Appendix 1 handling difficulties in airplanes with very low wing loading (the ratio between the wing area and the weight of the airplane). Wind across the line of flight, known as crosswind, causes the windward side of the airplane to generate more lift and drag, causing the airplane to roll, yaw, and pitch off its intended flightpath. As a result, many light airplanes have limits on how strong the crosswind can be while using flaps. Furthermore, once the airplane is on the ground, the flaps may decrease the effectiveness of the brakes since the wing is still generating lift and preventing the entire weight of the airplane from resting on the tires, thus increasing stopping distance, particularly in wet or icy conditions. Thereby, usually the pilot will raise the flaps as soon as possible for airplanes without spoilers to prevent this from occurring under wet or icy conditions, and done with caution. Notwithstanding the possible effect of a full flap landing, achieving the target airspeed over the threshold, and flying the slowest approach speed for the wind conditions, selecting the maximum flap setting approved by the airplane manufacturer will provide the shortest operational landing distance.
i. Threshold Crossing Height (TCH) and Threshold Crossing Airspeed. As previously discussed, the correct threshold crossing airspeed is specifically defined. Therefore it is essential to understand the impact of deviations in threshold crossing airspeeds.
j. Critical Condition Combinations. The most critical conditions of landing performance are combinations of:
• High gross weight,
• High DA,
• Wet/contaminated runway,
• Tailwind landing,
• Downhill slope,
• Less than maximum landing flap, and
• Short runway.
NOTE: In all cases, it is necessary to make an accurate prediction of minimum landing distance to compare with the available runway.
5. LANDING AND BRAKING TECHNIQUE. The flare, touchdown, and the braking technique are also critical factors in completing a successful approach and landing maneuver.
Landing and braking techniques are discussed below from a point at the beginning of the approach flare through a point at which the airplane decelerates to normal taxi speed or has been brought to a full stop.
a. The Flare. A proper flare reduces the aircraft’s rate of descent to achieve the desired firm landing. If the flare is extended while additional speed is bled off, additional runway will be used. An extended flare may also result in an increase in pitch attitude, which may lead to a tail strike. A firm landing does not mean a hard landing, but rather a deliberate or positive touchdown at the desired touchdown point. A landing executed solely for passenger comfort considerations, which extends the touchdown point beyond the touchdown zone (TDZ), is not impressive, desirable, or consistent with safety or regulations. It is essential to fly the airplane onto the runway at the target touchdown point. See Figure 1-5, Effects of an Extended Flare, as an example of the results of an extended flare.
Appendix 1 FIGURE 1-5. EFFECTS OF AN EXTENDED FLARE
b. T ouchdown. A proper approach and flare positions the airplane to touch down at the target touchdown point. Once the main wheels have contacted the runway, the pilot must maintain directional control and initiate the stopping process. The runway distance available to stop is longest if the touchdown was on target. Once the airplane is on the ground, ground spoilers, wheel brakes, and reversers (depending on airplane equipage) are much more effective in slowing the airplane than the aerodynamic drag produced in the flare maneuver.
c. Non-Turbojet Airplanes Braking After Touchdown. Putting maximum weight on the wheels after touchdown is an important factor in obtaining optimum braking performance.
During the early part of rollout, some lift continues to be generated by the wing. After touchdown, the nosewheel is lowered to the runway to maintain directional control. During deceleration, the nose may pitch down due to braking, transferring the weight from the main wheels to the nosewheel. This does not aid in braking action, so back pressure is applied to the controls without lifting the nosewheel off the runway. This enables directional control while keeping weight on the main wheels. Careful application of the brakes is initiated after the nosewheel is on the ground and directional control is established. Maximum brake effectiveness Page 13
Appendix 1 is just short of the point where skidding occurs. If the brakes are applied so hard that skidding takes place, braking becomes ineffective. (Refer to the FAA Airplane Flying Handbook.)
d. Braking. A distinction should be made between the procedures for minimum landing distance and an ordinary landing roll with considerable excess runway available. Minimum landing distance will be obtained by creating a continuous peak deceleration of the airplane;
that is, extensive use of the brakes for maximum deceleration. On the other hand, an ordinary landing roll with considerable excess runway may allow extensive use of aerodynamic drag to minimize wear and tear on the tires and brakes. If aerodynamic drag is sufficient to cause deceleration, it can be used in deference to the brakes in the early stages of the landing roll.
