Landing Performance

Introduction:

Descent Planning:

  • Planning for descent ensures the accurate and timely arrival at the desired space in time, generally to set up for landing
  • While differences in aircraft, avionics, and operation flown, and various aircraft, the 60:1 rule allows for basic in-flight calculations in preparation for descent
  • The 60-to-1 Rule:

    • The 60 to 1 rule states for every 1 degree of shift (up/down/left/right), an offset of 100 feet per 1 Nautical Mile (NM) occurs
    • As it relates to descent planning, this means for every 1 degree the pitch is lowered (relative to level flight), you will lose 100 feet every NM
  • Practical Application:

    • Example: A pilot is 20 NM away from an airport at 6,000' MSL to enter the downwind at 1000' Mean Sea Level (MSL)
    • Determining Pitch Angle:

      • We have 5,000' to lose over 20 NM
      • If we take the altitude to lose and divide it by the distance (5,000'/20NM), we see that we need to lose 250 feet every 1 NM
      • Referencing the 60-to-1 rule, if a 1-degree pitch down is 100' per 1 NM, then a 2.5-degree pitch down is 250' per 1 NM
    • Determining Vertical Speed:

      • We determined that our pitch must be 2.5 degrees down, and since we're flying at some constant speed, this translates to a predictable vertical speed, which we can calculate
      • If you multiply the descent angle (2.5 degrees in the above example) by the Nautical Miles-per-minute and then multiply that number by 100, you get the Feet Per Minute (FPM) descent rate
        • Shown another way: (NM per Min * Pitch down * 100 = Descent in Feet per Minute)
      • First, determine nautical miles per minute:
        • Divide the airspeed (NM per Hour) by 60 (Minutes per hour) to get Nautical Miles per minute
        • If we intend to descend at 90 knots, we divide that by 60 and get 1.5 NM per minute
      • Second, determine vertical speed:
        • If we then multiply our NM per minute by our pitch-down of 2.5 degrees, we get 3.75
        • Multiply 3.75 by 100, and you get 375 feet per minute
      • Another way of doing the same thing is knowing we have 20 NM to travel at 1.5 NM/Minute; we can determine that we have approximate 13 minutes to lose 5,000 feet
        • 5000 divided by 13 = 384 FPM
    • Working Backwards:

      • We can work backward too, for example, let's say we wish to descend at 200 FPM for passenger comfort
        • If we are traveling at 1.5 NM per minute, and we have 5,000' to lose at 200 FPM, it will take 25 minutes to accomplish this descent (5,000'/200 FPM)
          • Since we calculated ~375 FPM was required for being 20 NM out, and we plan to reduce our rate of descent, we need to begin descending earlier
          • Assuming we are flying at 1.5 NM per minute, and we need 25 minutes to descend, we need to begin our descent at 37.5 NM away (25* 1.5)
        • Additionally, instrumentation may display how far away the aircraft is from a known point
          • If instrumentation displays 5 minutes to a point, and the pilot needs to descend 1000' in that time, then feet (1,000) divided by minutes (5) equals the required feet per minute, in this case 200, feet per minute
      • Note the 60-to-1 rule is a rule of thumb and not an exact science, but accurate enough to guide basic decisions and cross-check expected performance
      • Vertical Descent Angle and Threshold Crossing Height
        Vertical Descent Angle/
        Threshold Crossing Height
      • When flying a non-precision approach, a Vertical Descent Angle (VDA) and Threshold Crossing Height (TCH) may be published. For Copter approach procedures, a Heliport Crossing Height (HCH) will be depicted in place of the TCH. The VDA is strictly advisory and provides a means to establish a stabilized descent to the Minimum Descent Altitude (MDA). The presence of a VDA does not guarantee obstacle protection in the visual segment. If there are obstacles in the visual segment that could cause an aircraft to destabilize the approach between MDA and touchdown, the profile will not show a VDA and will instead show a note that states "Visual Segment-Obstacles" [Figure 1]
        • When descending to MDA, consider the time desired to level-off and stabilize to see the landing environment and "break out" to land
      • Pay attention to shock cooling, whereby operations at a lower engine setting for extended periods can quickly cool an engine
      • Bold Method as several great articles that discuss descent planning to include: "How the 60-1 rule helps you plan a perfect descent and know when to reduce the throttle when given a "descent at pilot's discretion" and "How To Calculate Your Descent Rate To MDA"

Approach Performance:

  • Compensating for Winds on Approach:

    • Compensating for winds on approach will change based on wind direction, approach angle sought, and other factors
    • Pilots will generally elect to conduct a crab, followed by a slip to landing
    • In those instances where a stepper approach is sought, a forward-slip may be used
    • Crab:

      • Coordinated flight whereby you are pointing the nose of the aircraft upwind enough to keep the airplane's ground track straight
      • The angle by which the aircraft is flying relative to the runway is considered 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, a crab to sideslip transition for the landing flare and touchdown should be made
    • 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 are using "wing-low, top-rudder" to track the aircraft straight for altitude loss (forward-slip) or crosswind compensation (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
        • Simply stated, the higher the angle of bank, the lower the nose must be
      • Forward-slip:

        • A forward slip allows pilots to increase the aircraft's rate of descent without increasing airspeed in the process
        • 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 track to be maintained while steepening the descent without adding excessive airspeed
        • This is accomplished by applying full rudder and utilizing the angle of bank to maintain a ground track
        • Since the heading is not aligned with the runway, the slip must be removed before touchdown to avoid excessive side loading on the landing gear, and if a crosswind is present, an appropriate side-slip may be necessary 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 greater (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, airspeed indicator error may be observed when performing slips
          • Recognize a properly performed slip by the airplane's attitude, sound of the airflow, and flight control feel
        • Prior to 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 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 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, which may result in fuel ports becoming 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 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 normally 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 over-speed condition
  • Tailwinds:

    • Tailwinds increase an aircraft's ground speed
    • Flying the same indicated approach speed therefore results in a faster than normal ground speed
    • As a result of a faster approach, the approach angle must be steeper than normal if flying a normal pattern
    • Further, landing distance will be increased, as there is more energy that must be bled off to stop
    • Unless there forced to land in a tailwind, pilots should always opt for landing in the opposite direction, thereby having a headwind

Determining Landing Distances:

  • Locate the aircraft performance charts within the Pilot Operating Handbook/Pilot Information Manual
  • Note that most manuals will include an example with which you can follow if you're unsure of how to begin
  • Pilots are required by FAR to know the lengths of runways they intend to use, but sometimes plans have to change
    • If for whatever reason you cannot determine a runway length (uncontrolled field, not data), you can calculate it by conducting a low approach
    • Take your airspeed, divide by 60 for NM per minute. Divide by 60 again to get NM per second. Multiply NM per second by the time it took to fly the length of the runway. The result is a number in NM, multiplied by 6076 feet is the length of the runway in feet
  • Landing Distance Variables:

    • 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, approach speeds
    • Temperature:

      • As temperature increases (pressure being 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 let the aircraft "feel" like it is flying at a speed faster than it is moving across the ground
        • This reduces inertia and makes an aircraft easier (faster) to stop
      • Tailwinds do the exact opposite and require extra speed to achieve the desired lift required 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 roll-out distance while landing uphill reduces distance
      • Remember, varying runway slopes can induce illusions
    • Windshear:

      • Wind shear is a change in wind speed and/or direction over a short distance
      • It can occur either 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:

      • Pavement, grass, gravel, rubber slicks
      • Hydroplaning:
        • When a fast moving object contacts water, the water is difficult to penetrate, often compared to the properties of concrete
        • When a fast moving tire contacts standing water, it may not penetrate the film to the pavement, and this results in hydrplaning
        • Runways are therefore constructed to remove water and may have groves to "raise" the paved surface
        • Similarly, deep treads or channels in tires may allow up to 2 inches of water before hydroplaning occurs
        • Despite these mitigations, hydroplaning speed depends on tire pressure, not weight
          • This is because a heavier airplane creates a larger "footprint," spreading the load
          • Minimum total hydroplaning speed (knots) = 9 x square root of tire inflation pressure (psi)
        • 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
          • Vhydroplane = 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
          • 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 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
            • This condition could 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 & Systems

      • Loss of flaps means high approach speeds and generally lower approach angles, increasing landing distance

Use of Flaps on Landing:

  • Flaps are considered high-lift devices
  • The use of flaps allow for 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"
    • This ballooning is recognized with a gain in altitude and 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 lower the nose

Landing Performance Best Practices:

