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4.3.6.6 Δtact 3 is the demonstrated time interval between initiation of the second and third pilot actions to stop the airplane.
4.3.6.7 Δtact 4→n is the demonstrated time interval between initiation of the third and fourth (and any subsequent) pilot actions to stop the airplane. For AFM expansion, a one-second reaction time delay to account for inservice variations should be added to the demonstrated time interval between the third and fourth (and any subsequent) pilot actions. If a command is required for another crewmember to initiate an action to stop the airplane, a two-second delay, in lieu of the one-second delay, should be applied for each action. For automatic deceleration devices that are approved for performance credit for AFM data expansion, established systems actuation times determined during certification testing may be used without the application of the additional time delays required by this paragraph.
4.3.6.8 The sequence of pilot actions may be selected by the applicant, but it must match the sequence established for operation in service, as prescribed by § 25.101(f). If, on occasion, the specified sequence is not achieved during testing, the test need not be discarded; however, sufficient testing should be conducted to establish acceptable values of Δtact.
4.3.6.9 Section 25.109(a)(1)(iv) and (a)(2)(iii) require the one-engine-inoperative and all-engines-operating accelerate-stop distances, respectively, to include a distance increment equivalent to two seconds at V1. (Although the requirement for the distance increment equivalent to two seconds at V1 is explicitly stated in the “dry runway” criteria of § 25.109, it is also applied to the “wet runway” accelerate-stop distances by reference in § 25.109(b).) This distance increment is represented pictorially on the right side of the “Flight Manual Expansion Time Delays” presentation in figure 4-1 above, and in the speed versus distance plot of figure 4-2 below, on the following page. The two-second time period is only provided as a method to calculate the required distance increment and is not considered to be a part of the accelerate-stop braking transition sequence.
Consequently, no credit for pilot actions, or engine and systems transient responses (e.g., engine spin-down) may be taken during this two-second time period. Similarly, the two-second time period may not be reduced for airplanes incorporating automated systems that decrease the number of pilot actions required to obtain the full braking configuration (e.g., automatic spoiler systems).
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AC 25-7D CHG 1 Figure 4-2. Accelerate-Stop Speed versus Distance
DISTANCE 3rd Action 2nd Action V1 (1st Action) VEF AFM model must accurately represent demonstrated energy state of the airplane after V1.
} Distance for 2 secs. at V1 SPEED
4.3.6.10 Section 25.109(a)(1)(ii) requires that any residual acceleration causing the airplane to exceed V1, while the airplane and its systems become stabilized in the braking configuration, be included in the accelerate-stop distance.
The effects of system transients, such as engine spin-down, brake pressure ramp-up, spoiler actuation times, etc., should be accounted for in this time period. The area of interest is noted at the top of the graphical representation of the speed versus distance relationship in figure 4-2 above.
4.3.6.11 All-Engine Accelerate-Stop Distance.
For the all-engines-operating accelerate-stop distance prescribed by § 25.109(a)(2), apply the demonstrated time intervals and associated delays, of paragraphs 4.3.6.5 through 4.3.6.7 of this AC, after the airplane has accelerated to V1.
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4.3.6.12 Describe the procedures used to determine the accelerate-stop distance in the performance section of the AFM.
4.3.7 Procedures: Wet Runway Accelerate-Stop Distance.
4.3.7.1 The following guidance is provided for showing compliance with the requirements stated in § 25.109(b) through (d) for determining accelerate-stop distances applicable to wet runways. In general, the wet runway accelerate-stop distance is determined in a similar manner to the dry runway accelerate-stop distance. The only differences are in reflecting the reduced stopping force available from the wheel brakes on the wet surface and in provisions for performance credit for the use of reverse thrust as an additional decelerating means. The general method for determining the reduced stopping capability of the wheel brakes on a smooth wet runway is as follows: First, determine the maximum tire-toground wet runway braking coefficient of friction versus ground speed from the relationships provided in § 25.109(c)(1). Then, adjust this braking coefficient to take into account the efficiency of the anti-skid system. (See paragraph 4.3.7.3 of this AC for a definition of anti-skid efficiency. See paragraphs 4.3.7.5 and 4.3.7.6 for material on how to determine the wet runway anti-skid efficiency.) Next, determine the resulting braking force and adjust this force for the effect of the distribution of the normal load between braked and unbraked wheels at the most adverse CG position approved for takeoff, as prescribed by § 25.109(b)(2)(ii). (See paragraph 4.3.7.8 for a discussion of normal load distribution.) In accordance with § 25.109(b)(2)(i), apply further adjustments, if necessary, to ensure that the resulting stopping force attributed to the wheel brakes on a wet runway never exceeds (i.e., during the entire stop) the wheel brakes stopping force used to determine the dry runway accelerate-stop distance (under § 25.109(a)). Neither the dry runway brake torque limit nor the dry runway friction (i.e., anti-skid) limit should be exceeded. Alternative methods of determining the wet runway 4-32
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wheel brakes stopping force may be acceptable as long as that force does not exceed the force determined using the method just described.
