. VMO and MMO protect different parts of the envelope, so staying below one does not prove compliance with the other. Use the published restrictions, turbulence and configuration speeds as well.

Protect buffet margins

Descending usually expands the available speed envelope relative to the high-altitude cruise case, but a chosen constant Mach still needs monitoring against low- and high-speed buffet and the operating limits. Temperature deviations, mass, bank and thrust setting influence the actual boundaries. A transient acceleration or upset can erase an assumed margin. The appropriate schedule is an approved aircraft output, not the intersection of two approximate lines drawn from memory.

Why can you be high and fast with idle thrust?

Fuel carries chemical energy; height carries potential energy mgh; speed carries kinetic energy ½mV². In descent, potential energy can become speed or be dissipated by drag. Idle thrust still supplies some power. A continuous descent reduces level-off and thrust changes and can save fuel/noise when the route and energy state permit.

Track three energy stores

Fuel carries chemical energy; height carries potential energy mgh; speed carries kinetic energy ½mV². In descent, potential energy can become speed or be dissipated by drag. Idle thrust still supplies some power. A continuous descent reduces level-off and thrust changes and can save fuel/noise when the route and energy state permit.

At a fixed aerodynamic descent condition, a tailwind increases ground distance covered during the same height loss; a headwind shortens it. If a planned profile assumed more headwind than actually occurs, the aircraft can arrive high because it has less time per ground mile to descend. A changing wind field can also produce transients, so the steady model is only a planning picture.

Plan both slowing and descending

A constant-IAS descent tends to release kinetic energy as TAS decreases, altering the height/distance balance; the Mach-to-IAS transition changes that energy allocation. Late descent clearance, track shortening, higher-than-expected tailwind, restrictive speed requirements and delayed configuration can leave excess height or speed.

Earlier descent or more track creates time/distance; approved speed changes, speedbrakes and configuration change the drag/energy budget. Reducing speed can initially flatten the descent while energy is exchanged, and inappropriate flap/gear deployment can exceed limits. Choose a permitted strategy early, monitor the result, and use the operator’s stabilised-approach/go-around criteria if the state cannot be recovered.

Can you make the performance decision?

Four original questions. Work from the stated conditions and review the reasoning.