Steady Level Flight and Maximum Endurance - ATPL 032 — Performance

An aeroplane in steady level flight is an equilibrium with wings: lift equals weight, and thrust equals drag. This lesson works that balance against the curves from the previous lesson to answer three questions the syllabus asks again and again: between which speeds can the aeroplane fly level, what happens when the balance is disturbed, and at what speed does staying airborne cost the least fuel?

The equilibrium of level flight

In straight and level flight at a steady speed, the thrust exactly matches the drag, which is why the drag curve is also called the thrust-required curve. Both drag and power required follow their bucket shapes against speed: high at the slow end (induced drag dominating), high again at the fast end (parasite drag dominating), cheapest in the middle. Now disturb the balance. If the thrust set exceeds the drag at the current speed, the surplus is excess thrust, and with the pilot holding altitude it accelerates the aeroplane; the speed rises, the drag rises with it, and the acceleration ends when drag has grown to meet the thrust again. Set less thrust than drag and the same logic runs backwards, and the power picture tells the same story: power available above power required accelerates the aeroplane at constant altitude until the required power catches up. Level flight at a steady speed is always a meeting point of the two curves.

Reading the curves

Draw the thrust-available line across the thrust-required bucket and the geometry answers the speed-envelope question directly. The curves cross twice: the left intersection is the minimum speed for level flight that thrust can sustain, and the right intersection is the maximum speed for level flight. Between them the available line sits above the bucket, and the gap is excess thrust available for accelerating or climbing; outside them level flight cannot be held. In practice the stall usually intervenes before the thrust-limited minimum is reached, so the left intersection is more theory than technique; the right one is very real.

Altitude works both ends of that envelope. The available line sinks as the air thins, so the maximum level-flight IAS falls steadily with altitude. Expressed in TAS the story has a twist: because the drag bucket slides toward higher true airspeeds with altitude while the available thrust falls only gradually, the maximum TAS often rises at first, peaks at some optimum altitude, and then falls as the thrust decay takes over. At the absolute ceiling the envelope has closed to a single speed and there is nothing left over.

Speed stability

The two sides of the drag bucket behave very differently when a gust or a moment of inattention nudges the speed. Above the minimum-drag speed, on the front side of the curve, a speed increase raises the drag above the set thrust and the aeroplane decelerates back; a speed loss lowers the drag and it accelerates back. The aeroplane polices its own speed. Below Vmd, on the back side, the logic inverts: lose a little speed and the drag rises, which slows you further, which raises the drag again. The disturbance feeds itself, and only the pilot (or autothrottle) breaks the loop.

Endurance or range?

Sometimes the mission is not to go anywhere. A hold over a fix while a runway is cleared, a delay absorbed while weather passes, time bought while a failure is worked through: these want maximum endurance, the most minutes airborne per kilogram of fuel. A diversion on thin reserves, or any leg where the fuel must cover distance, wants maximum range instead: the most nautical miles per kilogram. The two call for different speeds, and the exam expects you to know which situation calls for which.

Holding speeds: Vmd and Vmp

Maximum endurance means minimum fuel flow, and what sets the fuel flow differs by powerplant:

Turbojet — fuel flow follows thrust, and in level flight thrust is drag, so the fuel flow is least where the drag is least: maximum endurance is flown at Vmd, the minimum-drag speed.
Propeller — fuel flow follows power, so the fuel flow is least where the power required is least: maximum endurance is flown at Vmp, the minimum-power speed, which sits noticeably below Vmd.

Vmp comes with small print, and the syllabus quotes it: the speed sits below Vmd, on the back side of the drag curve, so it is speed-unstable and demands constant attention; the angle of attack is high, the margin to the stall reduced, and the controls feel sluggish. Real holds are therefore usually flown somewhat faster than the theoretical minimum, trading a little fuel for stability and margin.

Wind, altitude and the hold

Endurance is time, not distance, and that single fact settles the wind question: wind has no effect on endurance. The fuel buys minutes aloft regardless of what the air mass does over the ground. Altitude does matter, and differently per powerplant. A jet's SFC improves in the colder air aloft, so its endurance improves slightly with altitude, best near the tropopause; its endurance IAS, Vmd, is essentially the same IAS at any level. A propeller aeroplane reads the other way: at a given IAS the TAS grows with altitude, the power required grows with it, and the fuel flow follows, so prop endurance is best at low altitude. Its endurance IAS, Vmp, also barely moves with level.

Absorbing delay before it becomes a hold

When the delay at destination is known early, the cheapest place to absorb it is en route, not in the stack. Slowing toward the endurance-side speeds during the last portion of cruise converts expensive holding minutes into cheap cruising minutes: the aeroplane arrives later, burns less doing so, and may find its slot open and the hold gone entirely. The technique can be elected by the pilot or commanded by ATC when arrival delays build, and modern flow management uses it routinely for exactly the reason this lesson prices out: a minute absorbed at cruise costs less fuel than the same minute flown in circles at low altitude.

Explain how drag and power required vary with speed, and the effect of excess thrust and power on speed in level flight

Interpret the thrust/power required and available curves in straight and level flight

Describe how the maximum level-flight IAS and TAS vary with altitude

Describe situations calling for maximum endurance or maximum range

Explain fuel flow against TAS and thrust for jet and propeller aeroplanes, and state the maximum-endurance speeds Vmd and Vmp with the disadvantages of holding at Vmp

Explain the effect of wind and altitude on endurance and on the maximum-endurance speed, for both powerplants

Describe managing en-route airspeed to reduce or avoid holding, and when it is used