Instrument Approach - Basic Principles

Basic Principles

Instrument approaches are generally designed such that a pilot of an aircraft in instrument meteorological conditions (IMC), by the means of radio, GPS or INS navigation with no assistance from air traffic control, can navigate to the airport, hold in the vicinity of the airport if required, then fly to a position from where he or she can obtain sufficient visual reference of the runway for a safe landing to be made, or execute a missed approach if the visibility is below the minimums required to execute a safe landing. The whole of the approach is defined and published in this way so that aircraft can land if they suffer from radio failure; it also allows instrument approaches to be made procedurally at airports where air traffic control does not use radar or in the case of radar failure.

An instrument approach procedure may have as many as five separate segments depending on how the approach procedure is structured. These segments include:

• Arrival segment: The segment from when the aircraft leaves to en-route airway to the initial approach fix (IAF).
• Initial approach: The segment between the initial approach fix (IAF) and the intermediate fix (IF), or the point where the aircraft is established on the intermediate course or final approach course.
• Intermediate Approach: The segment between the IF or point, and the final approach fix (FAF).
• Final approach: The segment between the FAF or point, and the runway, airport, or missed approach point (MAP).
• Missed approach: The segment between the MAP or the point of arrival at decision height and the missed approach fix at the prescribed altitude.

When aircraft are under radar control, air traffic controllers may replace some or all of these phases of the approach with radar vectors (the provision of headings on which the controller expects the pilot to navigate his aircraft) to the final approach, to allow traffic levels to be increased over those of which a fully procedural approach is capable. It is very common for air traffic controllers to vector aircraft to the final approach aid, e.g. the ILS, which is then used for the final approach. In the case of the rarely used Ground-Controlled Approach (GCA), the instrumentation (normally Precision Approach Radar) is on the ground and monitored by a controller, who then relays precise instructions for adjustment of heading and altitude to the pilot in the approaching aircraft.