to altitude (PE); the airspeed decreases to something less  than 120 knots as the altitude increases.  There are two primary reasons to evaluate climb performance.  First, aircraft must climb over obstacles to avoid hitting  them. Second, climbing to higher altitudes can provide  better weather, fuel economy, and other benefits. Maximum  Angle of Climb (AOC), obtained at V$_{X}$, may provide climb  performance to ensure an aircraft will clear obstacles.  Maximum Rate of Climb (ROC), obtained at V$_{Y}$, provides  climb performance to achieve the greatest altitude gain over  time. Maximum ROC may not be sufficient to avoid obstacles  in some situations, while maximum AOC may be sufficient  to avoid the same obstacles.  [Figure 11-7]  AOC is a comparison of altitude gained relative to distance  traveled. AOC is the inclination (angle) of the flight path. For  maximum AOC performance, a pilot flies the aircraft at V$_{X }$ so as to achieve maximum altitude increase with minimum  horizontal travel over the ground. A good use of maximum  AOC is when taking off from a short airfield surrounded by  high obstacles, such as trees or power lines. The objective is  to gain sufficient altitude to clear the obstacle while traveling  the least horizontal distance over the surface.  One method to climb (have positive AOC performance) is  to have excess thrust available. Essentially, the greater the  force that pushes the aircraft upward, the steeper it can climb.  Maximum AOC occurs at the airspeed and angle of attack  (AOA) combination which allows the maximum excess  thrust. The airspeed and AOA combination where excess  thrust exists varies amongst aircraft types. As an example,  Figure 11-8  provides a comparison between jet and propeller  airplanes as to where maximum excess thrust (for maximum  AOC) occurs. In a jet, maximum excess thrust normally  occurs at the airspeed where the thrust required is at a  minimum (approximately L/D$_{MAX}$). In a propeller airplane,  maximum excess thrust normally occurs at an airspeed below  L/D$_{MAX}$ and frequently just above stall speed.  ROC is a comparison of altitude gained relative to the time  needed to reach that altitude. ROC is simply the vertical  component of the aircraft’s flight path velocity vector. For  maximum ROC performance, a pilot flies the aircraft at V$_{Y }$ so as to achieve a maximum gain in altitude over a given  period of time.  Maximum ROC expedites a climb to an assigned altitude.  This gains the greatest vertical distance over a period of  time. For example, in a maximum AOC profile, a certain  aircraft takes 30 seconds to reach 1,000 feet AGL, but  covers only 3,000 feet over the ground. By comparison,  using its maximum ROC profile, the same aircraft climbs  Figure 11-8. Comparison of maximum AOC between jet and propeller airplanes.  Figure 11-7. Maximum angle of climb (AOC) versus maximum rate  of climb (ROC).  Rate of Climb (ROC)  Angle of Climb (AOC)  11-7  Altitude  Distance  Max AOC Max ROC  Thrust  Velocity (TAS)  Thrust  Velocity (TAS)  (Full PCL)  (Full Throttle)  L/D $_{MAX }$ L/D $_{MAX }$ T$_{E }$ T$_{E }$ T$_{A }$ T$_{A }$ T$_{R }$ T$_{R }$ LEGEND  T$_{E }$ thrust  excess T$_{A}$ thrust available T$_{R }$ thrust  required AOC  angle of  climb TAS  true  airspeed L/D $_{MAX }$ lift to drag  ratio  maximum PCL power control lever  Max AOC (jet)  Max AOC (prop) 