2-1 CHAPTER 2. PREDEPARTURE FLIGHT PLANNING
2.1 Background.
2.1.1 D ispatch Requirements . Large transport category airplanes are subject to specific regulations with the intent of ensuring that a flight does not begin that cannot be safely concluded. Title 14 CFR parts 91 subpart K (part 91K), 121, and 135 operators must not plan to arrive at either the destination or the alternate airport at a weight that would require them to use more runway than is available (and 14 CFR part 125, § 125.49(a) requires part 125 operators to only use an airport if it is adequate for the proposed operation). These predeparture planning procedures are also recommended, although not required, for part 91 operators.
Note: D uring a discussion of predeparture flight planning requirements, it is important to keep in mind that the regulations specify flight-planning requirements, not landing requirements. In other words, upon arrival, all (100 percent) of the runway available may be used for landing.
2.1.2 T itle 14 CFR Compliance . It is not the intent of this AC to provide all the predeparture landing distance regulatory requirements. However, pilots must be knowledgeable of the regulations applicable to their type of operation, so they can both comply with such regulations and fully understand regulatory shortcomings. Parts 91K, 121, and 135 require operators of large transport category airplanes to comply with certain landing distance requirements at the time of takeoff. These requirements are sometimes referred to as “dispatch requirements.” The requirements limit the allowable takeoff weight to that which would allow the airplane to land within a specified percentage of the Landing Distance Available (LDA) on: (1) the most favorable runway at the destination airport under still air conditions; and (2) the most suitable runway in the expected wind conditions. Part 91, § 91.1037(e); p art 121, § 121.195(e); and part 135, § 135.385(d) further require an additional 15 percent be added to the landing distance required when the runway is wet or slippery, unless a shorter distance can be shown using operational landing techniques on wet runways.
2.1.2.1 S ections 91.1037(b), 121.195(b), and 135.385(b). Regulations require that during preflight planning, the flightcrew may not depart on the flight unless it has been determined that the aircraft will be able to perform a full stop landing within 60 percent of the effective length of the runway. To determine the landing distance required, multiply the AFM approved landing distance by 1.667 or divide the AFM approved landing distance by 0.60. This factor assumes a dry runway condition at TOA.
Example: AFM approved landing distance = 3000 ft (3000’ x 1.667 = 5000’ or 3000’ ÷ 0.60 = 5000’.) In this example, to comply with the 60 percent rule, the runway LDA at the planned destination would need to be at least 5000 ft long.
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2.1.2.2 Sections 91.1037(e), 121.195(e), and 135.385(d). If weather reports or forecasts indicate that the runways at the destination airport may be wet or slippery, unless a shorter distance can be shown using operational landing techniques, that the aircraft may not depart unless the operator can show that the effective runway length at the destination airport is at least 115 percent of the runway length required for a dry runway (as calculated above).
Example: If it was determined that the runway at the planned destination would need to be 5000 ft to comply with the 60 percent rule in dry conditions (as in the first example, above), then in forecast wet conditions, the runway LDA would need to be at least 5750 ft long (5000’ x 1.15 = 5750’).
Note 1: The additional 15 percent increase required by the regulations above is based only on the previously calculated distances used for a dry runway. Caution: The increase is not based on demonstrated wet runway stopping performance; therefore, wet runway factored landing distance may not provide enough runway for the aircraft to stop.
Note 2: There exists a common misconception that grooved runways or runways covered in PFC are considered dry for predeparture-planning calculations. Pilots and dispatchers must remember that the 15 percent increase is still required if the runway will be wet on landing, even if it is grooved or PFC.
2.1.2.3 Sections 91.1037(c), (d), (e), and 135.385(f). Parts 91 and 135 operators that are authorized via operations specifications (OpSpecs) or management specifications (MSpecs) to use a Destination Airport Analysis program (such as a part 135 eligible-on-demand operator) may depart on a flight if preflight planning indicates that the aircraft will be able to perform a full stop landing within 80 percent of the effective runway length available. To determine the landing distance required, multiply the AFM approved landing distance by
1.25 or divide the AFM approved landing distance by 0.80. This factor assumes a dry runway condition at TOA.
Example: AFM approved landing distance = 3000 ft.
(3000’ x 1.25 = 3750’ or 3000’ ÷ 0.80 = 3750’.) In this example, to comply with the 80 percent rule, the appropriately authorized operator would need to confirm that the runway LDA at the planned destination is at least 3750 ft long.
Note: The 80 percent factored distance in most cases will not provide a safe landing margin if the runway is either wet or contaminated.
Operators must exercise extreme caution when preflight planning if the destination airport runway is either wet or contaminated.
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2.1.3 Overview. Table 2-1 below provides an overview of predeparture landing distance requirements.
Table 2-1. Regulatory Planned Dispatch Destination Airport Requirements Regulation Type of Airplane Runway Condition Percentage of Effective Runway Length § 121.195(b) § 135.375(a) § 135.385(b) § 91.1037(b) Large turbine/Large reciprocating engine powered Dry 60% § 121.195(c) § 135.385(c) Turboprop, Large turbine engine powered (excluding turbojet) Dry 70% § 121.195(d) § 135.385(d) § 91.1037(d) Turbojet without an approved AFM wet runway landing technique Wet/Slippery 115% of § 121.195(b) or § 135.385(b), or § 91.1037(d) AFM factored dry landing distance § 135.385(f) § 91.1037(c)(2) Large turbine powered, eligible on demand OpSpecs/MSpecs Authorized 80% § 121.197 § 135.387(a) § 91.1037(b) Large turbine engine powered, alternate airport Dry Turbojet airplanes: 60% Turbo-propeller: 70% § 121.197 § 135.387(a) § 135.375(b) Large turboprop/Large reciprocating engine, alternate airport Dry 70% § 135.387(b) § 91.1037(d) Large turbine powered, eligible on demand, alternate airport OpSpecs/MSpecs Authorized 80%
2.1.4 Part 91 Recommendations. Preflight planning requirements for part 91 operators are governed by §§ 91.103 and 91.605. (Section 91.1037 is only applicable to part 91K operators.) Although not required by regulation, we recommend that part 91 operators and pilots calculate predeparture landing distance performance requirements based on the guidance contained in their AFM.
2.2 AFM Landing Distance Data. Operators and pilots should flight plan using the best available information concerning expected destination runway conditions. The regulatory requirements referenced in this chapter are intended to represent a minimum level of compliance. Compliance with preflight planning regulations is accomplished by using approved data from the AFM.
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2.2.1 It is important for pilots and operators to keep in mind that AFM landing performance data is determined during certification flight tests using analysis criteria that are not representative of everyday operational practices. Landing distances determined in compliance with 14 CFR part 25 and published in the AFM do not reflect normal operational landing distances. The goal of determining landing distances during certification tests is to demonstrate the shortest landing distances for a given airplane weight with a test pilot at the controls. Flight test and data analysis techniques employed by some airplane manufacturers for determining landing distances can result in the use of high touchdown sink rates (as high as 480 ft per minute) and approach angles of negative
3.5 degrees to minimize the airborne portion of the landing distance. Maximum manual braking, initiated as soon as possible after landing, is used to minimize the braking portion of the landing distance. Therefore, the landing distances determined under part 25, § 25.125 and published in the AFM may be shorter than and not reflect normal operational landing distances.
2.2.2 With respect to landing performance data for wet or contaminated runways, the landing distances provided in the AFM may not represent performance that is operationally achievable. This is because the wet or contaminated runway data is usually the result of applying an algorithm to the dry, smooth, hard surface runway data, not on demonstrated aircraft ability. In other words, wet and contaminated runway data also may not represent performance that you should expect to achieve in normal operations. The fact that the short distances achieved during flight testing are not usually repeatable during normal operations, along with many other variables affecting landing distance, is taken into consideration in the preflight landing performance calculations by requiring a significant safety margin in excess of the unfactored AFM landing distance that would be required under those conditions.
2.2.3 To summarize, landing distance information provided by some manufacturers in the AFM, determined in compliance with part 25 during aircraft flight testing is aimed at achieving and recording the shortest landing distance possible and is not representative of everyday operational practices. The distances reported have been demonstrated in flight tests with a test pilot at the controls using techniques not typically used by pilots in normal operations. Both NTSB and FAA data identify the failure of pilots and operators to assess actual required landing distances to account for slippery or contaminated conditions or any other changed condition existing at the time of landing as a significant causal factor in landing overruns. Even when predeparture flight planning requirements have been satisfied, we strongly recommend that pilots perform a TOA landing distance assessment prior to landing.
3-1 CHAPTER 3. TIME OF ARRIVAL (TOA) LANDING DISTANCE ASSESSMENT
3.1 Background. A TOA assessment is critical when the landing runway is not the same runway analyzed for dispatch or when runway conditions and/or type and depth of contamination have changed. In addition, current weather, actual airplane weight, and available braking systems to be used should be considered. Once the operational landing distance is determined, an additional safety margin of at least 15 percent should be added to that distance. Except under emergency conditions, flightcrews should not attempt to land on runways that do not meet the TOA assessment criteria and safety margin.
Note: The FAA acknowledges that there are situations when pilots need to know the absolute performance capability of the airplane. These situations include emergencies or abnormal and irregular configurations of the airplane such as engine failure or flight control malfunctions. In these circumstances, the pilot must consider whether it is safer to remain in the air or to land immediately and must know the actual landing performance capability, without an added safety margin, when making these evaluations. This guidance is not intended to curtail such evaluations from being made for these situations.
3.2 General. Per Title 49 of the United States Code (49 U.S.C.) § 44702(b), operators engaged in air transportation have a statutory obligation to operate with the highest possible degree of safety in the public interest. Although current regulations do not require a landing distance assessment be performed at the TOA, operators are required to restrict or suspend operations when conditions are hazardous. Sections 91.3, 91.1011,
121.533, 121.535, 121.537, 125.371, and 135.69 place the responsibility for the safe operation of the flight jointly with the operator, pilot in command (PIC), and dispatcher, as appropriate to the type of operation being conducted.
3.2.1 A s explained in Chapter 2, compliance with regulations applicable at the time of dispatch or takeoff does not guarantee that the airplane can land safely within the distance available on the runway used for landing at the TOA, particularly if the runway, runway surface condition, meteorological conditions, airplane configuration, airplane weight, or the intended use of airplane ground deceleration devices is different than that used in the preflight calculation.
3.2.2 A dditionally, §§ 121.195(e) , 135.375(b), and 135.385(c) and (e) allow use of an alternate airport to meet the requirements if forecast conditions at the destination airport are inadequate. These provisions imply, but do not mandate, that a landing distance assessment is accomplished prior to conducting an approach to determine if it is safe to land at the destination or if it is necessary to divert to an alternate airport.
3.3 C onducting the Assessment. Operators and pilots should use the most adverse reliable braking action report, if available, or the most adverse expected conditions for the runway or portion of the runway that will be used for landing, when making a TOA landing distance assessment. Operators and pilots should consider the following factors in determining the operational landing distance: the age of the report, meteorological conditions present since the report was issued, type of airplane or device used to obtain
3-2 the report, whether the runway surface was treated since the report, and the methods used for that treatment. Operators should create guidance for flightcrews that enable adequate briefings, articulate pilot flying and pilot monitoring duties, and ensure clear policies and procedures. Pilots who operate their aircraft independently should ensure decision points regarding TOA landing criteria are mapped out prior to the flight.
3.3.1 Wh en to Conduct the Assessment . A TOA landing distance assessment is initially performed when landing weather and field conditions are obtained, usually prior to beginning the descent from cruise altitude (top of descent). The assessment includes a consideration of how much deterioration in field conditions can be tolerated so that a quick decision can be made just prior to landing if new information is obtained that indicates that field conditions are worse than expected (e.g., the preceding aircraft provides a worse-than-expected braking action report).
3.3.2 Source of Data.
3.3.2.1 When available, the operational landing distance data used is advisory data based on the recommendations of AC 25-32. This data may be provided by the manufacturer or developed by a performance data provider.
3.3.2.2 If advisory data for a landing distance assessment at TOA is not available from the manufacturer or from a performance data provider, the Landing Distance Factors (LDF) from Table 3-1, Landing Distance Factors, may be used. To find the landing distance required, multiply the AFM dry, unfactored landing distance by the applicable LDF in Table 3-1 for the runway conditions existing at the TOA. If the AFM landing distances are presented as factored landing distances, then that data must be adjusted to remove the applicable preflight factors that were applied. The LDFs given in Table 3-1 include a 15 percent safety margin; an air distance representative of normal operational practices; a reasonable accounting for temperature; the effect of increased approach speed, reduced wheel braking, and thrust usage (reverse or not); and the additional effect of reduced wheel braking capability on altitude and wind distance adjustment.
3.3.2.3 C urrently, there are no plans to provide aircraft manufacturers of normal, utility, aerobatic, and commuter category airplanes (14 CFR part 23 airplanes) with advisory information similar to AC 25-32 (which is applicable to transport category airplanes). In the absence of guidance to manufacturers of part 23 aircraft, operational landing distance data may be based on the recommendations of AC 25-32. In the absence of guidance to part 23 aircraft manufacturers, the manufacturer or data provider should consider the recommendations in AC 25-32 when creating data for a TOA assessment.
3.3.3 Using the LDF Table. As stated above, when manufacturer-p roduced or third-party provider data is not available, the LDFs from Table 3-1 may be used for the TOA landing distance assessment.
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1. To find the landing distance required, multiply the unfactored AFM landing distance by the applicable LDF in Table 3-1 for the runway conditions existing at the TOA.
2. If the AFM landing distances are presented as factored landing distances, first adjust the data to remove the preflight factors that were applied prior to multiplying the distance by the LDF. (See appropriate examples in Chapter 2.)
3. The LDFs given in Table 3-1 include a 15 percent safety margin.
Table 3-1. Landing Distance Factors The following factors are multipliers to the unfactored AFM demonstrated landing distances:
Runway Condition Code 6 5 Grooved/ PFC 5 Smooth 4 3 2 1 Braking Action Dry Good Good Good to Medium Medium Medium to Poor Poor Turbojet, No Reverse 1.67 2.3 2.6 2.8 3.2 4.0 5.1 Turbojet, With Reverse 1.67 1.92 2.2 2.3 2.5 2.9 3.4 Turboprop (see Note) 1.67 1.92 2.0 2.2 2.4 2.7 2.9 Reciprocating 1.67 2.3 2.6 2.8 3.2 4.0 5.1 Note: These LDFs apply only to turboprops when the AFM provides for a landing distance credit for the use of ground idle power lever position. Turboprops without this credit should use the “Turbojet, No Reverse” LDFs.
3.4 Runway Condition Considerations. Airport assessments should be based on information that most accurately represents the conditions anticipated. When available for the portion of the runway that will be used for landing, the following are considered:
• RwyCCs,
• Expected runway condition (contaminant type and depth), and
• Braking action reports.
3.4.1 Timeliness. It is important to note the time of the latest RwyCC and any associated reliable braking action reports. Several overruns have occurred when pilots were provided with a runway condition that was no longer reliable given changes in meteorological conditions.
