Flight Level or Altitude Confusion

Flight Level or Altitude Confusion


Flight level or altitude confusion occurs when a pilot is cleared to fly at a particular level and correctly acknowledges this clearance, yet levels at a different flight level or altitude.

Contributing Factors

Flight level or altitude confusion is usually the result of the combination of two or more of the following factors:

  • Read-back/hear-back error because of similar sounding phrases;
  • Non-standard phraseology;
  • Mindset tending to focus on two digits, e.g. “one zero” and thus to understand more easily "FLIGHT LEVEL ONE ZERO ZERO" when the clearance was to FL110;
  • Failing to question the unusual (e.g. bias of expectation on a familiar standard terminal arrival (STAR); and/or,
  • Subconsciously interpreting a request to slow down to 250 kt as a clearance to descend to FL100.
  • Failure to set the standard QNE altimeter setting of 29.92 (hg) or 1013.2 (mb) when climbing through the transition altitude. If crew members are distracted as they climb through transition altitude, they can leave their altimeter settings at a local QNH reading, resulting in a level bust when the aircraft levels off at the wrong altitude.

The correct transmission of FL 100 (i.e. "FLIGHT LEVEL ONE ZERO ZERO" or "FLIGHT LEVEL ONE ONE ZERO") differs by country and has also changed over time. Currently, ICAO standard phraseology (as specified in Annex 10, Volume II, Chapter 5, 2018) prescribes the use of whole hundreds for flight levels. However, older ICAO phraseology related documents (such as Doc 9432 Manual of Radiotelephony, 2007) state that each digit of the flight level is to be pronounced separately. There was a period of time when some countries (e.g. the UK and the EU member states) used the whole hundreds rule contrary to the ICAO standard. Based on the time of their basic training and the contents of the following refresher training, some pilots and air traffic controllers still use the rule for pronouncing each digit seperately for flight levels containing whole hundreds.

Similar confusion can occur at other flight levels or between altitudes, although it is much less common and FL100/110 confusion is both the most common and the most hazardous flight level confusion seen in Europe and North America.


Sound Standard Operating Procedures (SOPs), especially with regard to:

  • Radio Discipline;
  • Adherence to the pilot-controller confirmation/correction process (communication loop); and,
  • Cross-checking between flight crew to ensure that the selected altitude is the cleared altitude.

Accidents and Incidents

The following events on the SKYbrary database include "accepted ATC clearance not followed" as a factor:

On 29 April 2021, the crew of an Airbus A319 that had just taken off from runway 18 at St Petersburg saw a Cessna M337 about to cross their intended track close to the same height, and levelled off to ensure they passed beneath it, achieving a 100-foot separation. The conflict was attributed to the controller’s general loss of situational awareness and his failure to properly scan airspace in the vicinity of the airport. The Cessna 337 pilot’s failure to fly at the standard circuit height or track downwind at the standard distance from the runway was deemed contributory.

On 12 January 2022, an Embraer 170 and a Cessna 525 crossed tracks without the prescribed minimum separation, with neither ATC nor the Embraer crew being aware. Although ATC had issued acknowledged clearances to keep the Embraer 1,000 feet above the Cessna, it actually passed beneath it, violating minimum lateral separation. The underlying cause of the event was found to be an unrectified recurrent intermittent fault in one of the Cessna’s air data systems. Poor Cessna crew/controller communication during the event, systemically poor safety culture at its operator, and shortcomings in the Textron Aircraft Maintenance Manual were considered contributory.

On 6 November 2018, an Airbus A340-600 in cruise northbound over the Swiss Alps received an overspeed warning after encountering an unexpected wind velocity change. However, the crew failed to follow the prescribed response procedure. This led initially to a climb above their cleared level, and further inappropriate actions were then followed by PAN and MAYDAY declarations as control of the aircraft was briefly lost in a high-speed descent below their cleared level. The operator subsequently enhanced pilot training realism by providing it in a simulator configured for the aircraft variant operated by the company. The operator also introduced upset recovery training.

On 6 January 2018, a Boeing 737-900 and an Airbus A320 both inbound to Surabaya with similar estimated arrival times were cleared to hold at the same waypoint at FL100 and FL110 respectively but separation was lost when the A320 continued below FL110. Proximity was limited to 1.9nm laterally and 600 feet vertically following correct responses to coordinated TCAS RAs. The Investigation found that all clearances / readbacks had been correct but that the A320 crew had set FL100 instead of their FL110 clearance and attributed this to diminished performance due to the passive distraction of one of the pilots.

On 2 May 2015, a Boeing 777-200 deviating very significantly north of its normal route from Malabo to Douala at night because of convective weather had just turned towards Douala very close to 13,202 feet high Mount Cameroon whilst descending through 5000 feet, when an EGPWS TERRAIN AHEAD alert and ‘PULL UP’ warning prompted an 8,000 foot climb which passed within 2,100 feet of terrain when close to and still below the summit. The Investigation attributed the dangerous event primarily to the augmented crew’s absence of situational awareness and the operator’s failure to risk-assess the route involved.

Related Articles

Further Reading

  • 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

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