For instance, the use of aerodynamic drag is applicable only for deceleration to 60 to 70 percent of the touchdown speed. At speeds less than 60 to 70 percent of the touchdown speed, aerodynamic drag is so slight as to be of little use, and braking must be utilized to produce continued deceleration. Since the objective during the landing roll is to decelerate, the powerplant thrust should be the smallest possible positive value (or largest possible negative value in the case of thrust reversers).
NOTE: In addition to proper procedures, many other variables affect the landing performance. Any item that alters the landing speed or deceleration rate during the landing roll will affect the landing distance.
(1) There are three primary forces available for deceleration during the rollout process:
wheel braking, aerodynamic drag, and reverse thrust/propeller reversing, if available. Spoilers are designed to reduce lift and transfer aircraft weight to the landing gear. Deployment of drag devices is most effective at higher speeds and is not affected by runway surface conditions. In all cases, the pilot must ensure the automatic deployment of the deceleration devices occurs. If not, the pilot must immediately manually deploy the devices to minimize the time lost to achieve as close as possible the assessed landing distance.
(2) Timely deployment of spoilers will increase drag by 50 to 60 percent, but more importantly, deployment of the spoilers increases wheel loading by as much as 200 percent in the landing flap configuration. This increases the tire-to-ground friction force, making the maximum tire braking forces available. Many airplanes with autospoilers installed require weight on wheels to deploy the spoilers, which reinforces the requirement for a positive touchdown, as a soft touchdown can delay the automatic deployment. Spoiler deployment immediately after touchdown provides the greatest benefit.
(3) When minimum landing distances are considered, braking friction forces predominate during the landing roll and, for the majority of airplane configurations, braking friction is the main source of deceleration when the runway is dry.
e. Wet and Contaminated Runway Surface. When the runway is wet or slippery, reverse thrust (if the airplane is equipped) may be the dominant deceleration force just after touchdown, and throughout the deceleration if the runway has poor or worse braking conditions. As the airplane slows down, the wheel brakes become the dominant deceleration force. When the runway length is limited, for airplanes equipped with an antiskid system, maximum wheel braking should be applied immediately after touchdown. For airplanes without an antiskid Page 14
Appendix 1 system, slow back pressure should be applied to the yoke such that it will not raise the nose of a nose gear airplane for aerodynamic braking while maximum braking that will not cause skidding is applied. In all situations, braking should be maintained until the airplane slows to a safe taxi speed for the conditions.
f. Autobrakes. If the airplane is equipped with autobrakes, manufacturers recommend the use of the autobrakes for all landings on contaminated runways. Autobrakes are applied earlier in the landing roll, and to the level selected by the pilot(s) or flightcrew for the anticipated landing condition, and runway available, and provide the most efficient and timely use of braking action.
However, the pilot must ensure that if the autobrakes do not engage upon touchdown, then manual braking must be applied.
g. Antiskid. Application of brakes is different for airplanes equipped with a functioning antiskid braking system than for airplanes without such a system.
(1) For airplanes without an antiskid system, brakes should be applied progressively throughout the deceleration process, and the pilot must recognize the point that wheel skid occurs. Maximum braking effectiveness occurs just prior to the point where wheel skidding occurs. However, should a skid occur, releasing brake pressure can stop skidding and then maximum braking can be reestablished until the deceleration process is completed. Pilots should be aware that a skid is most likely at higher speeds and at that point may not be perceptible.
(2) For airplanes with an antiskid system, to achieve the benefits of antiskid, the brakes must be applied firmly throughout the deceleration process. When maximum braking is required, it is accomplished by holding maximum brake application pressure and allowing the antiskid system to operate. Letting up on the brakes (unless required to regain directional control) defeats the purpose of the antiskid system. The pulsation caused by the modulation of the brake pressure by the antiskid system indicates that the antiskid system is operating normally, although the pulsation may be disconcerting to the pilot.
h. Touchdown Technique. A firm touchdown at the target touchdown point, followed by, for those airplanes so equipped, the deployment of ground spoilers, the timely selection of thrust reverse (if installed), and the application of max/progressively applied firm braking will result in the shortest landing distance ground roll, particularly on wet or contaminated runway surfaces.
i. Directional Control. The nosewheel should be lowered onto the runway immediately after touchdown. Placing the nosewheel on the runway will assist in maintaining directional control. It also decreases the wing angle of attack, thereby decreasing lift and placing more load onto the tires, which increases tire-to-ground friction.
j. Thrust Reversers. In the event of an asymmetric deployment, the nosewheel on the ground will aid in directional control. If the thrust reversers deploy asymmetrically, or if the airplane begins to drift due to a crosswind, close the thrust reversers and reestablish directional control. Once the airplane’s track down the runway is reestablished, redeploy the thrust reversers. Use airplane steering in accordance with the AFM procedures.
Appendix 1
6. SUMMARY. A stabilized approach terminating with a landing in the TDZ, timely deployment of airplane deceleration devices, and braking technique are critical elements to mitigating the landing runway overrun risk. It is a responsibility of operators to consider the factors presented, and incorporate these items, as well as the extensive research and safety information available regarding avoiding a runway overrun into training programs and operations manuals. It is the pilot’s responsibility to apply the landing assessment process, exercise conservative aeronautical decision-making (ADM), be proficient in the landing techniques for the conditions to be encountered, and that a go-around or diversion are mitigations to prevent a runway overrun.
Appendix 2 APPENDIX 2. REGULATORY CONSIDERATIONS AND RECOMMENDED
OPERATIONAL PRACTICES
1. REGULATORY REQUIREMENTS FOR LANDING FOCUS ON PREDEPARTURE FLIGHT PLANNING. The intent is to ensure that a flight operation does not begin that cannot reasonably be safely concluded upon reaching the destination or alternate airport, as applicable.
Regarding regulations, there are differences in the various operating regulations concerning landing performance. Compliance with these regulations is shown using approved data from the Airplane Flight Manual (AFM).
NOTE: Some manufacturers supply advisory or supplemental data that provides landing performance data based on landing weight, airport elevation and temperature, corrected for wind (headwind, tailwind), and slope. Manufacturers may also supply advisory or supplemental data that accounts for runway condition (wet, dry, and contaminated) and braking degradation (i.e., antiskid is inoperative, thrust reversers are inoperative, etc.). Pilots and operators should be familiar with the data presented in their AFM, and they should be capable of applying both the approved data and advisory or supplemental data to make operational decisions.
2. REGULATORY CONSIDERATIONS. It is not the intent of this advisory circular (AC) to provide the regulatory requirements. However, operators and pilots must be knowledgeable of the operational regulations applicable to their type of operation. Figure 2-1, Sections 121.195(b),
135.385(b), and 91.1037(b) 60-Percent Rule; Figure 2-2, Sections 121.195(d), 135.385(d), and 91.1037 Wet Runway 115-Percent Rule; and Table 2-1, Regulatory Planned Landing Distance Calculations Overview, provide a regulatory overview of airplane landing performance requirements applicable to flight operations conducted in accordance with Title 14 of the Code of Federal Regulations (14 CFR) parts 91K, 121, and 135.
FIGURE 2-1. SECTIONS 121.195(B), 135.385(B), AND 91.1037(B) 60-PERCENT RULE
Appendix 2 NOTE: Part 91, § 91.1037 states, in part: “(e) Unless, based on a showing of actual operating landing techniques on wet runways, a shorter landing distance (but never less than that required by paragraph (b) or (c) of this section) has been approved for a specific type and model airplane and included in the Airplane Flight Manual, no person may take off a turbojet airplane when the appropriate weather reports or forecasts, or any combination of them, indicate that the runways at the destination or alternate airport may be wet or slippery at the estimated time of arrival unless the effective runway length at the destination airport is at least 115 percent of the runway length required under paragraph (b) or (c) of this section.”
FIGURE 2-2. SECTIONS 121.195(D), 135.385(D), AND 91.1037 WET RUNWAY 115-PERCENT RULE
NOTE: Calculations depicted in Table 2-1 are preflight planning calculations. When the flight arrives at the destination, 100 percent of the effective runway length is available for landing. It should be noted that operators with operations specifications (OpSpecs) and management specifications (MSpecs) must complete these calculations in accordance with those directives.
Appendix 2 TABLE 2-1. REGULATORY PLANNED LANDING DISTANCE
CALCULATIONS OVERVIEW Regulation Type of Airplane Runway Condition Percentage of Effective Runway Length § 121.195(b) § 135.385(b) § 91.1037(a) Large turbine powered Dry 60 percent § 121.195(c) § 135.385(c) Turbopropeller alternate airport Dry 70 percent § 121.195(d) § 135.385(d) § 91.1037(e) Turbojet without an approved AFM wet runway landing technique Wet/Slippery 115 percent of § 121.195(b) or § 135.385(b), or § 91.1037(e) AFM factored dry landing distance § 135.385(f) § 91.1037(c)(2) Large turbine powered, eligible on demand OpSpecs/MSpecs Authorized 80 percent § 121.197 § 135.387(a) Large turbine powered turbojet alternate airport Dry 60 percent § 121.197 § 135.387(a) Large turbine powered turbopropeller alternate airport Dry 70 percent § 135.387(b) § 91.1037(d) Large turbine powered, eligible on demand alternate airport OpSpecs/MSpecs Authorized 80 percent NOTE: For 14 CFR part 125 operators, it is highly recommended that they adopt the stated regulatory requirements contained in Table 2-1 above.
a. Part 91 Operational Recommendations.
(1) Preflight planning requirements for part 91 operators are governed by §§ 91.103 and 91.605. It is highly recommended that part 91 operators and pilots calculate predeparture landing distance performance requirements based on the guidance contained in their AFM, and employ the landing assessment process before initiating the approach landing phase of the flight, and consider the factors presented in this AC to avoid a runway overrun.
(2) To ensure that an acceptable landing distance safety margin exists at time of arrival (TOA), the FAA recommends a 15-percent safety margin be applied to the actual airplane landing distance. The 15-percent safety margin is a minimum safety margin to be applied after accounting for all known variables, such as the meteorological conditions, runway surface conditions, landing with a tailwind, airplane configuration and weight, runway slope, threshold crossing height and airspeed, and the timely utilization of ground deceleration devices.
Be prepared, know the landing conditions, divert to an alternate, or go-around, but do not risk a runway overrun.
Pages 3 (and 4)
Appendix 3 APPENDIX 3. OPERATIONAL AWARENESS OF WET AND CONTAMINATED
RUNWAYS LANDING DISTANCE DATA
1. MANUFACTURERS AND AIRPLANE FLIGHT MANUAL (AFM) DATA.
In accordance with Federal Aviation Administration (FAA) certification rules, the FAA-approved AFM data are determined only for dry runway conditions. Some manufacturers provide supplemental FAA-approved AFM data for operation on wet grooved runways.
Manufacturers may also provide supplemental advisory landing distance data for conditions beyond those required by regulation; however, they are not used in lieu of the advised 15-percent safety margin. The data contained in these supplements, although not FAA approved, are based on the same flight test data used to generate the FAA-approved dry runway takeoff and landing performance presented in the AFM. Performance is calculated using analytical corrections to dry runway performance utilizing methods appropriate for aircraft certification outside the United States. Again, at least a 15-percent safety margin should be applied to these data.
2. METHODS USED TO ADJUST DRY RUNWAY PERFORMANCE. The analytical methods employed to adjust the dry runway performance to account for conditions encountered on wet and contaminated runways are complex and comprehensive. The methods utilized include adjustments for changes in the braking coefficient, precipitation drag, and hydroplaning, as applicable.
