Multiple Aircraft on Localizer

[M Invernoz]

Hello everyone:

Talking to one of my parters both of us got in the question about Multiple aircraft stablished on the ILS.
I Know vectoring to the LOC can be done in Radar Control airspaces and the minimums can be reduced in sequence, but the real question goes beyond that. It´s more technical. about the ILS equipment itself. What about the interference of the ILS signal? This signal will reflect on the first plane and this will interfere to the second one and give him untrusted or unreliable readings?
In procedural control one aircraft may approach at any one time, But let´s suppose an airport in VMC conditions where the aircrafts are in visual contact by the TWR but they still doing the ILS procedure. The signal will be unreliable for the second aircraft? or lets put another scenery where in VMC one IFR aircraft is on the ILS with 5100 feet,15NM from touchdown and a VFR aircraft is crossing from left to right with 2000 feet 5 miles ahead. The chart waypoint at 10NM out states the aircraft must have 3700 feet and here is the big one. The VFR aircraft will not be on the approach path of the IFR one but......The VFR aircraft makes any interefence on the ILS signal to the IFR one? does the signal get weak? unreliable? hope to be clear on the examples and questions, my english is a little rusty.

[Talkdownman]

It's worked alright at Heathrow for several decades...

[Dont Hang Up]

With the frequencies used and the nature of the signal the distortions tend to be very limited.

Remembering that the localiser signal is generated by an antenna array beyond the far end of the runway, the main risk of interference is when a landing aircraft reaches the end of the runway and turns off. It is the effect of the aircraft turning (changing its profile) and moving off the centre-line that creates the distortion.
That distortion, while it may be observed as a wobble all the way out on the approach, is only likely to be critical for a landing aircraft in the final stage of approach - maybe the last half-mile.
While it is improbable that the distortion would be sufficient to destabilise the approach, I understand that normal practice, under low visibility conditions, would never allow landing sequencing to be that tightly spaced anyway.

[10 DME ARC]

This takes me back to my CAA ATCA interview at Manchester Airport back in 1981. One of the board members, who was an engineer, asked me if I thought a helicopter flying an ILS would disrupt the localiser with its rotor blades! I bumbled something like it would have to be very close to the localiser aerial for this to happen. He then gave me and the other two board members a lecture on how he thought it would!! I got the ATCA post and went on to do my ATCO cadetship and now all these years later have never heard another thing about this!!
In ATC we give larger spacing during LVO's because of the possibility of disruption of localiser/glidepath but I do remember once being told by a very senior BA captain that these distances could be greatly reduced and best leave it to aircrafts sensors to decide on go-around rather than sending aircraft round due to less than required spacing!

[Gonzo]

This takes me back to my CAA ATCA interview at Manchester Airport back in 1981. One of the board members, who was an engineer, asked me if I thought a helicopter flying an ILS would disrupt the localiser with its rotor blades! I bumbled something like it would have to be very close to the localiser aerial for this to happen. He then gave me and the other two board members a lecture on how he thought it would!! I got the ATCA post and went on to do my ATCO cadetship and now all these years later have never heard another thing about this!!
In ATC we give larger spacing during LVO's because of the possibility of disruption of localiser/glidepath but I do remember once being told by a very senior BA captain that these distances could be greatly reduced and best leave it to aircrafts sensors to decide on go-around rather than sending aircraft round due to less than required spacing!

Modern autoland systems are very good at ignoring transient fluctuations, so in many ways there is merit in this approach (pardon the pun). The difficulty is in determining the capability of the equipment on the aircraft in order to permit this.

When we get to type-specific localiser sensitive areas, with ATC tool support, one could see this procedure being used, though it would require a great deal of assurance work.

[M Invernoz]

It's mainly to do with the size of the aircraft relative to the ILS signal. When an aircraft is close (within a few hundred metres) to the LLZ antenna it can have a significant effect on the signal for aircraft flying the approach because it it blocking and reflecting much of the signal that is being transmitted, at five miles from touchdown, for example, the size of an aircraft relative to the signal is tiny and will have no noticeable impact on the signal for following aircraft. ATC procedures, particularly those used in LVP, ensure that preceding landing aircraft have vacated the runway (and so are out of the main area that can affect the signal) and departing aircraft have passed over the LLZ antenna before an approaching aircraft reaches a point where the impact on the signal is operationally significant. In practise, it is not uncommon for there to be little 'blips' or wobbles in the LLZ in good weather conditions because the LVP restrictions are not in force and aircraft/vehicles do have an effect on the LLZ signal.

All ILS antennas have what are known as sensitive areas and critical areas and it is these that are protected and, if aircraft or vehicles go into them which may result in a noticeable effect on the LLZ signal. Size of the aircraft is also relevant - in some cases the areas to be protected are larger for B747 and A380 aircraft than for other types. There's lots of good information on ILS protection on Wikipedia and Skybrary so I won't try to replicate it here.

Yes, I know there´s a lot of info about the critical and sensitive areas of the ILS. But they all refer to the vehicles or aircraft ¨on ground¨, not about several Aircraft on the LOC. I know there are tons of airports like Zurich or Heathrow where they have 4 o 5 planes on final But is there any advisory or information about the interference on LOC signals when this approaches are made? any aircraft in the world performed a go around due to signal lost?

[Dont Hang Up]

Yes, I know there´s a lot of info about the critical and sensitive areas of the ILS. But they all refer to the vehicles or aircraft ¨on ground¨, not about several Aircraft on the LOC. I know there are tons of airports like Zurich or Heathrow where they have 4 o 5 planes on final But is there any advisory or information about the interference on LOC signals when this approaches are made? any aircraft in the world performed a go around due to signal lost?

Perhaps you are thinking of the ILS as some pencil-thin guidance beam that the aircraft flies down. In which case you may think that aircraft ahead might "block" the signal. In reality that is not how it works. The Localiser is actually two transmission beams with quite a wide spread. The narrow centre-line guidance is just where the two signals exactly balance. That is a simplification but good enough for here I think. Remember also that the localiser equipment is some way beyond the far end of the runway, so any aircraft ahead may be between you and the runway threshold but are not between you and the localiser antennas.

As said before, the main risk is when an aircraft at the far end of the runway turns off and creates a non-symmetrical distortion of the radio beams (it is now quite close to the antennas) and this potentially shifts the balance point of the two signals as seen by the approaching aircraft.