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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). 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.
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, they will always be allowed to bypass the holding pattern and go directly to the airport (if possible). Obviously, this causes more delays for other aircraft already in the stack.
Aircraft flying in circles is an inefficient (and hence costly) usage of time and fuel, so measures are taken to limit the amount of holding necessary. Air traffic flow management is used to delay aircraft while grounded at their departure points when delays are expected at their destinations.
Many aircraft have a specific holding speed published by the manufacturer; this is a lower speed at which the aircraft uses less fuel per hour than normal cruise speeds. A typical holding speed for transport category aircraft is 210 to 265 knots (491 km/h). Holding speeds are a function of aircraft weight at the point of holding. If possible, a holding pattern is flown with flaps and landing gear up to save fuel.
The entry to a holding pattern is often the hardest part for a novice pilot to grasp, and determining and executing the proper entry while simultaneously controlling the aircraft, navigating and communicating with ATC requires practice. There are three standard types of entries: direct, parallel, and offset (teardrop). The proper entry procedure is determined by the angle difference between the direction the aircraft flies to arrive at the beacon and the direction of the inbound leg of the holding pattern.
The parallel and teardrop entry are mirrored in case of a left-hand holding pattern.
Maximum holding speeds are established to keep aircraft within the protected holding area during their one-minute (one-minute and a half above 14,000 ft MSL) inbound and outbound legs. For civil aircraft (not military) in the United States, these airspeeds are:
The ICAO Maximum holding speeds:
With their higher performance characteristics, military aircraft have higher holding speed limits.
In Canada the speeds are:
†MHA – Minimum Holding Altitude
To achieve a one-minute inbound leg, there are two key ways to modify timings: