Why are there delays in fog?
 

Thought I'd write a post on why delays increase in fog.

Mods, if you want to merge this with the recent fog disruption thread, please do.

Dealys go up because the landing rate goes down.

Right, let's start with good weather to establish the baseline.


The attached picture shows the usual exit (N6) taken by 747s and 777s etc from the southern runway on westerly operations - 27L (ie. landing towards the west). In normal operations, ATC can give a landing clearance when the aircraft ahead has crossed the runway edge (25m from the runway centreline - green line on the picture) and is continuing to move away from the runway. The landing clearance must be given before the arriving aircraft reaches the beginning of the paved runway surface. This results in a minimum of 3nm spacing on approach (2.5nm in a headwind of 10kts), and a landing rate of around 40-42 aircraft per hour.

When the visibility gets bad enough so that ATC cannot see certain runway exits from the control tower, we go into what we call Vis2 procedures (more colloquially termed 'Tower in Cloud'). Now we can only give a landing clearance when the aircraft ahead is more than 77.5m from the runway centreline (light green line). This means that we increase arrival spacing to achieve this; 4nm behind a Medium (737, A320 series etc), 5nm behind a Heavy (747, 777, A350). Results in a landing rate of 30-36.

In both cases, the aircraft will be flying a CAT I approach, which means that the flight crew must be visual with the runway (or at least the approach lights) at 200ft height above the runway, 2/3nm from touchdown. If they can't see anything, they go around.

When the visibility gets below 600m, we go into ATC Low Visibility Procedures (LVPs). This means we have to protect the radio signals of the Instrument Landing System to a much greater degree, to CAT III, to permit aircraft to perform autolands. As aircraft turn off the runway, the tail can 'bend' the ILS beam so much it would endanger aircraft following it on approach. During good weather, the fact that crew are visual below 200ft is the mitigation for this. In less than 600m visibility, there is no mitigation, so we have to ensure this beam bending does not occur when an aircraft close to the ground and following the ILS signal. This means that all aircraft have to be more than 137m from the runway centreline (orange line) before we give a landing clearance. Also, we have to give the landing clearance before the arriving aircraft reaches 1nm from touchdown. To achieve this, we have to have 6nm minimum spacing between aircraft, which results in a landing rate of 24 per hour.

However, LHR is in the forefront of optimising operations in poor weather. We have the Microwave Landing System, which is less susceptible to the bending described above. If an MLS-equipped aircraft is on approach in LVPs, then it only needs the aircraft ahead of it to be 82m away from the runway centreline (blue line), so we can place a 5nm, instead of a 6nm, gap ahead of it.

Also, we are commissioning replacement ILS systems, which again are less susceptible to bending, so that any Medium (737, A320 etc) aircraft only needs to be 82m away from the runway centreline (blue line). Again, this means a 5nm gap behind any Medium. Landing rate now is 28-30 rather than 24.

The bottleneck in fog is not aircraft finding their way from the runway to terminal and vice versa, or ATC losing track of them, it's all about how far from the runway the lead aircraft has to be before ATC can clear the following aircraft to land.

Any questions, please fire away!
https://cimg6.ibsrv.net/gimg/www.fly...b8c5079985.jpg




To see what happens when an ILS beam is bent and an aircraft follows it, have a look here, a Singapore Airlines 777 landing at Munich was doing a practice autoland but the ILS wasn't protected:

Edit: I have added back in the new picture. Also, we have improved things since I wrote this post, anything but widebodies and 757s can now be 77.5m away from the runway (light green) even in the poorest visbility, rather than all requiring to be 137m from the centreline.

Fascinating to hear the real reason behind it all, there's been a lot of untruths posted here recently - thanks for the corrections.

Thanks for this.

Same as driving a car, keep more distance in adverse conditions which will mean slower traffic.

And as Heathrow is running close to Capacity there is no chance of recovering. Only option is some cancellation.

Very nice, thanks for this, and please don't merge this thread with any others let alone the disruption thread!

Good read indeed. ^

I'll second this. Thanks very much for posting.

Fascinating - great to get this insight. ^

Thanks Heathrow Tower. Fascinating. I'd always thought the lower rate in fog had to do with leaving a bigger gap for safety, like you would in car. Interesting to learn that it's more scientific than that.

That video looks worrying. Am I understanding correctly that the aircraft tried to follow the beam round a bend?!

Yes, the aircraft followed the ILS beam that was being bent by another aircraft in the protected area.

Fascinating - thanks for taking the time to explain

Great thread. Thank you so much Heathrow Tower. I’ve added a link to the dashboard so we can find this thread whenever we need it in the future

Excellent reading! I knew the overall reasons for it, but it is always great to read learn more about it.

Great read thank you Heathrow Tower

Really interesting information. Thanks.