Thrust

• Planes have to provide thrust, as a force, to move them through the resistance of air and overcome gravity. The amount of thrust in relation to the weight of the aircraft has a direct bearing on how fast it can fly. The more horsepower thrust a plane develops, the faster it can move through the atmosphere. Given two aircraft of equal weight, with one plane having 30 percent more thrust horsepower, the higher horsepower craft will fly faster and attain cruising speed much quicker.
  
  

Lift and Wing Profile

• The angle of a plane's wing can determine how fast it can move through the air. Wings have varying thicknesses, sizes and shapes, which correlate to the amount of lift they produce. Speeding air over the curved surface of a wing produces a pressure drop above the the wing, generating lift. The angle of the wing, known as "angle of attack," as it passes through the air affects lift. The higher the angle, the slower the movement through the air.
  
  

Induced Drag

• Induced drag involves the backward pulling force on a plane as it moves through the air. A plane collides with air as a necessary means to move through it. Therefore, induced drag is always present and affects the speed of all aircraft. Induced drag only disappears if the plane becomes oriented in a nose-down position to overcome it.
  
  

Friction and Parasitic Drag

• Airplanes have an outer shell known as the "skin." The smoother the skin, the faster the plane passes through the air. If the plane has numerous protuberances on the fuselage, such as rivets, bumps, seams and joints, it causes extra drag. Old bi-wing planes of WWI vintage had non-retractable landing gear, exposed gun turrets, struts and strut wires, which slowed the planes down because of skin drag. Modern jet liners have sleek and sharp profiles, designed to slice through the air. Aircraft pick up several MPH in speed after they have received a coat of wax.
  
  

Air Motion

• All aircraft must fly through moving air currents at different altitudes. These masses of flowing air can have a direct effect on the speed of an aircraft. A headwind will create high resistance as a plane passes through it; and although the craft speed will read normal in relation to the air mass it moves through, its ground speed will show a marked reduction. The ground speed reduction will affect arrival time by lengthening it. Conversely, a tailwind will increase aircraft ground speed with the same thrust setting and shorten arrival time.
  
  

Humidity

• Humidity in the air changes the pressure in the atmosphere, affecting the way a plane flies. Air pressure goes down when the humidity rises. In high humidity conditions, the wings of an airplane have less air molecules to "grip" in the air, and this slows the aircraft down. To overcome the loss of lift, more thrust must be added. Humid air also contains less oxygen, which affects both piston, turbo prop and jet engine performance. Running into humid conditions will actually slow a plane's engine down.
  
  

Altitude Density

• With an increase in altitude, the air density decreases, affecting the performance of non-jet engine aircraft. Less dense air at high threshold altitudes can also not fully support the control surfaces of a plane, which makes them more sluggish and hard to control. The result is a reduction in speed due to lack of thrust and increased skin friction.
  
  

Temperature

• When the temperature of the air is increased, the air density drops. The colder the temperature, the higher the density of the air. Aircraft that take off on very hot days must increase their thrust to provide enough lift to the wing surfaces to make a climb. A colder day will provide denser air, allowing an aircraft to take off with less speed but at the same thrust setting.