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.

Implementation
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.

Usage

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  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.

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.

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. Typical holding speeds for transport category aircraft are . 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, coupled with flight management systems, can 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 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.

 A direct entry is performed just as its name would suggest: 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, re-intercepting the inbound track, and continues in the hold from there.
 In an offset or teardrop entry, the aircraft flies to the holding fix, turns into the protected area, flies for one minute, and then turns back inbound, proceeding to the fix and continuing from there.

The parallel and teardrop entry are mirrored in case of a left-hand holding pattern.

Speed limits

Maximum holding speeds are established to keep aircraft within the protected holding area during their one-minute (one-minute and a half above  MSL) inbound and outbound legs. For civil aircraft (not military) in the United States, these airspeeds are:
 Up to  MSL: 200 KIAS
 From  MSL: 230 KIAS
  MSL and above: 265 KIAS

The ICAO Maximum holding speeds:

 Up to : 
 : 
 : 
 Above : M0.83
With their higher performance characteristics, military aircraft have higher holding speed limits.

In Canada, the speeds are:
 All propeller including turboprop aircraft – MHA† to : 
 Civilian Jet – MHA to : 
 Above : 

Climbing during the hold:
 Turboprop – normal climb speed
 Jet aircraft –  maximum

†MHA – Minimum Holding Altitude

Timing corrections
To achieve a one-minute inbound leg, there are two commonly used ways to modify timings:
Simple method: If inbound leg is less than one minute, add the same number of seconds to the outbound leg. If the inbound time is more than one minute, subtract the same number of seconds from the outbound leg.
E.g., inbound time is 0:55 → outbound time is 1:05.
E.g., inbound time is 1:06 → outbound time is 0:54.
More precise method: Subtract 2/3 of the error (in seconds) for inbound legs more than one minute, and add 3/2 of the error (in seconds) for inbound legs of less than one minute.
E.g., inbound time is 0:56 → error is 4 seconds. Thus +3/2*4=+6. 1:00+0:06=1:06. Fly 1:06 outbound.
E.g., inbound time is 1:06 → error is 6 seconds. Thus −2/3*6=−4. 1:00−0:04=0:56. Fly 0:56 outbound.
Additionally, for an initial estimate, add the headwind or subtract the tailwind component's speed in knots.
E.g., initial outbound with a tailwind component of 7 knots → initial outbound time is 0:53.
E.g., initial outbound with a headwind component of 20 knots → initial outbound time is 1:20.

See also

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

References

Aeronautical Information Manual (AIM) paragraph 5-3-8 https://www.faa.gov/air_traffic/publications/atpubs/aim_html/

External links

 Interactive Flash tutorial and holding entry trainer.
 Langley Flying School – IFR Rules and Procedures – En Route and Holds
Flight Crew Guide - Entry into the Holding
Flight Crew Guide - Flying the Holding pattern – Wind correction

Category:Flight phases
Category:Air traffic control