ILS Approach Categories
There are three general classifications of ILS approaches: CAT I, CAT II, and CAT III (autoland). The basic ILS approach is a CAT I approach and requires only that pilots be instrument rated and current, and that the aircraft be equipped appropriately. CAT II and CAT III ILS approaches have lower minimums and require special certification for operators, pilots, aircraft, and airborne/ground equipment. Because of the complexity and high cost of the equipment, CAT III ILS approaches are used primarily in air carrier and military operations. [Figure 4-41]
CAT II and III Approaches
The primary authorization and minimum RVRs allowed for an air carrier to conduct CAT II and III approaches can be found in OpSpecs Part C. CAT II and III operations allow authorized pilots to make instrument approaches in weather that would otherwise be prohibitive.
While CAT I ILS operations permit substitution of midfield RVR for TDZ RVR (when TDZ RVR is not available), CAT II ILS operations do not permit any substitutions for TDZ RVR. The TDZ RVR system is required and must be used. The TDZ RVR is controlling for all CAT II ILS operations.
The weather conditions encountered in CAT III operations range from an area where visual references are adequate for manual rollout in CAT IIIa, to an area where visual references are inadequate even for taxi operations in CAT IIIc. To date, no U.S. operator has received approval for CAT IIIc in OpSpecs. Depending on the auto-flight systems, some aircraft require a DH to ensure that the aircraft is going to land in the TDZ and some require an Alert Height as a final cross-check of the performance of the auto-flight systems. These heights are based on radio altitude (RA) and can be found in the specific aircraft’s AFM. [Figure 4-42]
Both CAT II and III approaches require special ground and airborne equipment to be installed and operational, as well as special aircrew training and authorization. The OpSpecs of individual air carriers detail the requirements of these types of approaches, as well as their performance criteria. Lists of locations where each operator is approved to conduct CAT II and III approaches can also be found in the OpSpecs.
Special Authorization approaches are designed to take advantage of advances in flight deck avionics and technologies like Head-Up Displays (HUD) and automatic landings. There are extensive ground infrastructures and lighting requirements for standard CAT II/III, and the Special Authorization approaches mitigate the lack of some lighting with the modern avionics found in many aircraft today. Similar to standard CAT II/III, an air carrier must be specifically authorized to conduct Special Authorization CAT I/II in OpSpecs Part C.
Simultaneous Approaches To Parallel Runways
Airports that have two or more parallel runways may be authorized to use simultaneous parallel approaches to maximize the capacity of the airport. Depending on the runway centerline separation and ATC procedures, there are three classifications of simultaneous parallel approaches: Simultaneous dependent approaches, simultaneous independent approaches and simultaneous independent close parallel approaches. A simultaneous dependent approach differs from a simultaneous independent approach in that the minimum distance between parallel runway centerlines may be less. A staggered separation of aircraft on the adjacent final approach course is required; but there is no requirement for a No Transgression Zone (NTZ) or Final Monitor Controllers. An independent approach eliminates the need for staggered approaches and aircraft may be side by side or pass if speeds are different.
- Simultaneous approaches involving an RNAV approach may only be conducted when (GPS) appears in the approach title or a chart note states that GPS is required. See the “ILS Approaches” paragraph above for information about pilot responsibilities when simultaneous approaches are in use.
- Flight Director or Autopilot requirements for simultaneous operations will be annotated on the approach chart.
- Simultaneous approaches may only be conducted where instrument approach charts specifically authorize simultaneous approaches.
Simultaneous Dependent Approaches [Figure 4-46]
When simultaneous dependent approaches are provided, ATC applies specific minimum diagonal separation criteria, depending on the runway separation, between aircraft on adjacent final approach courses. Aircraft will be staggered by a minimum of 1 NM diagonally on final, depending on the distance between runway centerlines. Greater separation standards are applied when the distance between runway centerlines is greater. [Figure 4-43]
At some airports, simultaneous dependent instrument approaches can be conducted with runways spaced less than 2,500 feet with specific centerline separations and threshold staggers. ATC is permitted to apply reduced diagonal separation and special wake turbulence procedures. The lead aircraft of the dependent pair is restricted to being small or large aircraft weight type and is cleared to the lower approach. The design of the approach, aircraft weight type, and lateral separation between the two approaches provide necessary wake turbulence avoidance for this type of operation. An example of approach design to help avoid wake turbulence is that some locations use different glide slope angles on adjacent approaches; also, if applicable, staggered thresholds help. An ATIS example is: “Simultaneous ILS Runway 28 Left and ILS Runway 28 Right in use.” For further information, see FAA Orders JO 7110.65 and JO 7110.308.
Where a simultaneous approach operation is approved, sometimes each approach chart indicates the other runway(s) with which simultaneous approaches can be conducted. For example, “Simultaneous approaches authorized with runway 12L”. As procedures are revised, the chart note will be modified to indicate “Simultaneous approach authorized” but will not list the other runways or approach types as that detailed information will normally be transmitted in the ATIS or by ATC. For example, pilots flying into Sacramento, California, may encounter parallel approach procedures. [Figure 4-44] When there is no chart note stating, “Simultaneous approaches authorized”, standard separation is used between aircraft on parallel approaches.