Holding (aviation)
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For other uses, see Holding (disambiguation).
In aviation, a holding (or hold) is an area of airspace used to delay aircraft already in flight. Because fixed-wing aircraft cannot stop in midair, they fly in circles, which keeps them near their destination airport until it is their turn to land.
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[edit] Implementation
A holding for IFR aircraft is usually a racetrack pattern based on a holding fix. This fix can be a radio beacon such as an NDB or 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 for aircraft with DME may be assigned holds with legs defined in nautical miles rather than minutes. Less frequent turns are more comfortable for passengers and crew.
Apart from a radio beacon, the holding fix can be any fixed point in the air, and can be created using two crossing VOR radials, or it can be at a specific distance from a VOR using a coupled DME. When DME is used, the inbound turn of the racetrack may be permanently defined by distance limits rather than in minutes. Left-hand turns may be assigned to some holding patterns if there are airspace restrictions nearby.
A holding for VFR aircraft is usually a (smaller) racetrack pattern flown over something easily recognizable on the ground (such as a bridge, highway intersection or lake).
[edit] Usage
The primary use of a holding is delaying aircraft that have arrived over their destination but cannot land yet because of traffic congestion, poor weather, or unavailability of the runway (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 holding stack or 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. ATC will control the whole process, in some cases using a dedicated controller (called a stack controller) for each holding.
One airport may have several holdings; depending on where aircraft arrive from or which runway is in use, or because of vertical airspace limitations.
An aircraft with an emergency will be allowed to bypass the holding and go directly to the airport (if possible). Obviously, this causes more delays for other aircraft already in the stack.
[edit] Limiting usage of holdings
Because aircraft flying in circles is an inefficient usage of time and fuel (and hence costly), measures are taken to limit the amount of holdings flown. Air traffic flow management is used to delay aircraft not yet airborne at their departure point when delays are expected at their destination. Despite this regulation, aircraft are required to carry extra fuel specifically for holding, should it become necessary.
[edit] Flying a holding
Most aircraft have a specific holding speed published by the manufacturer, this is a speed at which the aircraft uses the least amount of fuel per hour, this a relatively low speed compared to enroute flying. A typical figure for airline aircraft is 170 to 210 knots. If possible, a holding is flown with flaps and landing gear up to save fuel.
Holding entry procedures (which differ depending on the direction the aircraft enters the holding from), and accurately flying the holding, is an essential part of IFR pilot training, and will always be tested on examination flights.
Modern autopilots can be programmed to enter and fly holdings automatically.
[edit] Holding entry procedures
The entry to a hold 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 teardrop. The type is defined by the angle difference between the direction the aircraft flies to go to the beacon and the direction of the holding's racetrack.
- A direct entry is exactly what it sounds like: the aircraft flies directly to the holding fix, and immediately begins the first turn outbound.
- In a parallel entry, the aircraft flies to the holding fix, parallels the inbound course for one minute outbound, and then turns back, flies directly to the fix, and proceeds in the hold from there.
- In a teardrop (or offset) entry, the aircraft flies to the holding fix, turns into the protected area, flies for one minute, and then turns back inbound, proceeds to the fix and continues from there.
| Standard holding entry diagrams | |||
 Direct entry |
 Parallel entry |
 Teardrop entry |
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The parallel and teardrop entry are mirrored in case of a left-hand holding pattern.
[edit] Speed limits
Maximum holding speeds are established in order to keep aircraft within the protected holding area during their one-minute inbound and outbound legs. For civil aircraft (not military) in the United States, these airspeeds are:
- At 6,000' MSL and below: 200 knots
- From 6,001' to 14,000' MSL: 230 knots
- At and above 14,001' MSL: 265 knots
With their higher performance characteristics, military aircraft have higher holding speed limits.
[edit] See also
- Bovingdon stack, a holding pattern in the United Kingdom
- Flight planning