3. BRAKING COEFFICIENT. The braking coefficient is a measure of the braking efficiency at the contact point between the airplane main tires and the runway. The braking coefficient is the ratio of braking force divided by the weight on the tire. A tire supporting 5,000 pounds with a braking coefficient of 0.5 would be capable of producing 2,500 pounds of stopping force.
As contaminants are introduced onto the runway, the braking coefficient is reduced, decreasing the stopping capability of the airplane.
NOTE: For example, a wet runway will typically reduce the braking coefficient by about 50 percent. Runways contaminated with dry snow typically can reduce the braking coefficient down to about 0.10, or only 20 to 25 percent of the braking capability of a dry runway. In other words, the 2,500-pound stopping force on a dry runway may now be only 500 to 600 pounds on dry snow.
4. PRECIPITATION DRAG. Precipitation drag comprises two components: precipitation impingement drag and precipitation displacement drag.
a. Precipitation Impingement Drag. Precipitation impingement drag is the force created as contaminant spray from the tires impacts parts of the airframe, such as the flaps behind the main gear. Impingement drag assists in slowing the airplane and resists acceleration.
b. Precipitation Displacement Drag. Precipitation displacement drag is a resistance force created as the tires push runway contaminant out of the way while moving along the runway. It is the force you feel when trying to move your hand rapidly through water. As your hand moves the water is forced out of the way, and the faster you try to move your hand, the stronger the resistance. Runway contaminants act much the same way. The resultant resistance force assists Page 1
Appendix 3 in slowing the airplane during landing or a rejected takeoff, and also resists acceleration during takeoff.
NOTE: Some manufacturers do not include the effects of precipitation drag on landing distances. However, if the effect of precipitation drag is included in the landing distance performance for the airplane, it has the non-intuitive characteristic. This means that (given a conservative approach to assessing the airplane’s landing distance due to precipitation drag) if a deeper contaminant level is selected with the intention of being conservative, the result will be the opposite—it will be non-conservative. If less contaminant is actually there, it will result in a longer distance than calculated.
5. DETERMINE THE RUNWAY SURFACE CONDITION. Appropriate determination of the runway surface condition is basic to the mitigation of a runway overrun. Once the surface condition is determined, it is a pilot’s responsibility to decide whether the standard AFM dry runway performance charts are appropriate or whether the wet and contaminated performance data needs to be consulted. In preparing for takeoff, this decision is considerably easier since the pilot can visually survey the runway. The standard dry runway charts are for a totally dry runway.
NOTE: For example, if the ramp is too damp to sit on due to morning dew, the runway is probably the same. The pilot should use the wet or contaminated performance data.
6. DATA REVIEW. When reviewing takeoff and landing data, the pilot should pay attention to the details, and select the correct chart. In many cases this is obvious, but there may be conditions, or combinations of conditions, that can make the selection problematic. Careful review of Aviation Routine Weather Reports (METAR) or other airport reports may provide sufficient information to make the proper selection. At airports where Automatic Terminal Information Service (ATIS) is available, surface contamination reports may be included that can be used as a guide. At other airports, no information may be available. Regardless of the source, it is prudent to utilize the information that yields the most conservative performance. If reports indicate mixed runway conditions such as snow and slush, use the chart for the contaminant that results in the longest distance. If no reliable information is available, use the chart for the reasonably likely condition that provides the longest distance.
NOTE: If you choose to land with a tailwind, a tailwind is a significant factor that increases an airplane’s landing distance. Selection of a runway condition that renders a shorter distance should only be based on factual information. The pilot should resist the temptation to use a less severe runway contamination condition simply because it produces a landing distance that is equal to or less than the available runway length.
Appendix 3 NOTE: Runway overrun mitigation implies, if the landing assessment reveals that the runway length available is not long enough to safely land and stop the airplane, change to a longer runway, or even divert to a different airport.
