Low Visibility Approaches
Many instrument approaches allow for landing in conditions of low visibility. ICAO classifies ILS approaches as being in one of the following categories:
| Category | Decision Height (above threshold) | RVR limit | Visibility |
|---|---|---|---|
| I | 200 ft | 550 m or 2400 ft (1200 ft is approved at some airports),
increased to 800m for single crew operations || 800 m |
|
| II | 100 ft | 300 m or 1000 ft | N/A |
| IIIa | 50 ft < DH < 100 ft | 200 m | N/A |
| IIIb | 0 < DH < 50 ft | less than 200 m but not less than 75 m | N/A |
| IIIc | No DH | No RVR | N/A |
CAT III minima depend on roll-out control and redundancy of the autopilot. Smaller aircraft will generally only be equipped to fly a CAT I ILS. For larger aircraft it is typical that these approaches are under the control of the flight control system with the flight crew providing a supervisory role.
A head-up display (HUD) allows the flight crew to fly the aircraft using the guidance cues from the ILS sensors so that if such a large deviation were seen, the pilot would be able to respond in an appropriate and safe manner. This is becoming increasingly popular with "feeder" airlines and most manufacturers of regional jets are now offering HUDs as either standard or optional equipment. In addition a HUD can provide a low visibility take-off capability.
For both automatic and HUD landing systems, the equipment requires special approval for its design and also for each individual installation. The design takes into consideration all of the additional safety requirements for operating an aircraft close to the ground and takes into consideration the ability of the flight crew to react to a "system anomaly." Once installed, the equipment also has additional maintenance requirements to ensure that it is fully capable of supporting reduced visibility operations.
For all CAT II or III approaches, additional crew training is required for such approaches, and a certain number of low visibility approaches must either be performed or simulated in a set period of time for pilots to stay 'current' in performing them.
For practical reasons Category IIIc approaches are rare, but Category IIIb approaches are relatively common at major airports.
There are also air traffic control considerations with low visibility approaches: when using ILS, the integrity of the signal must be protected, which requires that certain areas of the airport close to the installations being free of other aircraft and vehicles. Also there must be bigger gaps between aircraft on final approach to both protect the ILS signal and to cope with slower runway vacation times. In addition, the airport itself has special considerations for low visibility operations including different lighting for approach area, runways, and taxiways as well as the location of emergency equipment.
Read more about this topic: Instrument Approach
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