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Level Bust with more than 1000 ft

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Category: Safety Alert Safety Alerts
Content source: EUROCONTROL Safety Alerts EUROCONTROL

Request for Support Message

Level Bust with more than 1000 ft

Date: 09 May 2008 Request for Support Message.gif

This information was received from DFS - the German Air Navigation Service Provider

  • An Occurrence with a military fighter aircraft busting a level has drawn our attention to a possible deficiency with procedures associated with level bust incidents.
  • The question is: What is to be expected from pilots busting a level by more than 1000ft, when, depending on the position of other traffic, immediate return to the cleared level could be more dangerous than busting the level itself?
  • The situation is especially dangerous with high performance aircraft on busy ATC frequencies, where perhaps a pilot suddenly recognizes his error and does not delay his return back to cleared level until he is able to confirm it with ATC. Level bust by high performance aircraft, like military fighters, can be easily greater than 1000ft in a very short time, where the controller is not able to detect this level bust in a timely manner.
  • In our particular incident with the military fighter aircraft, the Altitude information was even lost on radar because of garbling with radar data from other traffic in close proximity, only a few hundred metres apart.

A Possible Solution and a Request

A possible solution could be the procedure that pilots, after recognizing a level bust by more than 1000ft themselves, would change level to the next closest 500ft step intermediate altitude/level (for example 2500ft or FL 115) to establish at least emergency separation until there is definite clarification with ATC about level to descent or climb to.

Support Requested

Readers were asked whether there was a need for a procedure or a recommendation for pilots in this kind of level bust incident? What could the possible solution look like?


Responses were received from 15 European airlines and 10 non-European airlines; most of these were from pilots with responsibility for flight safety or standards. There were 6 responses from ANSPs, 2 from pilots, 3 from EUROCONTROL staff members and 1 from IATA. The following is a brief summary of comments.

  1. Level busts exceeding 1000 ft above/below cleared level in controlled airspace are currently very unusual.
  2. Most instances of level bust >1000 ft involve military aircraft; these instances are uncommon because as with civil aircraft, great emphasis is placed on this during training; return to assigned level is usually rapid - probably before the ATCO has an opportunity to intervene.
  3. Avoiding action by military aircraft may appear dangerous to non-military aircraft and controllers, but seldom is.
  4. Many cases of loss of separation from military aircraft occur due to airspace infringement rather than level bust.
  5. Most military aircraft monitor the emergency frequency continuously, and so can be contacted readily if required.
  6. Military aircraft are not equipped with TCAS, but usually have powerful equipment capable of identifying and avoiding other aircraft.
  7. Level busts by commercial aircraft resulting from pilot error seldom exceed 500 ft before the error is noticed by a pilot or by the ATCO.
  8. Most, but not all commercial aircraft are equipped with TCAS, though MELs may permit aircraft to operate for up to 10 days with unserviceable TCAS.
  9. Some existing civil aircraft and (currently) VLJ aircraft are not required to carry TCAS.
  10. All civil and the majority of military flight rules currently require aircraft to return to their assigned level as soon as a deviation is identified.
  11. Civil and military operators recommend or require reduction in climb/descent rate when approaching the cleared level. An amendment to ICAO Annex 6 to reflect this is in course of preparation.
  12. Rate of climb/descent within controlled airspace should be restricted, e.g. to a maximum of 4000 ft/min.
  13. A possible solution to the level bust >1000 ft problem would be the procedure suggested by DFS (to fly to an intermediate altitude). Some respondents suggested refinements to this procedure including suggested phraseology.
  14. The majority of respondents agreed that introducing a new procedure to deal with an infrequent problem would be liable to cause confusion for a wide variety of reasons. Most believed that the best solution to this problem would be:
    • to make no change to current procedures;
    • to continue to emphasise the need for careful cross-checking of level clearance and instrument settings;
    • to monitor carefully TCAS indications and alerts and take appropriate avoiding action.
  15. Meanwhile, further study is indicated to determine the extent and gravity of the problem identified, and to meet the challenge that will result from growing numbers of non-TCAS equipped VLJs and other aircraft.

The full response


© European Organisation for Safety of Air Navigation (EUROCONTROL) May 2008. This alert is published by EUROCONTROL for information purposes. It may be copied in whole or in part, provided that EUROCONTROL is mentioned as the source and to the extent justified by the non-commercial use (not for sale). The information in this document may not be modified without prior written permission from EUROCONTROL. The use of the document is at the user’s sole risk and responsibility. EUROCONTROL expressly disclaim any and all warranties with respect to any content within the alert, express or implied.