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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
  • 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
  • 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
  • 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 if a factor of tire pressure
    • Pressure also impacts the speed at which hydroplaning can occur
  • Inoperative Equipment

Determining Takeoff Distances:

  • Takeoff distance is calculated using performance charts which can be found in your Pilot Operating Handbook/Airplane Flying Manual

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

References: