A level bust, also known as an altitude deviation, occurs when an aircraft fails to fly at the level for which it has been cleared. A level bust is defined by EUROCONTROL as: "Any unauthorised vertical deviation of more than 300 feet from an ATC

Air traffic control

Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...

 flight clearance."

This may take one of three different forms:

1. An aircraft in level flight climbs or descends without clearance
2. An aircraft climbing or descending fails to level off accurately at the correct level (either passing through and continuing the climb or descent, or passing through and then returning to the correct level)
3. An aircraft levelling off at the correct level or altitude, but with an incorrect altimeter
   Altimeter
   An altimeter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater.-Pressure altimeter:...
   
    setting

Causal factors

Mis-hear: An ATCO member failed to detect an incorrect readback. Mis hear errors are recorded when an ATCO fails to detect and correct an incorrect pilot R/T read back which is audible. These errors are more common at the ATC centres with high R/T workload.
Correct pilot readback followed by incorrect action: The pilot readback the controllers instruction correctly however performed an incorrect action (e.g., climbed to incorrect FL). Some events in this category will involve occasions where crews have received a clearance to a level which is known to cause confusion such as FL100/FL110 or FL200/220. The UK have introduced non-standard R/T phraseology to overcome this difficulty but the problem is still with us; in 2004 there are 8 recorded occasions where a crew have correctly acknowledged a decent clearance to FL110 but have then descended to FL100. Other events will involve a breakdown in cockpit SOPs; It is not fully understand why this type of event occurs but it is possible that high R/T loading, high cockpit workload and communication issues are all contributory factors.
Incorrect pilot readback by correct aircraft: The pilot's readback of the controller instruction was erroneous (e.g., wrong level)
Pilot readback by incorrect aircraft: A pilot read back an instruction that had been issued to another aircraft.
Failed to follow cleared SID: The pilot failed to follow comply with the level restrictions include in a SID

Standard Instrument Departure

Standard instrument departure routes, also known as departure procedures are published flight procedures followed by aircraft on an IFR flight plan immediately after take-off from an airport.- Introduction :...

. Failed to follow cleared SID is a particular issue with departures from Gatwick, Stansted, and Luton although other airports are affected. Gatwick, Stansted and Luton all have step climb SIDs and these errors normally involve an aircraft failing to stop at the first stop altitude.
Altimeter setting error: The crew did not have the correct pressure set. Altimeter setting error is a problem mainly reported in the London TMA, 80% of the errors occur when the aircraft is in the climb, is above the transition altitude/level and the standard pressure setting isn't set.
Poor manual handling: The pilot's manual flying of the aircraft led to aircraft failing to comply with clearance.
Aircraft technical problem: Technical problems exist with the aircraft
Failure to follow ATC instruction: The pilot did not comply with an ATC instruction (e.g., the aircraft failed to hold when instructed).

Accidents and serious incidents involving level bust

• 12 November 1996, an IL76 collided in mid air
  Mid-air collision
  A mid-air collision is an aviation accident in which two or more aircraft come into contact during flight. Owing to the relatively high velocities involved and any subsequent impact on the ground or sea, very severe damage or the total destruction of at least one of the aircraft involved usually...
  
   with a Saudi B747 near New Delhi India, following a level bust by the IL76.
• 12 November 1996, a B737 descended below its assigned holding level in the LHR stack, in IMC
  Instrument meteorological conditions
  Instrument meteorological conditions is an aviation flight category that describes weather conditions that require pilots to fly primarily by reference to instruments, and therefore under Instrument Flight Rules , rather than by outside visual references under Visual Flight Rules . Typically, this...
  
  , to within 100 feet vertically and between 680 and 820 metres horizontally of a MD-81 at its correct level. Neither aircraft was fitted with ACAS
  Airborne Collision Avoidance System
  An airborne collision avoidance system is an aircraft system that operates independently of ground-based equipment and air traffic control in warning pilots of the presence of other aircraft that may present a threat of collision. If the risk of collision is imminent, the system indicates a...
  
  .
• 21 February 2001, a level bust 10 nm north of Oslo 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.
• 24 November 2006, an A310 descended significantly below cleared altitude during radar vectored approach positioning, as a result of the flight crew's failure to set the QNH
  QNH
  QNH is one of the many Q codes. It is defined as, "barometric pressure adjusted to sea level." It is a pressure setting used by pilots, air traffic control , and low frequency weather beacons to refer to the barometric setting which, when set on an aircraft's altimeter, will cause the altimeter to...
  
  , which was very low.

See also

Level bust awareness e-learning toolkit