Descent & Landing Performance

Crab:

• Crabbing is a technique where you point the aircraft's nose in coordinated flight upwind enough to keep the airplane's ground track straight.
• The angle by which the aircraft is flying relative to the runway is the crosswind correction.
• It is preferable, in general aviation, to fly a crab and transition to a slip for landing to avoid side-loading the landing gear.
• At some point during the final approach, the crab must transition to a side-slip for the landing flare and touchdown.

Slips to Landing:

WARNING:
All procedures are GENERALIZED.
Always fly per Pilot Operating Handbook procedures,
observing any relevant Standard Operating Procedures (SOPs)

• A slip is a cross-control procedure where you use "wing-low, top-rudder" to track the aircraft straight while dropping altitude (forward slip) or to compensate for crosswind (side slip).
  • In doing this, you will need to lower the nose as the increase in drag without an increase in thrust will cause a rapid loss of airspeed, risking a stall.
  • The higher the angle of bank, the lower the nose must be.

• Forward-slip:

  • A forward slip enables pilots to increase the aircraft's rate of descent without simultaneously increasing airspeed..
  • The pilot accomplishes a forward slip by hanging as much of the fuselage (increasing drag) in the breeze as possible.
  • This increase in drag bleeds energy.
  • Assuming proper runway alignment, the forward slip will allow the aircraft's track to be maintained while steepening the descent without adding excessive airspeed.
    • Accomplished by applying full rudder and utilizing the angle of bank to maintain a ground track.
  • Since the aircraft's heading does not align with the runway, the pilot must remove the slip before touchdown to avoid excessive side loading on the landing gear. If a crosswind is present, the pilot may need to apply an appropriate side-slip at touchdown, as described below.
  • Using the maximum amount of rudder deflection possible will create only one variable (the aileron).
  • With the flaps set to the final setting, set the throttle to idle.
  • Initiate the slip by simultaneously providing aileron input (bank) to lower a wing (upwind wing in a crosswind condition) and rudder input (yaw) in the opposite direction so that the longitudinal axis is at an angle to the original flight path.
  • Maintain the appropriate amount of bank and yaw to maintain the extended runway centerline.
  • Maintain the appropriate amount of bank and yaw to maintain the extended runway centerline.
  • Note that the amount of slip (sink rate) depends on the bank angle: the steeper the bank, the greater the descent rate, the steeper the descent angle, the greater the need for opposite direction yaw (rudder) up to the "practical slip limit" (banking capacity exceeds rudder effectiveness).
  • Adjust the pitch attitude as appropriate to maintain airspeed.
    • Trim as necessary.
  • Note that because of the location of the pitot tube and static course, pilots may encounter airspeed indicator errors when performing slips.
    • Recognize a properly performed slip by the airplane's attitude, the sound of the airflow, and the feel of the flight controls.
  • Before the roundout, discontinue the forward slip.
  • Complete the appropriate approach and landing procedure.

• Side-slip:

  • A side-slip allows pilots to compensate for a crosswind on final approach.
    • Think side-wind, side-slip, as described here.
  • First, you apply aileron into the wind to compensate for the crosswind blowing you off the centerline.
  • Next, you use the rudder to maintain alignment with the runway centerline.
  • The horizontal component of lift forces the airplane to move sideways toward the low wing.
  • Use the aircraft's rudder to align the aircraft to the runway centerline while dipping the wings (toward the wind) to maintain track (drift).
  • Held to touchdown, this will result in the low-side wheel touching down first, followed by the high-side wheel, and lastly, the nose/tail wheel.
  • Note that when performing a slip, the Pilot Operating Handbook may impose certain restrictions, such as:
    • Avoiding slips with full flaps.
    • Avoiding slips for prolonged periods can cause fuel ports to become uncovered.
    • Airspeed indications may vary due to static ports receiving direct wind.
      • Suppose your static port is on the left side of the fuselage. In that case, a slip using the right rudder will cause the perceived static pressure to be higher than actual, as ram air is forced into the static port, resulting in your indicated airspeed indicating less than actual. Therefore, it would generally be advisable to maintain an airspeed comfortably within the middle range of the white arc (flap operating range) to avoid being either too close to a cross-control stall or a flap overspeed condition.

Flight Profile Flown:

• Landing profiles are procedures and settings as recommended by your Pilot Operating Handbook/Pilot Information Manual.
• These conditions and settings for performance data are labeled on the chart and consist of factors such as weight, flap settings, and approach speeds.

• Use of Flaps on Landing:

  • Flaps are considered high-lift devices.
  • The use of flaps allows the aircraft to fly a slower, steeper approach.
  • 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 create more lift suddenly, and therefore, the aircraft will "balloon."
    • Pilots recognize this ballooning by a gain in altitude and a pitch down.
    • Flaps that create the most lift with the least drag to offset will cause the most dramatic pitching moment.
  • When lowering flaps, anticipate this balloon effect by being ready to drop the nose.

