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.

A common example of this is confusion between FL 100 and FL 110 (i.e. the pilot is cleared to fly at FL 110 but levels at FL 100, or vice-versa).


Alternative non-standard phraseology used with success by a number of European air navigation service providers (ANSPs) is "FLIGHT LEVEL ONE HUNDRED", and some states have extended this phraseology to include "FLIGHT LEVEL TWO HUNDRED" and "FLIGHT LEVEL THREE HUNDRED". As a result, Regulation 2016/1185 stated that flight levels containing whole hundreds are to be pronounced as "FLIGHT LEVEL (NUMBER) HUNDRED".

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 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.

On 29 February 2020, an Airbus A320 inbound to Delhi lost separation against an outbound A320 from Delhi on a reciprocal track and the conflict was resolved by TCAS RA activation. The Investigation found that the inbound aircraft had correctly read back its descent clearance but then set a different selected altitude. Air Traffic Control had not reacted to the annunciated conflict alert and was unable to resolve it when the corresponding warning followed and it was noted that convective weather meant most aircraft were requesting deviations from their standard routes which was leading to abnormally complex workload.

On 12 November 1996, an Ilyushin IL76TD and an opposite direction Boeing 747-100 collided head on at the same level in controlled airspace resulting in the destruction of both aircraft and the loss of 349 lives. The Investigation concluded that the IL76 had descended one thousand feet below its cleared level after its crew had interpreted ATC advice of opposite direction traffic one thousand feet below as the reason to remain at FL150 as re-clearance to descend to this lower level. Fifteen Safety Recommendations relating to English language proficiency, crew resource management, collision avoidance systems and ATC procedures were made.

On 10 September 2017, an Airbus A380-800 cleared for an ILS approach at Moscow Domodedovo in visual daylight conditions descended below its cleared altitude and reached 395 feet agl whilst still 7nm from the landing runway threshold with a resultant EGPWS ‘PULL UP’ warning. Recovery was followed by an inadequately prepared second approach which was discontinued and then a third approach to a landing. The Investigation attributed the crew’s difficulties primarily to failure to follow various routine operating procedures relating to use of automation but noted that there had been scope for better presentation of some of these procedures.

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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