4.3.7.2 Maximum Tire-to-Ground Braking Coefficient of Friction.
The values specified in § 25.109(c)(1) were derived from data contained in Engineering Sciences Data Unit (ESDU) 71026, Frictional and Retarding Forces on Aircraft Types, Part II: Estimation of Braking Force, dated August 1981). The data in ESDU 71026 is a compilation from many different sources, including the National Aeronautics and Space Administration, the British Ministry of Aviation, and others. ESDU 71026 contains curves of wet runway braking coefficients versus speed for smooth and treaded tires at varying inflation pressures. These data are presented for runways of various surface roughness, including grooved and porous friction course runways. Included in the data presentation are bands about each of the curves, which represent variations in water depths from damp to flooded, runway surface texture within the defined texture levels, tire characteristics, and experimental methods. In defining the standard curves of wet runway braking coefficient versus speed that are prescribed by the equations in § 25.109(c)(1), the effects of the following variables were considered: tire pressure, tire tread depth, runway surface texture, and the depth of the water on the runway.
4.3.7.2.1 Tire Pressure.
Lower tire pressures tend to improve the airplane’s stopping capability on a wet runway. The effect of tire pressure is taken into account by providing separate curves (equations) in § 25.109(c)(1) for several tire pressures. As stated in the rule, the tire pressure used to determine the maximum tire-to-ground braking coefficient of friction must be the maximum tire pressure approved for operation. Linear interpolation may be used for tire pressures other than those listed.
4.3.7.2.2 Tire Tread Depth.
The degree to which water can be channeled out from under the tires significantly affects wet runway stopping capability. The standard curves of braking coefficient versus speed prescribed in § 25.109(c)(1) are based on a tire tread depth of 2 mm. This tread depth is consistent with tire removal and retread practices reported by airplane and tire manufacturers and tire retreaders. It is also consistent with FAA guidance provided in AC 121.195(d)-1A, regarding the tread depth for tires used in flight tests to determine operational landing distances on wet runways. Although operation with zero tread depth is not prohibited, it is unlikely that all of the tires on an airplane would be worn to the same extent.
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4.3.7.2.3 Runway Surface Texture.
ESDU 71026 groups runways into five categories. These categories are labeled “A” through “E,” with “A” being the smoothest and “C” the most heavily textured ungrooved runways. Categories “D” and “E” represent grooved and other open textured surfaces. Category A represents a very smooth texture (an average texture depth of less than 0.004 inches) and is not very prevalent in runways used by transport category airplanes. The majority of ungrooved runways fall into the category C grouping. The curves represented in § 25.109(c)(1) represent a texture midway between categories B and C.
4.3.7.2.4 Depth of Water on the Runway.
Obviously, the greater the water depth, the greater the degradation in braking capability. The curves prescribed in § 25.109(c)(1) represent a well-soaked runway, but with no significant areas of standing water.
4.3.7.3 Anti-Skid System Efficiency.
Section 25.109(c)(2) requires adjusting the maximum tire-to-ground braking coefficient determined in § 25.109(c)(1) to take into account the efficiency of the anti-skid system. The anti-skid system efficiency is defined as the relative capability of the anti-skid system to obtain the maximum friction available between the tire and the runway surface. It is expressed as either a percentage or a factor based on that percentage
(e.g., 85 percent or 0.85). Applicants can either use one of the anti-skid efficiency values specified in § 25.109(c)(2) or derive the efficiency from flight tests on a wet runway. Regardless of which method is used, § 25.109(c)(2) requires that an appropriate level of flight testing be performed to verify that the anti-skid system operates in a manner consistent with the efficiency value used, and that the system has been properly tuned for operation on wet runways.
4.3.7.4 Classification of Types of Anti-Skid Systems.
4.3.7.4.1 The efficiency values specified in § 25.109(c)(2) are a function of the type of anti-skid system installed on the airplane. Three broad system types are identified in the rule: on/off, quasi-modulating, and fully modulating.
