Base Effect

Base Effect


The term "Base Effect", in the context of a specific aerodrome, refers to the potential for a gradual erosion of safety margins in the day-to-day flight procedures of locally based operators.


Virtually every aerodrome has locally-based aircraft, pilots and, in many cases, air traffic controllers. Over time, these individuals become very familiar with the aerodrome environment, traffic flows and ATM procedures. This knowledge, combined with a degree of complacency, can, over time, lead to the acceptance of localised deviations to standard procedures and protocols. To a large extent, this "deviance normalisation" results from an enhanced level of familiarity and "mutual trust" amongst the local operators and between the operators and local air traffic control. Although these modified local procedures might work reasonably well on a day to day basis, they may be deficient should transient aircraft be present, in the event that there is a mixture of Visual Flight Rules (VFR) and Instrument Flight Rules (IFR) traffic or if an unplanned event or emergency occurs.


Examples of these localised deviations are numerous and can vary from location to location. They include, but are not limited to, failure to follow published arrival or departure procedures, failure to fly at appropriate airspeeds, failure to comply with standard circuit joining procedures, adoption of non-standard circuit patterns, failure to use standard communication phraseology, omission of position reports, flight at inappropriate altitudes, failure to yield right of way, and so forth. Over time, these non-standard procedures become the norm for local traffic and, in the context of a controlled aerodrome, the ATS expectation. Thus, the introduction of transient traffic, marginal weather, an emergency or other abnormal situation can lead to confusion, increased controller workload, reduced situational awareness and an increased potential for a loss of separation.


Prevention of Base Effect hinges on good airmanship and discipline. Tenets of good discipline include:

  • Not accepting the perspective that rules must sometimes be bent to get the job done;
  • Rejecting the notion that Standard Operating Procedures (SOPs) apply only to those with less ability and experience;
  • Rejecting shortcuts and refusing to abandon the rules to do things that appear to be “better”, "easier" or "more efficient"; and,
  • Planning and preparing for problems before they arise.

A well-disciplined pilot will follow the appropriate rules and procedures, both at his/her home aerodrome and while away, unless an emergency situation compels a deviation. A disciplined controller will follow standard procedures and protocols and take steps to ensure that operators within their area of responsibility are compliant with all applicable rules, regulations and procedures.

The benefits of elimination, or even reduction, of Base Effect are significant in the context of enhancing safety of flight.

Accidents and Incidents

  • A319 / AS32, vicinity Marseille France, 2016 On 27 June 2016, an Airbus A319 narrowly avoided a mid-air collision with an AS532 Cougar helicopter whose single transponder had failed earlier whilst conducting a local pre-delivery test flight whilst both were positioning visually as cleared to land at Marseille and after the helicopter had also temporarily disappeared from primary radar. Neither aircraft crew had detected the other prior to their tracks crossing at a similar altitude. The Investigation attributed the conflict to an inappropriate ATC response to the temporary loss of radar contact with the helicopter aggravated by inaccurate position reports and non-compliance with the aerodrome circuit altitude by the helicopter crew.

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