3-4 ATTENTION: Pilots are strongly advised to review the weather conditions and compare that to the time of the latest braking action report.
3.4.2 S ituational Awareness. Throughout the descent and approach, pilots should remain vigilant for any deterioration in conditions. You should use all available resources to determine what condition you may expect upon landing to include ATC reports, field condition reports, flight visibility, and onboard weather radar.
3.5 R isks Associated With Moderate or Hea vy Rain. Several runway excursions and overruns have raised concerns with wet runway stopping performance assumptions.
Analysis of the stopping data from these incidents and accidents indicates the braking coefficient in each case was significantly lower than expected for a wet runway than would be predicted by § 25.109 and AC 25-7. These incidents and accidents occurred on both grooved and ungrooved runways. The data indicates that applying a 15 percent safety margin to wet runway advisory data, as recommended (see Figure 5-1, Effects of Compound Factors), may be inadequate in certain wet runway conditions.
3.5.1 Rainfa ll Amount. In a rainfall event, such as an active thunderstorm, the amount of water or moisture on a runway can change rapidly. The dynamic nature of rainfall can present challenges for the timely and accurate reporting of water or moisture on a runway.
3.5.2 Risk Mitigation. It is recommended that if it is anticipated that more than 1/8 inch of water will be on the runway, the LDF associated with medium to poor braking, or a Rwy CC of 2, be used during the TOA assessment. In addition, when planning to land on a smooth runway under conditions of moderate or heavy rain, or when landing on a grooved or PFC runway under heavy rain, pilots should consider that the surface may be contaminated with water at depth greater than 1/8 inch and adjust their landing distance assessment accordingly.
Note: A Special Weather Observation (SPECI) will only be generated if a thunderstorm begins. A SPECI is not generated when rainfall rates simply change.
In addition, ATC radar is not optimized for the detection of rainfall and during an active or fast-moving weather event, PIREPs can rapidly become obsolete.
3.6 Risk Associated With Standing Water. When standing water is present, aircraft braking can rapidly degrade as the contact area of the tire is lifted away from the pavement by the effects of hydroplaning. When this occurs, pilots have reported that the aircraft feels like it accelerates. This change in sensation is because braking performance can degrade in a matter of seconds, resulting in up to a 77-percent decrease in braking force. This rapid decrease in friction also affects cornering ability, making lateral drift more difficult to control, if not impossible. Unexpected handling characteristics due to standing water have been shown to lead to pilot confusion and a possible delay in the use of thrust reversers or ground spoilers.
3.6.1 RwyCC Limitations. Note on the RCAM (see Table 3-2, Pilot/Operator Runway Condition Assessment Matrix (RCAM)) that only 1/8 inch of water separates a wet r unway (with a RwyCC of 5 and “good” braking action) from a runway contaminated
3-5 with standing water (and a RwyCC of 2 and “medium to poor” braking action). This dramatic difference is due to the possibility of dynamic hydroplaning that may occur any time water depth exceeds 1/8 inch.
3.6.2 O perations at Foreign (Non-U.S.) Airports.
3.6.2.1 Pilots should be aware that the design and maintenance standards referenced in this AC might not be met in other countries. Standards for design, construction, and maintenance of runways are based on ICAO Annex 14, Aerodromes, standards; however, they may lack oversight in implementation of those standards.
3.6.2.2 O utside of the United States, there is often less usage of grooving or PFC overlay which, when present, will normally aid in drainage, mitigate the risk of hydroplaning during active precipitation, and improve braking action.
Unless the pilot is knowledgeable of an international airport’s runway maintenance program, and sure that the runway has improved drainage capabilities with grooved or PFC surfaces, they should consider basing their TOA assessment on the recommendations in paragraph 3.6.1.
3.7 Aircraft Performance Considerations. In addition to runway surface conditions, the following considerations may also impact operational landing distance calculations:
• Runway slope,
• Airport pressure altitude,
• Wind,
• Temperature,
• Airplane weight and configuration,
• Approach speed at threshold,
• Adjustment to landing distance (such as autoland), and
• Planned use of airplane ground deceleration devices.
Note: Risk of overrun linked to a runway being more slippery than expected is amplified in tailwind conditions (increased ground speed at touchdown and potential floating aircraft). This potential situation needs particular attention.
3.8 Autobrake and Manual Brake Usage. When autobrakes are a part of the aircraft’s landing configuration, the landing distance assessment is not intended to force a higher than necessary autobrake selection. Autobrakes normally target a deceleration rate and may not require the employment of the full braking capabilities of the aircraft.
Conversely, landing calculations using manual brakes assume that the full capability of the aircraft’s wheel brake system is employed during the rollout phase.
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3.8.1 Safety Margin. To accommodate for real-world operational variations, the manual wheel braking landing distance used for a TOA assessment should include a safety margin of at least 15 percent. For operations when the runway is dry, or when the runway is wet, grooved, or PFC, if the manual braking distance provides a 15 percent safety margin, then the braking technique may include a combination of autobrakes and manual brakes even if the selected autobrake landing data does not provide a 15 percent safety margin.
The operator will need to ensure that proper procedures and crew training are in place when the selected autobrake landing data does not provide a 15 percent safety margin.
3.8.2 C ontaminated Runways. For contaminated runways, the minimum 15 percent safety margin should always be based on either autobraking or manual braking to be used
(i.e., should correspond to the selected level of autobrake, if used).
3.9 Touchdown Point. The touchdown point used in the landing distance assessment reflects the assumed air distance. Operational landing data usually includes an allowance for 1,500 ft or 7 seconds of air distance from the threshold to touchdown. An air distance as short as 1,000 ft may be used provided an operator’s landing assessment procedures include enhancements to minimize the risk of overruns or undershoots, including:
1. Training in touchdown control and short field landing techniques.
2. Identification of required touchdown point and training to assure go-around procedures are initiated if unable to achieve a suitable touchdown point.
3. Approach guidance and runway markings on the specific runway are consistent with a shorter air distance.
4. Operational data provided to the crew for the specific runway, conditions, and aircraft-landing configuration without the need for interpolation.
5. The flight techniques assumed in the creation of the performance data used for shorter air distances are appropriately trained and documented. For example, the assumed speed bleed-off used in the performance data needs to be consistent with the trained flight techniques for flaring the aircraft.
3.10 Crosswind Considerations. During operations when standing water may be present, operators should be aware of the effect of crosswind landing on a contaminated runway, given most aircraft only provide demonstrated crosswind and not a maximum crosswind limit. Tire cornering limitations can reduce wheel braking capability. As a result, crosswind limits can become a dominant factor in TOA landing distance assessments.
Pilots should always be vigilant of these limits, especially when braking action is less than good.
3.11 Assessment Based on Preflight (Dispatch) Criteria. When the runway is dry, the assessment may be as simple as confirming that the runway meets the criteria used for dispatch. To use an assessment based on preflight (dispatch) criteria, pilots need to:
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1. Ensure that the runway used for landing is the same runway that was assessed during preflight or for dispatch;
2. Confirm that the wind velocity and other factors that could affect landing distance are the same as those considered during preflight or the dispatch assessment; and
3. When applicable, consider the possibility of a late runway change such as a side step to a parallel runway.
3.12 Risk Management Considerations. Landing on a contaminated runway can involve multiple hazards, including cognitive bias, changing weather conditions, and high task loading. Because these hazards occur at a time with pressing time constraints, they can create an environment where errors are likely to occur. Managing such risks requires mitigation in the form of organizational policies, effective training, and well-designed procedures.
