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Takeoff Performance

Introduction:

Climb Planning:

  • Climb Planning is necessary for several reasons which include flight planning and obstacle clearance

Takeoff Distance Variables:

  • Gross Weight:

    • As gross weight increases, the difference between nose-wheel lift-off and takeoff speed decreases
    • When an instructor is not in the plane, the pitch attitude may differ
      • The aircraft will be airborne sooner, climb more rapidly, and have higher performance
  • Center of Gravity:

    • The farther forward the CG the longer the takeoff roll
    • More authority is required to lift a heavy nose
    • This can be amplified with heavy takeoff weight
    • As CG moves forward, the difference between nose-wheel lift-off and takeoff speed decreases
  • Nose Strut:

    • Nose-wheels do more than for just taxi and shock absorption but also to aid in bouncing the aircraft upward
      • If the nose-wheel is improperly serviced:
        • If the oil level is high, the springboard effect is reduced; but the change in shock absorber effect is minimal (strut compression on landing)
        • If the oil level is low, the reverse is true; springboard effect is essentially normal, but shock absorbing is poor
  • Power Settings:

    • Applying power to quickly may yaw the aircraft to the left due to torque, most apparent in high-powered engines
  • Flight Profile Flown:

    • The Pilot Operating Handbook/Airplane Flight Manual will specify different configurations and procedures with which to fly
    • Flaps:

      • Lowering the flaps increase drag, but also increases lift, allowing for quicker rotation into ground effect
      • Aircraft must accelerate sufficiently in ground effect however, before continuing a climb
  • Outside Air Temperature:

    • Temperature is a key variable in determining density altitude
    • As temperature rises, so does density altitude
    • Conversely, density altitude drops with temperature
  • Field Elevation/Density Altitude:

    • The field elevation is irrelevant besides the fact that it correlates to starting at a higher density altitude
    • While density altitude can actually be lower than field elevation, an aircraft on that same day at a lower altitude would almost definitely experience a lower density altitude as well, barring any sort of environmental phenomena
  • Surface Winds:

    • The winds impact how air flows over the wing of an aircraft
    • Headwinds work with the motion of the aircraft to increase flow while tailwinds push against the normal flow of air
    • As a result, with a headwind, the airplane already feels some sort of airflow over the wings before it starts to roll, thereby increasing lift and resulting in a shorter takeoff roll
    • With a tailwind you would have increased speed to develop minimum lift causing stress on tire and increased takeoff distance
  • Runway Slope:

    • Airports are not perfectly flat and they will have some variance in altitude from one end to another, especially at large airfields
    • Much like when driving a car, moving an airplane uphill requires the engine to work harder to accelerate which results in a longer time to reach rotation speeds, increasing takeoff roll
    • Conversely, taking off down hill allows for faster acceleration resulting in a shorter takeoff roll
    • When available, runway slope data will be provided. Runway slope will be shown only when it is 0.3 percent or greater. On runways less than 8000 feet, the direction of the slope up will be indicated, e.g., 0.3% up NW. On runways 8000 feet or greater, the slope will be shown (up or down) on the runway end line, e.g., RWY 13: 0.3% up., RWY 31: Pole. Rgt tfc. 0.4% down
    • Aircraft slope can be determined by referencing the airport diagram found in the Chart Supplement U.S. or online at FAA.gov, or commercial sources
  • Runway Surface Condition:

    • Pavement, grass, gravel, rubber slicks
    • Hydroplaning:

      • Dynamic Hydroplaning:
        • Dynamic hydroplaning occurs when standing water on a wet runway is not displaced from under the tires fast enough to allow the tire to make pavement contact over its total footprint area
        • The tire rides on a wedge of water under part of the tire surface
        • It can be partial or total hydroplaning, meaning the tire is no longer in contact with the runway surface area
        • It is possible that as the tire breaks contact with the runway that the center of pressure in the tire footprint area could move forward
        • At this point, total spin-down could occur and the wheel stops rotating, which results in total loss of braking action
        • The speed at which this happens is called minimum total hydroplaning speed
      • Viscous Hydroplaning:
        • Viscous hydroplaning can cause complete loss of braking action at a lower speed if the wet runway is contaminated with a film of oil, dust, grease, rubber or the runway is smooth
        • The contamination combines with the water and creates a more viscous (slippery) mixture
        • It should be noted that viscous hydroplaning can occur with a water depth less than dynamic hydroplaning, and skidding can occur at lower speeds, like taxiing during light rain, applying the brakes and rolling over an oil spill
        • With regards to rubber, consider that rubber is found primarily on the approach and departure end of the runway
      • Rubber Reversion Hydroplaning:
        • Rubber reversion hydroplaning is less known and is caused by the friction-generated heat that produces superheated steam at high pressure in the tire footprint area
        • The high temperature causes the rubber to revert to its uncured state and form a seal around the tire area that traps the high-pressure steam
        • It is theorized that this condition would occur on damp runways or when touchdown occurs on an isolated damp spot of a dry runway, which results in no spin-up of the tires and a reverted rubber skid
  • Tire Pressure

    • Braking effectiveness is a factor of tire pressure
    • Pressure also impacts the speed at which hydroplaning can occur
    • Fly in accordance with the aircraft's Pilot Operating Handbook
  • Inoperative Equipment

Determining Takeoff Distances:

  • Takeoff distance is calculated using performance charts which can be found in your Pilot Operating Handbook/Airplane Flying Manual
  • Once calculated, cross-check required takeoff distance against runways available to see what is, or is not acceptable

Use of Flaps on Takeoff:

  • Flaps are considered high-lift devices
  • Use of flaps allow for the aircraft to create more lift on takeoff to reduce takeoff distance
  • When lowering flaps, you are changing the chord line which increases the angle of attack (AoA)
  • This increase in AoA causes the aircraft's wing to suddenly create more lift, and therefore the aircraft will "balloon"
  • When lowering flaps anticipate this balloon effect by being ready to lower the nose

Suggestions:

  • Takeoffs are optional but landings are mandatory
    • Make decisions early as to whether or not a takeoff under existing conditions is wise

Case Studies:

Conclusion:

  • Pilots must be familiar with their aircraft's performance in accordance with Federal Aviation Regulations
  • Although general aviation charts found in the POH/AFM do not consider every variable, it is important to have an understanding of the various conditions that may exist
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