Lateral and directional static stability

The complex inter-relationship between the roll and yaw channels of a typical ASE make conventional lateral and directional static stability testing both difficult and unwarranted. It might actually be impossible to perform a steady-heading sideslip without over-riding part of the ASE system. For example, if lateral cyclic immediately initiates a turn the pilot will have to apply yaw pedal to hold a given sideslip angle thereby effectively disabling the yaw channel of the ASE and negating the test. For unaugmented helicopters measurement of LV and NV help to quantify the aircraft’s tendency towards spiral instability, its natural turn co-ordination characteristics and its propensity to oscillate laterally and directionally. All these qualities are heavily affected by the ASE further negating any assessment of static stability. For helicopters without bank angle initiation of turn co-ordination, turn-on-one-control tests may still be appropriate as methods of assessing the performance of the heading hold and roll attitude hold.

7.5.2.5 Spiral stability

As with manoeuvre stability the actual spiral stability characteristics of a helicopter may be completely suppressed when the ASE is engaged. The amount of in-turn stick required during a level turn can no longer be used as an indication of the helicopter’s natural spiral stability since the lateral cyclic deflection from trim may be derived purely from the requirements of the ASE system. Some systems are engineered to display neutral stability with the stick returning to the centre once a turn has been initiated. Alternatively other systems show the characteristics of strong stability, with the angle of bank and rate of turn being directly proportional to the amount of in­turn stick.

7.5.2 Autopilot testing

There are three main aspects to testing autopilots: the interface with the pilot, the performance of the system, and the safety of operation.

7.5.3.1 Pilotlautopilot interface

As the autopilot is directly controlling the flight path of the aircraft it is extremely important that the pilot is fully aware of which modes are engaged and what those modes are directing the aircraft to do. The assessment of the interface examines the display of autopilot status, the method of mode selection/deselection, and the requirement on the crew to monitor the system operation. A particularly important issue is the warning provided to the pilot that a function has been deselected or has failed. The pilot must be able to assimilate with ease which aspects of the flight path are no longer under automatic control. Of equal importance is the ease with which the pilot can ‘fly through’ an autopilot mode if he or she elects to fly the aircraft manually rather than disengage the mode. This situation usually arises when there is a sudden requirement to manoeuvre the aircraft.