Descent Performance

Descent performance is about how an airplane comes down: how steep its path is, how quickly it loses height, and what the pilot can do to change either. Most jet descents are flown with the engines at idle, so the airplane is spending the energy stored in its height and speed, and the questions are the ones a glider pilot asks: how far, how fast, and at what airspeed.

The subject mirrors climb performance. A climb is paid for by excess thrust; a descent is governed by excess drag. The same drag curve and the same equations, with the signs reversed, give the gradient and the rate. They sit behind everyday decisions on the line: where to start down, when to use the speedbrakes, and how fast to fly an emergency descent.

Forces in a steady descent

In a steady descent the airplane flies a straight path inclined at the flight path angle γ below the horizontal, at constant speed. Lift acts perpendicular to the path, drag and thrust along it, and weight vertically down. Weight resolves into W cos γ perpendicular to the path and W sin γ along it, pointing forward.

Resolving along the path gives the basic equation of the descent:

D = T + W sin γ

Drag is balanced by thrust plus the forward component of weight. Perpendicular to the path, L = W cos γ, so in any steady descent lift is slightly less than weight, exactly as in a climb.

This is the drag-thrust balance in descent. In level flight thrust equals drag. Reduce the thrust at the same speed and drag exceeds it; only the weight component can make up the deficit, D − T, so the airplane descends at the angle where W sin γ equals it. The FAA's Pilot's Handbook of Aeronautical Knowledge puts it practically: to descend at the level-flight airspeed, power must be reduced as the descent is entered, because the forward component of weight grows as the descent steepens.

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Descent gradient and rate of descent

Rearranging the force balance gives the descent gradient:

sin γ = (D − T) ÷ W

Anything that raises drag or lowers thrust steepens the descent; adding thrust at the same speed makes it shallower. With the thrust at idle the excess drag is close to the whole drag, and since lift is close to weight the gradient is close to drag ÷ lift: the reciprocal of the lift/drag ratio, as in a glide.

The rate of descent (ROD) is the vertical speed:

ROD = TAS × sin γ

At a given gradient, the rate of descent is proportional to the true airspeed. Over the ground, the rate of descent in feet per minute equals the gradient in feet per nautical mile multiplied by the groundspeed in knots, divided by 60. Seen through the power curves, it is the power deficit divided by the weight.

A 3° path, the standard instrument approach angle, is a gradient of about 5.2 percent, or 318 ft per nautical mile. At 140 kt groundspeed that needs 318 × 140 ÷ 60, about 740 ft/min; the shortcut, five times the groundspeed, gives 700.

Effect of mass, wind and altitude

Mass acts in two different ways, and exam questions test both.

• In a glide at the best lift/drag speed, mass does not change the angle, which depends on the maximum lift/drag ratio, a property of the configuration and the angle of attack. The speed that gives it rises with the square root of the mass, so a heavier airplane follows the same path faster, with a higher rate of descent, and reaches the ground sooner. A 10 percent lighter airplane should glide about 5 percent slower.
• In an idle descent at a fixed speed schedule, such as a jet's planned Mach number and IAS, mass changes the angle. The schedule is faster than the minimum drag speed. A heavier airplane has a higher minimum drag speed, so the scheduled speed lies closer to it, the lift/drag ratio is better and drag is a smaller fraction of the weight. The path is shallower, the descent needs more distance, and the top of descent moves further from the destination.

A steady wind changes neither the rate of descent nor the gradient through the air, only the gradient over the ground. A headwind lowers the groundspeed, so the same height is lost over less ground; a tailwind stretches the path. For the greatest ground distance in a glide, fly slightly faster than the minimum drag speed into a headwind and slightly slower with a tailwind.

Altitude works through the true airspeed. At a given IAS the TAS is roughly IAS ÷ √σ, where σ is the relative density; at FL400, where σ is about 0.25, the TAS is about twice the IAS. At the same IAS and angle the airplane therefore descends faster high up: the gradient and still-air distance are unchanged, but the rate of descent is higher.

A jet's schedule adds a twist. Above the crossover altitude it descends at constant Mach number. Below the tropopause the air warms as the airplane descends and the speed of sound rises, so the TAS rises, and with it the rate of descent at a given gradient, while the IAS increases. Below the crossover altitude the airplane holds a constant IAS; the rising density lowers the TAS, and the Mach number falls faster still, so the airplane moves away from its Mach limits (see airspeed definitions).

Maximum angle and minimum rate of descent

The minimum drag speed (VMD) is the bottom of the total drag curve, where induced and parasite drag are equal and the lift/drag ratio is greatest; as an IAS it hardly changes with altitude. On the A320 it is shown as green dot, which below 20,000 ft is about 2 × the weight in metric tons + 85 kt: about 205 kt at 60 t. Around VMD the drag curve gives four useful descents.

