ILS SENSITIVE/CRITICAL AREAS

[terry1261]

I am reliably informed that putting anything in these ILS areas will cause misleading cockpit indications. I am also reliably informed that some atc units taxi aircraft through these areas while aircraft are flying the ILS. Is such practice sensible, safe??

[IRTRAINER]

Happens regularly at bournemouth, without warning, slightly less worrying when CAVOK. I guess it wouldn't happens in very bad weather:confused:

[chevvron]

Last time I was at Cranfield, the localiser was off the airport across a busy main road, so I bet you get lots of interference there!
All IFR airports should have low vis procedures to mitigate the problem you state though.

[John Doe II]

I think normal practice is clear the area before an arriving aircraft reaches 2 NM or until a departing aircraft is airborne during LVP

[goddammit]

So, aicraft attempt to fly an ILS while atc cause the indications to be unreliable, do the pilots try to follow the cockpit indications or do they assume the indications are wrong? If the glidepath is not accurate, does that have obstacle clearance implications? Does one get a refund or is one still charged for the full price of an ils approach??

[IRTRAINER]

Refund? I'd rather pay full price and always fly an accurate ils

[SixDelta]

It depends on the category of the installation and whether or not LVPs are in force.

An airfield having only a CAT 1 installation will still have procedures to ensure that the ILS is fully protected during use in LVPs.

The worst variations in ILS signal occur as an aircraft fly's over the Localiser antenna on departure, this will be visible in the cockpit as a rapid fluctuation of the CDI lasting a second or so, nothing major.

[HEATHROW DIRECTOR]

There appear to be a few ignorant people on this thread - and no offence intended.

Terry 1261.. You do not say what you do on your profile but it seems you are not too familiar with aviation ops. Why do you say "I am also reliably informed that some ATC units taxi aircraft through these areas while aircraft are flying the ILS. Is such practice sensible, safe??" Anyone on here - pilot or controller - would confirm that that is precisely what happens at every ILS-equipped airport in the world thousands of times a day. At busy airports aircraft will be taxiing across runways up to the point when landing aircraft are only a mile or less from touchdown.

Under LVPs (low visibility procedures) the airfield authority will arrange for the Localiser Sensitive Area to be protected. Under any other operations they will not. ATC will adjust operations, both in the air and on the ground, to provide adequate spacing and separation to preserve the integrity of the ILS.

The only time restrictions are required is during bad weather. At other times it is perfectly normal procedure because during fair weather conditions aircraft will usually land "visually". If a pilot informs ATC that he will be carrying out a practice autoland in good weather, ATC will usually remind him that the ILS will not be fully protected.

[chevvron]

HD - His profile mentions Cranfield, that's why I put the bit about Cranfield LLZ in. Surprisingly it's Cat 1 approved, in spite of the probability of the traffic on the uncontrolled external road causing fluctuations in the signal.

[vintage ATCO]

The sensitive areas and crtical areas around ILS installations are different. It is the sensitive areas we protect in LVPs for Cat II/III ops. This is why runway holding points are further back and outside the respective SA. When operating at Cat I we use the normal holding points which are inside the Localiser Sensitive Area. (I know LVPs are not just for Cat II/III :D )

Critical areas are much smaller and are just around the LLZ and GP antennas and usually marked by small fences/warning signs. If anything goes into a critical area for more than a second or so the monitors will turn the kit off. Therefore acft are not taxied through critical areas (in fact I don't know anywhere where a critical area would impinge on a taxiway.)

It does intrigue when you have a public road between the LLZ and the runway. I noticed this at Venice. It would be outside the CA and presumably the installation is not used for better than Cat I.

[JustaFew]

Placing a vehicle, or any larger object, in the critical area will affect the cockpit indications. If carrying out an autoland in such a situation, the plane will attempt to follow the signals and produce an irregular flight path. Pilots usually then disconnect the autopilot and hand-fly the approach, or go-around. Airfields should have within their MATS part 2 procedures which prevent vehicles, planes being within the critical area during an ILS approach, thus protecting the integrity of the ILS signals.
Outside the critical area but within the sensitive area, there will often be planes taxiing, taking off, or landing. This does not appear to upset the cockpit indications, and allows more aircraft movements.

[vintage ATCO]

Placing a vehicle or any larger object, or even smaller, inside a critical area will knock the kit off, believe me. As soon as the near field monitor detects any irregularities with the signal it tries standby and if that is the same the kit is shut down. This happens in very short order.

Critical areas are protected at all times otherwise the kit will keep failing. In order to permit, say, grass cutting inside critical areas we have to turn the particular kit off.

[Dizzee Rascal]

Therefore acft are not taxied through critical areas (in fact I don't know anywhere where a critical area would impinge on a taxiway.)


The little place I work at with a CAT1 ILS has a taxiway (and holding point) in between the GP and the RWY, infringing the GP critical area. Aircraft on the taxiway are held at an intermediate holding point outside the critical area until the aircraft using the ILS has reported visual, going-around or has landed.

[vintage ATCO]

I see there is a taxiway between the GP and the runway but I suspect the GP crtical area does not actually impinge on the taxiway, otherwise the kit would fail if someone taxied past it. I can see the desireability of not holding at D1 with someone on the ILS if this is what you mean.