The Bovingdon stack is a section of airspace to the north west of London where inbound planes to London Heathrow Airport, which is 20 miles (30 km) to the south, are held. It is a busy example of a hold. It extends above the village of Bovingdon and the town of Chesham, and requires the VOR navigational beacon BNN which is situated on the former RAF Bovingdon airfield. At busy times on a clear day a dozen planes may be circling overhead. Other holding patterns serving Heathrow are at Biggin Hill, Kent (BIG - SE Arrivals), Lambourne, Essex (LAM - NE Arrivals) and Ockham, Surrey (OCK - SW Arrivals), where inbound aircraft will normally use the pattern closest to their arrival route. They can be visualised as invisible helter skelters in the sky.