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COLONIAL AIRLINES, INC. TO: All Pilots, Dispatchers and Flight Radio Operators Date: 12-3-47 FROM: Director of Communications SUBJECT: Airborne Quoted below is a letter received from Mr. Andrew C. Miller, Chief Air Carrier Operations Division, Civil Aeronautics Administration, which is self-explanatory: "It has been brought to our attention that interaction may occur between airborne ILS receiving equipment and airborne HF transmitting equipment, resulting in an erratic needle indication of the "cross pointer" indicator. In those indicators utilizing the flag alarm, the interaction usually also causes the alarm to be actuated. This matter is receiving attention from the various airline and other, however results to date indicate that the difficulty is caused by harmonic relationship between the ILS channels and HF transmitting frequencies and is not likely to be eliminated insofar as the presently used ILS receiving equipment is concerned. Inasmuch as the interaction takes place only while signals are being emitted from the transmitter, a condition which can be controlled by the plane commander, it is not felt that it is too serious provided that pilots are aware of the possibility of interaction. It is the purpose of this letter therefore to bring the matter to your attention and to request that action be taken to insure that all pilot personnel are given instructions concerning the possibility of this interaction." EW/cew E. Werzyn