What is Aircraft holding pattern?


Aircraft Holding pattern

Imagine that you are on an aircraft and about to reach your destination but suddenly you hear the voice of the pilot saying "because of holding we would be about 15 minutes ahead of schedule". This mainly happens either due to busy runways or if a runway is undergoing snow removal procedure. What does an aircraft do during this time, this question shall be addressed through this article in a systematic procedure.

Action taken by ATC (Air Traffic Controller):

What would happen if a traffic light stops working? It would definitely result in a huge traffic Jam. Similarly, if one of the runways of a busy airport gets contaminated due to heavy snowfall, there would be a Traffic Jam in the sky.  Helicopters could easily stay at the same position but the same is not possible for an aircraft, so how would the aircraft be able to stay in the sky and how would they get the command to do so.

As soon as the ATC recognizes the unavailability of a particular runway, it orders the aircraft to enter a holding pattern to prevent a potential Traffic Jam. After this request from ATC, the aircraft starts to loiter and wait for its turn to land. Holding is the time period during which the aircraft is loitering in mid-air and waiting for its turn to land. The procedure used to assure a safe and orderly flow of traffic into and out of busy terminal is called holding.

The shape of a holding pattern:

There could be many ways to enter a holding pattern but all the holding patterns are designed in a similar fashion. The top view of any holding pattern looks similar to that of a race track, with two parallel and two semi-circular paths. These patterns are designed to maintain the aircraft within a specified space for a specific period of time.

Most of the holding patterns are published on an airway chart or approach and landing chart, but sometimes they are unpublished and specified by the ATC. There are various stationary points which could be considered as a holding fix, which is an essential part of the holding pattern. These stationary points include navigational aids like VOR, NDB, an airway intersection and a GPS waypoint. 
Components of a holding pattern:
Hold Parallel Entry
Hold Parallel Entry
There are mainly three types of entries possible to a holding pattern; these include teardrop entry, parallel entry and direct entry. To understand about the entry procedure to a holding pattern one should be clear about various components of a holding pattern.

Hold Teardrop Entry
Hold Teardrop Entry
Holding pattern consists of four components. The fixed end is a 180 degree turn and is started at the holding fix. It is then followed by an outbound leg, outbound end and inbound leg which is again joined to the holding fix. In this way, a holding pattern is constructed which resembles a racing track with outbound and inbound legs parallel to each other.

Hold Direct Entry
Hold Direct Entry

Understanding Missed Approach:

The landing approach is terminated if the pilot determines before the decision height or missed approach point, that the runway is not clearly visible and a safe landing could not be accomplished. In such cases, pilot elaborates the situation to the ATC.

ATC then analysis the situation immediately and describes a remedy plan. ATC would either command the aircraft to loiter and wait for land on a different runway or to move to a nearby airport to land. Therefore in case of a missed approach, aircraft might have to enter a holding pattern to wait for its turn to land on a different runway.        

Rules monitoring the motion in a holding pattern:

The inbound leg should be one minute long, and the aircraft should be loitering with the holding speed of 230 knots. If the effects of air are neglected, both inbound and outbound legs should be one minute long.

The pilot has to include wind correction angle and time calculation, to counter to the effects of wind. The type of entry to the holding pattern depends upon the current position of an aircraft. A standard holding pattern consists of rights turns.

To conclude, holding could be visualised as a queue where aircraft is waiting for its turn to land.

Thanks for reading!

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