Notwithstanding emerging RNAV technology, the ILS
is the most precise and accurate approach NAVAID
currently in use throughout the NAS. An ILS CAT I
precision approach allows approaches to be made to
200 feet above the TDZE and with visibilities as low
as 1,800 RVR; with CAT II and CAT III approaches
allowing descents and visibility minimums that are
even lower. Nonprecision approach alternatives cannot
begin to offer the precision or flexibility offered by an
ILS. In order to further increase the approach capacity
of busy airports and exploit the maximum potential of
ILS technology, many different applications are in use.
A single ILS system can accommodate 29 arrivals
per hour on a single runway. Two or three parallel
runways operating consecutively can double or triple
the capacity of the airport. For air commerce this
means greater flexibility in scheduling passenger
and cargo service. Capacity is increased through the
use of parallel (dependent) ILS, simultaneous parallel
(independent) ILS, simultaneous close parallel
(independent) ILS, precision runway monitor
(PRM), and converging ILS approaches. A parallel
(dependent) approach differs from a simultaneous
(independent) approach in that the minimum distance
between parallel runway centerlines is reduced; there is
no requirement for radar monitoring or advisories; and a
staggered separation of aircraft on the adjacent
localizer/azimuth course is required.
In order to successfully accomplish parallel,
simultaneous parallel, and converging ILS
approaches, flight crews and air traffic controllers
have additional responsibilities. When multiple
instrument approaches are in use, ATC will advise
flight crews either directly or through ATIS. It is
the pilot’s responsibility to inform ATC if unable or
unwilling to execute a simultaneous approach.
Pilots must comply with all ATC requests in a
timely manner, and maintain strict radio discipline,
including using complete aircraft call signs. It is
also incumbent upon the flight crew to notify ATC
immediately of any problems relating to aircraft
communications or navigation systems. At the very
least, the approach procedure briefing should cover
the entire approach procedure including the
approach name, runway number, frequencies, final
approach course, glide slope intercept altitude, DA
or DH, and the missed approach instructions. The
review of autopilot procedures is also appropriate
when making coupled ILS or MLS approaches.
As with all approaches, the primary navigation
responsibility falls upon the pilot in command. ATC
instructions will be limited to ensuring aircraft separation.
Additionally, missed approach procedures are
normally designed to diverge in order to protect all
involved aircraft. ILS approaches of all types are
afforded the same obstacle clearance protection and
design criteria, no matter how capacity is affected by
multiple ILS approaches. [Figure 5-37]
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 typically 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.
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 OpsSpecs – Part C. CAT II
and III operations allow authorized pilots to make
instrument approaches in weather that would otherwise
While CAT I ILS operations permit substitution of
midfield RVR for TDZ RVR (when TDZ RVR is not
Figure 5-37. ILS Final Approach Segment Design Criteria.
available), CAT II ILS operations do not permit any
substitutions for TDZ RVR. The touchdown zone
RVR system is required and must be used. Touchdown
zone RVR is controlling for all CAT II ILS operations.
Figure 5-38. ILS Approach Categories.
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 OpsSpecs.
Depending on the auto-flight systems, some airplanes
require a DH to ensure that the airplane is going to land
in the touchdown zone and some require an Alert
Height as a final crosscheck 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 5-39]
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 OpsSpecs 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 OpsSpecs.