Criticisms of the system

The three main problems are:

• "Overbooking"
  Some of the slots allocated at Conferences are eventually left unused, and other airlines that want them may not be able to use them at short notice. Since plans may be changed after Conferences due to circumstances that the planner cannot reasonably be expected to anticipate, it is difficult to establish whether or not slot requests at Conferences are genuine.
  
  Airlines are trying to mitigate the problem by monitoring plans with actuals, establishing overbooking profiles, and encouraging prompt release of slots no longer required.
  
  
• New entrants
  After historical slots have been allocated at Conferences, 50% of the remaining slots available are first allocated to new entrants. It is argued that this does not give sufficient competitive scope for new entrants at airports close to saturation.
  
  The size of this problem should be viewed in the context that new entrants are generally able to obtain slots at, or tolerably near, the times they want at all but a dozen of the approximately 120 airports worldwide where schedules are coordinated, provided the airports expand capacities within the capabilities.
  
  Transferring historical slots from incumbents to accommodate new entrants would fatally attack the fundamental basis of the schedule coordination system. Such actions, if necessary, should therefore be taken by the appropriate authorities outside of the system, and notified in advance of the Conferences, leaving them to function normally thereafter.
  

• Reciprocity
  Historical precedence appears to have led to an imbalance of opportunities for airlines of States with adequate airport capacity wishing to operate to States with airports which are close to full.
  
  However, the problem is not due to the slot allocation process, but rather to the process of negotiating air services agreements. These agreements have dealt with routes, frequencies and aircraft capacities, but have often ignored the availability of slots at the airports at which traffic rights were exchanged. The remedy would seem to lie in taking account of slot availability when negotiating air services agreements.

Changing the system
In attempting to deal with the above problems, various solutions have been proposed. The earliest, and one that still gains academic support, is to scrap the present system in favour of some kind of market mechanism.

This idea was first floated in a study sponsored by the United States government in 1979. It was shelved in the face of almost unanimous international governmental opposition. Foreign governments objected to paying a second time for rights already negotiated. The idea also conflicted with the recommendation by the International Civil Aviation Organizatiion that airport user charges should be cost-related.

Another solution under consideration was government regulation of slot allocation. This ran into the same problems as mentioned above, with respect to international services.

Regulation presently applies in the U.S.: international services are guaranteed slots, if committed under international obligations, within a tolerable limit of alternative offers; if necessary, domestic slots are withdrawn by lottery. To correct inevitable quirks in domestic allocations by regulation, domestic slots may be bought and sold. Experience to date suggest this has led to strengthening the position of dominant airlines, and not of new entrants.

A third alternative has been considered: maintaining the present system, but modifying procedures and priorities by law. However, recourse to law could inhibit the present ability of Coordinators and airlines to quickly resolve conflicting requirements in a spirit of cooperation within the time available, before schedules must be published and flown.

The present system is open to suggestions for change from any quarter. Various proposals have been brought up for discussion –  e.g., priority for large over small aircraft, long over short routes, passenger over freighter services, frequency capping, etc. Each involves some groups receiving preference over others. Each proposal also requires careful consideration in the context of worldwide acceptability, which is the key to a continuation of consensus based solutions to congestion problems.

In its 1989 review of civil aviation policy, the United Kingdom's Civil Aviation Authority stated that the present system "should not be lightly discarded". It works, and before discarding it, one would have to be well assured that an alternative will work better. To date, no such alternative has been found.