MOC VTOL.2300(a)(1) Function and operation of Fly-by-Wire flight control system

(a)     Flight crew awareness of the modes of operation

(1)     If the design of the flight control system has multiple modes of operation (e.g. hover, transition, cruise modes) and/or includes degraded modes following failures, a means should be provided to indicate to the crew any mode that significantly changes or degrades the handling or operational characteristics of the aircraft.

(2)     The sub-modes of operation (both in nominal and degraded mode) and the transition between them should be smooth, and should be evaluated to determine whether or not they are intuitive. If these sub-modes or the transition between them are not intuitive, an indication to the flight crew may be required. This indication may be different from the classic “failure alerting”.

(3)     In case of several flight control modes, limitations should be clearly annunciated and the definition of a Training Area of Special Emphasis (TASE) in the Flight Crew Data (FCD) may need to be established during the certification of the Operational Suitability Data (OSD).

Compliance should be shown in conjunction with other paragraphs (such as VTOL.2445), where failures could lead to flight control mode degradation.

(b)     Flight envelope protection

If Flight Envelope Protection (FEP) features are implemented, then these should follow the following principles:

(1)     Onset characteristics of each envelope protection feature should be smooth, appropriate to the phase of flight and type of manoeuvre; and not be in conflict with the ability of the pilot to satisfactorily change the aircraft flight path (e.g. speed, attitude) within the approved flight envelope.

(2)     Limit values of protected flight parameters (and if applicable, associated warning thresholds) should be compatible with:

(i)      the aircraft structural limits;

(ii)     the required safe and controllable manoeuvring of the aircraft;

(iii)     the margins to critical conditions;

(iv)     dynamic manoeuvring, airframe and system tolerances (both from manufacturing and in-service), and non-steady atmospheric conditions - in any appropriate combination and phase of flight - should not result in a limited flight parameter beyond the nominal design limit value that would cause unsafe flight characteristics;

(v)      the rotor rotational speed limits;

(vi)     the blade stall limits;

(vii)    the engine and transmission torque limits; and/or

(viii)   any other operation limitations for the aircraft and lift/thrust system installation.

(3)     The aircraft should be responsive to pilot commanded dynamic manoeuvring within a suitable range of the parameter limits that define the approved flight envelope.

(4)     The FEP system and any failure condition not shown to be extremely improbable should be analysed per MOC VTOL.2135 MHQRM (including the effect on flight envelope probabilities) and VTOL.2510.

(5)     When simultaneous envelope limiting is active this should not result in adverse coupling or adverse priority (e.g. if two or more envelope limitations could exist simultaneously, this consequence should not be a wrong priority).

Adherence to the above principles should be shown in conjunction with the demonstration of compliance with the following requirements:  VTOL.2110, VTOL.2425(a), VTOL.2500, VTOL.2510(a)(b) and VTOL.2135 with MOC VTOL.2135.

(c)      Flight control and critical displays:

The following apply at all attitudes and in all modes of operation:

(1)     The flight control system should be designed to continue to operate and not hinder aircraft recovery from any attitude.

(2)     Control systems for essential services should be designed so that when a movement to one position has been selected, a different position can be selected without having to wait for the completion of the initially selected movement, and the system should arrive at the finally selected position without further attention. The movements that follow and the time taken by the system to allow the required sequence of selection should not be such as to adversely affect the airworthiness of the aircraft.

(3)     Compliance should be shown by evaluation of the closed loop flight control system. This evaluation is intended to ensure that there are no features or unique characteristics (including numerical singularities) which would restrict the pilot’s ability to recover from any attitude. The intent is not to limit the use of envelope protection features or other systems that augment the control characteristics of the aircraft.

(4)     The following conditions that might occur due to pilot action, system failures or external events should be considered:

(i)      Abnormal attitude (including the aircraft becoming inverted;)

(ii)     Excursion of any other flight parameter; and

(iii)     Flight conditions that may result in higher than normal pitch, roll or yaw rates.

(5)     For each of the conditions in (c)(4):

(i)      The flight control system should continue to operate;

(ii)     The design of the flight control laws, including any automatic protection function should not hinder aircraft recovery; and

(iii)     Critical flight displays should continue to provide accurate indications and any other information that the pilot may require to execute recovery from the unusual attitude and/or arrest the higher than normal pitch, roll or yaw rates.