The shallowest descent is flown at VMD, where drag, and so the excess of drag over thrust, is least. The minimum rate of descent comes at the slower minimum power speed, VMP, about 0.76 VMD on the idealized drag curve, because power required is least there. On that curve, gliding at VMP gives about 12 percent less sink than at VMD but about 13 percent less distance: VMP buys time, VMD distance.

The maximum angle of descent needs the largest excess drag. Above VMD parasite drag grows with the square of the speed, so at idle the path steepens as the speed rises, and speedbrakes, gear and flap steepen it further. Flying slower than VMD also steepens the path, but it erodes the stall margin and puts the airplane on the back of the drag curve, where speed is unstable, so steep descents are flown fast. The highest rate of descent adds a high true airspeed to the steep gradient.

Idle descents and speedbrakes

A jet normally descends from the top of descent at idle thrust, the most fuel-efficient descent and the design case for FMS planning: the flight management system computes the idle descent path backward from the end of the descent (see climb and descent planning). Idle is not zero thrust. On the A320 the FADEC raises idle automatically when bleed demand is high, for example with wing anti-ice on, and more thrust means a shallower idle path.

Speedbrakes, flight spoilers raised symmetrically, add drag and dump some lift. At a given speed and thrust they steepen the path and raise the rate of descent; on a given path they let the airplane slow down. For the intermediate approach, the A320 training manual gives a mean deceleration of about 10 kt per NM in level flight, extending the next configuration when the speed reaches the current configuration's maneuvering speed plus 10 kt, and about 20 kt per NM with the speedbrakes. Their limits vary by type (see spoilers and speed brakes):

• On the A320, extension raises VLS, so a speed margin is needed first; among other conditions it is inhibited in CONF FULL, with the thrust levers above MCT and at alpha floor, and the crew retract the speedbrakes before selecting flaps full.
• On the Boeing 737 NG the speed brake lever must not go beyond the FLIGHT DETENT in flight, and the speedbrakes must not be deployed below 1,000 ft radio altitude.

The landing gear is also a drag device, within VLO and VLE. In the landing configuration drag is so high that thrust must be added to hold a 3° path.

Piston airplanes reduce power gradually to avoid shock cooling, a sharp fall in cylinder head temperature that stresses cylinders and valves; FAA AC 61-107B notes that light twins need some power in the descent, whereas most jets can descend rapidly at idle. Light airplanes usually descend at a chosen rate near cruise speed: the CAP 697 specimen airplane SEP1 descends at 145 KIAS and 1,000 ft/min, gear and flaps up.

Emergency descent profiles

An emergency descent is a descent at the highest practicable rate, made when the airplane must reach a lower altitude at once: after a decompression, to reach breathable air before hypoxia incapacitates anyone, or with fire or smoke.

In a transport jet the crew put on their oxygen masks first. On the A320 the memory items continue with signs on and emergency descent initiate; the procedure then calls for thrust levers idle if the autothrust is not active, speedbrakes full and, once the descent is established, speed maximum or appropriate; many procedures use a speed at or near MMO/VMO. If structural damage is suspected the airplane is maneuvered with care, and the A320 procedure calls for considering gear extension, which adds drag at the lower speed. The level-off is at 10,000 ft or the minimum safe altitude, whichever is higher; the A320 states it as FL100 or MEA/MORA.

Speed matters because oxygen supplies are sized around a quick descent. Under 14 CFR 121.333, the minimum two-hour supply for each flight crew member on flight deck duty covers a constant-rate descent from the maximum certificated altitude to 10,000 ft in 10 minutes, then 110 minutes at 10,000 ft: for an airplane certificated to 41,000 ft, an average of 3,100 ft/min. Passenger chemical oxygen generators typically run for only about 12 to 22 minutes.

In a light airplane the FAA's emergency descent uses idle power, the gear down if VLE allows, flaps as the POH specifies, a speed up to VNE in smooth air or VA in turbulence, and a bank of 30° to 45°, which clears the airspace below and increases the rate of descent.

An engine failure in the cruise of a multi-engine jet calls for the opposite: a slow drift-down at the one-engine-inoperative minimum drag speed, the shallowest path available, to the highest altitude the remaining engines can hold (see one engine inoperative en-route performance).

Descent planning rules of thumb

Pilots check the FMS with a few numbers:

• Distance to descend, jet: about 3 NM for every 1,000 ft to lose. From FL350 to sea level, 35 × 3 = 105 NM, plus distance to slow down.
• Distance to descend, light airplane: distance per 1,000 ft = groundspeed × (1,000 ÷ ROD) ÷ 60. At 500 ft/min and 120 kt, each 1,000 ft takes 2 minutes and 4 NM.
• Wind and mass: a tailwind moves the top of descent further out and a headwind brings it in; in an idle descent a heavier airplane starts down earlier.

A worked case: a jet must lose 12,000 ft within 30 NM. That is 400 ft per NM, about 6.6 percent or 3.8°. The 3 NM rule implies an idle path of about 333 ft per NM, so the airplane is high and needs speedbrakes or more track miles. At a groundspeed of 300 kt, 400 ft per NM means 2,000 ft/min.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.