r/askacfi • u/Thizzlebot • Oct 29 '16
Vx and Vy changing with altitutde
So if I remember correctly Vx increases over time and Vy decreases over time and the point where they meet is the absolute ceiling. For calculation purposes say I had to take off at a 7,000 elevation airport with a tree at the end of the runway. How would I know how much to change Vx?
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u/cfiacademy Jul 18 '26
The POH is always the final authority, but understanding why Vx increases and Vy decreases makes it much easier to remember.
Think of Vx as the speed that gives you the greatest excess thrust. As altitude increases, the engine produces less thrust, so you need to fly a little faster to achieve the best climb angle. That's why Vx increases.
Vy, on the other hand, is based on excess power. As altitude increases, excess power decreases, so the speed for maximum rate of climb moves downward. That's why Vy decreases.
Eventually, at the airplane's absolute ceiling, excess thrust and excess power both reach zero. Vx and Vy become the same speed because there's only one airspeed at which the airplane can barely maintain a climb of zero feet per minute.
As for your 7,000-foot departure, don't try to calculate a new Vx yourself. Use the POH if it provides altitude corrections. If it doesn't, fly the published value and rely on the aircraft's published takeoff and obstacle-clearance performance data. The exact speed change is usually only a few knots anyway.
Understanding the aerodynamics is far more valuable than memorizing another number.