These classifications represent evolving levels of technology and differing performance capabilities on dry and wet runways. The classification of anti-skid system types and the assigned efficiency values are based on information contained in Society of Automotive Engineers (SAE) Aerospace Information Report (AIR) 1739, Information on Anti-Skid Systems.
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4.3.7.4.2 On/off systems are the simplest of the three types of anti-skid systems. For these systems, full-metered brake pressure (as commanded by the pilot) is applied until wheel locking is sensed. Brake pressure is then released to allow the wheel to spin back up. When the system senses that the wheel is accelerating back to synchronous speed (i.e., ground speed), full-metered pressure is again applied. The cycle of full pressure application/complete pressure release is repeated throughout the stop (or until the wheel ceases to skid with pressure applied).
4.3.7.4.3 Quasi-modulating systems attempt to continuously regulate brake pressure as a function of wheel speed. Typically, brake pressure is released when the wheel deceleration rate exceeds a pre-selected value. Brake pressure is re-applied at a lower level after a length of time appropriate to the depth of the skid. Brake pressure is then gradually increased until another incipient skid condition is sensed. In general, the corrective actions taken by these systems to exit the skid condition are based on a pre-programmed sequence rather than the wheel speed time history.
4.3.7.4.4 Fully modulating systems are a further refinement of the quasi-modulating systems. The major difference between these two types of anti-skid systems is in the implementation of the skid control logic. During a skid, corrective action is based on the sensed wheel speed signal, rather than a pre-programmed response. Specifically, the amount of pressure reduction or reapplication is based on the rate at which the wheel is going into or recovering from a skid.
4.3.7.4.5 In addition to examining the control system for the differences noted above, a time history of the response characteristics of the anti-skid system during a wet runway stop should be used to help identify the type of anti-skid system. Comparing the response characteristics between wet and dry runway stops can also be helpful.
4.3.7.4.6 Figure 4-3 and figure 4-4 below show an example of the response characteristics of a typical on-off system on both dry and wet runways, respectively. In general, the on-off system exhibits a cyclic behavior of brake pressure application until a skid is sensed, followed by the complete release of brake pressure to allow the wheel to spin back up. Full-metered pressure (as commanded by the pilot) is then re-applied, starting the cycle over again. The wheel speed trace exhibits deep and frequent skids (the troughs in the wheel speed trace), and the average wheel speed is significantly less than the synchronous speed (which is represented by the flat-topped portions of the wheel speed trace). Note that the skids are deeper and more frequent on a wet runway than on a dry runway. For the particular example shown in figure 4-3, the brake becomes torque-limited 4-35
AC 25-7D CHG 1 toward the end of the dry runway stop, and it is unable to generate enough torque to cause further skidding Figure 4-3. Anti-Skid System Response Characteristics: On-Off System on Dry Runway 100 WHEEL SPEED (knots) 80 60 40 20 BRAKE PRESSURE = 1000 500 10 20 25 30 TIME (seconds) Figure 4-4. Anti-Skid System Response Characteristics: On-Off System on Wet Runway 100 WHEEL SPEED (knots) 80 60 40 20 BRAKE PRESSURE = 1000 500 Nupt 8 10 20 25 30 TIME (seconds) 4-36
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4.3.7.4.7 The effectiveness of quasi-modulating systems can vary significantly depending on the slipperiness of the runway and the design of the particular control system. On dry runways, these systems typically perform very well; however, on wet runways their performance is highly dependent on the design and tuning of the particular system. An example of the response characteristics of one such system on dry and wet runways is shown in figure 4-5 and figure 4-6 below, respectively. On both dry and wet runways, brake pressure is released to the extent necessary to control skidding. As the wheel returns to the synchronous speed, brake pressure is quickly increased to a pre-determined level and then gradually ramped up to the full-metered brake pressure. On a dry runway, this type of response reduces the depth and frequency of skidding compared to an on-off system. However, on a wet runway, skidding occurs at a pressure below that at which the gradual ramping of brake pressure occurs. As a result, on wet runways the particular system shown in figure 4-6 operates very similarly to an on-off system.
Figure 4-5. Anti-Skid System Response Characteristics: Quasi-Modulating System on Dry Runway 4-37
AC 25-7D CHG 1 Figure 4-6 Anti-Skid System Response Characteristics: Quasi-Modulating System on Wet Runway
4.3.7.4.8 When properly tuned, fully modulating systems are characterized by much smaller variations in brake pressure around a fairly high average value.
