CS 25.331 Symmetric manoeuvring conditions

ED Decision 2016/010/R

(See AMC 25.331)

(a)     Procedure. For the analysis of the manoeuvring flight conditions specified in sub-paragraphs (b) and (c) of this paragraph, the following provisions apply:

(1)     Where sudden displacement of a control is specified, the assumed rate of control surface displacement may not be less than the rate that could be applied by the pilot through the control system.

(2)     In determining elevator angles and chordwise load distribution in the manoeuvring conditions of sub-paragraphs (b) and (c) of this paragraph, the effect of corresponding pitching velocities must be taken into account. The in-trim and out-of-trim flight conditions specified in CS 25.255 must be considered.

(b)     Manoeuvring balanced conditions. Assuming the aeroplane to be in equilibrium with zero pitching acceleration, the manoeuvring conditions A through I on the manoeuvring envelope in CS 25.333(b) must be investigated.

(c)      Manoeuvring pitching conditions. The following conditions must be investigated:

(1)     Maximum pitch control displacement at V_A. The aeroplane is assumed to be flying in steady level flight (point A1, CS 25.333(b)) and the cockpit pitch control is suddenly moved to obtain extreme nose up pitching acceleration. In defining the tail load, the response of the aeroplane must be taken into account. Aeroplane loads which occur subsequent to the time when normal acceleration at the c.g. exceeds the positive limit manoeuvring load factor (at point A2 in CS 25.333(b)), or the resulting tailplane normal load reaches its maximum, whichever occurs first, need not be considered (See AMC 25.331(c)(1)).

(2)     Checked manoeuvre between V_A and V_D. Nose up checked pitching manoeuvres must be analysed in which the positive limit load factor prescribed in CS 25.337 is achieved. As a separate condition, nose down checked pitching manoeuvres must be analysed in which a limit load factor of 0 is achieved. In defining the aeroplane loads the cockpit pitch control motions described in sub-paragraphs (i), (ii), (iii) and (iv) of this paragraph must be used:

(See AMC 25.331(c)(2))

(i)      The aeroplane is assumed to be flying in steady level flight at any speed between V_A and V_D and the cockpit pitch control is moved in accordance with the following formula:

where:

δ1 =   the maximum available displacement of the cockpit pitch control in the initial direction, as limited by the control system stops, control surface stops, or by pilot effort in accordance with CS 25.397(b);

δ(t) = the displacement of the cockpit pitch control as a function of time. In the initial direction δ(t) is limited to δ₁. In the reverse direction, δ(t) may be truncated at the maximum available displacement of the cockpit pitch control as limited by the control system stops, control surface stops, or by pilot effort in accordance with CS 25.397(b);

t_max = 3π/2ω;

ω =    the circular frequency (radians/second) of the control deflection taken equal to the undamped natural frequency of the short period rigid mode of the aeroplane, with active control system effects included where appropriate; but not less than:

where:

V =     the speed of the aeroplane at entry to the manoeuvre.

V_A =    the design manoeuvring speed prescribed in CS 25.335(c)

(ii)     For nose-up pitching manoeuvres the complete cockpit pitch control displacement history may be scaled down in amplitude to the extent just necessary to ensure that the positive limit load factor prescribed in CS 25.337 is not exceeded. For nose-down pitching manoeuvres the complete cockpit control displacement history may be scaled down in amplitude to the extent just necessary to ensure that the normal acceleration at the c.g. does not go below 0g.

(iii)     In addition, for cases where the aeroplane response to the specified cockpit pitch control motion does not achieve the prescribed limit load factors then the following cockpit pitch control motion must be used:

where:

t₁   =   π/2ω

t₂   =   t₁ + ∆t

t_max = t₂ + π/ω;

∆t  =   the minimum period of time necessary to allow the prescribed limit load factor to be achieved in the initial direction, but it need not exceed five seconds (see figure below).

(iv)     In cases where the cockpit pitch control motion may be affected by inputs from systems (for example, by a stick pusher that can operate at high load factor as well as at 1g) then the effects of those systems must be taken into account.

(v)      Aeroplane loads that occur beyond the following times need not be considered:

(A)     For the nose-up pitching manoeuvre, the time at which the normal acceleration at the c.g. goes below 0g;

(B)     For the nose-down pitching manoeuvre, the time at which the normal acceleration at the c.g. goes above the positive limit load factor prescribed in CS 25.337;

(C)     t_max.

[Amdt 25/13]

[Amdt 25/18]