Holding (aeronautics)

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. The primary use of a holding pattern is to delay 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 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. One airport may have several holding patterns; depending on where aircraft arrive from or which runway is in use, or because of vertical airspace limitations. Since an aircraft with an emergency has priority over all other air traffic, it will always be allowed to bypass the holding pattern and go directly to the airport (if possible).