AMC 25.255 Out-of-trim characteristics

ED Decision 2003/2/RM

1       Amount of Out-of-trim Required

1.1     The equivalent degree of trim, specified in CS 25.255(a)(1) for aeroplanes which do not have a power-operated longitudinal trim system, has not been specified in quantitative terms, and the particular characteristics of each type of aeroplane must be considered. The intent of the requirement is that a reasonable amount of out-of-trim should be investigated, such as might occasionally be applied by a pilot.

1.2     In establishing the maximum mistrim that can be sustained by the autopilot the normal operation of the autopilot and associated systems should be taken into consideration. Where the autopilot is equipped with an auto-trim function the amount of mistrim which can be sustained will generally be small or zero. If there is no auto-trim function, consideration should be given to the maximum amount of out-of-trim which can be sustained by the elevator servo without causing autopilot disconnect.

2       Datum Trim Setting

2.1     For showing compliance with CS 25.255(b)(1) for speeds up to V_MO/M_MO, the datum trim setting should be the trim setting required for trimmed flight at the particular speed at which the demonstration is to be made.

2.2     For showing compliance with CS 25.255(b)(1) for speeds from V_MO/M_MO to V_FC/M_FC, and for showing compliance with CS 25.255(b)(2) and (f), the datum trim setting should be the trim setting required for trimmed flight at V_MO/M_MO.

3       Reversal of Primary Longitudinal Control Force at Speeds greater than V_FC/M_FC

3.1     CS 25.255(b)(2) requires that the direction of the primary longitudinal control force may not reverse when the normal acceleration is varied, for +1 g to the positive and negative values specified, at speeds above V_FC/M_FC. The intent of the requirement is that it is permissible that there is a value of g for which the stick force is zero, provided that the stick force versus g curve has a positive slope at that point (see Figure 1).

FIGURE 1

3.2     If stick force characteristics are marginally acceptable, it is desirable that there should be no reversal of normal control sensing, i.e. an aft movement of the control column should produce an aircraft motion in the nose-up direction and a change in aircraft load factor in the positive direction, and a forward movement of the control column should change the aircraft load factor in the negative direction.

3.3     It is further intended that reversals of direction of stick force with negative stick-force gradients should not be permitted in any mistrim condition within the specified range of mistrim. If test results indicate that the curves of stick force versus normal acceleration with the maximum required mistrim have a negative gradient of speeds above V_FC/M_FC then additional tests may be necessary. The additional tests should verify that the curves of stick force versus load factor with mistrim less than the maximum required do not unacceptably reverse, as illustrated in the upper curve of Figure 2. Control force characteristics as shown in Figure 3, may be considered acceptable, provided that the control sensing does not reverse (see paragraph 3.2)

4       Probable Inadvertent Excursions beyond the Boundaries of the Buffet Onset Envelopes. CS 25.255(e) states that manoeuvring load factors associated with probable inadvertent excursions beyond the boundaries of the buffet onset envelopes determined under CS 25.251(e) need not be exceeded. It is intended that test flights need not be continued beyond a level of buffet which is sufficiently severe that a pilot would be reluctant to apply any further increase in load factor.

5       Use of the Longitudinal Trim System to Assist Recovery

5.1     CS 25.255(f) requires the ability to produce at least 1·5 g for recovery from an overspeed condition of V_DF/M_DF, using either the primary longitudinal control alone or the primary longitudinal control and the longitudinal trim system. Although the longitudinal trim system may be used to assist in producing the required normal acceleration, it is not acceptable for recovery to be completely dependent upon the use of this system. It should be possible to produce 1·2 g by applying not more than 556 N (125 lbf) of longitudinal control force using the primary longitudinal control alone.

5.2     Recovery capability is generally critical at altitudes where airspeed (V_DF) is limiting. If at higher altitudes (on the M_DF boundary) the manoeuvre capability is limited by buffeting of such an intensity that it is a strong deterrent to further increase in normal acceleration, some reduction of manoeuvre capability will be acceptable, provided that it does not reduce to below 1·3 g. The entry speed for flight test demonstrations of compliance with this requirement should be limited to the extent necessary to accomplish a recovery without exceeding V_DF/M_DF, and the normal acceleration should be measured as near to V_DF/M_DF as is practical.