Automatic stabilization equipment (ASE)

7.5.2.1 Longitudinal long-term, LDO and falling leaf modes

The attitude stabilization provided by an ASE should completely suppress these nuisance modes. The attitude holds should ensure long-term datum holding with the tightness of this hold improved by pseudo-rate feedback obtained from differentiating the output from the vertical gyros. The magnitude of any underlying dynamic instability can be gauged by observing series actuator activity. What constitutes appropriate testing must be carefully considered since movement of the cyclic stick by the pilot in order to generate a disturbance may alter the attitude datum being used by the ASE. The attitude hold performance during protracted flight in role representative turbulence is by far the most appropriate test. Additional tests to document heading hold performance, and altitude hold performance (if operative), are also to be conducted.

It should be remembered that tight control of pitch and roll attitude may not give satisfactory handling qualities whilst hovering in turbulent conditions. If the helicopter is hovering in a wind of varying strength then even though the aircraft attitude may be being maintained it will drift relative to the ground and so plan-position keeping may be poor. To overcome this deficiency and alleviate the pilot from the workload associated with continual re-positioning of the rotorcraft during extended hovering it is common for AFCS system designers to substitute a Doppler-based hover hold for the attitude hold used in forward flight. Increasingly these auto-hover functions make use of a mix of Doppler, inertial and satellite signals. Assessment of such a hold would normally be part of an autopilot assessment and therefore the assessment of the attitude hold whilst in the low-speed regime needs to be approached with these additional tests in mind.

7.5.2.2 Control response

The rate feedback included in most ASE systems can be used to improve the control response characteristics of the helicopter. A step cyclic input in the cockpit will, however, cause a discrete attitude change rather than generate a rate response. Since most ASE systems use foot pressure within the unlock logic for the heading hold it is likely that in the low-speed regime a step yaw pedal input will still generate a yaw rate. The method used to unlock the attitude hold during pilot inputs may cause anomalies in the control response or lead to the adoption of non-standard piloting techniques. It is worth noting that the ACAH nature of the typical ASE may result in excessive control activity during aggressive tactical flight as the pilot exercises the controls in an attempt to generate sustained angular rates. In such situations a switch allowing a change to a SAS mode providing RCAH may be warranted.