7. RUNWAY SURFACE CONDITIONS BEYOND DATA COVERAGE. There are runway surface conditions, such as wet ice, that are worse than the wet and contaminated data provided in the airplane’s landing performance AFM. Even the most effective braking systems are ineffective in the presence of a near-zero braking coefficient.
NOTE: For example, with a zero or near-zero braking coefficient, the rolling friction coefficient of all the tires is near zero. Nosewheel steering will be ineffective, and the pilot will not be able to correct for any side force on the airplane, such as a crosswind. Given this reported runway condition, the flight should divert to an airport where the better landing conditions are reported.
8. OPERATIONS AT NON-TOWERED AIRPORTS. Operations at non-towered airports require pilots to gather available information on expected runway conditions prior to initiating an approach.
NOTE: For example, if there is no clear report of runway condition(s), but the pilot knows rain has been in the area, that pilot should assume the runway is wet. Also, if there is rain actively falling on the runway, standing water should be assumed. If there is any doubt as to the amount of water on the runway, assume the most conservative condition that requires the longest landing distance.
9. DECELERATION. Once committed to an operation on a wet or contaminated runway, the pilot should expect a lower level of deceleration than experienced on a dry runway.
NOTE: For example, consider the landing distances for wet and contaminated surfaces, assuming zero wind and a zero runway gradient.
Then add the distance effect of a tailwind, downhill runway gradient, additional speed, and delayed use of deceleration devices. Under these conditions, a wet runway increases the landing distance over a dry runway by approximately 26 percent. If standing water is present, the landing distance increases approximately 52 percent.
NOTE: The contaminant only affects the ground roll and braking. It has no impact on the air distance from 50 ft to touchdown, which is included in the landing distance. In the presence of standing water, for the total landing distance to increase 52 percent, the ground braking distance increases 100 percent. This situation will surprise a pilot the first time it is encountered because the deceleration rate on this surface will be only one-half of what the pilot has experienced on a dry runway.
Appendix 3
10. CONTAMINANT PROCEDURES. For any contaminant, the pilot should expect a relatively low deceleration rate in the initial phase of braking. A wet runway may be less severe than other contaminants, but the pilot must remember that any increase in total landing distance will occur entirely in the landing ground roll braking segment.
11. HYDROPLANING. Hydroplaning is defined as a condition that exists when the tires ride on the surface of the contaminant, much like a water ski. When the tires are hydroplaning, directional control and braking action are virtually impossible. An effective antiskid system can minimize the effects of hydroplaning. See Figure 3-1 below.
FIGURE 3-1. HYDROPLANING
a.
Hydroplaning Causal Factors. Hydroplaning is a condition that can exist when an airplane has landed on a runway surface contaminated with standing water, slush, and/or wet snow. Hydroplaning can have serious adverse effects on ground controllability and braking efficiency. The three basic types of hydroplaning are dynamic, reverted rubber, and viscous.
Any one of the three can render an airplane partially or totally uncontrollable anytime during the landing roll. Hydroplaning is a significant factor that should be considered in determining stopping or steering capability on a contaminated or otherwise flooded runway. Hydroplaning depends on wheel speed and tire pressure. The key item to remember when the airplane is hydroplaning is that the tires are no longer in contact with the runway, although the airplane is not airborne. Though at high speeds, aerodynamic drag can provide a significant decelerating force. Reverse thrust/propeller reversing, if installed, provides nearly all the decelerating force when hydroplaning since the brakes are essentially ineffective. These characteristics are included in the distances presented in the contaminated runway supplemental data when provided by the airplane manufacturer.
(1) Dynamic hydroplaning is related to tire inflation pressure. Data obtained during hydroplaning tests have shown the minimum dynamic hydroplaning speed (VP) of a tire to be
8.6 times the square root of the tire pressure in pounds per square inch (psi). For an airplane with a main tire pressure of 24 pounds, the calculated hydroplaning speed would be approximately 42 knots (kts). It is important to note that the calculated speed referred to above is for the start of dynamic hydroplaning. Once hydroplaning has started, it may persist to a significantly slower speed depending on the type being experienced. (Refer to FAA-H-8083-3, Airplane Flying Handbook.)