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              COLONIAL AIRLINES, INC.
        

                         December 14, 1948
TO: All Pilots

FROM: G. R. Janas

SUBJECT: Two-Letter Compass Locator Identifier System

The Civil Aeronautics Administration has announced that as rapidly as possible two-letter identifiers will be assigned compass locators associated with the ILs. The following is quoted from the Administration's announcement to explain how assignment will be made:

  "Without exception, the compass locator identifiers shall be derived in the following manner:
   
    The three-letter location identifier shall be considered as a base.

    Forward Course Compass Locators* -  The first and second letters of the three-letter location identifier shall be assigned to the forward course outer marker compass locator (Class OMK), and the second and third letters shall be assigned to the forward course middle marker compass locator (Class MMK).

        Example: Chicago, Ill. - Chi
                          OMK  - CH
                          MMK  - HI

Back Course Compass Locators* - The third and second letters of the three-letter location identifier shall be assigned to the back course outer marker compass locator (Class OMK) and the second and first letters shall be assigned to the back course middle marker compass locator (Class MMK).

         Example:  Chicago, Ill. - CHI
                Back Course OMK  - IH
                Back Course MMK  - HC

*This designation terminology is utilized herein for clarity, pending official establishment of differentiating classifications, as may be required in the future. Inasmuch as only one "back course" compass locator facility is now operative, at Chicago Municipal Airport, the designation "back course" may be applied to this facility, with the designation "forward course" not used, in connection with instrument landing systems which include no more than two single-approach facilities of this type.

This system of identifier assignment will provide 100% association of the compass locator facilities to the instrument landing system of which they are an integral part and thus increase the accuracy with which the airmen may determine and report his position when making an approach under instrument conditions. The same method of assignment shall be made in those locations where only one compass locator may be installed.