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In strong headwinds, aircraft fly more slowly over the ground – effectively it is like walking the wrong way along a moving walkway, so maintaining a fixed separation distance in accordance with ICAO rules results in a reduced landing rate. For example with a Heavy – Heavy wake pair such as a 777 following an A340 the ICAO wake separation distance is 4nm. In light headwinds it takes the aircraft about 90 seconds to cover this distance. But with a 35 knot headwind at 3000ft which is not particularly strong, it would take the aircraft 107 seconds to cover the 4 miles, effectively losing 17 seconds worth of runway capacity for that wake pair. Click Next Block The new Time Based Separation Rules keep the time interval between arrivals the same irrespective of headwind conditions and allow us to close up the gap – so in the case of the heavy to heavy wake vortex pair the gap is reduced to 3.4nm in a 35 knot headwind thereby recovering the lost capacity. And the data that we have shows that it is safe to do that as the wake vortex decays mush faster in headwinds. Where wake is an issue is in still air or light quartering tailwinds and under those circumstances the tools that we have developed will increase separation. And the benefits are clear – with distance based separation Heathrow’s landing rate in calm winds is between 40-45 dependent on the mix of arrivals. In strong headwinds this falls to between 32-38 resulting in delays, increased holding and can result in cancellations. With Time Based Separation we recover more than half of this lost capacity back. The reason why it is not 100% recovery is that we do not drop below minimum radar separation of 3nm.
Went live LOS 24 MAR 2015, CAA Signed off 1 MAY. Won IET Award. Multiple articles written over the past year, been an outstanding success.
TBS 15 Nov 15, 60 knots headwind at 3000ft (267/60), 4000ft wind 80kts at 270 Arrival Regulation, 36/60
The difference between the target minimum and delivered separation was smaller, suggesting that separations were being delivered consistently closer to the target minimum than in the past.
One year on: Results of Time Based Separation at LHR
One year on:
Results of Time Based
Separation at LHR
General Manager of Customer Affairs
Aviation Chief Technologist
Runway Throughput Benefits (May – July)
• Across all wind conditions
– Saving is 78NM per day
– Increase of 20 movements per day
• In strong wind conditions
– Saving is 178NM per day
– Increase of 49 movements per day
All AWE 20 1.2
Wind related ATFM delay reduced by 62%
Estimate over 25,000 minutes of
delay saved in November versus
distance based separation
15 days of strong headwinds
Nov 10th: Zero minutes
ATFM delay with 40 knot
Summary of Findings
No Increase in Go Arounds or
Wake Turbulence Encounters
reduction in ATFM
of spacing saved per day
in strong wind conditions (>20kts)
per hour in strong wind conditions
of aircraft experienced
of spacing on approach
saved per day, on average
per hour across all wind conditions
No pilot inquires or comments