Holding Areas

28.10.2004

Submission:

I came across your web site and have a question for you guys. What is the amount of protected airspace around a hold that ATC assigns to you? I know this varies with speed and aircraft.

Korey USA

Answer or Commentary:

(I.W.

This question raises 3 points. Firstly ATC does not actively in real time assign an aircraft any protected airspace other than the procedural separation of 5 minutes from the hold. However saying that, protected airspace is provided and can be divided into 2 types-

Protection from obstacles or terrain, from which usually stems-

Protection (i.e. procedural ATC separation) from other aircraft.

Obstacle protection areas for a hold are laid down in ICAO document 8168 (PANS-OPS) and take into account

Holding fix tolerances
Speed
Altitude
Wind
Tracking tolerances
Type of entry


These parameters determine the primary obstacle protection areas. Then around this primary protection area is provided a 5 nm buffer area. Within this buffer area the obstacle clearance reduces from 1000 ft to 200 ft at the outer edge.

When looking at procedural ATC separation from other aircraft there are no laid down criteria other than the 5 minutes time separation. However it is a generally accepted theory that if an aircraft in the hold is clear of obstacles (i.e. the obstacles are outside the possible area that an aircraft could be when holding), then the aircraft would be clear of another aircraft that could be in a similar. This can be summed up by:

"If the obstacle protection areas of 2 adjacent holding patterns, at a certain altitude, do not overlap then they are considered to be procedurally separated up to that altitude"

At this stage the general idea diverges into 2 different camps. The conservative camp uses the entire obstacle protection areas including the buffer areas. The other camp reasons that the buffer area is more an allowance for wind and can be ignored. The reasoning is that if one aircraft is on the edge of the buffer area due to wind then the other aircraft in the other hold would also be effected by the same wind pushing it to the opposite edge of the hold. So the buffer areas can be disregarded in the determination of the procedural separation of two adjacent holding patterns.

At this point the conservative camp counter argues with the case of one aircraft effected by the wind on the edge of the buffer area. While in the other holding pattern the pilot of the second aircraft has taken the wind into account and is only being effected by the navigational tolerances (i.e. remaining inside the primary protection area). The debate goes on..

What do other countries use in determining procedural ATC separation, Primary areas only or Secondary areas as well? Or has some one else comments on this area.)