In short: As altitude increases, both Vx and Vy increase as true airspeeds, but Vx rises a little faster, so the two converge. Converted to indicated airspeed, the lines still close in on each other and meet at the absolute ceiling, where the airplane can no longer climb. Rod Machado explains it with a set of rate-of-climb curves.
Video: “Why Vx & Vy Change With Altitude” by Rod Machado, from his YouTube channel.
What you’ll learn
- How rate-of-climb curves at sea level, 5,000 and 10,000 feet compare
- Why maximum rate of climb decreases as altitude increases
- How Vy is found at the top of each curve and Vx at a tangent line from the origin
- Why both speeds increase as true airspeeds, with Vx increasing faster
- How converting to indicated airspeed produces the familiar converging chart
- Why Vx and Vy meet at the airplane’s absolute ceiling
Reading the rate-of-climb curves
Rod starts with three curves showing rate of climb versus true airspeed for a typical small airplane at sea level, 5,000 feet and 10,000 feet. The peak of each curve is the maximum rate of climb at that altitude. Two things stand out: the peaks get lower as altitude increases, and they shift slightly to the right. That means Vy, the best rate-of-climb speed, increases slightly as a true airspeed as you climb.
He uses true airspeed on the horizontal axis because it represents the airplane’s performance at different altitudes more accurately than indicated airspeed.
Finding Vx with a tangent line
To find Vx, Rod draws a line from the graph’s origin that just touches each curve. The touch point marks the best angle-of-climb speed, because that tangent line has the steepest slope, the most altitude gained for a given horizontal distance. Those tangent points also move to the right with altitude, and they move a bit faster than the Vy points. So as true airspeeds, Vx and Vy converge as you climb.
Switching to indicated airspeed
Pilots fly indicated airspeed, so Rod converts the true airspeed values to indicated airspeeds using an E6B. Because true airspeed grows relative to indicated as you climb, the indicated airspeed needed for Vy decreases with altitude, which is why the Vy line on the familiar performance chart tilts to the left and closes in on the Vx line.
Where the two lines meet is the absolute ceiling, the altitude at which the airplane’s rate of climb is zero. It’s a neat way to see why the speeds you fly in a climb aren’t fixed numbers.
Go deeper with Rod Machado
Rod’s Private/Commercial Pilot Handbook explains climb performance, airspeeds and the charts behind them in the same approachable way. Learn more in Rod Machado’s Private/Commercial Pilot Handbook.
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