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Protection Of The ILS Critical Area  
User currently offlineR12055p From United States of America, joined Jul 2008, 43 posts, RR: 0
Posted (5 years 10 months 3 weeks 2 days 16 hours ago) and read 13498 times:

I was listening to KSEA ATC during the recent Seattle winter weather spell and heard an interesting conversation between a snow plow and the tower. The snow plow had entered the ILS critical area and the tower asked them to leave the critical area.

My question is, just how critical is the ILS critical area? I know ILS approaches are very sensitive and a small disruption could throw off an aircraft's approach and landing. While a fully loaded 744 with all of its advanced electrical systems might have a substancial influence on the radio signals used in ILS approaches, I find it difficult to understand how something like a snow plow could cause enough interference to make any real difference.

I have seen other posts on the accuracies of ILS approaches but none on the actual significance of keeping the critical area protected... please correct me if I am incorrect.

Thanks so much for any input,

Ross

20 replies: All unread, jump to last
 
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 1, posted (5 years 10 months 3 weeks 2 days 14 hours ago) and read 13480 times:



Quoting R12055p (Thread starter):
I have seen other posts on the accuracies of ILS approaches but none on the actual significance of keeping the critical area protected... please correct me if I am incorrect.

Here is a pretty good presentation on the importance of the ILS critical area.

http://corpapps.navcanada.ca/Content...ication_of_Critical_Areas_Quin.pdf


User currently offlineDogBreath From United Kingdom, joined Mar 2008, 265 posts, RR: 0
Reply 2, posted (5 years 10 months 3 weeks 2 days 14 hours ago) and read 13473 times:

Hope I don't bore you too much with this info.

The ILS system uses two separate antennae's to provide a horizontal and vertical guidance to the aircraft's VOR/ILS receiver. The horizontal antenna (or Localiser) is located at the far end of the runway (ie. the upwind end of the runway) and not affected too much by interference from taxying aircraft, vehicles due to the location of this antennae.
However the vertical (glidepath) antennae is located on either side of the runway (approx 120 metres from the centreline) adjacent to the touchdown point (normally 300 metres in from the threshold). Due to it's location and the fact that the antennae phasing is more critical because it is operating at a higher frequency and thus the wavelengths involved are much shorter, it can be greatly affected by taxying aircraft at the holding point for departure, and of course vehicles such as snowploughs, etc.
These two antennae's are very precise and since prone to interference, that translates into oscillations or scalloping of the signal received on the aircraft receivers and instruments. Normally during an approach the aircraft's autopilot/flight director is coupled to the ILS receiver and if these signals become disturbed (mainly to the glidepath due to it's location) results in the aircraft porpoising as the autopilot follows the signal.

Depending on the airport (it's equipment type) the ILS can be graded Category 1, 2 and 3. The levels of category are dependent on the airports location, prevailing weather and status (ie. type of aircraft operations, etc). As I mentioned the glidepath antennae is located adjacent to the touchdown point and the taxyiways leading to the departure end of the runway can pose a problem for this antennae. Therefore there are a number of holding point limits, such as a Cat 1 Hold point and Cat 2/3 Hold point. ATC can and will restrict the positions of taxying aircraft, vehicles to protect the ILS critical area/s.

On a Category 1 approach if an aircraft or vehicle moves into the ILS critical area, this isn't normally too much of a problem as the flying pilot can disengage the autopilot and fly the
approach manually if Visual or if still in IMC fly 'raw data' if able or conduct a missed approach if required. Some airports I've flown into in Europe will ask you on finals (when VMC) if it' OK for them to taxi an aircraft inside the critical area (as the Cat1 Holding point is quite a long way from the threshold - Berlin Schonefeld being an example). If it's visual then no problems as I'll fly the rest of the approach manually.
However in poor weather conditions and when low visibility procedures are in force, the ILS is upgraded to Cat 2 or 3 and the ILS critical areas must be clear at all times when aircraft are on approach. Hence the warning to the snowplough in your case.
As Cat 2 and 3 approaches require the use of Autopilot and Autoland (in the case of my Companies SOP's) it is critical that the ILS signals have no interference as the approach must be terminated and a missed approach carried out if there are any unusual autopilot/flight director inputs (ie. ILS interference).

So in a nutshell, yes, the snowplough can most likely interfere with the glidepath signal if it encroaches within the critical area.
Hope that helps.



Truth, Honour, Loyalty
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 3, posted (5 years 10 months 3 weeks 2 days 11 hours ago) and read 13430 times:

The ideal glide path transmitter antenna would be an array centered at the touch down point, with half the elements above ground level and half below ground level. This would be rather impractical, to say the least, as it would involve significant excavation and interfere with landing aircraft.

Instead, you place the GP antenna next to the runway and (to some degree) compensate for this displacement by antenna placement to get a glide path leading to the touch down point rather than the base of the antenna.

You also keep all the transmitter antenna elements above ground. Through clever phasing, you use the ground in front of the antenna as a mirror (the ground is a good reflector in this part of the electromagnetic spectrum) to create a mirror image of the antennas above ground which sets up the glide path signal in space.

Looking from the vantage point of the aircraft GP antenna, you have two or three antennas above ground, mirrored in the ground in front of the GP mast to form an image of two or three antennas below ground level as well.

With this in mind, it is easy to understand that driving anything, such as a snow plow, across the area causing the reflection will disturb the signal.

The most critical area is the area closest to the GP mast. Walking through this area will often be enough to disturb the glide path signal enough to trip the near field monitor and take the GP off the air.

The disturbance caused is not due to electrical systems in the vehicles but rather due to their physical characteristics. Snow, moisture, layered snow, snow walls, vehicles, mounds of earth from digging, equipment... the potential sources are many!

The further you get from the GP mast, the less sensitive the GP is to objects in the reflection area. It is hard to say just how long the area you have to consider is, but if you are going to do any serious work within 700 m or so in front of the GP mast, you better consider the implications. That's why you have critical areas and protected/sensitive areas.

Rgds,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineEMBQA From United States of America, joined Oct 2003, 9364 posts, RR: 11
Reply 4, posted (5 years 10 months 3 weeks 2 days 11 hours ago) and read 13418 times:



Quoting R12055p (Thread starter):

My question is, just how critical is the ILS critical area?

How critical is the critical area...? Pretty critical. Big grin OK.. serious now... yea, going into that area with out Ground Control approval during LS landings will get your hand smacked pretty hard. There is a BIG yellow line painted on the taxiway and a big sign that says ILS Critical Area.



"It's not the size of the dog in the fight, but the size of the fight in the dog"
User currently offlineSaab2000 From Switzerland, joined Jun 2001, 1618 posts, RR: 11
Reply 5, posted (5 years 10 months 3 weeks 2 days 9 hours ago) and read 13383 times:

I have been on approach to ORD when they taxied something across the area and the glide slope became unstable as the transmission became disrupted. The A/P tried to track what it thought was the glideslope and pitched up and down rather severely. Took the autopilot off and flew by hand. Not a huge deal as we were still about 7 miles out, but yeah, that critical area is there for a reason.


smrtrthnu
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 6, posted (5 years 10 months 3 weeks 2 days 7 hours ago) and read 13329 times:

You don't even want to know what the GP signal looks like if you have a big friggin' five blade transport helo hovering in front of the GP mast. It's not pretty! Big grin


I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineAbnormal From UK - England, joined Aug 2007, 81 posts, RR: 0
Reply 7, posted (5 years 10 months 3 weeks 2 days 6 hours ago) and read 13310 times:

One of the bugs about Airbus Autoland is that F/Os can't get any line experience using it in CAT 1 or better weather because of the FCOM autoland restriction about ensuring ILS sensitive areas are protected.

Anybody ever find a work around? Would ATC agree to protect the area if requested in advance?


User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 8, posted (5 years 10 months 3 weeks 2 days 4 hours ago) and read 13278 times:



Quoting Abnormal (Reply 7):
Anybody ever find a work around? Would ATC agree to protect the area if requested in advance?

Have never had a problem doing an auto land in CAT I wx or better. No real need to protect the critical area since you are not IMC. We have to do a auto land on all our aricraft every 28 days and it'a never a problem.


User currently offlineFlipdewaf From United Kingdom, joined Jul 2006, 1578 posts, RR: 0
Reply 9, posted (5 years 10 months 3 weeks 2 days 4 hours ago) and read 13276 times:
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Here is EGNV on Google maps, You can see a place called urlay nook on the approach to runway 23.

http://maps.google.co.uk/?ie=UTF8&ll...503&spn=0.035723,0.077248&t=h&z=14

At urlay nook there is a factory we call smokey joes and the the ILS signal seems to get bent slightly around the factory but it is always the same so no one really bothers about it.

Fred


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 10, posted (5 years 10 months 3 weeks 2 days 3 hours ago) and read 13243 times:



Quoting Flipdewaf (Reply 9):
At urlay nook there is a factory we call smokey joes and the the ILS signal seems to get bent slightly around the factory but it is always the same so no one really bothers about it.

That's interesting. GP or LOC? Bent how, how does it show?



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineVikkyvik From United States of America, joined Jul 2003, 10239 posts, RR: 26
Reply 11, posted (5 years 10 months 3 weeks 1 day 19 hours ago) and read 13088 times:
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Quoting Abnormal (Reply 7):
One of the bugs about Airbus Autoland is that F/Os can't get any line experience using it in CAT 1 or better weather because of the FCOM autoland restriction about ensuring ILS sensitive areas are protected.

Could you explain this to a layperson?

Thanks  Smile



How can I be an admiral without my cap??!
User currently offlinePilotpip From United States of America, joined Sep 2003, 3151 posts, RR: 11
Reply 12, posted (5 years 10 months 3 weeks 1 day 18 hours ago) and read 13071 times:

Snow kills the glideslope. The antenna is actually some feet beyond the touchdown point and the signal is bounced off the ground, and up into the air. When the ground is snow covered the radio waves are scattered everywhere and as a result the signal is often degraded or completely unusable. This often results in getting a LOC only approach when visibility is already low because of snow. Good times.


DMI
User currently offlineAbnormal From UK - England, joined Aug 2007, 81 posts, RR: 0
Reply 13, posted (5 years 10 months 3 weeks 1 day 10 hours ago) and read 12984 times:



Quoting Vikkyvik (Reply 11):

ILS approaches in CAT II or III conditions may be flown by either pilot but only the Capt is authorized to use the Autoland capability of the aircraft. That leaves CAT I or better as the only opportunity for F/Os (first officer) to get actual real aircraft flying experience/practice with Autoland.

Unfortunately, in CAT I or better conditions ATC does not neccesarily protect the ILS critical areas as they do when CAT II or CAT II weather conditions are in effect and interference from surface vehicles with LOC and G/S beams is common in CAT I conditions.

So if the F/O is flying the aircraft in CAT I or better conditions to gain experience with autoland, even though the conditions are CAT I, the crew are practicing as if it were CAT II or III and both pilots are heads down, monitoring the aircraft, and nobody is looking outside. If there is interference bending the beam(s) then the aircraft may be put into a dangerous situation.

We could modify our procedures to have one pilot maintain a heads up watch outside to watch out for glidepath deviations but interference may not happen until the aircraft is close to the ground where it poses undue risk. For that reason our regulator won't permit us to do them.


User currently offlineVikkyvik From United States of America, joined Jul 2003, 10239 posts, RR: 26
Reply 14, posted (5 years 10 months 3 weeks 1 day 7 hours ago) and read 12924 times:
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Thanks for your reply, Abnormal.

Quoting Abnormal (Reply 13):
ILS approaches in CAT II or III conditions may be flown by either pilot but only the Capt is authorized to use the Autoland capability of the aircraft. That leaves CAT I or better as the only opportunity for F/Os (first officer) to get actual real aircraft flying experience/practice with Autoland.

So if I've got this straight:

CAT 1 - Capt (hand-fly or autoland) / FO (hand-fly or autoland)
CAT 2/3 - Capt (hand-fly or autoland) / FO (hand-fly)

Can you hand-fly a CAT 2/3 approach without a heads-up guidance system?



How can I be an admiral without my cap??!
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 15, posted (5 years 10 months 3 weeks 1 day 6 hours ago) and read 12893 times:



Quoting Pilotpip (Reply 12):
Snow kills the glideslope. The antenna is actually some feet beyond the touchdown point and the signal is bounced off the ground, and up into the air. When the ground is snow covered the radio waves are scattered everywhere and as a result the signal is often degraded or completely unusable. This often results in getting a LOC only approach when visibility is already low because of snow. Good times.

Nitpick: The antenna is not beyond the touch down point. As described above it uses the ground as a mirror to create a mirror image of the mast. The GP originates at the base of the mast, or rather at the runway centerline at the point at which the direction to the GP mast is perpendicular to the runway. Ehm. I think that came out a bit confuscated. Oh well.

Snow will first of all cause the glide slope to drop and go asymmetrical, i e the upper half sector will become wider than the lower half sector. When the snow is layered, such as you will get if there are periods of thawing alternated with snowfall, it is especially damaging to the signal characteristics of the GP. This is due to a phenomenon known as ducting, where the signal will bounce between the layers for a distance before emerging from the muck and reflecting up towards the aircraft.

Eventually this will trip the near field monitor and bring the GP off the air. It can be very bad before this happens though.

Then there's snow and ice accumulation on the antennas. In regions where snow is rare, the antennas may be completely unprotected. That will just wreak havoc on the signal, but my relatively amateur guess (there are people who analyse this stuff for a living) is that it'll be worse for the LOC antennas.



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineMD11Engineer From Germany, joined Oct 2003, 14127 posts, RR: 62
Reply 16, posted (5 years 10 months 3 weeks 16 hours ago) and read 12722 times:



Quoting Flipdewaf (Reply 9):
Here is EGNV on Google maps, You can see a place called urlay nook on the approach to runway 23.

http://maps.google.co.uk/?ie=UTF8&ll...503&spn=0.035723,0.077248&t=h&z=14

At urlay nook there is a factory we call smokey joes and the the ILS signal seems to get bent slightly around the factory but it is always the same so no one really bothers about it.

Fred

In HHN, our pilots told us that there is a significant bumpin the glideslope (coming from the east) and that the aircraft tend to land a bit left of the centerline (AFAIK, this is due to the wid coming mostly from the west and the HHN runway runs in a north-east-soth-west direction, and the aircraft strightening out just before touchdown). AFAIK, there is also a difference if we have our (metal) hangar doors opened or closed, since they act like a reflector for radio waves of this part of the spectrum.

Jan


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 17, posted (5 years 10 months 3 weeks 8 hours ago) and read 12640 times:



Quoting MD11Engineer (Reply 16):
In HHN, our pilots told us that there is a significant bumpin the glideslope

Do you know what type of GP system is installed at EDFH?

Most GP installations today are dual-frequency. In other words, you are transmitting a clearance signal below the GP beam on a slightly different frequency, giving a strong fly-up signal. Due to the capture effect (google it) the ILS receivers will almost exclusively register the strongest signal even when they are receiving both. The clearance signal ensures that aircraft will have fly up when below the GP beam in the entire GP coverage volume (which is a requirement).

Problems will arise when there are reflectors on the ground in the coverage area. They may reflect the clearance signal up through the GP beam and overwhelm it with a fly up signal strong enough to register in the aircraft receiver. This will be very local. Receivers have a strong damping built into them which smoothens such disturbances, turning it into a slow "speed bump".

Quoting MD11Engineer (Reply 16):
AFAIK, there is also a difference if we have our (metal) hangar doors opened or closed, since they act like a reflector for radio waves of this part of the spectrum.

Building hangars near the runway can often mean having to invest money in a better LOC antenna array. Generally speaking, the more transmitter elements there are in a LOC antenna array, the narrower and less sensitive to disturbances from hangars, taxiing aircraft etc it will be. A six element localizer will require all hangars etc to be quite far removed from the runway. At the other end of the spectrum I know of one 64 element localizer, required due to a forest which could not be cut down being located close to the threshold end of the serviced runway.

Quoting MD11Engineer (Reply 16):
and that the aircraft tend to land a bit left of the centerline

As you alluded to, this is hardly due to the localizer being off center. Even low-spec localizers will rarely be more than one meter or so offset in my experience and that is at a point a distance out from the threshold . I think you are absolutely correct in attributing it to other environmental factors such as wind or maybe a visual picture which throws the pilots off.

It is a lot easier to monitor a LOC than a GP, as you can have a far field monitor on the ground at the threshold end of the runway. It will immediately show if the LOC is off.

Rgds,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineMD11Engineer From Germany, joined Oct 2003, 14127 posts, RR: 62
Reply 18, posted (5 years 10 months 3 weeks 8 hours ago) and read 12626 times:



Quoting FredT (Reply 17):
Quoting MD11Engineer (Reply 16):
and that the aircraft tend to land a bit left of the centerline

As you alluded to, this is hardly due to the localizer being off center. Even low-spec localizers will rarely be more than one meter or so offset in my experience and that is at a point a distance out from the threshold . I think you are absolutely correct in attributing it to other environmental factors such as wind or maybe a visual picture which throws the pilots off.

The problem in HHN is, that unlike most other airports, we often have a combination of CAT3 fog PLUS strong cross winds (due to the fact that HHN is sited on a hilltop almost 500m above sea level and during bad weather periods we are actually already inside the clouds). I think that most pilots are not used to such conditions. Last week we had CAT3B conditions and at the same time 35 kts wind in gusts. Most other places have either low visibility OR high winds, but rarely both at the same time.

Jan


User currently offlineSpeedbird001 From United Kingdom, joined May 2000, 30 posts, RR: 0
Reply 19, posted (5 years 10 months 3 weeks 3 hours ago) and read 12573 times:

Could someone please clarify something for me as regards using the coupled ILS.

I fly the 737 and we arm the autpilot mode VOR/LOC to capture and track the localiser and APP to capture the follow the glideslope.
There has been some debate about when to engage these modes, most common is the idea that you shouldn't couple to the glisdeslope until within 10nm. I'm quite new to the aircraft and I'm not sure if this is an aircraft recommendation or to do with the reliablity of the ILS signal and protection as mentioned above.
Could anyone shed some light on the distances where the signals are approved for use or how to find this info?

Many thanks


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 20, posted (5 years 10 months 2 weeks 6 days 7 hours ago) and read 12459 times:

Speedbird,
the glide path service volume typically only extends to 10 nm out from the touch down point. Outside of the service volume the glide path is not flight checked, nor is it designed to provide correct information further out. In other words, you have no guarantee at all that you will receive correct information from the correct glide path (yes, you could theoretically receive the glide path from another facility operating on the same frequency) when further out.

Regards,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
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