Vx and Vy, the Two Climb Speeds That Converge at Your Ceiling, and the Departure Calculation That Changes Every Time You Leave Sea Level

Vx and Vy are the two climb speeds published in every Pilot’s Operating Handbook. Knowing their definitions is not the same as understanding when to use each one - or how altitude changes both numbers every time you fly above sea level.

What Is the Difference Between Vx and Vy?

Vx is best angle of climb speed. At Vx, the airplane gains the most altitude for the least horizontal distance covered. Geometrically, it produces the steepest line between your liftoff point and any obstacle at the departure end. That makes it the correct speed when terrain or obstacles are directly in the departure path.

Vy is best rate of climb speed. At Vy, the airplane gains the most altitude per unit of time - the highest reading on your vertical speed indicator. The airplane covers more ground per minute than at Vx, but it reaches cruise altitude faster, maintains better airflow through the engine cowling, and burns less fuel during the climb.

On most Cessna 172 variants, Vx falls in the low sixties in knots and Vy falls in the low to mid seventies. On a Piper Cherokee 160, both speeds sit slightly higher and closer together. The exact figures vary by model year and configuration, so always use the numbers in your specific airplane’s POH - not a generic chart on the flight school wall.

Why Are Vx and Vy Different Airspeeds?

A climbing airplane has more performance available than it needs to maintain level flight. That surplus appears in two distinct forms: excess thrust (the engine-propeller combination produces more forward force than drag requires) and excess power (horsepower delivered exceeds what level flight demands).

In a naturally aspirated piston trainer, maximum excess thrust peaks at a lower airspeed than maximum excess power. Vx corresponds to maximum excess thrust. Vy corresponds to maximum excess power. That is why Vx is always the slower of the two - they represent different performance peaks, not two arbitrary points on the same curve.

When Should I Use Vx vs. Vy on Departure?

The decision comes down to what the departure picture requires.

If there is an obstacle - a tree line, a ridge, a structure - at the departure end, the problem is geometric. The question is where the airplane is in space as it crosses that point on the ground. Use Vx.

If there is no obstacle and the runway is long at a low-elevation airport, the problem is efficiency. How quickly does the airplane reach altitude, and how much fuel does the climb cost? Use Vy.

Brief this decision before you line up. Decide which airspeed you are targeting on the initial climb before the throttle goes forward. That question should be answered on the ground, not at sixty knots.

How Do Vx and Vy Change With Altitude?

As altitude increases, air density decreases. A naturally aspirated engine draws in thinner air and produces less power. The propeller generates less thrust. As total available performance shrinks, both surplus peaks shift - but in opposite directions.

Vx increases with altitude. The speed required to achieve best angle of climb creeps upward as you climb higher.

Vy decreases with altitude. The speed required to achieve best rate of climb creeps downward.

They are converging. At sea level, Vx and Vy on a light trainer are often 15 to 20 knots apart. At 8,000 feet density altitude, the gap narrows to roughly 10 knots. At 12,000 feet density altitude, perhaps 5 knots. Above that, the gap closes to zero.

What Is an Airplane’s Absolute Ceiling?

The absolute ceiling is the altitude at which Vx and Vy converge to the same airspeed. At that point, the engine is producing everything it has just to overcome drag. There is no surplus thrust and no surplus power - nothing remaining for climbing.

On the Cessna 172, the published absolute ceiling is approximately 14,900 feet. That altitude is not a destination in primary training. But the convergence happening below it - at five, eight, or ten thousand feet density altitude - has real and immediate consequences.

At 5,000 feet density altitude, Vx and Vy are already closer together than their sea-level values. Your climb angle at Vx is shallower; your rate at Vy is fewer feet per minute. At 10,000 feet density altitude, rate of climb at Vy might be 300 feet per minute rather than the 700 feet per minute you experienced at sea level. Vx angle is also meaningfully shallower at that altitude.

If you are departing a mountain strip with rising terrain on the departure end, that shrinking margin is the difference between comfortable clearance and a serious problem.

How Do I Use the Climb Performance Charts in My POH?

Your POH includes a climb performance chart or table requiring two inputs: pressure altitude and outside air temperature. Those numbers return your expected climb rate - and sometimes your expected climb gradient - for the actual conditions on the day you fly.

Use this chart before any departure where performance has real consequences: a mountain airport, a short strip with terrain off the end, a hot afternoon at an airfield above 3,000 feet elevation. Treat the output as a real number you are comparing to whatever is in your departure path.

