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.FAA-H-8083-15A, "Instrument Flying Handbook", Chapter 10. http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/]

Implementation

A holding pattern 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 aircraft with 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 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 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. 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 VFR aircraft is usually a (smaller) racetrack pattern flown over something easily recognizable on the ground, such as a bridge, highway intersection or lake.

Usage

The primary use of a holding pattern 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. 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. [14 CFR 91.3 - "Responsibility and authority of the pilot in command." http://www.access.gpo.gov/nara/cfr/waisidx_01/14cfr91_01.html]

Limiting usage of holdings

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. Despite this regulation, aircraft are required to carry extra fuel specifically for holding, should it become necessary.

Flying a holding pattern

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 convert|250|kn|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.

Entry procedures and accurate flying of the holding procedure are essential parts of IFR pilot training, and will always be tested on examination flights.

Modern autopilots can be programmed to enter and fly holding patterns automatically.

Entry procedures

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 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.FAA-H-8083-15A, "Instrument Flying Handbook", Standard Entry Procedures, page 10-12. http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/]

* A "direct" entry is performed exactly as it sounds: 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 continues 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.

Direct_entry_(Sector_3)Image:HoldParallelEntry.svg|Parallel_entry_(Sector_1)Image:HoldTeardropEntry.svg|Teardrop_entry_(Sector_2)The parallel and teardrop entry are mirrored in case of a left-hand holding pattern.

peed 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 convert|6000|ft|m MSL and below: convert|200|kn|0|mph|km/h|lk=in
* From 6,001 to convert|14000|ft|m|abbr=on MSL: convert|230|kn|0|mph|km/h|abbr=on
* Above 14000 ft MSL: convert|265|kn|0|mph|km/h|abbr=on

With their higher performance characteristics, military aircraft have higher holding speed limits.

In Canada the speeds are:
* All propeller including turboprop aircraft - MHA^† to convert|30000|ft|m|abbr=on: convert|175|kn|0|mph|km/h|abbr=on
* Civilian Jet - MHA to convert|14000|ft|m|abbr=on: convert|230|kn|0|mph|km/h|abbr=on
* Above 14000 ft: convert|265|kn|0|mph|km/h|abbr=on

:Climbing during the hold:
*turboprop - normal climb speed
*Jet aircraft - convert|310|kn|0|mph|km/h|abbr=on maximum

^†"MHA – Minimum Holding Altitude"

ee also

* Bovingdon stack, a holding pattern in the United Kingdom
* Flight planning

References