I think we all know there are two fundamental measures of climb–rate and angle. Actually I don’t like the term angle of climb, but that’s the normal terminology used for piston airplanes.
Rate of climb is the amount of altitude gained in a specific amount of time, usually expressed in feet per minute. For trip planning rate of climb is the most useful and using the appropriate airspeed to achieve optimum rate of climb for your weight is usually most efficient.
We are told to use the best angle of climb airspeed–Vx–when we want to climb as steeply as possible. A steep climb is one that achieves the highest altitude over the shortest distance.
Steep or angle are pretty useless terms when planning a takeoff or go-around. What you really need to know is how much altitude you will gain in a certain distance. That’s because the terrain or obstruction you are trying to clear is a fixed distance away and you need to know if you can clear the obstacle.
That measure, altitude gained over distance, is called climb gradient. It’s the takeoff performance requirement transport pilots must consult and be certain the airplane can achieve given weight and all other performance factors prior to any takeoff.
Climb gradient–also called simply grade by railroaders or highway designers–is normally expressed as a percentage. A 1% climb gradient in aviation terms is an altitude increase of 1% percent during a nautical mile flown, or about 60 feet.
So you know that hill is two miles away and 120 feet higher than the runway so a 1% positive climb gradient will just barely clear the ridge. You could do the math to work backward from a known rate of climb to find the gradient. Or you could apply math to find the altitude gained versus distance if you know the climb angle. But applying available climb gradient to the charted obstructions around an airport is so easy.
Some piston airplane POH climb charts include gradient along with rate, which is useful. But in the POH that do that, at least the ones I am familiar with, the data is presented only for climbing at Vy, best rate airspeed. For planning purposes you can assume that if the available climb gradient data clears obstructions when holding Vy you would have more clearance margin by flying Vx. But you wouldn’t know how much.
So the best we can do with the available data is climb initially at Vx until above obstructions and then accelerate to Vy to continue the en route climb.
Another reason to climb at the highest gradient initially is to gain the most altitude while remaining as close as possible to the runway. Wouldn’t it be wonderful if we could climb straight up to a safe altitude right over the airport. Then if an engine quit or even sputtered a return to the runway would be a piece of cake.
But we can’t climb straight up, and in most cases even helicopters can’t, but making our initial climb at Vx comes as close as possible.
Once clear of obstructions and at a safe altitude accelerating to Vy expands our options because it puts the most air under us in the shortest possible time.
If there are obstacles not far from your takeoff runway it’s worth the time to consider the maximum climb gradient your airplane can achieve. Even if the POH doesn’t have gradient data you can calculate it by factoring in the Vx airspeed and rate of climb data. Remember to correct the airspeed to groundspeed because that’s what matters so wind, temperature and elevation are all variables to consider.
After you calculate available climb gradient be sure to build in a fudge factor. In transport airplanes the fudge factor is at least 0.8%. The climb gradient measured in flight test–almost always with the most critical engine failed–is reduced by the 0.8% to determine net gradient. It is only the engine-out net gradient that transport pilots are allowed to use when planning obstacle clearance for takeoff. So it seems logical to me we should fudge at least 1% when calculating climb gradient in a piston airplane takeoff.
The lighter the airplane the closer Vx and Vy indicated airspeeds are likely to be. But even in my airplane which weighs 5,400 pounds maximum for takeoff Vy is 18 knots faster than Vx, a pretty wide margin. Whatever the airspeed difference it’s worth it to know when you need maximum climb gradient and how to achieve it and what percent it is likely to be.