Conditional Clearance

In air traffic management, conditional clearance is a specialized form of clearance issued by air traffic control (ATC). Unlike a standard clearance that is immediately effective, a conditional clearance only becomes valid when a specific condition is met. This procedure allows controllers to manage traffic flow efficiently while maintaining separation between aircraft or vehicles on or near an active runway.

Conditional clearances are most often used in surface movement control, particularly at airports with high traffic density. When applied correctly, they improve efficiency and reduce delays. However, if misunderstood or misapplied, they can create ambiguity and safety risks.

Conditional Clearance

Definition

According to ICAO, a conditional clearance is:

“A clearance issued by an air traffic controller which does not become effective until a specified condition has been satisfied.”

This type of clearance always contains a reference condition (e.g., another aircraft or vehicle movement) and must follow a strict communication format to prevent misinterpretation.

Common Examples

Conditional clearances often refer to movements relative to another aircraft or vehicle:

• “AFTER THE DEPARTING AIRCRAFT…”
• “BEHIND THE LANDING AIRCRAFT…”
• “BEHIND THE LUFTHANSA A340…”

For instance, ATC might instruct:
“SAS 941, BEHIND DC9 ON SHORT FINAL, LINE UP BEHIND.”

This means SAS 941 may line up on the runway only after the DC9 on short final has passed.

Operational Use

Conditional clearances are usually employed in:

1. Runway Operations
   • Line-up clearances when traffic is already on approach or departure.
   • Expedite departures during peak hours without compromising safety.
2. Taxiway Operations
   • Movement instructions relative to other aircraft or vehicles (“Behind the A320 crossing left to right, taxi to holding point runway 27”).
3. Ground Vehicle Movement
   • Instructions for airport vehicles to enter/cross taxiways or runways only after an aircraft has passed.

Associated Threats and Risks

While conditional clearances improve efficiency, they introduce risks when misunderstood by pilots or miscommunicated by controllers. Key threats include:

1. Ambiguity in Traffic Identification
   • More than one similar aircraft (same type/operator) visible at once.
   • Aircraft in unusual or unfamiliar liveries (special paint schemes).
2. Positional Misunderstandings
   • If the aircraft is at a different holding point than the controller assumes.
   • Confusion about which traffic is the reference (e.g., two aircraft approaching simultaneously).
3. Timing Issues
   • If the pilot interprets the clearance as valid before the condition has been fully satisfied.
4. Runway Incursions
   • The most severe threat: an aircraft or vehicle enters an active runway while another aircraft is still landing or departing.

ICAO Rules and Format

To reduce risks, ICAO sets strict rules for conditional clearance usage:

• Runway Restriction
  Conditional phrases such as “behind landing aircraft” or “after departing aircraft” must not be used for active runway movements unless:
  • Both the aircraft/vehicle concerned are visible to the controller and
  • The pilot has visually identified the reference aircraft/vehicle.
• Order of Transmission
  ICAO specifies the sequence in which conditional clearances must be delivered:
  1. Aircraft Identification
  2. The Condition
  3. The Clearance
  4. Reiteration of the Condition

Example:
“SAS 941, BEHIND DC9 ON SHORT FINAL, LINE UP BEHIND.”

This format ensures the pilot hears the condition before the clearance and is reminded again at the end.

• Requirement of Identification
  The pilot must positively identify the reference aircraft or vehicle before acting on the clearance.

(Reference: ICAO Doc 4444 – PANS-ATM, Chapter 12, para 12.2.7)

Safety Considerations

1. Pilot Responsibilities
   • Must clearly identify the reference aircraft or vehicle.
   • Must not act until the condition is fully satisfied.
   • Should request clarification if in doubt.
2. Controller Responsibilities
   • Must issue clear and unambiguous instructions.
   • Ensure that the condition aircraft/vehicle will be the first to pass in front of the subject aircraft.
   • Use plain, standard ICAO phraseology—never non-standard wording.
3. Best Practices
   • Avoid issuing conditional clearances when multiple aircraft are in close proximity and could be confused.
   • Use conditional clearances primarily during visual meteorological conditions (VMC), not in low visibility.
   • Maintain extra vigilance during peak operations and when unusual traffic (special flights, charter operations) is present.

Accidents and Incidents

Several accidents and serious incidents have been linked to the misuse or misinterpretation of conditional clearances. These typically involve:

• Runway Incursions caused by an aircraft lining up before the condition was satisfied.
• Misidentification of the reference aircraft, particularly at night or in poor visibility.
• Breakdown in Communication where the pilot assumed the clearance was unconditional.

Example Case Studies (to be detailed for training purposes):

• A crew lining up prematurely while another aircraft was landing.
• Taxiway collisions when pilots followed the wrong reference aircraft after a conditional clearance.

These incidents underscore the importance of strict adherence to ICAO phraseology and clear visual identification.

Conditional clearance is a valuable ATC tool that balances efficiency with safety. When issued and executed correctly, it enables faster sequencing of aircraft and smoother operations, especially at busy airports. However, because of the potential for misinterpretation, it requires:

• Strict compliance with ICAO format.
• Mutual confirmation of traffic identification between pilots and controllers.
• Caution in complex traffic situations to avoid ambiguity.

As with all aspects of air traffic management, the priority remains safety over efficiency. For both controllers and pilots, conditional clearance should be treated as a tool of convenience, but never at the expense of clarity or safety.