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A level bust occurs when an aircraft fails to fly at the level to which it has been cleared, regardless of whether actual loss of separation from other aircraft or the ground results. Level busts are also known as Altitude Deviations.
A level bust is defined as: Any unauthorised vertical deviation of more than 300 feet from an ATC flight clearance. (EUROCONTROL)
Within Reduced Vertical Separation Minima (RVSM) airspace this limit is reduced to 200 feet.
Definitions applied by other organisations are similar but sometimes refer to a deviation of 300 feet or more.
Types of Level Bust
- An aircraft in level flight climbs or descends without clearance.
- An aircraft climbing or descending fails to level off accurately at the correct level (either levelling before the correct level is reached, or passing through and continuing the climb or descent, or passing through and then returning to the correct level).
- An aircraft levels off at the correct indicated level or altitude, but with an incorrect altimeter setting.
- Loss of Separation from other aircraft, which may result in collision. This is especially likely when Reduced Vertical Separation Minima (RVSM) procedures are in force.
- Collision with an obstacle or the ground. This is known as Controlled Flight Into Terrain (Controlled Flight Into Terrain (CFIT)) and is a major cause of fatal aircraft accidents.
- Injury, especially to cabin crew or passengers, occasioned by violent manoeuvres to avoid collision with other aircraft or the ground.
- Standard Operating Procedures, both on the flight deck and in the ATCU, which detail procedures to be followed to reduce the chance of level bust.
- Onboard aircraft equipment designed to warn of potential collision with other aircraft (Airborne Collision Avoidance System (ACAS)/TCAS) or with the ground (GPWS or EGPWS).
- Ground-based equipment designed to warn of potential collision with other aircraft (Short Term Conflict Alert (STCA) or Medium Term Conflict Detection (MTCD)) or with the ground (Minimum Safe Altitude Warning (MSAW)).
- Air-ground communications, for example:
- The pilot mishears the level clearance, the pilot does not read back the clearance and the ATCO does not challenge the absence of a read-back;
- The pilot reads back an incorrect level but the ATCO does not hear the erroneous read-back and does not correct the pilot’s read-back; or,
- The pilot accepts a level clearance intended for another aircraft (call-sign confusion).
- ATCO-induced situations, for example:
- Late re-clearance;
- The ATCO assigns an altitude after the pilot was cleared to a flight level (climbing);
- The ATCO assigns a flight level after the pilot was cleared to an altitude (descending).
- Note: potential loss of separation resulting from the ATCO assigning an inappropriate altitude or flight level in a flight clearance does not constitute a level bust because no deviation from the flight clearance occurs; nevertheless, this situation should be considered in this discussion.
- Pilot-induced situations, for example, the pilot understands and reads back the correct altitude or flight level but selects an incorrect altitude or flight level because of:
- Aircraft Technical Equipment designed to prevent a Level Bust (e.g. autopilot or altitude alert) does not operate as designed.
- Pilot-equipment interface. The flight crew sets an incorrect altimeter pressure setting or mis-sets other aircraft technical equipment or fails to respond correctly to aural or visual warnings.
- Pilot Workload;
- ATCO work-load;
- High Rates of Climb or Descent;
- Holding Patterns;
- Airspace Procedure/Design;
- Volume of traffic.
- Improve standard of pilot training, especially in:
- Improve standard of ATCO training, especially in:
- Improved technical equipment.
- HindSight 1: Level Bust – Advice to the ATCO;
- HindSight 2: Level Bust – Analysis of AIRPROX in Japan;
EUROCONTROL Safety Letters:
- Level Bust: A Shared Issue;
- Presentation of Level Bust Toolkit and Action Plan;
- Reducing Level Bust: Seeking Solutions Today to Tomorrows’ Challenges;
EUROCONTROL ACAS Bulletins:
- No 1: Follow the RA!
- No 2: RAs and 1000 ft Level-Off Manoeuvres;
- No 3: Wrong Reaction to "Adjust Vertical Speed" RAs;
- No 4: TCAS II and VFR Traffic;
- No 5: Controller and Pilot ACAS Regulation and Training;
- No 6: Incorrect Use of the TCAS Traffic Display;
- No 7: The Dos and Don'ts of TCAS II Operations;
- No 8: TCAS II Operation if European RVSM Airspace;
EUROCONTROL Level Bust Toolkit.