# How do preceding aircraft affect the ILS signals?

(airliners.net) Planes on approach in sequence to London Heathrow.

When LVP is in effect, the ILS area is protected.

I'm wondering, when there is a queue to a runway, how does the long queue affect the signals for those behind?

I'm guessing it does somehow, and the answer may shed some light on the new Enhanced ILS.

I've never noticed any perturbation of the ILS signal from aircraft ahead in flight. Aircraft on the ground crossing the runway, absolutely yes. Something big enough (747 or A-380) exiting at the far end, perhaps briefly, but not generally something you notice.

I'll let somebody else explain the theory, but that's my observed results.

The photo in the question is actually what you get when ATC issues visual approaches. Visual approaches allow many more planes per hour and relieve ATC workload. For a true ILS approach the absolute minimum radar separation is 3 miles (even after the lead plane slows for landing) and up to 6 miles when following a super. In addition, for aircraft over 12500 pounds [leading or following] the runway must be cleared before the following plane crosses the threshold.

Now the effect of aircraft on the ground is different because they may be very close to the transmitter and so can block, absorb, or distort a large percentage of the transmission. Similar to blocking a flashlight with your hand; very close will block a large portion of the light but at several feet away you can only block few percent. On top of this there is diffraction lensing around the lead aircraft so that it doesn't make a solid shadow, this effect is overpowers by the wide angles associated with an aircraft on the ground immediate to the transmitter.(see Aragoscope for more details) In short Aircraft on approach are a long distance from the transmitter so they have little effect and they exit the runway fairly quick after landing. The effect is roughly the inverse of the distance from the transmitter cubed. $$\frac{1}{dist^3}$$ So an object at 6000 feet(one N.mile) will have about $$\frac{1}{216}$$ of the effect that the object has at 1000 feet. (0.46%)

• Visual approaches is something you get in US, but in Europe they are much less common. I seriously doubt they are issuing visuals in EGLL that much and if then on pilot's request. – Vladimir F Mar 12 '19 at 15:35
• The picture in the question may have the perspective distorted from photographic effects, but assuming it is accurate ATC would not keep that close spacing without pilots visually separating themselves. At least in the USA it is a matter of legal responsibility, if minimums are violated even a little bit while under radar separation(flagged by computer for review) then either the controller is in violation for loss of separation or the pilot is in violation for deviating from ATC instructions, as such controllers leave extra space for unexpected speed changes. – Max Power Mar 28 '19 at 16:19
• If controller and pilot agree to pilot visual separation the pilot is responsible for avoiding wake and collision with the specific leading aircraft; minimum positive separation(3 miles) for radar control no longer applies. The USA also uses tower visual separation, in which the controller in the tower out of the window an talks to two airplanes, this is used both at small airports without radar and for the first few miles after takeoff at busy airports to achieve radar minimums before radar control accepts the hand-off. (plane A may depart left and B depart right but only 2 minutes after) – Max Power Mar 28 '19 at 16:22
• Busy airports in the USA use visual separation anytime the weather permits as it allows many more planes per hour and less control work which allows the controller to handle more planes at once and reduces radio congestion. At least 7 of these airports move more planes per year than EGLL.(sorry for three posts, character limits combined with my verbosity) – Max Power Mar 28 '19 at 16:34
• There is a difference between visual separation and a visual approach. The saparation of traffic can be visual even on the ILS approach. – Vladimir F Mar 28 '19 at 17:14