Temperature:

• As the temperature increases (while pressure remains constant), the air becomes less dense, and it is as though you're flying at a higher altitude.

Field Elevation/Altitude:

• More specifically, density altitude.

Winds

• Headwinds provide airflow (lift) over an airfoil before adding forward motion.
• These conditions allow the aircraft to "feel" as if it is flying at a speed faster than it is moving across the ground.
  • Headwinds therefore reduce inertia and make an aircraft easier (faster) to stop.
• Tailwinds do the exact opposite and require extra speed to achieve the desired lift needed to maintain approach parameters.

Runway Slope

• Same as if you were driving a car or, for that matter, walking, it is easier to keep up speed going downhill than up.
• Landing downhill increases the roll-out distance, while landing uphill reduces it.
• Remember, varying runway slopes can induce illusions.

Wind Shear:

• Wind shear is a wind speed and direction change over a short distance.
• It can occur horizontally or vertically and is most often associated with strong temperature inversions or density gradients.
• Four common sources of low-level wind shear are:
  • Frontal activity.
  • Thunderstorms.
  • Temperature inversions.
  • Surface obstructions.
• Read more here.
• If available, utilize information from Low-Level Wind Shear and Microburst Detection Systems.

Runway Surface Condition:

• Runway conditions will vary, but the typical surface conditions are pavement, grass, gravel, and rubber slicks.

• Grass Runway Surface:

  • Grass can be slippery when wet, as it absorbs and retains moisture.
  • Landing distance on grass increases due to reduced braking effectiveness, particularly when the surface is wet.
  • Additionally, the use of a wet runway may cause treads to dig into the surface, making it uneven.
  • Grass runways will necessitate the use of a soft-field approach & landing procedure.

• Hydroplaning:

  • When a fast-moving object collides with water, the water is difficult to penetrate, often likened to the properties of concrete.
  • When a fast-moving tire contacts standing water, it may not penetrate the film on the pavement, resulting in hydroplaning.
  • The three types of hydroplaning are dynamic, viscous, and rubber reversion.

  • Dynamic Hydroplaning:

    • Dynamic hydroplaning occurs when standing water on a wet runway is not displaced from under the tires quickly enough to allow the tire to make contact with the pavement over its entire 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.
    • As the tire breaks contact with the runway, the center of pressure in the tire footprint area could move forward.
    • At this point, total spin-down could occur, causing the wheel to stop rotating and resulting in a complete loss of braking action.
    • The speed at which this happens is the minimum total hydroplaning speed.
    • V_hydroplane = 9 √ (tire pressure).

  • Viscous Hydroplaning:

    • A wet runway contaminated with a film of oil, dust, grease, rubber, or if the runway is just smooth, can cause viscous hydroplaning and, therefore, a complete loss of braking action at lower speeds.
      • Realize that wheels take time to spin up to match the aircraft's speed, meaning they drag on the runway and deposit rubber in the touchdown zone.
    • The contamination combines with the water and creates a more viscous (slippery) mixture.
    • Note that viscous hydroplaning can occur with a water depth less than that required for dynamic hydroplaning, and skidding can occur at lower speeds, such as taxiing during light rain, applying the brakes, or rolling over an oil spill.
    • Regarding rubber, note that it is typically found primarily on the approach and departure ends of the runway.

  • Rubber Reversion Hydroplaning:

    • Rubber reversion hydroplaning is less known, 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, forming a seal around the tire area that traps the high-pressure steam.
      • This condition can occur on damp runways or when touchdown occurs on an isolated damp spot of a dry runway, resulting in no spin-up of the tires and a reverted rubber skid.

  • Hydroplaning Mitigation:

    • Utilization of graded runways removes water, while grooves in pavement help to "raise" the paved surface above where water gathers.
    • Similarly, deep treads or channels in tires may allow up to 2 inches of water before hydroplaning occurs.
    • Touchdown speed should be as low as practical, consistent with safety.
    • Braking should be light to assess performance initially.
    • Aerodynamic braking by raising the nosewheel may cause a hydroplane to cease.
    • Identify the hotspots in your field where water tends to accumulate or rubber may compromise the integrity of the runway's surface.
    • Despite these mitigations, the speed of hydroplaning depends on tire pressure, not weight.
      • In the case of heavier airplanes, a larger tire "footprint" spreads the load.
      • Minimum total hydroplaning speed (knots) = 9 x square root of tire inflation pressure (psi).

Tire Pressure:

• Braking effectiveness is a factor of tire pressure.
• Pressure also impacts the speed at which hydroplaning can occur.

Inoperative Equipment & Systems

• Loss of flaps means higher approach speeds and generally lower approach angles, which increase landing distance.