Now that the helicopter is capable of airspeed, altitude, heading and vertical speed control it is possible to use these modes in combination to provide automatic navigation and approach to a landing site. Navigation modes include heading selection, VOR, ILS, Back Course, Go-around and On-route Navigation.
Heading selection. The heading selection (HDG) mode differs from heading hold in that large heading changes may be required and, therefore, the mode is achieved through the roll channel. As mentioned above, heading selection may be a mode performed by the autopilot separate from the ASE heading hold function. Alternatively both modes can be performed by the autopilot. The heading error signal may be scheduled as a function of TAS in order to ensure the helicopter executes ‘Rate 1’ turns. In addition a bank angle limit may be included to prevent excessive roll attitude changes at high speed.
VOR tracking. VOR tracking is a mode common to most autopilots and is designed to provide automatic intercept, capture and tracking of a selected VOR radial. Typically the pilot will tune the navigation receiver to the desired VOR frequency, select a VOR radial and using the HDG mode set a heading that will intercept the desired course. As the helicopter approaches the beacon the VOR signal is monitored for beam deviation, beam rate and validity. At the appropriate time the helicopter is steered towards the beacon and eventually the radial will be captured, with the helicopter flying in the desired direction. Steerage on to the radial is usually achieved in stages either by using a 45° ‘cut’ or successive cuts of up to 30°.
This feature is designed to avoid overshooting the beam when large intercepts are used at high speeds. Some autopilots automatically reduce the bank angle and roll rate limits to avoid over-controlling as the beam centre is approached. On entering the cone of confusion located overhead the beacon an Over Station Sensor (OSS) warning will be given to the pilot. The link between the radio receiver and the autopilot is severed and the system will usually revert to heading hold. Once the signal becomes usable again the link is restored and radial tracking is resumed. Some systems will increase the bank angle and roll rate limits on re-engagement to ensure rapid reacquirement of the beam centreline in gusty conditions. If the pilot selects a new course, or radial, whilst over-flying the beacon the autopilot will usually steer the helicopter on to this heading using the HDG mode. On leaving the cone of confusion when the VOR signal is reacquired the system will adjust the heading to track the centreline of the new outbound radial. Consequently the VOR tracking logic may cut the corner avoiding the need to overfly the actual VOR beacon. If this logic is also applied to GPS or IN based waypoint tracking problems can occur if the pilot wishes to use the automatic navigation function to overfly a particular point of interest and then execute a turn immediately afterwards.
ILS mode. Initially the ILS mode operates in a similar manner to the VOR mode described above. The navigation receiver is tuned to the appropriate localizer frequency and a beam intercept course acquired using the HDG mode. As the helicopter closes on the localizer beam the radio signal is monitored and at the appropriate time a steerage signal is sent to the roll channel. The helicopter will thus capture and track the beam. Once again the helicopter may be steered on to the correct heading through a series of ‘cuts’. Additionally the heading error signal may be modified as a function of TAS to avoid an overshoot at high speed.
Typically overcontrolling is avoided by reducing the bank angle and roll rate limits as the beam centre is approached. The helicopter will continue along the localizer beam until the glide slope beam is acquired. Following acquisition of this signal existing holds are disconnected. If the autopilot operates in only three channels any longitudinal mode will need to be decoupled as the pitch channel is used for glide slope maintenance. Such decoupling usually takes place automatically leaving the pilot to monitor the airspeed and adjust the collective lever position as appropriate. If, however, a 4-channel system is fitted then airspeed hold can be retained since the glide slope command will be fed to the collective channel.
It is usual for the gain applied to the glide slope signal to be varied as a function of altitude as sensed by the radio altimeter and perhaps passage over the middle marker. The reduction in gain is designed to avoid overcontrolling as the glide slope beam narrows towards the landing site. At a certain radio altitude the helicopter will ‘autolevel’, through the pitch and/or collective channel, to avoid ground contact. The helicopter will then continue to fly along the runway under localizer guidance awaiting pilot action to either disengage the hold, and commit to a landing, or abort the landing by engaging a Go-Around mode.
Go-around mode. Although not a navigation mode, Go-Around (GA) is often associated with an automatic ILS mode. GA is activated, in most cases, by means of a switch placed on the collective lever. The GA mode enables the pilot to abort an automatic approach with the autopilot causing the helicopter to adopt a positive rate of climb. In 3-channel systems the pilot will have to raise the collective to maintain the airspeed. With a 4-channel system the airspeed can also be controlled and the AFCS would usually program a speed change to set and hold the airspeed at the speed for best rate of climb, VY.
Back-course mode. The Back-Course (BC) mode operates in a similar manner to the initial phase of the VOR mode. The BC mode provides for automatic intercept, capture and tracking of the back course localizer signal. The control law gains may be adjusted since the helicopter will be closer to the localizer by the length of the runway. Capture of the glide slope is usually inhibited to prevent any possibility of a rate of descent being commanded.
On-route navigation modes. The coupling of a navigation computer to the autopilot provides on-route navigation modes. The navigation computer will use signals from internal or external sources to determine the helicopter’s position in relation to a preprogrammed track. On receipt of an error signal the autopilot will use the HDG logic to maintain the desired ground track. Sources of ground position data include, Doppler, GPS, or an INS. Modern systems feature combinations of these systems, the signals of which are mixed and filtered to generate a very accurate fix of the present position of the helicopter. Navigation systems intended for ASW or SAR usually allow the pilot to program combinations of waypoints, or leg-lengths, in order to execute set search patterns over a target area. Navigation modes such as these help the pilot maintain a good lookout whilst ensuring a precise ground track.