Holding patterns pose particular problems in relation to level busts because several aircraft are packed into a small volume of airspace and are constantly manoeuvring and changing their levels. Descent below the cleared level immediately reduces vertical separation from the aircraft below, and is difficult for an ATCO to detect and correct quickly.
Two aircraft in the hold are issued with descent clearances. The lower aircraft descends too slowly or the higher aircraft descends too quickly. (controllers should note that, in the managed mode, FMS provides for 1000 fpm rate of descent)
Pilot is slow to initiate descent following a reclearance.
Accurate pilot-ATCO communication and adherence to flight clearance is essential when descending in a holding pattern because the overlay of aircraft data labels on the ATCO’s radar display may not allow the immediate detection of an impending traffic conflict. The usual ATC safety-net of STCA is likely to be less effective.
Properly used, ACAS provides an effective safety net. However, if a RA in the hold is not accurately followed and/or a return to the cleared level is not made immediately "clear of conflict" is annunciated, then the likelihood of a secondary loss of separation is increased by the probable presence of other nearby traffic 1000ft above and 1000ft below the assigned level. Multiple aircraft ACAS events are possible in a stack and have been recorded. It is crucial that all aircraft receiving a TCAS RA when in a holding pattern follow the RA strictly.
Commencement of Descent: Descent should commence on acknowledgement of clearance.
Mode S Selected Altitude DAP: Use of Mode S information, in tools such as the Vertical Stack List (VSL), can dramatically improve a controller's situational awareness. The image below shows the VSL display in operation in the London Terminal Control. On the left is the normal surveillance picture of the Bovingdon hold with a lot of garbling. On the right is the VSL showing level occupancy, actual altitude and in orange the Selected Altitude DAP. The tool not only enhances controllers’ vertical stack awareness but provides a warning of a potential level bust.
HindSight 10: The tenth edition of HindSight, titled "Level Bust or... Altitude Deviation ?", published in December 2009, contains a variety or articles addressing different aspects of the Level Bust issue including: