An instrument approach is a procedure for bringing an aircraft into the vicinity of, and then landing on, a particular airport runway. It is generally made known to the pilot via the publication of instrument approach procedure charts, or 'approach plates.' Note that in this case, a 'runway' specifically means a numbered runway, so is only usable from one direction. As an example, runway 24 is the reciprocal of runway 06; it's the same piece of tarmac but used in different directions. An instrument approach is tied to that piece of tarmac and one of those two directions.
There are several different types of instrument approach. They come in precision and non-precision flavors. They differ based on the type of instrumentation on the airfield and in the aircraft. A precision approach provides both horizontal (lateral) guidance and vertical (glideslope) guidance. In other words, at any point during the approach there will be instruments to tell you where you are in relation to the glidepath both horizontally and vertically. Non-precision approaches only provide instrumented lateral guidance; vertical guidance is provided based on a list of permissible and target altitudes indexed to the aircraft's position. Landmarks, radio beacons, satellite navigation and the pilot's dead reckoning may all used to determine the position.
Some of the non-precision approach types are:
On the precision approach front, we have
- ILS is one of the most popular
- GCA again - if the controller is using a PAR (Precision Approach Radar) and providing both vertical and horizontal guidance to the aircraft, this is a precision approach. Generally used more by military aicraft and controllers.
- MLS - Microwave Landing System, a replacement for ILS using scanning microwave beams. Supplanted to some degree by GPS, it is nevertheless still in use in some locations. It is able to handle a steeper glidepath than ILS systems.
This isn't an exhaustive list either.