Descent and Glide Performance - ATPL 032 — Performance

Descending is the climb chapter run in reverse, with one satisfying difference: the weight changes sides. On the way up, W·sin γ was a bill the thrust had to pay; on the way down it becomes income, driving the aeroplane along the path, and in the pure glide it is the only engine the aeroplane has. This lesson resolves the descent forces, reads the descent from the same curves as ever, and works through what moves the glide, the most-examined descent there is.

The forces coming down

Tilt the path down at angle γ and resolve again. Across the path nothing new happens: L = W·cos γ, so the lift is once more slightly less than the weight. Along the path the ledger flips: thrust and the weight component now pull together against the drag, T + W·sin γ = D. A steady descent therefore exists exactly when the drag exceeds the thrust, and the syllabus names the deficit both ways: the excess thrust required (or excess drag), D − T, is what the weight component pays along the path, and the excess power required, Pr − Pa, is paid in height per minute:

The glide

Set the thrust to zero and the descent becomes a glide: W·sin γ = D, and dividing the two force balances gives the cleanest result in the subject, tan γ = D/L. The glide angle depends on nothing but the lift-to-drag ratio: shallowest where L/D is greatest, which is the minimum-drag speed Vmd. The same ratio measures the reward on the ground: the still-air glide range is the height multiplied by L/D. An airliner with an L/D of 17 turns each mile of height into 17 miles of distance, which is why engines-out arithmetic is less desperate than instinct suggests.

Reading a descent from the curves

On the familiar picture, a descent is what happens below the lines. Pull the thrust back and the available line drops beneath the required curve: at each speed the shortfall in the thrust picture, divided by the weight, is the descent gradient, and the shortfall in the power picture is the rate of descent. Idle thrust makes the deficit nearly the whole drag bill, which is the steep end; adding thrust shrinks the deficit and shallows the path, until at T = D the descent has flattened into level flight.

What moves the glide

Mass — the exam's favourite trap: the glide angle and still-air range are unchanged, because L/D does not depend on weight. The heavier aeroplane flies its best glide at a higher IAS, sinks faster and arrives sooner, but covers the same distance.
Altitude — no effect on the angle or on the range per unit of height: the best-glide IAS is the same at any level, flown at a higher TAS up high.
Wind — the air-mass glide is untouched, but the path over the ground tilts: a headwind steepens it and cuts the range, a tailwind stretches it. The refinement: fly a little faster than best-glide speed into a headwind, a little slower with a tailwind.
Speed — Vmd is the shallowest glide; anything faster or slower sits at a worse L/D and comes down more steeply.
Configuration — flaps, gear and speedbrakes all cut L/D: the glide steepens and the range shrinks, which is exactly what they are extended for on final.

The powered descent

With the engines above idle, thrust becomes the descent's throttle in the literal sense: more thrust shrinks the D − T deficit, shallowing the path and cutting the rate of descent; less steepens it. Around that lever, the glide's cast performs unchanged: more mass means a higher speed along a similar path, altitude sets TAS but not the picture, wind bends the ground track, speed away from the optimum steepens things, and drag devices steepen them deliberately. A modern arrival is the managed version of this lesson: idle where possible, thrust where the path or ATC demands it.

Resolve the forces in a steady descent and in the glide, and explain excess thrust required and excess power required

Explain descent performance from the available and required curves, and interpret them in a steady descent

Explain the effect of mass, altitude, wind, speed and configuration on the glide descent

Explain the effect of mass, altitude, wind, speed and configuration on the powered descent

The forces coming down

Check you understood

The glide

Reading a descent from the curves

What moves the glide

The powered descent