Narrative:

This report is not for a specific event; but for a common ATC instruction which is not defined and causes confusion. With VFR aircraft in VMC; a common ATC instruction for an inbound aircraft is to 'enter (or report) a 3 mi base' or 'enter (or report) a 3 mi final' (or some other specified distance); with the generally accepted meaning being to not fly a pattern; but to fly directly to the point described. The issue is what is that point? The more troublesome of the two is the instruction for a '3 mi base' (or some other distance). These terms are not defined in the aim or the pilot/controller glossary; so the meaning of this phrase is interpreted by each pilot. During recent poll of a local pilot's association; there was a roughly even split among pilots. About half interpreted the instruction '3 mi base' to mean enter a base leg; with the base leg being the standard distance from the threshold; merely extended outward three miles. The other half interpreted the term '3 mi base' to mean enter a base leg with the turn to final 3 mi from the threshold. I question the second interpretation; as I would expect the instruction for that to be 'enter base for a 3 mi final;' but that is irrelevant. Among a group of very intelligent and proficient pilots; there was a 50/50 split on the meaning of a common instruction; meaning about half the time the pilot issued that instruction is not where the controller expects him/her to be. The difference between a 3-MI-long standard base leg and a base leg to a 3 mi final means an aircraft is about 4 mi from the location the controller is expecting. A controller expecting a 3-MI-long base could lose separation for an IFR inbound; and could even create a collision hazard. I strongly suggest a revision to the pilot/controller glossary and/or aim to define these common instructions.

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Original NASA ASRS Text

Title: PLT EXPRESSED CONCERN REGARDING ATC'S USE OF BASE ENTRY INSTRUCTIONS AND THE VARIOUS INTERPRETATIONS OF SAME.

Narrative: THIS RPT IS NOT FOR A SPECIFIC EVENT; BUT FOR A COMMON ATC INSTRUCTION WHICH IS NOT DEFINED AND CAUSES CONFUSION. WITH VFR ACFT IN VMC; A COMMON ATC INSTRUCTION FOR AN INBOUND ACFT IS TO 'ENTER (OR RPT) A 3 MI BASE' OR 'ENTER (OR RPT) A 3 MI FINAL' (OR SOME OTHER SPECIFIED DISTANCE); WITH THE GENERALLY ACCEPTED MEANING BEING TO NOT FLY A PATTERN; BUT TO FLY DIRECTLY TO THE POINT DESCRIBED. THE ISSUE IS WHAT IS THAT POINT? THE MORE TROUBLESOME OF THE TWO IS THE INSTRUCTION FOR A '3 MI BASE' (OR SOME OTHER DISTANCE). THESE TERMS ARE NOT DEFINED IN THE AIM OR THE PLT/CTLR GLOSSARY; SO THE MEANING OF THIS PHRASE IS INTERPED BY EACH PLT. DURING RECENT POLL OF A LCL PLT'S ASSOCIATION; THERE WAS A ROUGHLY EVEN SPLIT AMONG PLTS. ABOUT HALF INTERPED THE INSTRUCTION '3 MI BASE' TO MEAN ENTER A BASE LEG; WITH THE BASE LEG BEING THE STANDARD DISTANCE FROM THE THRESHOLD; MERELY EXTENDED OUTWARD THREE MILES. THE OTHER HALF INTERPRETED THE TERM '3 MI BASE' TO MEAN ENTER A BASE LEG WITH THE TURN TO FINAL 3 MI FROM THE THRESHOLD. I QUESTION THE SECOND INTERP; AS I WOULD EXPECT THE INSTRUCTION FOR THAT TO BE 'ENTER BASE FOR A 3 MI FINAL;' BUT THAT IS IRRELEVANT. AMONG A GROUP OF VERY INTELLIGENT AND PROFICIENT PLTS; THERE WAS A 50/50 SPLIT ON THE MEANING OF A COMMON INSTRUCTION; MEANING ABOUT HALF THE TIME THE PLT ISSUED THAT INSTRUCTION IS NOT WHERE THE CTLR EXPECTS HIM/HER TO BE. THE DIFFERENCE BTWN A 3-MI-LONG STANDARD BASE LEG AND A BASE LEG TO A 3 MI FINAL MEANS AN ACFT IS ABOUT 4 MI FROM THE LOCATION THE CTLR IS EXPECTING. A CTLR EXPECTING A 3-MI-LONG BASE COULD LOSE SEPARATION FOR AN IFR INBOUND; AND COULD EVEN CREATE A COLLISION HAZARD. I STRONGLY SUGGEST A REVISION TO THE PLT/CTLR GLOSSARY AND/OR AIM TO DEFINE THESE COMMON INSTRUCTIONS.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.