During busy periods, arriving aircraft may be required to delay their approach because of weather or traffic congestion, and this is known as ‘holding’. Where delays are expected, flights may be held at high level many miles from the airport until they are able to descend and join the airfield holding pattern. Various holding patterns are available near main airports, and aircraft fly a racetrack pattern until an approach is possible, or alternatively a decision to divert is made. Standard holding patterns may themselves become congested and in such cases aircraft may be required to hold by flying between navigation beacons, as for example between Land’s End and Southampton in southern England.
Holding patterns near airports are sometimes referred to as ‘stacks’ because aircraft are stacked one above the other with 1,000 ft separation. As the lowest flight is cleared to leave the hold, other flights descend in 1,000 ft steps. Full details of holding patterns are given on arrival charts for individual airfields. If a short delay is expected the controller will inform the pilot that the delay is less than 20 min. If the delay is longer, pilots will be given an indication of the length of time they can be expected to hold and they will also be given an Expected Approach Time (EAT) which is the time the flight can expect to leave the lowest level of the holding stack to commence an approach to the destination airfield.