  • Do not be afraid to delay landing
    • Under zero wind conditions, most runways have adequate cross-fall (rounding of the runway surfaces or crown) to provide drainage under high rates of precipitation
    • It appears that drainage can be seriously affected in crosswinds above 10 knots; however, a 15- to 20-minute waiting period after a downpour is usually sufficient to drain the water
  • Be knowledgeable of the many variables associated with landing under wet runway conditions:
    • Landing weather forecast
    • Aircraft weight and approach speed
    • Hydroplaning speed
    • Conditions of tires - if the tread depth of the tires on an aircraft is greater than the depth of the water on the runway, then hydroplaning will not occur. Knowledge of the general condition of the tires (why we do pre-flights) should be helpful in a qualitative sense when potential hydroplaning conditions exist
    • Brake characteristics
    • Wind effects on the aircraft while landing on a wet runway (crabbing)
    • Runway length and slope
    • Glide path angle
  • Do not exceed 1.3 Vs plus wind additives at the runway threshold
  • Establish and maintain a stabilized approach
  • Maximum flaps provide minimum approach speeds
  • Be prepared to go around from the threshold
  • Do not perform a long flare
  • Do not allow the aircraft to drift during the flare
  • Touch down firmly and do not allow the aircraft to bounce
  • If a crosswind exists, apply lateral wheel control into the wind
  • Keep the aircraft centerline aligned with the runway centerline
  • Anti-skid braking should be applied steadily to full pedal deflection when automatic ground spoilers deploy, and main wheel spin-up occurs. Do not modulate brake pressure
    • The anti-skid system will not operate until the main wheels of the aircraft spin; don't lock your brakes before touchdown
  • Be prepared to deploy ground spoilers manually if automatic deployment does not occur. Spoiler deployment greatly assists wheel spin-up during wet runway operations by materially reducing the wing lift and increasing the weight on the wheels, thus shortening your stopping distance
  • Apply maximum reverse thrust as soon as possible after main gear touchdown; this is when it is most effective
  • Get the nose of the aircraft down quickly
    • Do not attempt to hold the nose off for aerodynamic braking
  • Apply forward column pressure as soon as the nose-wheel is on the runway to increase the nose-wheel weight for improved steering effectiveness. Do not, however, apply excessive forward column pressure because the down elevator will, to some extent, unload the main wheels and decrease braking effectiveness
  • If the aircraft is in a skid, align the aircraft centerline with the runway centerline if you can. Get off the brakes to maximize cornering capability and bring the aircraft back to the runway center
    • If you are in a crab and cannot align aircraft centerline with the runway centerline and attempted cornering is not effective, get out of reverse thrust to eliminate reverse thrust component side forces tending to push the aircraft off the side of the runway

Determining Approach Rate of Descent:

  • It is necessary to determine the descent rate for a non-precision approach so that the aircraft reaches the MDA at a distance from the threshold that will allow landing in the touchdown zone
  • To determine the required rate of descent:
    • Subtract the Touchdown Zone Elevation (TDZE) from the Final Approach Fix (FAF) altitude
    • Divide the result by the time inbound
  • For example: If the FAF altitude is 2000' MSL, the TDZE is 400' MSL, and the estimated time inbound is two minutes, then a rate of descent of 800 FPM should be used [(2000-400)/2 = 800]
  • To verify the descent point from MDA (on a 3° glide path) to a landing on the intended runway can be made:
    • Subtract the MDA from the TDZE
    • Divide the result by 300
  • For example: given a 800' MSL MDA and 400' MSL TDZE, the position from which a descent from MDA to a landing should be initiated is approximately 1.3 NM from the threshold [(800-400)/300 = 1.3]
  • Note pilots should be crossing the runway threshold at a nominal height of 50' above the TDZE
  • To determine an approximate rate of descent to maintain a glideslope (precision approach), divide groundspeed by 2, and then multiply the result by 10
  • For example: 90 knots/2 = 45, 45 x 10 = 450 fpm

Case Studies:

  • National Transportation Safety Board Identification: ERA20CA139:
    • The NTSB determines the probable cause(s) of this accident to be: The airplane hydroplaning while landing on a wet runway, which degraded its braking capability and resulted in a runway overrun onto grass and mud and the nose landing gear collapsing. Contributing to the accident was the pilot's improper decision to land the airplane until it was near the runway midpoint due to fog over the approach end of the runway.

Landing Performance Case Studies:

  • NTSB Identification: ERA13CA394: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot's loss of directional control during takeoff due to right main landing gear contact with a pool of standing water on the runway, which resulted in a runway excursion
  • NTSB Identification: CEN13LA02: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot's decision to continue the landing after touching down long and on a wet runway that reduced the airplane's braking capability, which resulted in an overrun

Landing Performance Knowledge Quiz:

Conclusion:

  • The flight is not over until the aircraft is chocked and the engine(s) turned off
  • Concerning viscous hydroplaning, consider where you will see that rubber build-up
  • Hydroplaning is an extremely under-appreciated and dangerous hazard that exists not only during but after inclement weather
  • Rubber is not only on the approach end but on the departure end of the runway
    • If you find yourself behind the aircraft scrambling to stop and you slam on the brakes over this rubber, you may do more harm than good!
  • Always be sure to compare your landing distance expected to the available landing distance in the Chart Supplement, and not just the length of the runway as displaced thresholds may significantly reduce the available runway for landing
  • If you experience hydroplaning, GO-AROUND!
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References: