Holding (aviation)

In aviation, holding (or flying a hold) is a maneuver designed to delay an aircraft already in flight while keeping it within a specified airspace.

A holding pattern for instrument flight rules (IFR) aircraft is usually a racetrack pattern based on a holding fix. This fix can be a radio beacon such as a non-directional beacon (NDB) or VHF omnidirectional range (VOR). The fix is the start of the first turn of the racetrack pattern. Aircraft will fly towards the fix, and once there will enter a predefined racetrack pattern. A standard holding pattern uses right-hand turns and takes approximately 4 minutes to complete (one minute for each 180 degree turn, and two one-minute straight ahead sections). Deviations from this pattern can happen if long delays are expected; longer legs (usually two or three minutes) may be used, or aircraft with distance measuring equipment (DME) may be assigned patterns with legs defined in nautical miles rather than minutes. Less frequent turns are more comfortable for passengers and crew. Additionally, left-hand turns may be assigned to some holding patterns if there are airspace or traffic restrictions nearby.

In the absence of a radio beacon, the holding fix can be any fixed point in the air, and can be created using two crossing VHF omnidirectional range radials (also called intersection), or it can be at a specific distance from a VOR using a coupled distance measuring equipment. When DME is used, the inbound turn of the racetrack may be permanently defined by distance limits rather than in minutes. Furthermore, in appropriately equipped aircraft, GPS waypoints may be used to define the holding pattern, eliminating the need for ground-based navigational aids entirely.

A hold for visual flight rules aircraft is usually a (smaller) racetrack pattern flown over something easily recognizable on the ground, such as a bridge, highway intersection or lake.

The primary use of a holding is delaying aircraft that have arrived at their destination but cannot land yet because of traffic congestion, poor weather, or runway unavailability (for instance, during snow removal or emergencies). Several aircraft may fly the same holding pattern at the same time, separated vertically by 1,000 feet or more. This is generally described as a stack or holding stack. As a rule, 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, and so on. Air traffic control (ATC) will control the whole process, in some cases using a dedicated controller (called a stack controller) for each individual pattern.

...
Wikipedia