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.

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

  • 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:

  • 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 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 3 February 2019, two aircraft which had just landed on adjacent parallel runways almost collided during their taxi in after one failed to give way to the other at an intersection as instructed, causing the other to perform an emergency stop which was achieved just in time to avoid a collision. Whilst not attributing direct cause to other than the crew of the aircraft which continued high speed taxiing as the intersection was approached, having noted that all taxiway lighting at Amsterdam is permanently lit at night, a range of factors were identified which had facilitated the error made.

On 6 November 2017, an Embraer E190 cleared for a normal visibility night takeoff at Nice began it on a parallel taxiway without ATC awareness until it had exceeded 80 knots when ATC noticed and a rejected takeoff was instructed and accomplished without any consequences. The Investigation found that although both pilots were familiar with Nice, their position monitoring relative to taxi clearance was inadequate and both had demonstrated a crucial lack of awareness of the colour difference between taxiway and runway lighting. Use of non-standard communications phraseology by both controllers and flight crew was also found to be contributory.

On 7 December 1983, a Boeing 727-200 taking off from Madrid in thick fog collided at high speed with a Douglas DC-9 which had not followed its departure taxi clearance to the beginning of the same runway. The DC-9 crew did not advise ATC of their uncertain location until asked for their position after non-receipt of an expected position report. The Investigation concluded that flight deck coordination on the DC-9 had been deficient and noted that gross error checks using the aircraft compasses had not been conducted. The airport was without any surface movement radar.

On 22 November 1994 a McDonnell Douglas MD 82 flight crew taking off from Lambert- St. Louis at night in excellent visibility suddenly became aware of a stationary Cessna 441 on the runway ahead and was unable to avoid a high speed collision. The collision destroyed the Cessna but allowed the MD82 to be brought to a controlled stop without occupant injury. The Investigation found that the Cessna 441 pilot had mistakenly believed his departure would be from the runway he had recently landed on and had entered that runway without clearance whilst still on GND frequency.

On 3 December 1990 a Douglas DC9-10 flight crew taxiing for departure at Detroit in thick fog got lost and ended up stopped to one side of an active runway where, shortly after reporting their position, their aircraft was hit by a departing Boeing 727-200 and destroyed by the impact and subsequent fire. The Investigation concluded that the DC9 crew had failed to communicate positional uncertainty quickly enough but that their difficulties had been compounded by deficiencies in both the standard of air traffic service and airport surface markings, signage and lighting undetected by safety regulator oversight.

On 22 December 2013, a Boeing 747-400 taxiing for departure at Johannesburg at night with an augmented crew failed to follow its correctly-acknowledged taxi clearance and one wing hit a building resulting in substantial damage to both aircraft and building and a significant fuel leak. The aircraft occupants were all uninjured but four people in the building sustained minor injuries. The accident was attributed to crew error both in respect of an inadequate briefing and failure to monitor aircraft position using available charts and visual reference. Some minor contributory factors relating to the provision of airport lighting and signage were noted.

On 11 October 2013, the commander of a Boeing 737-400 taxiing on wet taxiways at night after landing at Zurich became uncertain of his position in relation to the clearance received and when he attempted to manoeuvre the aircraft off the taxiway centreline onto what was believed to be adjacent paved surface, it became bogged down in soft ground. The Investigation considered the direct cause of the taxiway excursion was not following the green centreline lights but it recommended improvements in the provision of clear and consistent taxi instructions and in taxiway designations in the area of the event.

On 29 March 2014, a Beech 1900D being taxied by maintenance personnel at Calgary entered the active runway without clearance in good visibility at night as a Boeing 737-700 was taking off. The 737 passed safely overhead. The Investigation found that the taxiing aircraft had taken a route completely contrary to the accepted clearance and that the engineer on control of the aircraft had not received any relevant training. Although the airport had ASDE in operation, a transponder code was not issued to the taxiing aircraft as required and stop bar crossing detection was not enabled at the time.

n 22 December 2003, a Boeing 737-700 being operated by UK Operator Easyjet on a scheduled passenger flight from Amsterdam to London Gatwick was taxiing for departure at night in normal visibility and took a different route to that instructed by ATC. The alternative route was, unknown to the flight crew, covered with ice and as a consequence, an attempt to maintain directional control during a turn was unsuccessful and the aircraft left wing collided with a lamp-post. The collision seriously damaged the aircraft and the lamp post. One passenger sustained slight injuries because of the impact. The diagram below taken from the official investigation report shows the area where the collision occurred.

On 2 February 2006, a Boeing 747-400 was taxiing for a departure at Melbourne Airport. At the same time, a Boeing 767-300 was stationary on taxiway Echo and waiting in line to depart from runway 16. The left wing tip of the Boeing 747 collided with the right horizontal stabiliser of the Boeing 767 as the first aircraft passed behind. Both aircraft were on scheduled passenger services from Melbourne to Sydney. No one was injured during the incident.

On 17 March 2002, at Ted Stevens Anchorage Airport, a McDonnell Douglas MD82 operated by Alaska Airlines, on a night pushback in snow conditions collided with an inbound taxiing McDonnell Douglas MD-11. The MD82 suffered substantial rudder damage although the impacting MD11 winglet was undamaged.

On 27 March 1977, a KLM Boeing 747-200 began its low visibility take-off at Tenerife without requesting or receiving take-off clearance and a collision with a Boeing 747-100 backtracking the same runway subsequently occurred. Both aircraft were destroyed by the impact and consequential fire and 583 people died. The Investigation attributed the crash primarily to the actions and inactions of the KLM Captain, who was the Operator's Chief Flying Instructor. Safety Recommendations made emphasised the importance of standard phraseology in all normal radio communications and avoidance of the phrase take-off in ATC Departure Clearances.

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

SKYbrary Partners:

Safety knowledge contributed by: