For those that don’t know, holding stacks are the airport’s ‘waiting rooms’ for arriving aircraft. Whenever aircraft cannot proceed to land immediately, flights enter at the top of the stack – usually at around 11 or 12 thousand feet before gradually spiralling down to 7,000ft.
The NATS air traffic controllers in London Terminal Control at Swanwick and at airport towers around the country, then direct the aircraft onto the final approach path for the airport. The separation between arriving aircraft pairs is largely dependent on their size. A heavy A380, which whips up a lot of air turbulence in its wake, followed by the lighter B737 requires 7 miles separation, whereas the gap between two A380s is just 4 miles.
Choosing the optimum combination of arrivals is the key to making the most of the available runway capacity and at Heathrow, which is scheduled to 98% capacity, having a constant stream of arrivals allows optimum sequencing to take place and is absolutely vital to the smooth running of the operation.
It will be many years before aircraft can arrive at an airport ‘just in time’ so in the meantime we need to have a slight oversupply of aircraft to ensure that runway capacity is not wasted with unnecessary gaps. As such holding stacks are incredibly useful, but they are also noisy, inflexible and mean aircraft burn more fuel at lower levels and emit more CO2.
With the advent of more sophisticated satellite based navigation systems it’s now possible for aircraft to follow routes with an incredible level of accuracy. That means we’re able to be smarter about airport arrivals using a concept called Linear Holding.
There are two different types of linear hold, the trombone and the point merge. Both work by keeping all the arriving aircraft at the same level, but separated in the horizontal plane by satellite navigation tracks. At exactly the right moment – to the second – the aircraft is vectored off the linear hold and onto final approach. The big difference is that these linear holds can be much higher than a traditional stack, potentially up to 20,000 feet, and are therefore quieter for people living underneath and more fuel efficient for the airlines.
The traditional stack holds would remain for use in exceptional circumstances but they would be moved further out and raised up.
We’ve actually introduced one linear in the UK already; with the implementation of a point merge hold for arrivals into London City Airport. Instead of flying over land, arrivals now join the point merge arc out over the North Sea before being peeled off in the optimum order for a continuous decent approach into the airport.
The introduction of linear holds is just one example of the benefits of modernising the structure of UK airspace, much of which was designed in the 1950s and 60s. Air traffic is forecast to increase by 40% by 2030 and we need to act now if we’re going to keep pace with rising demand, while at the same time working to take advantage of the technologies that can help us minimise the impact of noise on the ground.
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