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Level Bust is defined as any unauthorised vertical deviation of more than 300 feet from an ATC flight clearance.
Within Reduced Vertical Separation Minima (RVSM) airspace this limit is reduced to 200 feet.(EUROCONTROL - HEIDI)
Definitions applied by other organisations are similar but sometimes refer to a deviation of 300 feet or more.
The level bust issue only relates to aircraft in controlled airspace or a designated ATZ outside controlled airspace and under either radar or procedural ATC control.
A Level Bust or Altitude Deviation 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 becoming less dangerous because improvements in technology such as better STCA and Mode S have improved the ability of controllers to safely manage any consequent loss of separation. Furthermore, the availability and proper use of ACAS provides a final safety net which significantly reduces the risk of a Mid-Air Collision, and TAWS has also reduced the risk of a level bust resulting in a CFIT accident.
The move to Flexible Use Airspace (FUA), the absence of ACAS on many military aircraft and the high performance of many military jet aircraft, means that the consequences of level busts involving military aircraft are more difficult to manage.
A 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; however for completeness, examples of situations in which the action of the ATCO could contribute to a level bust are listed in this article.
Types of Level Bust
The following types exclude involuntary transient departure from acquired levels attributable to the effects of turbulence:
- aircraft both accepts a clearance and sets/records it correctly but then does not follow it [flight management error (usual) or technical fault (rarely)]
- aircraft accepts a clearance correctly but then sets it incorrectly without the error being picked up by the crew [flight management error]
- aircraft reads back clearance incorrectly and this error is not picked up by ATC so it is then recorded/set and followed [ATC error]
- aircraft is unable to react fast enough to a late reclearance and passes through new cleared level [ATC error]. See the scenarios in the separate article Level Bust - ATCO Induced Situations.
- aircraft follows clearance with the wrong altimeter sub scale setting
- aircraft departs cleared flight level without clearance to do so.
- Loss of Separation from other aircraft, which may result in collision.
- Collision with an obstacle or the ground (Controlled Flight Into Terrain), especially as a result of having the wrong altimeter sub-scale setting. This can happen when an aircraft descends in a low pressure area with standard pressure (1013) set.
- 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 (ACAS/TCAS) or with the ground (GPWS or EGPWS).
- Ground-based equipment designed to warn of potential collision with other aircraft, such as STCA, or provide more information on intentions of the aircraft (e.g. Mode S)
- Air-ground communications, for example:
- The pilot mis-hears 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).
- Pilot-induced situations, for example:
- One pilot is off frequency when clearance is given by ATC; although it is read back correctly he/she mis-sets it on the FMS.
- PNF fails to spot that PF has set FL110 on the FMS not the FL100 that PNF has (correctly) read back to ATC.
- Flight crew used to a 6000ft Transition Altitude/Level set up a SID for a departure from an airport which has a 3000ft Transition Altitude and then fail, with a low barometric pressure, to change to standard pressure setting (1013) quickly enough.
- Uncertainty between two flight crew about a clearance given is resolved by reading back what they think it was to see if ATC then correct them. ATC don’t notice and it is not correct. [ATC hear-back error but flight crew should have asked for the clearance again not guessed it]
- A military fast jet without ACAS but with Transponder Mode C/S exceeds its correctly acknowledged climb clearance and very rapidly causes multiple civil transport RA sequences at three different successive flight levels which, because of the fast-jet's rate of climb, all consist of RAs which change too quickly to be capable of being actioned.
- the pilot understands and reads back the correct altitude or flight level but selects an incorrect altitude or flight level because of confusion of numbers with another element of the message (e.g. speed, heading or flight number), Expectation of another altitude or flight level, or Interruption or Distraction. [proper cross checking procedures on a multicrew flight deck should mitigate against these]
- Aircraft Technical Equipment designed to prevent a Level Bust (e.g. autopilot or altitude alert) does not operate as designed.
- ATCO-induced situations, for example:
- The ATCO is interrupted or distracted and does not take action in time to rectify an impending loss of separation;
- The ATCO assigns an incorrect altitude, or reassigns a FL after the aircraft has been cleared to an altitude;
- The ATCO's instructions are misunderstood because of inadequate English proficiency, or use of standard phraseology, or speed of transmission;
- The altitude clearance is passed late and re-clearance is not achievable without overshoot or undershoot;
- The ATCO issues an instruction for an altitude restriction when the aircraft is above the transition altitude (i.e., with altimeters set to standard pressure setting);
- The ATCO issues a complex transmission containing more than two instructions (e.g., speed, altitude and heading).
- 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.
Factors which contribute to the risk of a level bust occurring:
- Only one pilot/crew member on the ATC frequency;
- ATCO work-load;
- Holding Patterns;
- Airspace Procedure/Design;
- Pilot Workload;
Factors which increase the risk of collision following a level bust:
- Volume of traffic.
- High Rates of Climb or Descent;
Accidents and Incidents
A list of all LB-related accidents and incidents on SkyBrary can be found here.
- Avoiding Level Busts
- a focus by both flight crew and ATCOs on effective Air-Ground Communication especially in terms of radio discipline and the particular importance of the Cross-checking Process and the read-back/hear-back process. For flight crew, the overall principles of effective Crew Resource Management (CRM) are particular relevant as is the rigourous application of SOPs related to altimeter subscale setting and to the receipt and pre-selection of altitude re-clearances. For ATCOs, the principles of Team resource management are important.
- Mitigating the consequences of Level Busts is achieved by maximising the effective use of both flight crew and ATM Safety Nets.
- For Flight crew, Situational Awareness in respect of other traffic in the vicinity can be enhanced by passive monitoring of other aircraft on the COM frequency in use and by passive monitoring of the TCAS display and active monitoring of it if a TCAS TA or preventive TCAS RA occurs. If a corrective TCAS RA occurs then, it is essential that it is actioned as required, that ATC are advised of any deviation from clearance as soon as practicable and that any deviation is limited to the requirements of the RA.
- For ATCOs, Situational Awareness can be enhanced by actively assessing the most critical separation margins implied by the clearances given and by maximising the use of Mode S data if displayed. These actions may allow a likely level bust to be identified. If one or more TCAS RAs occur then a constant awareness of the general disposition of traffic may enable ATCOs to prevent secondary effects of RA deviation(s) following a level bust.
- TCAS mitigation of risk of collision resulting from a level bust, a HINDSIGHT 10 article
EUROCONTROL has produced a wide range of valuable material raising awareness of, and addresssing the causes of, the Level Bust issue, including Safety Letters, an important series of briefing notes (also part of the Level Bust Toolkit), a number of articles in HindSight - EUROCONTROL, an Action Plan, Posters and more. For a complete list of material available on the SKYbrary Bookshelf, see the article Level Bust Products.
Airbus Briefing Notes