1.Purpose. This AMC describes acceptable means of compliance with the specifications of CS 25.353. These specifications provide structural design load conditions that apply to the airframe, and that occur as a result of multiple cockpit rudder control (e.g. pedal) inputs.
2.Related CS-25 specifications. a. CS 25.351, Yaw manoeuvre conditions. b. CS 25.353, Rudder control reversal conditions.
3.Background. a. Specifications. CS 25.351 and CS 25.353 specify structural design load conditions that occur as a result of cockpit rudder control inputs. These conditions are intended to encompass all of the rudder manoeuvre loads expected to occur in service. b. Yaw manoeuvre conditions. The design load conditions specified in CS 25.351 are considered to be limit load conditions, and a safety factor of 1.5 is applied to obtain the ultimate loads. c. Rudder control reversal load conditions. The design load conditions specified in this CS 25.353 are more severe than those in CS 25.351, and include cockpit rudder control reversals. These conditions are anticipated to occur very rarely, and therefore these are considered to be ultimate load conditions, and no additional safety factor is applied. d. Overswing sideslip angle definition: the maximum (peak) sideslip angle reached by the aeroplane with the cockpit rudder control displaced as specified in paragraph 4.b below.
4.Application of the specifications. a. General
(1)The aeroplane must be designed for the cockpit rudder control reversal load conditions specified in CS 25.353. These are considered to be ultimate load conditions and, therefore, no additional safety factor is applied. However, any resulting permanent deformation must not prevent continued safe flight and landing.
(2)Design loads must be determined as specified in CS 25.321. The load conditions are considered from VMC to VC/MC. A pilot force of 890 N (200 lbf) is assumed to be applied for all conditions. These conditions are to be considered with the landing gear retracted and speed brakes (or spoilers when used as speed brakes) retracted. Flaps (or flaperons or any other aerodynamic devices when used as flaps) and slats-extended configurations are also to be considered if they are used in en-route conditions.
(3)System effects. System effects should be taken into account in the evaluation of this manoeuvre. For example, fly-by-wire aeroplanes should be analysed assuming that the aeroplane is in the normal control law mode. Any system function used to demonstrate compliance with these requirements should meet the following criteria:
(i)The system is normally operative during flight in accordance with the aeroplane flight manual procedures, although limited dispatch with the system inoperative could be allowed under applicable master minimum equipment list (MMEL) provisions, provided that the MMEL requirements are still complied with, taking into account the rudder reversal pedal inputs as the next critical event under dispatch configuration; and
(ii)Appropriate crew procedures should be provided in the event of a loss of function. If a loss of system function would not be detected by the flight crew, the probability of a loss of function (i.e. the failure rate multiplied by the maximum exposure period) should be less than 1/1000.
(4)Failure conditions. Assuming that the systems which are used to demonstrate compliance with CS 25.353 meet the criteria in 4.a(3)(i) and (ii) above, considering the very low probability of a full rudder control (e.g. pedal) doublet event, failure scenarios do not need to be addressed in combination with the rudder control reversal load conditions specified in CS 25.353. b. Yaw manoeuvre conditions Conditions (a) through (e) of CS 25.353 are intended to be a full displacement cockpit rudder control input followed by three cockpit rudder control reversals and a return to neutral. The aeroplane airspeed should be kept reasonably constant throughout the manoeuvre using pitch control. These conditions should be investigated assuming rational or conservative roll control input (pilot or system induced). Refer to the illustration in Figure 1 below.
(1)Rudder control input. In the context of CS 25.353, ‘suddenly’ means as fast as possible within human and system limitations. In the absence of a rational analysis, the initial rudder control displacement is achieved in no more than 0.2 seconds, and full cockpit rudder control reversal displacement is achieved in 0.4 seconds. Alternatively, the applicant may assume that the rudder control is displaced instantaneously. The resulting rudder displacement should take into account any additional displacement caused by sideslip build-up, and the effects of flexibility should be considered when relevant.
(2)Rudder control reversals. As soon as the maximum overswing yaw angle is achieved, full opposite rudder control input is applied. The achieved rudder deflection may be limited by control laws, system architecture, or air loads, and may not be of the same magnitude as the initial rudder deflection prior to the rudder control reversal. For a critically damped aeroplane response, the maximum overswing yaw angle may be assumed to occur when the sideslip angle is substantially stabilised. Two additional rudder control reversals are performed as defined in paragraph 4.b(1) above. After the second reversal, as soon as the aeroplane yaws to the opposite overswing yaw angle, the rudder control is suddenly returned to neutral. [Figure or form omitted from this preview — available in the Avioverse workspace library.] Figure 1: Illustration of the cockpit rudder control inputs
[Amdt No: 25/22]