AMC 25.109(d)(2) Accelerate-stop distance: anti-skid efficiency on grooved and porous friction course (PFC) runways.

• ED Decision 2003/2/RM found in: CS-25 Amdt 27 - Large Aeroplanes (Jan 2023)

AMC 25.109(d)(2) Accelerate-stop distance: anti-skid efficiency on grooved and porous friction course (PFC) runways. ED Decision 2003/2/RM Properly designed, constructed and maintained grooved and PFC runways can offer significant improvements in wet runway braking capability. A conservative level of performance credit is provided by [25.109(d)](#_DxCrossRefBm1709820677) to reflect this performance improvement and to provide an incentive for installing and maintaining such surfaces. In accordance with [CS 25.105(c)](#_DxCrossRefBm1709820679) and 25.109(d), applicants may optionally determine the acceleratestop distance applicable to wet grooved and PFC runways. These data would be included in the AFM in addition to the smooth runway accelerate-stop distance data. The braking coefficient for determining the accelerate-stop distance on grooved and PFC runways is defined in CS 25.109(d) as either 70% of the braking coefficient used to determine the dry runway accelerate-stop distances, or a curve based on ESDU 71026 data and derived in a manner consistent with that used for smooth runways. In either case, the brake torque limitations determined on a dry runway may not be exceeded. Using a simple factor applied to the dry runway braking coefficient is acceptable for grooved and PFC runways because the braking coefficient’s variation with speed is much lower on these types of runways. On smooth wet runways, the braking coefficient varies significantly with speed, which makes it inappropriate to apply a simple factor to the dry runway braking coefficient. For applicants who choose to determine the grooved/PFC wet runway accelerate-stop distances in a manner consistent with that used for smooth runways, [CS 25.109(d)(2)](#_DxCrossRefBm1709820677) provides the maximum tyre-to-ground braking coefficient applicable to grooved and PFC runways. This maximum tyre-to-ground braking coefficient must be adjusted for the anti-skid system efficiency, either by using the value specified in CS 25.109(c)(2) appropriate to the type of anti-skid system installed, or by using a specific efficiency established by the applicant. As anti-skid system performance depends on the characteristics of the runway surface, a system that has been tuned for optimum performance on a smooth surface may not achieve the same level of efficiency on a grooved or porous friction course runway, and vice versa. Consequently, if the applicant elects to establish a specific efficiency for use with grooved or PFC surfaces, anti-skid efficiency testing should be conducted on a wet runway with such a surface, in addition to testing on a smooth runway. Means other than flight testing may be acceptable, such as using the efficiency previously determined for smooth wet runways, if that efficiency is shown to be representative of, or conservative for, grooved and PFC runways. The resulting braking force for grooved/PFC wet runways must be adjusted for the effect of the distribution of the normal load between braked and unbraked wheels. This adjustment will be similar to that used for determining the braking force for smooth runways, except that the braking dynamics should be appropriate to the braking force achieved on grooved and PFC wet runways. Due to the increased braking force on grooved and PFC wet runways, an increased download on the nose wheel and corresponding reduction in the download on the main gear is expected.