AMC 25.177(c) Steady, Straight Sideslips

ED Decision 2011/004/R

1       CS 25.177(c) requires, in steady, straight sideslips throughout the range of sideslip angles appropriate to the operation of the aeroplane, that the aileron and rudder control movements and forces be proportional to the angle of sideslip. The factor of proportionality must lie between limits found necessary for safe operation. The range of sideslip angles evaluated must include those sideslip angles resulting from the lesser of: (1) one-half of the available rudder control input; and (2) a rudder control force of 180 pounds. CS 25.177(c) states, by cross-reference to CS 25.177(a), that these steady, straight sideslip criteria must be met for all landing gear and flap positions and symmetrical power conditions at speeds from 1.13 V_SR1 to V_FE, V_LE, or V_FC/M_FC, as appropriate for the configuration.

2       Sideslip Angles Appropriate to the Operation of the Aeroplane

2.1     Experience has shown that an acceptable method for determining the appropriate sideslip angle for the operation of a transport category aeroplane is provided by the following equation:

ß = arc sin (30/V)

where

ß = Sideslip angle, and

V = Airspeed (KCAS)

Recognising that smaller sideslip angles are appropriate as speed is increased, this equation provides sideslip angle as a function of airspeed. The equation is based on the theoretical sideslip value for a 56 km/h (30-knot) crosswind, but has been shown to conservatively represent (i.e., exceed) the sideslip angles achieved in maximum crosswind take-offs and landings and minimum static and dynamic control speed testing for a variety of transport category aeroplanes. Experience has also shown that a maximum sideslip angle of 15 degrees is generally appropriate for most transport category aeroplanes even though the equation may provide a higher sideslip angle. However, limiting the maximum sideslip angle to 15 degrees may not be appropriate for aeroplanes with low approach speeds or high crosswind capability.

2.2     A lower sideslip angle than that provided in paragraph 2.1 may be used if it is substantiated that the lower value conservatively covers all crosswind conditions, engine failure scenarios, and other conditions where sideslip may be experienced within the approved operating envelope. Conversely, a higher value should be used for aeroplanes where test evidence indicates that a higher value would be appropriate to the operation of the aeroplane.

3       For the purposes of showing compliance with the requirement out to sideslip angles associated with one-half of the available rudder control input, there is no need to consider a rudder control input beyond that corresponding to full available rudder surface travel or a rudder control force of 801 N (180 lbf). Some rudder control system designs may limit the available rudder surface deflection such that full deflection for the particular flight condition is reached before the rudder control reaches onehalf of its available travel. In such cases, further rudder control input would not result in additional rudder surface deflection.

4       Steady, straight sideslips

4.1     Steady, straight sideslips should be conducted in each direction to show that the aileron and rudder control movements and forces are substantially proportional to the angle of sideslip in a stable sense, and that the factor of proportionality is within the limits found necessary for safe operation. These tests should be conducted at progressively greater sideslip angles up to the sideslip angle appropriate to the operation of the aeroplane (see paragraph 2.1) or the sideslip angle associated with one-half of the available rudder control input, whichever is greater.

4.2     When determining the rudder and aileron control forces, the controls should be relaxed at each point to find the minimum force needed to maintain the control surface deflection. If excessive friction is present, the resulting low forces will indicate the aeroplane does not have acceptable stability characteristics.

4.3     In lieu of conducting each of the separate qualitative tests required by CS 25.177(a) and (b), the applicant may use recorded quantitative data showing aileron and rudder control force and position versus sideslip (left and right) to the appropriate limits in the steady heading sideslips conducted to show compliance with CS 25.177(c). If the control force and position versus sideslip indicates positive dihedral effect and positive directional stability, compliance with CS 25.177(a) and (b) will have been successfully demonstrated."

[Amdt 25/11]