FLC SQUAWKS A HIJACKING CODE INSTEAD OF A LOSS OF RADIO ON A B737-200 GOING INTO OMA.

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CRUISING AT FL310 NEAR IRK VOR; ENCOUNTERED AN EXTENSIVE AREA OF HIGH CLOUD; MODERATE TURB AND MODERATE ICING. REQUESTED DSCNT AND CLRED TO FL280. TURB AND ICING PERSISTED. REQUESTED LOWER ALT. LOUD AND CONTINUOUS RADIO STATIC CONTINUED FOR APPROX THE NEXT 20 MINS; PUNCTUATED BY LOUD SQUEAL DURING LIGHTNING FLASHES. HVY PRECIP AND ISOLATED TSTM CELLS DEPICTED ON RADAR. ATTEMPTED FURTHER DSCNT CLRNC WITH ZMP ON 119.6 BUT RECEIVED NO RESPONSE. AN AIRBORNE RELAY THROUGH AN ACR ACFT PROVIDED A CLRNC TO FL240. AT THIS POINT; NO FURTHER RADIO CONTACT WAS ACHIEVED FOR A PROLONGED PERIOD OF TIME. OMA ATIS (4 OVCST 2 1/2 NM RAIN AND FOG 53 DEGS/51 DEGS; 080 DEGS; 20 KTS; ALTIMETER 29.72 LNDG RWY 17) WAS OBTAINED PRIOR TO RADIO FAILURE. AT THIS POINT IT WAS DECIDED TO REFER TO MANUAL SECTION 9.6 FOR LOST COM PROCS. THE PLAN WAS TO FLY TO THE OMA VOR; DSND IN THE DEPICTED HOLDING PATTERN; FLY THE ILS FOR RWY 17 AND SQUAWK NORDO. IN IMC; TURB; ICING; AND FLASHES OF LIGHTNING PUNCTUATING THE DARKNESS. XPONDER WAS MISSET TO 7500 RATHER THAN 7600. FOLLOWING THIS PLAN AND DSNDING THROUGH THE FREEZING LEVEL AT APPROX 10000 FT MSL; THE RADIOS BEGAN TO FUNCTION. RADAR VECTORS WERE THEN FOLLOWED FOR AN ILS TO RWY 17 AT OMA IN HVY RAIN; LIGHTNING AND APPROX 25 KTS XWIND. THE XPONDER HAD BEEN FORGOTTEN UNDER MORE PRESSING CONCERNS OF WX AND NAV. AFTER LNDG WE WERE DIRECTED TO REMOTE PARKING LOCATION AND LEARNED OF THE XPONDER MISTAKE UPON CONTACTING COMPANY ON DISCRETE FREQ. THE SIT WAS THEN RESOLVED. FACTORS THAT LED TO THIS SIT ARE NUMEROUS: ICING WHICH CAUSED NORDO; TURB; LIGHTNING; DARKNESS; HVY RAINS; NUMEROUS WX RADAR RETURNS; AND RADIO NOISE. ALSO; IT IS A FULL 10 NUMBERED PARAGRAPHS IN THE COMMERCIAL CHART LOST COM PROCS BEFORE THE XPONDER CODE IS MENTIONED; AND THEN IT IS BURIED WITHIN THE PARAGRAPH IN SMALL TYPE. FINALLY; 7700; 7600; AND 7500; ARE ONLY 1 DIGIT APART AND CAN BE EASILY MISSET. RECOMMEND THAT EMER CODES BE SINGULARLY DISTINCTIVE. SUPPLEMENTAL INFO FROM ACN 336864: IN WX WITH NO RADIO; INADVERTENTLY PLACED 7500; RATHER THAN 7600; IN XPONDER. AFTER LNG TOLD BY OMA GND CTL THAT TAXI LIGHTS AND OUR GATE HAD BEEN RENDERED INOP BY WX AND DIRECTED TO PARK OFF THE END OF THE RWY. AFTER SEVERAL MINS AND NO RESPONSE FROM ATC I CALLED COMPANY DISPATCHER ON DISCRETE FREQ AND ADVISED THAT OMA AUTH ASSUMED WE WERE BEING HIJACKED. THIS ASSUMPTION BASED ON OUR 7500 CODE AND COM PROBS. I INFORMED OMA GND OF THE MISTAKEN XPONDER CODE AND WE WERE CLRED TO THE GATE. XPONDER CODE FOR LOST COM SHOULD BE IN BOLD LETTERS AND PRINTED AT FRONT OF ICAO PROCS OUTLINED IN MANUAL. PRESENTLY BURIED IN 9 PARAGRAPHS OF TEXT PRINTED IN VERY SMALL TYPE. THE HIJACK CODE SHOULD BE MORE DISCRETE REQUIRING A POSITIVE ACTION RATHER THAN SEPARATED BY ONLY 1 DIGIT FROM LOST COM. 7500 - HIJACK. 7600 - LOST COM.