Definition
Lines drawn on an airplane performance chart that connect points of equal specific excess power (PS). Specific excess power is the rate at which an airplane can gain energy — climbing, accelerating, or some combination of both — at a given airspeed, altitude, weight, and power setting. A PS contour traces every combination of airspeed and altitude where the airplane has the same energy-gain capability. A PS value of zero marks the boundary where the airplane can only maintain its current energy state; positive PS contours show where it can add energy, and negative PS contours show where energy is being lost.
Plain English
Curved lines on a performance chart that show how much spare energy the airplane has at each speed and altitude. Each line connects all the spots where the airplane has the same ability to climb, accelerate, or both. The zero line marks the edge of what the airplane can do — beyond it, the airplane cannot keep up its speed and altitude at the same time.
Context Anchor
Seen in discussions of energy management, especially when visualizing how an airplane can move between different speed and altitude conditions.
Derivation
PS stands for specific excess power. 'Specific' means 'per unit of weight,' so the value is normalized for the airplane's weight. 'Contour' comes from the Italian contorno, meaning outline — the same idea as contour lines on a topographic map, where each line connects points of equal elevation. Here, each line connects points of equal energy-gain capability instead.
Why Pilots Care
They allow a pilot to see at a glance whether the airplane can trade speed for altitude, climb while maintaining speed, or must lose energy in a given flight condition.
Analogy
Think of a topographic map where contour lines show elevation. PS contours work the same way, but instead of showing how high the ground is, each line shows how much extra energy the airplane has available at that speed and altitude.
Grounding Statement
If the airplane is fast enough and has power left over, it can move to a higher-energy state; if it does not, it may only be able to hold its state or lose energy.
Intuition Check
Do not read “contours” here as just the outside shape of the airplane or a drawing outline. In this context, contours are chart lines connecting equal PS values.
Example Sentence 1
Studying the PS contours for the airplane, the pilot saw that climb and acceleration capability dropped sharply above 8,000 feet at gross weight.
Example Sentence 2
Crossing a higher PS contour during level acceleration showed the aircraft still had surplus power available.