The NTSB accident record contains a repeating category of departures where a pilot leaves a high-elevation airfield at maximum gross weight on a warm afternoon, uses remembered sea-level performance numbers, and finds the terrain rising faster than the airplane. The airplane performed exactly as it was capable of. The pilot did not correctly model what that capability was on that specific day. The climb performance chart prevents this. It is already in your POH, and it takes five minutes before you taxi.

What Does the Checkride Require for Vx and Vy?

The Airman Certification Standards (ACS) require demonstration of both normal and short-field takeoffs and climbs. For the initial climb, the ACS airspeed tolerance is +5 knots / -0 knots from the target airspeed. That asymmetry is deliberate. Flying below Vx during an obstacle clearance scenario means your climb angle is shallower than the POH intended. Flying below Vy during a normal climb sacrifices rate and engine cooling airflow. There is no room below the target.

The examiner will establish the scenario before you line up. A simulated obstacle in the departure path means they expect Vx. No obstacle means they expect Vy. They are also watching the transition: once the obstacle is cleared, pitch the nose forward, allow the airspeed to build to Vy, and continue the climb there. That transition should be deliberate and clean.

Staying in the Vx attitude past the obstacle is not the safer choice - it is less efficient and signals to the examiner that you are uncertain when to switch modes. Climb toward the obstacle. Clear it. Transition to Vy. That is the professional sequence.

On the oral exam, an examiner may ask you to explain the difference between Vx and Vy in plain terms, what happens to each speed at altitude, or which you would use departing a mountain airport with terrain in the departure path. Know the reasoning behind your answer, not just the definition. The reasoning is what tells you what to do when the scenario does not match the textbook exactly.

How Do I Get Comfortable With the Vx Pitch Attitude?

Vx feels unfamiliar at first. On a Cessna 172, the Vx climb puts the nose noticeably high. Every hour of stall training primes the brain to treat a slow airspeed and high pitch attitude as a warning. The instinct to push the nose down slightly - toward something that feels more like normal flight - is real. Unaddressed, it erodes obstacle clearance margin on a departure that actually demands performance.

The fix is deliberate practice away from terrain and traffic. Climb to three or four thousand feet with no pressure. Reduce to Vx. Hold it for sixty seconds. Note what the pitch looks like from your seat, the visual reference on the horizon, and what the engine sounds like at full power and low airspeed. Then let the airplane accelerate to Vy and notice what that transition feels like. Repeat this across several flights.

The goal is to make both attitudes familiar by visual reference - not by scanning the airspeed indicator. When the wheels leave the runway on a real obstacle departure, the pitch attitude for Vx should already be in your muscle memory.

What Changes in a Flap-Extended Configuration?

If your short-field departure procedure calls for flaps - typically 10 degrees on many trainers - the Vx for that configuration is a different number from clean-aircraft Vx. The POH provides the complete procedure and the appropriate speeds for each configuration, including the flap retraction sequence after clearing the obstacle.

Do not mix flap-extended speeds with clean-aircraft speeds. When transitioning to any new airplane type, learn both configurations as a complete picture.

What Is the Pre-Departure Brief for a Performance Departure?

Before any departure from an airport where performance has consequences, build this habit:

1. Compute density altitude using real inputs: pressure altitude and outside air temperature.
2. Pull the climb performance chart and find your actual expected climb rate for those conditions.
3. Assess the margin - is there comfortable clearance, or is it worth reconsidering departure time, weight, or the plan?
4. Brief Vx or Vy before taxiing, consciously and out loud. Know where the transition point is.
5. Pick an abort point on the runway before you roll.

Make every one of those decisions while the airplane is still. You have thirty seconds on the ground to think clearly. You have none at sixty knots.

Key Takeaways

• Vx (best angle) and Vy (best rate) represent two different performance peaks - maximum excess thrust and maximum excess power, respectively. Vx is always the slower of the two.
• Use Vx when obstacles are in the departure path; use Vy for normal departures where efficiency and engine cooling matter more than climb angle.
• Both speeds change with altitude: Vx increases, Vy decreases. They converge and meet at the absolute ceiling, where no climb performance remains.
• The climb performance chart in your POH gives you real numbers for real conditions. Use it before any departure where altitude, temperature, or terrain make performance matter.
• Brief your climb speed - and your transition point - before you line up, not after the throttle goes forward.