Facciamo un paio di esempi pratici:
grosso aeroporto trafficato e con multiple runway ops, condizioni meteo buone (ben al di sopra dei 550m RVR e/o ceiling ben sopra i 300ft) nel quale si e' numeri XXX all'atterraggio ad un paio di NM dal traffico precedente e con vari altri traffici davanti a noi sul sentiero ILS ed altrettanti che decollano dalla stessa RWY dove andremo ad atterrare senza contare varie RWY crossing di traffici a terra.
Siamo tutti quanti con piano di volo IFR , nessuno ha chiesto di fare dei visual e gli atterraggi "land after" sono la norma.
Le interferenze possibili al segnale ILS sono numerose e di varia natura.
In questo scenario si presuppone pero' che le condizioni meteo permettano delle operazioni "a vista" sia da parte degli aeromobili che dell' ATC (TWR) ed eventuali interferenze al segnale ILS sono accettate e previste dalla regolamentazione visto che si presuppone una correzione visiva in caso di anomalie del segnale in fase d'atterraggio a basse quote.
Tutt'altro discorso in caso di LVP.
Detto cio' e' anche vero che di questi "visual holding points" se ne vedono pochissimi e nella maggior parte degli aeroporti dotati di ILS si dividono in CAT I,II e III e vengono specificati nella clearance dall' ATC in base alle condizioni operative del momento.
Qui di seguito una lettura con qualche dato sull'argomento:
USE OF AUTOLAND SYSTEM ON ILS CATEGORY I FACILITIES OR CATEGORY II/III
FACILITIES WHEN LOW VISIBILITY PROCEDURES (LVP) ARE NOT IN FORCE
1. Introduction
1.1 Most ILS installations are subject to signal interference by either surface vehicles or aircraft.
To prevent this interference ILS critical areas are established near each localizer and
glideslope antenna. For Category II and III ILS installations, additionally an ILS sensitive area
is established. The critical areas are restricted from all vehicle or aircraft operation when the
ILS is being used, or at least when an aircraft is inside the Outer Marker or equivalent position.
At US airports the LLZ and GP critical areas will be protected when weather at the airport is
reported less than 800 feet ceiling and/or visibility is less than 2 miles with an aircraft inside the
OM or equivalent position.
1.2 The ILS sensitive area is protected from all surface vehicles and aircraft when ILS Category II
or III operations are conducted or anticipated. This typically takes place when landing
operations are conducted with RVR less than 600 m or with a ceiling of less than 200 feet. At
US airports the sensitive area is called the ”ILS critical area” (not to be confused with the LLZ
or GP critical areas which are smaller). The US ILS critical area is protected during the
corresponding conditions. The operators need to inform their pilots about the differences in
terminology which are summarised in the following Table:
ICAO/JAA US
ILS Critical Area(s) LLZ & GP Critical Areas
ILS Sensitive Area ILS Critical Area
Note: Appendix 1 to JAR-OPS 1.455 at Sub-para (b)(2)(x) refers to the requirement for the Localiser Sensitive Area to
be protected. A future NPA will propose that this be changed to the ILS Sensitive Area
1.3 LVPs on an aerodrome basically have two main objectives: to prevent collisions involving an
aircraft on the aerodrome and to ensure that the ILS sensitive area is protected. To find out if
LVPs are in force, the pilots need to contact ATC or listen to the ATIS.
1.4 When the LVPs are not in force, ILS beam bends may occur because of vehicle or aircraft
interference. Sudden and unexpected flight control movements may occur at a very low
altitude or during the landing and rollout when the autopilot attempts to follow the beam bends.
A Category I ILS is not required to support autoland operations although it is recognised that
some Category I ILS facilities can support autoland operations. Unless specifically mentioned
in the AIP, it must be assumed that ILS signals to Category I runways are not flight inspected
below 100 feet AGL, and therefore guidance signal anomalies may be encountered below this
altitude without advance warnings.