ILS provides dedicated ground-based guidance
An instrument landing system transmits lateral and vertical signals that lead toward a specific runway. It has long been the standard precision-approach technology at major airports.
RNAV approaches use onboard area navigation
Satellite navigation and certified aircraft systems can guide flights along flexible paths without a dedicated radio transmitter beside the runway. Some procedures add vertical guidance comparable in use to a precision approach.
Non-precision approaches may provide only lateral guidance
Older VOR, NDB or some localizer procedures require crews to manage descent using published altitude steps or calculated vertical paths. Minimums are generally higher than on the best precision approaches.
Minimums protect terrain and obstacle clearance
The procedure designer evaluates terrain, towers and navigation accuracy. Crews cannot simply descend below the published minimum because they believe the runway should be nearby.
The missed approach is part of the procedure
If the required visual references are not obtained, pilots climb and follow a published route away from terrain and conflicting traffic. A missed approach is a planned option, not a failure.
Aircraft and crew approval determine what can be flown
A runway may publish several approaches, but the operator, aircraft equipment and crew qualification determine which ones are available on a given flight.
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