Let’s imagine for a moment the following hypothetical situation: Two airplanes are taking off at exactly the same moment from parallel runways. Airplane A accelerates to and climbs out at Vx while Airplane B accelerates to and climbs at Vy.
What will happen in each case?
Of course, we already know that Vx is the speed for best angle of climb while Vy is the speed for best rate of climb. (If you sometimes have a hard time remembering which is which, think of an X as having lots of angles.) The rule of thumb for which speed to fly is easy: We use Vx immediately after takeoff to clear obstacles, followed by Vy to reach our cruising altitude quickly.
But wait a second. If Vy gives us the best reading on our VSI, why would we ever use a lower rate of climb initially?
The answer has to do with distance vs. time – namely the fact that clearing obstacles is a distance problem (i.e. gaining the maximum altitude in the shortest space possible by flying at best-angle speed) while climbing to cruise altitude is a time problem (because every second at Vy puts the maximum space between the ground and our airplane).
The benefit in climbing at Vx initially is that it allows us to reach a sufficient altitude in the available horizontal distance between the start of our takeoff roll and those tall trees off the departure end of the runway. Once clear of obstacles, the next challenge lies in reaching our cruise altitude as soon as possible, hence the transition to Vy.
So let’s return to our hypothetical dual takeoffs. After passing the runway departure-end threshold, Airplane A climbing at Vx will be higher than Airplane B climbing at Vy. But after, say, 10 minutes, Airplane B climbing at Vy will be higher, and also farther from the departure airport.
For safety’s sake, the FAA Flying Handbook says a blend of Vx and Vy is the best choice at airports with obstacles. And yet many pilots on takeoff will shove the throttle forward, rotate and then immediately allow the airplane to accelerate to a speed much faster than Vx. For some, this undoubtedly has to do with the fact that they’re intimidated flying at such a slow speed so near to the ground. If you fall into this group, ask an instructor to demonstrate a proper Vx climbout with you. The next time you must fly at Vx to safely clear obstacles, you’ll be glad you did.