Vx vs Vy: best angle, best rate, and altitude
Vx is the airspeed for the greatest altitude gain per horizontal distance. Vy is the airspeed for the greatest altitude gain per unit of time. They answer different questions: Vx is about clearing distance-limited obstacles; Vy is about gaining altitude as quickly as possible.
Neither speed is a universal number. The applicable aircraft flight manual or pilot's operating handbook (AFM/POH) gives the speeds and procedures for a particular model and configuration. Weight, power available, temperature, pressure altitude, and configuration affect climb performance.
What the pilot is optimizing
Imagine plotting the airplane's climb performance at a given weight and atmospheric condition. Vx is the point that yields the steepest climb path, measured as altitude gained for horizontal distance traveled. Vy is the point that yields the largest vertical speed, measured in feet per minute. A steep path can gain altitude over less ground distance while taking longer to reach a particular altitude; a higher rate can reach that altitude sooner while covering more ground.
For a practical-test answer, say what the units mean: Vx is commonly taught as best angle of climb and Vy as best rate of climb. Angle refers to distance; rate refers to time. Avoid describing Vx as simply “slower” or Vy as always “faster”—the actual speeds depend on the airplane and conditions.
Why the speeds move with altitude
The FAA Airplane Flying Handbook explains the usual trend for propeller airplanes: as altitude increases, Vx increases and Vy decreases. In other words, their recommended speeds move toward one another as climb performance falls. At the absolute ceiling, they converge; the airplane has no excess performance available for a sustained climb.
The trend is useful for understanding the concept, but it is not a substitute for a table or chart. Do not estimate an airplane's climb speed by adding or subtracting a fixed number per thousand feet. Use the AFM/POH values and conditions for the airplane being flown.
Choosing a speed for obstacle clearance
Suppose a training question asks for an obstacle-clearance climb. First locate the correct short-field or climb-performance procedure in the airplane's approved manual. Confirm the chart's weight, pressure altitude, temperature, configuration, and any other stated assumptions. Then identify the published Vx for that condition and explain that its purpose is to maximize height gained over the distance available.
For a climb after the obstacle is clear, the applicable procedure may call for transitioning to Vy or a normal climb speed. The exact point, configuration changes, and speeds come from that airplane's procedure and instructor guidance. If the question instead asks to reach a specified altitude in the least time, Vy is the concept to explain. This process gives a defensible answer without inventing a generic speed for every airplane.
Vx and Vy are not climb gradient and climb rate
Vx and Vy describe the airspeed selected for a performance objective. Climb gradient describes altitude gained over horizontal distance, often expressed in feet per nautical mile or percent. Climb rate describes vertical speed, commonly feet per minute. Wind changes groundspeed and therefore affects the ground-referenced gradient; it does not turn a rule-of-thumb speed into approved performance data.
For the math relationships, review climb rate versus climb gradient and use the climb-gradient calculator or FPM-to-gradient calculator for arithmetic checks.
Common checkride misunderstandings
- “Vx is always the obstacle speed.” It is the best-angle concept; the aircraft manual supplies the procedure and speed for the specific operation.
- “Vy gives the steepest climb.” Vy maximizes altitude gained per unit time, not per unit distance.
- “The speeds stay fixed at all altitudes.” The usual trend is Vx upward and Vy downward with altitude, but use the published data rather than extrapolating.
- “Wind is part of the airspeed number.” Airspeed and groundspeed are distinct. Wind affects the ground path and obstacle picture.
Primary FAA references
The FAA Airplane Flying Handbook discusses the Vx/Vy definitions and altitude trend in Chapter 3, and their use in takeoff and climb procedures in Chapter 6. Check the current handbook edition and the AFM/POH for the aircraft you are studying.