3.12.1 Policy Considerations. The overall strategy for managing risk should involve structured briefings during times of low task loading to address contingencies should new information be received. Once in the terminal area, the pilots should focus on the execution of the briefed plan, not analysis of the risk. While flightcrews always need a decision space for unexpected events, primary decision points concerning acceptable risk should be clearly communicated by the operator.
3.12.2 F lightcrew Training Considerations . Ensure that briefings are complete, checklists are run correctly, and that plan continuation bias does not drive actions counter to established rules and SOPs. This should be the primary goals of the pilot monitoring.
3.12.3 P rocedure Considerations . Decisions regarding TOA landing distance assessments should be made using the best available information considering timeliness, reliability, and data precision. SOPs should require that when runway conditions are likely to change, crews brief the worst braking levels acceptable and the indications that would accompany them.
This briefing needs to take place prior to descent. Once the descent begins, pilots actively monitor conditions and when faced with a pre-briefed scenario, respond with the previously briefed set of actions.
3.13 Technical Considerations for Understanding Landing Performance and the RCAM.
Much like how aircrews must have a technical knowledge of their aircraft’s systems and not just checklists and procedures, a basic engineering background on wheel braking and runway friction can serve to greatly enhance the guidance in this AC. As an aid for good decision making, a knowledge of runway friction and training on the RCAM is provided.
3.13.1 Runway Friction. The ability of a runway to act as an agent for an aircraft to transfer its weight into wheel braking force during landing is a function of the runway surface friction characteristics and the wheel braking system of the aircraft.
3.13.1.1 Runway Friction Properties Runway pavement is a function of the large-scale and small-scale properties of the pavement, known as macro and micro texture. Airports construct and aircraft are certified for runways whose
3-8 pavement properties can be shown to support a minimum amount of friction when measured with a testing device. These tests are typically done in dry conditions with devices that have self-wetting capabilities.
3.13.1.2 Wet Runways. When there is a possibility that the runway surface is lubricated with water to a degree large enough to affect wheel braking performance, the runway is reported as “wet” and classified in the RCAM with the number 5. Airports are required to accomplish routine friction testing.
Should such a test result in a value below a specified threshold, the runway is listed with a Notice to Air Missions (NOTAM) of “Slippery when Wet.”
Runway friction is also affected by any substance that is found on top of the pavement. When this occurs, the runway is said to be “contaminated.”
3.13.1.3 Water Removal. Runways are normally constructed with a built-in crown so that water will drain towards each side. To aid in this drainage, pavement can be designed with lateral grooves or with a special pavement property known as PFC overlay. Grooved runways increase the rate of water drainage through the polished channels in the grooves while PFC surfaces allow water to drain through the surface. The primary purpose of each is to facilitate the dynamic drainage of water below the aircraft tire as it rolls over the surface. However, these runway improvements do not affect the basic lubricating properties water has on the texture of the pavement itself.
3.13.2 T ypes of Contamination. Any substance that rests on the surface of a pavement can have three distinct properties. These are:
3.13.2.1 So lid Contaminant. The substance represents a non-movable surface that has its own friction properties. These properties can exhibit a wide range of friction, with relatively high friction available when hot sand is bonded to very cold ice to very poor friction. While solid contaminant is mostly encountered as frozen water, it can occur as a result of rubber buildup or in rare occasions, growths such as lichen.
3.13.2.2 Loose Contaminant. A condition whereby a substance compresses directly under a rolling tire but has a depth to a degree that the tire must also displace it as well.
3.13.2.3 Deformable Contaminant. A condition whereby a substance resting above the surface of the pavement acts to deform away from underneath the tire as it rolls. Deformable contaminant can be extremely hazardous as it can lift the tire away from the surface and result in hydroplaning.
3.13.3 T akeoff Performance . During takeoff, aircraft performance may be limited with consideration of loose contaminant imparting additional drag due to the combination of displacement by the aircraft’s tires and impingement of the contaminant spray on the aircraft structures. AC 25-31 details the guidance recommended under these conditions when determining takeoff performance data.