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Flight Level or Altitude Confusion

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

Description

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;
  • Subconsciously interpreting a request to slow down to 250 kt463 km/h
    128.5 m/s
    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).

ICAO standard phraseology is "FLIGHT LEVEL ONE ZERO ZERO" and "FLIGHT LEVEL ONE ONE ZERO";

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".

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.

Solution

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:

  • A310 / B736, en-route, Southern Norway, 2001 (On 21 February 2001, a level bust 10 nm north of Oslo Airport by a climbing PIA A310 led to loss of separation with an SAS B736 in which response to a TCAS RA by the A310 not being in accordance with its likely activation (descend). The B736 received and correctly actioned a Climb RA.)
  • A310, vicinity Birmingham UK, 2006 (On 24 November 2006, an A310 descended significantly below cleared altitude during a radar vectored approach positioning, as a result of the flight crew's failure to set the QNH, which was unusually low.)
  • A319 / A321, en-route, west north west of Geneva, Switzerland 2011 (On 6 August 2011 an Easyjet Airbus A319 on which First Officer Line Training was in progress exceeded its cleared level during the climb after a different level to that correctly read back was set on the FMS. As a result, it came into conflict with an Alitalia A321 and this was resolved by responses to coordinated TCAS RAs. STCA alerts did not enable ATC resolution of the conflict and it was concluded that a lack of ATC capability to receive Mode S EHS DAPs - since rectified - was a contributory factor to the outcome.)
  • A319/B733, en-route, near Moutiers France, 2010 (On 8 July 2010 an Easyjet Airbus A319 on which line training was being conducted mis-set a descent level despite correctly reading it back and, after subsequently failing to notice an ATC re-iteration of the same cleared level, continued descent to 1000 feet below it in day VMC and into conflict with crossing traffic at that level, a Boeing 737. The 737 received and actioned a TCAS RA ‘CLIMB’ and the A319, which received only a TCAS TA, was given an emergency turn by ATC. The recorded CPA was 2.2 nm and 125 feet.)
  • A320, en-route, north of Swansea UK, 2012 (On 7 September 2012, the crew of an Aer Lingus Airbus A320-200 mis-set their descent clearance. When discovering this as the actual cleared level was being approached, the AP was disconnected and the unduly abrupt control input made led to an injury to one of the cabin crew. The original error was attributed to ineffective flight deck monitoring and the inappropriate corrective control input to insufficient appreciation of the aerodynamic handling aspects of flight at high altitude. A Safety Recommendation to the Operator to review relevant aspects of its flight crew training was made.)
  • A321 / B738, en-route, south eastern Bulgaria, 2016 (On 8 September 2016, an Airbus A321 en route in Bulgarian airspace at FL 350 was given and acknowledged a descent but then climbed and came within 1.2nm of a descending Boeing 737. The Investigation found that the inexperienced A321 First Officer had been temporarily alone when the instruction was given and had insufficient understanding of how to control the aircraft. It was also found that despite an STCA activation of the collision risk, the controller, influenced by a Mode ‘S’ downlink of the correctly-set A321 cleared altitude, had then added to the risk by instructing the 737 to descend.)

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Related Articles

Further Reading

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