AMC No. 2 to CS 25.143(g) Controllability and Manoeuvrability

ED Decision 2007/010/R

1       The objective of CS 25.143(g) is to ensure that the limit strength of any critical component on the aeroplane would not be exceeded in manoeuvring flight. In much of the structure the load sustained in manoeuvring flight can be assumed to be directly proportional to the load factor applied. However, this may not be the case for some parts of the structure, e.g., the tail and rear fuselage. Nevertheless, it is accepted that the aeroplane load factor will be a sufficient guide to the possibility of exceeding limit strength on any critical component if a structural investigation is undertaken whenever the design positive limit manoeuvring load factor is closely approached. If flight testing indicates that the design positive limit manoeuvring load factor could be exceeded in steady manoeuvring flight with a 222 N (50 lbf) stick force, the aeroplane structure should be evaluated for the anticipated load at a 222 N (50 lbf) stick force. The aeroplane will be considered to have been overstressed if limit strength has been exceeded in any critical component. For the purposes of this evaluation, limit strength is defined as the larger of either the limit design loads envelope increased by the available margins of safety, or the ultimate static test strength divided by 1·5.

2       Minimum Stick Force to Reach Limit Strength

2.1     A stick force of at least 222 N (50 lbf) to reach limit strength in steady manoeuvres or wind up turns is considered acceptable to demonstrate adequate minimum force at limit strength in the absence of deterrent buffeting. If heavy buffeting occurs before the limit strength condition is reached, a somewhat lower stick force at limit strength may be acceptable. The acceptability of a stick force of less than 222 N (50 lbf) at the limit strength condition will depend upon the intensity of the buffet, the adequacy of the warning margin (i.e., the load factor increment between the heavy buffet and the limit strength condition) and the stick force characteristics. In determining the limit strength condition for each critical component, the contribution of buffet loads to the overall manoeuvring loads should be taken into account.

2.2     This minimum stick force applies in the en-route configuration with the aeroplane trimmed for straight flight, at all speeds above the minimum speed at which the limit strength condition can be achieved without stalling. No minimum stick force is specified for other configurations, but the requirements of CS 25.143(g) are applicable in these conditions.

3       Stick Force Characteristics

3.1     At all points within the buffet onset boundary determined in accordance with CS 25.251(e), but not including speeds above V_FC/M_FC, the stick force should increase progressively with increasing load factor. Any reduction in stick force gradient with change of load factor should not be so large or abrupt as to impair significantly the ability of the pilot to maintain control over the load factor and pitch attitude of the aeroplane.

3.2     Beyond the buffet onset boundary, hazardous stick force characteristics should not be encountered within the permitted manoeuvring envelope as limited by paragraph 3.3. It should be possible, by use of the primary longitudinal control alone, to pitch the aeroplane rapidly nose down so as to regain the initial trimmed conditions. The stick force characteristics demonstrated should comply with the following:

a.       For normal acceleration increments of up to 0·3 g beyond buffet onset, where these can be achieved, local reversal of the stick force gradient may be acceptable provided that any tendency to pitch up is mild and easily controllable.

b.       For normal acceleration increments of more than 0·3 g beyond buffet onset, where these can be achieved, more marked reversals of the stick force gradient may be acceptable. It should be possible for any tendency to pitch up to be contained within the allowable manoeuvring limits without applying push forces to the control column and without making a large and rapid forward movement of the control column.

3.3     In flight tests to satisfy paragraph 3.1 and 3.2 the load factor should be increased until either –

a.       The level of buffet becomes sufficient to provide a strong and effective deterrent to further increase of load factor; or

b.       Further increase of load factor requires a stick force in excess of 667 N (150 lbf) (or in excess of 445 N (100 lbf) when beyond the buffet onset boundary) or is impossible because of the limitations of the control system; or

c.       The positive limit manoeuvring load factor established in compliance with CS 25.337(b) is achieved.

4        Negative Load Factors

It is not intended that a detailed flight test assessment of the manoeuvring characteristics under negative load factors should necessarily be made throughout the specified range of conditions. An assessment of the characteristics in the normal flight envelope involving normal accelerations from 1 g to 0 g will normally be sufficient. Stick forces should also be assessed during other required flight testing involving negative load factors. Where these assessments reveal stick force gradients that are unusually low, or that are subject to significant variation, a more detailed assessment, in the most critical of the specified conditions, will be required. This may be based on calculations provided these are supported by adequate flight test or wind tunnel data.

[Amdt 25/3]