What is a level bust incident?

The level bust incident, also known as altitude deviation, is an air traffic control term used to describe a situation where an aircraft momentarily or inadvertently deviates from its assigned altitude without ATC clearance or authorization. This deviation could result in a potential conflict with other aircraft in the vicinity, or with terrain or obstacles. The altitude deviation may occur due to several reasons such as pilot error, flight instrument malfunction, communication error, or air traffic control error. Altitude deviations can range from minor to severe and can result in loss of separation and possible mid-air collision if not quickly resolved by ATC. Such incidents are investigated by regulatory authorities to identify the cause and take measures to prevent future occurrences.

An example situation:

for example, an aircraft is assigned by ATC to maintain flight level 320. However, due to misinterpretation of the altitude instruction and lack of situational awareness, the pilot inadvertently climbs to flight level 340 without ATC clearance. This results in loss of vertical separation with another aircraft flying at flight level 330 in the same airway. When ATC notices the deviation, they immediately issue corrective action to avoid any potential conflict.

The level bust incident, in this case, is that the aircraft temporarily or unintentionally deviated from its assigned altitude without ATC clearance or authorization, resulting in a loss of separation with other aircraft in the vicinity, which could potentially lead to a mid-air collision.

A real-world example of a level bust incident: