Definition

The flight level at or to which an aircraft is authorised to proceed under conditions specified by an ATC unit.

Source: EUROCONTROL EATM Glossary of Terms

Decription

The cleared flight level (CFL) is the flight level that an air traffic controller has cleared a controlled flight to maintan or climb/descend to.

Note: CFL is part of the broader term "cleared level" which encompasses "cleared altitude" and "cleared height" as well. This article focuses on the representation and use of the cleared level in automated ATM systems which are the primary tools of approach and area controlers. Therefore, the more specific term (CFL) is used instead of the broader one (cleared level).

Ideally, the CFL would equal the requested flight level (RFL) during cruise as this is normally the optimal level for the flight. However, due to various reasons (e.g. conflicting traffic, SUAs, letters of agreement, ATFM regulations, etc.) it may not be available (yet).

The CFL value can be adjusted multiple times within a single sector in order to adapt to the air situation. Examples of this include:

• Clearing the aircraft to an intermediate safe level (e.g. 1000 ft below a conflicting traffic) during climb if climb to the requested level cannot be accommodated.
• Clearing the aircraft to an intermediate safe level (e.g. 1000 ft above a conflicting traffic) if continuous descent cannot be accommodated.
• Instructing the aircraft to leave its cruising level and climb/descend 1000/2000 ft in order to solve a conflict.
• Successive clearance(s) to higher/lower levels in case of multiple climbing/descending aircraft. When using this technique, the lowest climbing/highest descending aircraft is cleared to a safe level that has been vacated by another aircraft. This can be used as an alternative to assigning vertical speeds. The advantage is that the pilot is free to adjust the rate of climb/descent. This comes at the cost of increased frequency occupancy (due to multiple instructions and readbacks).

The CFL is an important parameter of every controlled flight and is therefore presented in the aircraft label. This allows the controller to quickly assess the situation during the routine structured scan of the airspace.

Timely update of the CFL (on the part of the controller) and adherence to it (on the part of the pilot) are essential for achieving safe and efficient flow of traffic. Increased traffic volume as well as airspace complexity have lead to the need of developing support tools that help the controller maintain situational awareness and quickly detect deviations and potentially dangerous situations. Examples of these include:

• Safety net inhibition if the current flight level is comliant with the CFL. For example, a descending aircraft would not trigger an MSAW alert if the CFL is above the minimum safe altitude and the current aircraft level is above the CFL even though the aircraft is descending. This helps reduce nuisance alerts and increases controller trust in the system. Similarly, a climbing/descending aircraft would not trigger an STCA warning if the CFL is 1000 ft below/above another aircraft in level flight as long as the CFL is complied with. Note that in this situation, a high enough rate of climb/descend may, in rare cases, trigger an ACAS RA.
• Similarly to safety nets, MTCD indications may differ depending on the CFL value. For example, a potential conflict alert can be indicated if the CFL provides safe separation (i.e. if the aicraft is cleared to a different level, this may lead to a conflict) and a real conflict alert can be displayed if the CFL does not provide adequate separation. This provides timely warning to the controller in case of conflicting CFL input.
• Comparison between the actual and cleared level. Automated ATM systems normally generate an alert (CLAM) in case of discrepancy between the current level and the CFL. These are inhibited when there is a tendency for reducing the gap between the two (to account for situations where the aircraft is climbing/descending to the CFL but has not yet reached it).
• Comparison between the Mode S selected level and the CFL. This can be used as an early warning to the controller for situations where the aircraft is likely to deviate from the cleared level. Exemples of such situations include:
  • Pilot acting on a clearance intended for another aircraft due to callsign confusion
  • Pilot mishearing the clearance
  • Correct readback followed by incorrect setting of the selected level.

In addition to the above, correct input of CFL and the exit fligth level (XFL, i.e. the level at which the aircraft is planned to exit the sector) provides the controller with a quick way to assess the situation. This also facilitates the handover-takeover process by showing the relieving contoller the plan and its current state of execution.

The picture below shows an example aircraft label (the image is generic and is not from an existing ATM system). The aircraft is climbing through FL 345 to a CFL of FL 360 and the controller plans to clear the flight to FL 380 before the end of their sector. The discrepancy between the CFL (FL 360) and the mode S selected level (FL 370) is displayed by changing the colour of the selected level field to orange. This would prompt the controller to verify the crew's intentions using the phrase "ASB49, CHECK SELECTED LEVEL. CLEARED LEVEL IS 360.".

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