39 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.
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 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.
(b) 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 (max). It is expressed as either a percentage of max or a factor based on that percentage (e.g., 85% 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 must 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.
1 Classification of Types of Anti-Skid Systems.
(aa) 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, titled “Information on Anti-Skid Systems.”
(bb) 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).
(cc) 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.
(dd) Fully modulating systems are a further refinement of the quasimodulating 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.
(ee) 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.
(ff) Figure 11-3 shows an example of the response characteristics of a typical on-off system on both dry and wet runways. 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 11-3, the brake becomes torque-limited toward the end of the dry runway stop, and is unable to generate enough torque to cause further skidding.
(gg) 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 is shown in figure 11-4. 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 11-4 operates very similarly to an on-off system.
Figure 11-3. Anti-Skid System Response Characteristics On-Off System Dry Runway 100 WHEEL SPEED (knots) 80 60 40 20 BRAKE PRESSURE (pst) 500 10 15 20 TIME (seconas) 100 Wet Runway WHEEL SPEED (knots) 20 BRAKE PRESSURE (psi) 1000
0. 2 4 8 10 15 20 25 30 TIME (seconds) 41
Figure 11-4. Anti-Skid System Response Characteristics Quasi-Modulating System Dry Runway 150 - Wheel Speed (ft/sec) 100 50 Time (sec) 2000- Brake Pressure (psi) Time (sec) Wet Runway Wheel Speed (ft/sec) Time (sec) 2000 - Brake Pressure 500 - (psi) Time (sec) 42
43 (hh) 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 11-5 illustrates an example of the response characteristics of a fully modulating system on dry and wet runways.
Figure 11-5. Anti-Skid System Response Characteristics Fully Modulating System Dry Runway
150.0
100.0-
50.0- 12 14 18 20 Anti-skid Current Wet Runway 100 80 90 100 44
2 Demonstration of Anti-Skid System Operation When Using the Anti-Skid Efficiency Values Specified in § 25.109(c)(2).
(aa) 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 antiskid system declared by the applicant, and that the anti-skid system has been properly tuned for operation on wet runways.
(bb) 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 well below the maximum brake energy condition.
(cc) 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.
(dd) 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.
(ee) 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.
(ff) 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:
(i) The speed of a representative number of wheels.
(ii) 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).
(iii) The hydraulic pressure at each brake metering valve (i.e., upstream of the anti-skid valve).
(gg) 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:
(i) Anti-skid releases are neither excessively deep nor prolonged;
(ii) The landing gear is free of unusual dynamics; and
(iii) The airplane tracks essentially straight, even though runway seams, water puddles, and wetter patches may not be uniformly distributed in location or extent.
3 Determination of a Specific Wet Runway Anti-Skid System Efficiency .
(aa) 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.
(bb) 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.
(cc) Since the objective of the test is to determine the efficiency of the anti-skid 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.
(dd) The testing should be conducted on a smooth (i.e., not grooved or porous friction course) runway. If the applicant chooses to determine accelerate-stop distances for grooved and porous friction course (PFC) surfaces under § 25.109(d)(2), testing should also be conducted on a grooved or porous friction course runway to determine the anti-skid efficiency value applicable to those surfaces. Other means for determining the anti-skid efficiency value for grooved and PFC surfaces may also be acceptable, such as using the efficiency value previously determined for smooth runways, if that value is shown to also be representative of or conservative for grooved and PFC runways.
(ee) 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.
47 (ff) 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.
(gg) 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.
(hh) 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:
(i) The landing gear is free of unusual dynamics; and
(ii) The airplane tracks essentially straight, even though runway seams, water puddles, and wetter patches may not be uniformly distributed in location or extent.
(i) Two acceptable methods, referred to as the torque method and the wheel slip method, for determining the wet runway anti-skid efficiency value from wet runway stopping tests are described below. Other methods may also be acceptable if they can be shown to give equivalent results. The test instrumentation and data collection should be consistent with the method used.
(i) Torque Method:
(a) Under the torque method, the anti-skid system efficiency is determined by comparing the energy absorbed by the brake during an actual wet runway stop to the energy that is determined by integrating, over the stopping distance, a curve defined by connecting the peaks of the instantaneous brake force curve (see figure 11-6). The energy absorbed by the brake during the actual wet runway stop is determined by integrating the curve of instantaneous brake force over the stopping distance.
Figure 11-6. Instantaneous Brake Force and Peak Brake Force
Time Instantaneous Brake Force Peak Brake Force Time Intervals Brake Force
(b) Using data obtained from the wet runway stopping tests of paragraph 11c(4)(b)(3), instantaneous brake force can be calculated from the following relationship:
tire b b R ITF
where: F b = brake force T b = brake torque = wheel acceleration I = wheel and tire moment of inertia and R tire = tire radius.
(c) For brake installations where measuring brake torque directly is impractical, torque may be determined from other parameters (e.g., brake pressure) if a suitable correlation is available. Wheel acceleration is obtained from the first derivative of wheel speed. Instrumentation recording rates and data analysis techniques for wheel speed and torque data should be well matched to the anti-skid response characteristics to avoid introducing noise and other artifacts of the instrumentation system into the data.
(d) Since the derivative of wheel speed is used in calculating brake force, smoothing of the wheel speed data is usually necessary to give good results. The smoothing algorithm should be carefully designed as it can affect the resulting efficiency calculation. Filtering or smoothing of the brake torque or brake force data should not normally be done. If conditioning is applied, it should be done in a conservative manner (i.e., result in a lower efficiency value) and should not misrepresent actual airplane/system dynamics.
(e) Both the instantaneous brake force and the peak brake force should be integrated over the stopping distance. The anti-skid efficiency value for determining the wet runway accelerate-stop distance is the ratio of the instantaneous brake force integral to the peak brake force integral:
where s = stopping distance
(f) The stopping distance is defined as the distance traveled during the specific wet runway stopping demonstration, beginning when the full braking configuration is obtained and ending at the lowest speed at which anti-skid cycling occurs (i.e., the brakes are not torque-limited), except that this speed need not be less than 10 knots. Any variation in the anti-skid efficiency with speed should also be investigated, which can be accomplished by determining the efficiency over segments of the total stopping distance. If significant variations are noted, this variation should be reflected in the braking force used to instantaneous brake forceds peak brake forceds anti-skid efficiency =
49 determine the accelerate-stop distances (either by using a variable efficiency or by using a conservative single value).
( i i ) Wheel Slip Method:
(a) At brake application, the tire begins to slip with respect to the runway surface (i.e., the wheel speed slows down with respect to the airplane’s ground speed). As the amount of tire slip increases, the brake force also increases until an optimal slip is reached. If the amount of slip continues to increase past the optimal slip, the braking force will decrease.
(b) Using the wheel slip method, the anti-skid efficiency is determined by comparing the actual wheel slip measured during a wet runway stop to the optimal slip. Since the wheel slip varies significantly during the stop, sufficient wheel and ground speed data should be obtained to determine the variation of both the actual wheel slip and the optimal wheel slip over the length of the stop. A sampling rate of at least 16 samples per second for both wheel speed and ground speed has been found to yield acceptable fidelity.
(c) For each wheel and ground speed data point, the instantaneous anti-skid efficiency value should be determined from the relationship shown in figure 11-7.
Figure 11-7. Anti-Skid Efficiency – Wheel Slip Relationship
Wheel Slip Ratio0 free rolling 1 locked wheel
1.0 0 Optimal Slip
0.5 Anti-Skid Efficiency
ratio slip optimal theis OPS and speed Ground speed Wheel1=ratio slip wheelthe= WSRwhere OPS1 WSR1+10.5=Efficiency OPS>WSR
1.0=Efficiency OPS=WSR OPS WSR5.0OPSWSR5.1=Efficiency OPS<for WSR 3
(d) To determine the overall anti-skid efficiency value for use in calculating the wet runway accelerate-stop distance, the instantaneous anti-skid efficiencies should be integrated with respect to distance and divided by the total stopping distance:
where s = stopping distance
(e) The stopping distance is defined as the distance traveled during the specific wet runway stopping demonstration, beginning when the full braking configuration is obtained and ending at the lowest speed at which anti-skid cycling occurs (i.e., the brakes are not torque-limited), except that this speed need not be less than 10 knots. Any variation in the anti-skid efficiency with speed should also be investigated, which can be accomplished by determining the efficiency over segments of the total stopping distance. If significant variations are noted, this variation should be reflected in the braking force used to determine the accelerate-stop distances (either by using a variable efficiency or by using a conservative single value).
(f) The applicant should provide substantiation of the optimal wheel slip value(s) used to determine the anti-skid efficiency value. An acceptable method for determining the optimal slip value(s) is to compare time history plots of the brake force and wheel slip data obtained during the wet runway stopping tests. For brake installations where measuring brake force directly is impractical, brake force may be determined from other parameters (e.g., brake pressure) if a suitable correlation is available. For those skids where wheel slip continues to increase after a reduction in the brake force, the optimal slip is the slip value corresponding to the brake force peak. See figure 11-8 for an example and note how both the actual wheel slip and the optimal wheel slip can vary during the stop.
instantaneous brake forceds s anti-skid efficiency =