These systems can respond quickly to developing skids and are capable of modulating brake pressure to reduce the frequency and depth of skidding.
As a result, the average wheel speed remains much closer to the synchronous wheel speed. Figure 4-7 below illustrates an example of the response characteristics of a fully modulating system on dry and wet runways.
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AC 25-7D CHG 1 Figure 4-7. Anti-Skid System Response Characteristics: Fully Modulating System Dry Runway
50.0- 20 Jalv 1€ Wet Runway 100 4-39
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4.3.7.5 Demonstration of Anti-Skid System Operation when Using the Anti-Skid Efficiency Values Specified in § 25.109(c)(2).
4.3.7.5.1 If the applicant elects to use one of the anti-skid efficiency values specified in § 25.109(c)(2), a limited amount of flight testing must still be conducted, per § 25.109(c)(2), to demonstrate anti-skid system operation on a wet runway. This testing should be used to verify that the anti-skid system operates in a manner consistent with the type of anti-skid system declared by the applicant, and that the anti-skid system has been properly tuned for operation on wet runways.
4.3.7.5.2 A minimum of one complete stop, or equivalent segmented stops, should be conducted on a smooth (i.e., not grooved or porous friction course) wet runway at an appropriate speed and energy to cover the critical operating mode of the anti-skid system. Since the objective of the test is to observe the operation (i.e., cycling) of the anti-skid system, this test will normally be conducted at an energy level well below the maximum brake energy condition.
4.3.7.5.3 The section of the runway used for braking should be well soaked (i.e., not just damp), but not flooded. The runway test section should be wet enough to result in a number of cycles of anti-skid activity, but should not cause hydroplaning.
4.3.7.5.4 Before taxi and with cold tires, the tire pressure should be set to the highest value appropriate to the takeoff weight for which approval is being sought.
4.3.7.5.5 The tires and brakes should not be new but need not be in the fully worn condition. They should be in a condition considered representative of typical in-service operations.
4.3.7.5.6 Sufficient data should be obtained to determine whether the system operates in a manner consistent with the type of anti-skid system declared by the applicant, provide evidence that full brake pressure is being applied upstream of the anti-skid valve during the flight test demonstration, determine whether the anti-skid valve is performing as intended, and show that the anti-skid system has been properly tuned for a wet runway.
Typically, the following parameters should be plotted versus time:
1. The speed of a representative number of wheels.
2. The hydraulic pressure at each brake (i.e., the hydraulic pressure downstream of the anti-skid valve or the electrical input to each anti-skid valve).
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3. The hydraulic pressure at each brake metering valve (i.e., upstream of the anti-skid valve).
4.3.7.5.7 A qualitative assessment of anti-skid system response and airplane controllability should be made by the test pilot(s). In particular, pilot observations should confirm that:
1. Anti-skid releases are neither excessively deep nor prolonged;
2. The landing gear is free of unusual dynamics; and
3. The airplane tracks essentially straight, even though runway seams, water puddles, and wetter patches may not be uniformly distributed in location or extent.
4.3.7.6 Determination of a Specific Wet Runway Anti-Skid System Efficiency.
4.3.7.6.1 If the applicant elects to derive the anti-skid system efficiency from flight test demonstrations, sufficient flight testing, with adequate instrumentation, should be conducted to ensure confidence in the value obtained. An anti-skid efficiency of 92 percent (i.e., a factor of 0.92) is considered to be the maximum efficiency on a wet runway normally achievable with fully modulating digital anti-skid systems.
4.3.7.6.2 A minimum of three complete stops, or equivalent segmented stops, should be conducted on a wet runway at appropriate speeds and energies to cover the critical operating modes of the anti-skid system. Alternatively, if the operation and efficiency of the anti-skid system on a wet runway can be predicted by laboratory simulation data and validated by flight test demonstrations, a lesser number of stops may be acceptable. In this case, as many complete stops, or equivalent segmented stops, as necessary to present six independent anti-skid efficiency calculations should be conducted on a wet runway at appropriate speeds and energies to cover the critical operating modes of the anti-skid system. An independent anti-skid efficiency calculation can be presented for each stop for each independently controlled wheel or set of wheels.
4.3.7.6.3 Since the objective of the test is to determine the efficiency of the antiskid system, these tests will normally be conducted at energies well below the maximum brake energy condition. A sufficient range of speeds should be covered to investigate any variation of the anti-skid efficiency with speed.
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