Instrument approach
In aviation, an instrument approach or instrument approach procedure is a series of predetermined maneuvers for the orderly transfer of an aircraft operating under instrument flight rules from the beginning of the initial approach to a landing, or to a point from which a landing may be made visually. These approaches are approved in the European Union by EASA and the respective country authorities, and in the United States by the FAA or the United States Department of Defense for the military. The ICAO defines an instrument approach as "a series of predetermined maneuvers by reference to flight instruments with specific protection from obstacles from the initial approach fix, or where applicable, from the beginning of a defined arrival route to a point from which a landing can be completed and thereafter, if landing is not completed, to a position at which holding or en route obstacle clearance criteria apply."
There are three categories of instrument approach procedures: precision approach, approach with vertical guidance, and non-precision approach. A precision approach uses a navigation system that provides course and glidepath guidance. Examples include precision approach radar, instrument landing system, and GBAS landing system. An approach with vertical guidance also uses a navigation system for course and glidepath deviation, just not to the same standards as a PA. Examples include baro-VNAV, localizer type directional aid with glidepath, LNAV/VNAV and LPV. A non-precision approach uses a navigation system for course deviation but does not provide glidepath information. These approaches include VOR, NDB, LP, and LNAV. PAs and APVs are flown to a decision height/altitude, while non-precision approaches are flown to a minimum descent altitude.
IAP charts are aeronautical charts that portray the aeronautical data that is required to execute an instrument approach to an airport. Besides depicting topographic features, hazards and obstructions, they depict the procedures and airport diagram. Each procedure chart uses a specific type of electronic navigation system such as an NDB, TACAN, VOR, ILS/MLS and RNAV. The chart name reflects the primary navigational aid, if there is more than one straight-in procedure or if it is just a circling-only procedure. A communication strip on the chart lists frequencies in the order they are used. Minimum, maximum and mandatory altitudes are depicted in addition to the minimum safe altitude for emergencies. A cross depicts the final approach fix altitude on NPAs while a lightning bolt does the same for PAs. NPAs depict the MDA while a PA shows both the decision altitude and decision height. Finally, the chart depicts the missed approach procedures in plan and profile view, besides listing the steps in sequence.
Before satellite navigation was available for civilian aviation, the requirement for large land-based navigation aid facilities generally limited the use of instrument approaches to land-based runways. GNSS technology allows, at least theoretically, to create instrument approaches to any point on the Earth's surface ; consequently, there are nowadays examples of water aerodromes that have GNSS-based approaches.
Instrument approach segments
An instrument approach procedure may contain up to five separate segments, which depict course, distance, and minimum altitude. These segments are- Feeder routes: A route for aircraft to proceed from the en route structure to the IAF, which includes the course and bearing to be flown, the distance, and the minimum altitude.
- Initial approach segment: This segment provides a method for aligning the aircraft with the intermediate or final approach segment and to permit descent during the alignment. It begins at an IAF and ends at the intermediate approach segment or intermediate fix. A DME arc, a procedure turn / teardrop turn, or holding pattern may be involved or the terminal route may simply intersect the final approach course.
- Intermediate approach segment: This segment positions the aircraft for the final descent to the airport. It begins at the IF and ends at the final approach segment.
- Final approach segment: For a PA or APV, this segment starts where the glideslope intercepts the glideslope-intercept altitude plane. For an NPA, this segment starts at the FAF, final approach point, or where the aircraft is established on the final approach course. This segment ends at either the designated missed approach point or upon landing.
- Missed approach segment: This segment starts at the MAP and ends at a point or fix where the initial or en route segment begins.
Types of approaches
Though ground-based NAVAID approaches still exist, the FAA is transitioning to approaches which are satellite-based. Additionally, in lieu of the published approach procedure, a flight may continue as an IFR flight to landing while increasing the efficiency of the arrival with either a contact or visual approach.Visual approach
A visual approach is an ATC authorization for an aircraft on an IFR flight plan to proceed visually to the airport of intended landing; it is not an instrument approach procedure.A visual approach may be requested by the pilot or offered by ATC. Visual approaches are possible when weather conditions permit continuous visual contact with the destination airport. They are issued in such weather conditions in order to expedite handling of IFR traffic. The ceiling must be reported or expected to be at least 1000 feet AGL and the visibility is at least 3 SM.
