Landing aids and automatic landing

Because of the very real difficulty and high pilot workload during landing this phase of flight has been the subject of rapid development in systems to aid the pilot in his task. As well as improving safety these systems allow for better aircraft utilisation because one of the obvious limitations for operation under

This wing low

Compensating rudder

causing drift

Sideslip relative to wind

Fig. 13.10 Use of sideslip to correct drift

Aircraft is rolled slightly to induce sideslip into wind Aircraft axis is kept aligned with runway by use of rudder

purely ‘visual flight’ is that low cloud may make the approach impossible even to attempt.

Here we shall very briefly describe some of these aids, the proper study of which is a separate discipline in its own right.

It is apparent from the above discussion that one of the main problems in landing is that of following an accurate glide path to the runway threshold. This can, of course, be particularly difficult in conditions of poor visibility par­ticularly for large aircraft where the glide path needs to be established several miles from the touch-down point. The main purpose of any landing aid is thus to aid accurate flying during this phase of the landing. For modern aircraft a number of such aids is available. Among these are radio beacons which can be used for general navigational purposes as well as landing aids such as the non-directional beacon (NDB) which supplies the aircraft with a directional ‘fix’ on a known ground location or the very high frequency omni-directional beacon (VOR) which supplies both directional and range information. The most common aid dedicated solely to landing is the Instrument Landing System (ILS). In this a pair of radio beams are arranged to cross on the glide path. Deviation from the glide path is then indicated by a cockpit instrument, the ILS indicator (Fig. 10.2). Satellite-based GPS systems are also used.

Automatic flight along the glide path can be achieved by adding an auto­matic throttle and flight control system, with accurate height information being obtained from a radio altimeter. Automatic flare is provided to bring the air­craft on to the runway. Fully automatic landing systems of this type have greatly increased the range of conditions under which safe aircraft operation is possible.

A newer alternative to the ILS system is the more accurate Microwave Landing System (MLS). This system is however being challenged by an even newer technology, the Global Navigation Satellite System (GNSS). The advan­tage of the latter is the that it relies on signals from satellites and does not require expensive ground installations at airports. After lengthy trials, a satel­lite landing system has been cleared for use at a small number of selected airfields.