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Level Bust - ATCO Induced Situations

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Article Information
Category: Level Bust Level Bust
Content source: SKYbrary About SKYbrary
Content control: EUROCONTROL EUROCONTROL

Description

Actual or potential loss of separation sometimes occurs because an ATC instruction is difficult or impossible to comply with in time, or is incorrect or inappropriate (e.g. the ATCO instructs the pilot to level at an altitude which creates a potential confliction). Strictly speaking, such an occurrence is not a Level Bust because the pilot does not fail to follow a received clearance. However, because the effect is the same, these situations are considered below for completeness.

The following detailed contributing factors are often cited in level bust attributed to ATM or ATC instructions or services :

  • Callsign confusion;
  • 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).

Situations sometimes occur in which instructions given by the ATCO are difficult or even impossible to follow without a deviation occurring. The following are some typical scenarios:

Typical Scenarios

  • Scenario 1. The ATCO issues a clearance to climb or descend to a specified flight level and the pilot follows this clearance. Subsequently, the ATCO instructs the pilot to level at an intermediate flight level but at the time of this re-clearance the flight has passed the re-cleared flight level.
Scenario 1.jpg


  • Scenario 2. The flight is cleared to climb from below the transition altitude to a flight level above it. The pilot sets standard pressure setting and commences the climb. The ATCO re-clears the flight to level at an altitude below the transition altitude. The pilot levels at the re-cleared level, but with the standard pressure setting still set.
Scenario 2.jpg


  • Scenario 3. The flight is cleared to descend from above the transition level to an altitude that is below it. The pilot sets QNH and commences the descent. The ATCO re-clears the flight to a flight level above the transition level. The pilot levels at the re-cleared level but with QNH still set.
Scenario 3.jpg

Contributory Factors

Solutions

  • Avoid issuing vertical re-clearances that, given the observed evidence of the rate of climb/descent (energy state) of the aircraft, cannot be achieved without exceeding the cleared level .
  • Observe strict radio discipline, especially standard phraseology speed and timeliness of communication and language;
  • Where appropriate, stress the need the need to revert to the relevant barometric sub scale setting in the message to pilots.

Further Reading

EUROCONTROL Level Bust Toolkit

HindSight Magazine

  • 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. These and other Level Bust products are listed in the article Level Bust Products

Airbus Briefing Note