MART Vol. II MO/MP - NESA - Civil Air Patrol

7.5 Wing shadow method (signal null)The signal null or wing shadow method is based on the assumption that themetal skin of the search aircraft’s wing and fuselage will block incoming ELTsignals from the receiving antenna during steep-banked turns. The observer canmake simple estimates of the magnetic bearing to the transmitter by checking theaircraft heading when the signal is blocked.Once the search aircraft completes several signal-blocking turns in differentsectors of the search area, the observer can establish the approximate location ofthe ELT by drawing magnetic bearings, or “null vectors,” on the sectional chart.The ELT and accident scene will be at or near the intersection of the null vectors.To use the null method, you must know the location of your receivingantenna. On a low-wing airplane, like the Piper Cherokee, the com antenna isoften mounted on the underside of the fuselage, in line with the wings. On a highwingairplane, like the Cessna 172, the com antenna is normally mounted on thetop of the airplane, again in line with the wings. [Note: You may also use thereceiver of your aircraft's DF unit, which is normally mounted on the bottom of theaircraft.]7.5.1 ProceduresFirst, verify the receiver is tuned to the proper ELT frequency and that youcan hear the warbling tone. Mark your position on the sectional chart, preferablyover a small but significant feature. Then the pilot will make a 360° steeplybanked (