A airport holding pattern are areas located a few miles away from an airport. Many larger airports like London Heathrow have four aircraft holding points, which are located North East, North West, South East and South West. There are several reasons for creating holding or entry points to the airport in the first instance. 

- Create a structed system for arriving aircraft to make an airport landing safely.

-To sequence the planes into an orderly flow as arriving planes can't all land on a single runway at once.

- Holding pattern around an airport area of control also have departing flight to co-ordinate. Having a safe landing system helps with the flow of aircraft arriving and departing in volume.

In order to have a safe functuring system to co-ordinate all this airport activity, you need to have a well-trained team of air traffic controls, electronic systems like Instrument Landing System (ILS) and of course high-tech radar. Aircraft guidance is provided by the ILS receivers in the plane by performing a modulation depth comparison.  An ILS consists of two independent sub-systems, one providing lateral guidance localizer, the other vertical guidance to aircraft approaching a runway on a glideslope.

The Holding Pattern and Approach Charts

Approaching a holding pattern new arrivals will be added at the top. The aircraft at the bottom of the stack will be taken out and allowed to make an approach first, after which all aircraft in the stack move down one level, usually in lots of 1000 feet.

All this movement of aircraft is managed in an traffic control center by air traffic control (ATC) but more specifically by a dedicated controller called a stack controller for each individual pattern.  Standard holding pattern uses right-hand turns which when completed takes roughly 4 minutes to complete. This is broken down into a minute for each 180 ' turn and two one-minute straight ahead flights.

The four holding stakes for London Heathrow Airport.

The holding pattern from stake to ILS approach.

The three types of entry to a holding stake.

A holding pattern basically consists of three standard types of entries:

- A direct entry is when the plane flies directly to the holding fix then immediately begins the first turn outbound.

- A parallel entry is when the plane flies to the holding fix, parallels the inbound course for a minute outbound, and then turns back, flies directly to the fix, and continues in the hold from there.

- An offset or teardrop entry is when the plane flies to the holding fix, turns into a protected area, flies for a minute, and then turns back inbound, proceeds to the fix then continues.