A pilot may accept a visual approach clearance as soon as the pilot has the destination airport in sight. According to ICAO Doc. 4444, it is enough for a pilot to see the terrain to accept a visual approach. The point is that if a pilot is familiar with the terrain in the vicinity of the airfield he/she may easily find the way to the airport having the surface in sight.
ATC must ensure that weather conditions at the airport are above certain minima before issuing the clearance. According to ICAO Doc. 4444, it is enough if the pilot reports that in his/her opinion the weather conditions allow a visual approach to be made. In general, the ATC gives the information about the weather but it's the pilot who makes a decision if the weather is suitable for landing. Once the pilot has accepted the clearance, he/she assumes responsibility for separation and wake turbulence avoidance and may navigate as necessary to complete the approach visually. According to ICAO Doc. 4444, ATC continues to provide separation between the aircraft making a visual approach and other arriving and departing aircraft. The pilot may get responsible for the separation with preceding aircraft in case he/she has the preceding aircraft in sight and is instructed so by ATC.
In the United States, it is required that an aircraft have the airport, the runway, or the preceding aircraft in sight. It is not enough to have the terrain in sight.
When a pilot accepts a visual approach, the pilot accepts responsibility for establishing a safe landing interval behind the preceding aircraft, as well as responsibility for wake-turbulence avoidance, and to remain clear of clouds.
Contact approach
A contact approach that may be asked for by the pilot in which the pilot has 1 SM flight visibility and is clear of clouds and is expected to be able to maintain those conditions all the way to the airport. Obstruction clearances and VFR traffic avoidance become the pilot's responsibility.Charted visual flight procedures (CVFP)
A visual approach that has a specified route the aircraft is to follow to the airport. Pilots must have a charted visual landmark or a preceding aircraft in sight, and weather must be at or above the published minimums. Pilots are responsible for maintaining a safe approach interval and wake turbulence separation.RNP approach (formerly RNAV approach)
These approaches include both ground-based and satellite-based systems and include criteria for terminal arrival areas, basic approach criteria, and final approach criteria. The TAA is a transition from the en route structure to the terminal environment which provides minimum altitudes for obstacle clearance. The TAA is a "T" or "basic T" design with left and right base leg IAFs on initial approach segments perpendicular to the intermediate approach segment where there is a dual purpose IF/IAF for a straight-in procedure, or hold-in-lieu-of procedure-turn course reversal. The base leg IAFs is 3 to 6 NM from the IF/IAF. The basic-T is aligned with the runway centerline, with the IF 5 NM from the FAF, and the FAF is 5 NM from the threshold.The RNP approach chart should have four lines of approach minimums corresponding to LPV, LNAV/VNAV, LNAV, and circling. This allows GPS or WAAS equipped aircraft to use the LNAV MDA using GPS only, if WAAS becomes unavailable.
ILS approach
These are the most precise and accurate approaches. A runway with an ILS can accommodate 29 arrivals per hour.ILS systems on two or three runways increase capacity with parallel ILS, simultaneous parallel ILS, precision runway monitor, and converging ILS approaches. ILS approaches have three classifications, CAT I, CAT II, and CAT III. CAT I SA, CAT II and CAT III require additional certification for operators, pilots, aircraft and equipment, with CAT III used mainly by air carriers and the military. Simultaneous parallel approaches require runway centerlines to be between 4,300 and 9,000 feet apart, plus a "dedicated final monitor controller" to monitor aircraft separation. Simultaneous close parallel PRM approaches must have runways separation to be between 3,400 and 4,300 feet. Simultaneous offset instrument approaches apply to runways separated by 750–3,000 feet. A SOIA uses an ILS/PRM on one runway and an LDA/PRM with glideslope for the other.