What is the maximum number of planes a controller can control?

Question

In a small and un-busy airport, the same controller can be the ground and tower at the same time. If the sector is un-busy (e.g. late at night), he may also be a sector, ground and tower all at the same time. At busy airports of course, ground and tower are two persons. Sometimes even ground is split to "Ground South" and "Ground North".

So there is a threshold where the communication has to be split into two, either because the frequency would be too busy (only one person can talk on the frequency at any time), or there are so many planes that a controller would start making mistakes.

What is that threshold? (Is it a concrete number, or some other conditions which can be objectively determined?)

Answer 1

There is no fix maximum a controller is allowed to have under control, as the other answer already pointed out. The airspace under control of ATC is divided up into sectors containing the airways to and from airports within that sector or in upper sectors, airways just for en-route traffic. (see also: Why do pilots use airways instead of GPS direct?).

Sectors can be merged during off-peak hours, split back into separate sectors during peak hours and in the case of TMA or approach sectors, the controller owning the sector can receive additional help by splitting the approach sector further and using different controllers for different tasks within that sector: Feeder/Pickup - Director - Departure Controller. The thresholds of when a sector is split is dependent on many factors, like weather, runway in use, outside influences like adjacent sectors having more traffic that usual, special occurances within that sector like parachute jumper drops.

Sectors also vary in size and capacity based on their intended use. While TMA/Approach sectors are rather small and need to accomodate arriving traffic into an airfield, an enroute or upper sector can be significantly larger in size, lateral and vertical. Again, this is individual to each sector, because some factors which hold true for TMA sectors do not apply to enroute upper sectors and vice versa. It also really depends on the controller working the sector and his personal limits, favorite procedures and work practice. A few examples:

• A TMA controller using vectors to the approach instead of standard arrival routes or transitions will have more workload to monitor and sequence aircraft.
• The minimum radar separation in TMA sectors and upper sectors can be either 3NM or 5NM. While upper sectors are larger in size, they have to maintain a greater separation, so in some cases, an upper sector twice or multiple times the size of a TMA sector will only be able to accomodate the same number of aircraft (ceteris paribus) before reaching saturation.
• A controller having to work with special situations such as aircraft in distress or student pilots requiring more attention than experienced pilots, will reduce the controllers personal capacity.

In summary, to answer your question: There is a situation and controller based threshold, at which saturation will be reached. Controllers are trained to anticipate this threshold and make their supervisor aware of the situation beforehand, so that measures can be taken. There is however no fixed numerical maximum, unless specified in the ARTCC/ACC's operating procedures.

Answer 2

Chuckling here, as this is a hotly argued topic in controller circles. There is no direct official number. One airplane can be all a person could work(major emergency lots of coordination, etc.), or they could work 20+ and not break a sweat, it heavily depends on the nature of what's occuring and the staffing at the facility, the positions that are designed at the facility(various sectors at a tracon, where there's designated airspace for the position and frequency).

During the design and course of life of facilities, they constantly evaluated the positions they need during peak traffic times, to slow times, and the required staffing levels for those positions. Sometimes all you need 99% of the time is two positions, so that's what they'll aim to staff for, other times, based on history and operators at the airport you can get a good idea when your peaks and valleys occur and plan position distribution accordingly, with the staffing you have.

Answer 3

There is no limit, really. The question is not whether the controller would "start making mistakes", it is how much attention would the controller give to each plane, which just grows less and less the more aircraft there are.

Even if the tower goes completely dark, the pilots will self-organize and land themselves. The controller is just there to make the process easier and safer.

Sometimes controllers do ignore you when they get busy with something else and what happens is you end up flying really LONG downwinds, so by the time the controller gets around to you, you turn onto a 5-mile final or something like that. If the controller completely blows you off, you can leave the pattern, do a U-turn, come around and re-enter the pattern to try again.

At a big airport with a lot of commercial aircraft, if the controllers started to go AWOL the pilots would probably just stack themselves up on the instrument track, and even start landing themselves if the tower became totally unresponsive. To do this, they just tell the other pilots where they are and their position in the queue ("Lufthansa One Echo Echo downwind for two four left, number three behind the Embraer", etc).