UK air navigation service provider NATS has just achieved a “world first” by winning approval to operate time-based aircraft separation on runway approaches at London Heathrow airport.
So what? Well, if your airport is not operating at full capacity, this doesn’t provide you with much of an advantage. But if, like Heathrow, you are operating at capacity almost all the time, it matters a lot.
Time-based – rather that distance-based – separation on approach increases the aircraft landing rate when the airport is affected by high winds, and reduces consequent flight cancellations and delay. High winds are the biggest single cause of delay and flight cancellation at Heathrow.
When there are strong winds, distance-based separation increases the time between aircraft as they pass overhead any given point on the approach, and thus, of course, the time between landings. This occurs because although the aircrafts’ speed through the air remains the same, their speed across the ground is reduced by the headwind velocity.
Departures have always been separated by time because brakes-off time is easy to control, but asking controllers to visualise a time delay between approaching aircraft in varying wind has always been considered too difficult and potentially dangerous.
NATS says flight cancellations caused by high winds can be “almost eliminated”, and the system will save 80,000 minutes of delay per year. The system goes fully operational at LHR in Spring next year.
One of the reasons aircraft have, at present, to be separated by distance has been the fact that wake vortices from the aircraft ahead have to be given time to dissipate some of their energy before the following aircraft becomes affected by them.
Dr Jennifer Sykes of NATS’ operations strategy division says the company has studied over 100,000 approaching flights at Heathrow using Eurocontrol’s Lidar wake vortex detector to measure accurately the behaviour of aircraft wake vortices in strong headwinds. The results show, she told me, that vortices dissipate more quickly in windy conditions, therefore allowing aircraft to be closer together on final approach and still be safe.
The enabling tool for the controllers is called an “indicator support tool”. Using Mode S data from each aircraft about its type, airspeed and groundspeed, the device puts moving marks on the radar display of the approach path that indicate the correct time separation between each pair of aircraft.
Film of the radar display with the indicator support tool in use can be found on NATS’s YouTube site at http://www.youtube.com/watch?v=2jBvbpbq0SM