DURING DSCNT ON A MAINT FLT; A TEST PLT TOOK EVASIVE ACTION TO AVOID ANOTHER ACFT.

Narrative

THE FLT WAS FOR A MAINT CHK OF A SENECA II TURBO-CHARGING SYS. THE TESTS HAD BEEN CARRIED OUT S AND W OF ROCKFORD ARPT (RFD) USING FLT FOLLOWING OF ZAU AND ROCKFORD APCH. A MECH WAS OCCUPYING THE COPLT SEAT TO RECORD ENG DATA AND HELP WATCH FOR TFC. HAVING COMPLETED THE TESTS; WE WERE RETURNING TO DPA ARPT. WE DSNDED TO 3500 FT MSL AND INTERCEPTED THE DPA RWY 10 LOC COURSE. RFD APCH ADVISED US THAT RADAR SVC WAS TERMINATED AND TO SQUAWK VFR. WE CHANGED FREQS AND I LISTENED TO DPA ATIS. BEGINNING THE DSCNT FROM 3500 FT MSL I TUNED DPA TWR. AT APPROX 12-15 NM WEST OF DPA ARPT AND PASSING THROUGH 3200 FT MSL; MY ATTN; AND IMMEDIATELY; THE ATTN OF THE MECH; WAS CAUGHT BY A SINGLE ENG CESSNA (I BELIEVE A 182RG OR 210) APCHING 12:00 O'CLOCK TOWARD US; AND SLIGHTLY BELOW. I PULLED BACK ON THE YOKE TO CLR THE TFC; BUT BECAUSE OF LATE SIGHTING AND RATE OF CLOSURE; IT IS DIFFICULT TO KNOW HOW MUCH THE EVASIVE ACTION HELPED TO CLEAR THE TFC. CONTRIBUTING FACTORS TO THIS INCIDENT INCLUDE: 1) FLYING IN UNFAMILIAR ACFT; WHICH REQUIRES MORE TIME LOCATING CTLS; INSTS; AND SWITCHES; AND DETRACTING FROM FOCUSING ATTN OUTSIDE. 2) THE LIGHT COLOR OF THE CONFLICTING TFC; BLENDING IN WITH THE SNOW COVERED TERRAIN. 3) THE APPROACH ANGLE GIVING LITTLE RELATIVE MOVEMENT FOR THE EYE TO SEE. CORRECTIVE ACTION INCLUDES MAINTAINING ACTIVE WATCH FOR OTHER TFC AND INCREASING THE SCAN DURING CLB AND DSCNTS IN BUSY TERMINAL AREAS. ON A VFR DSCNT; LIKE THIS ONE; GENTLE TURNS L AND R OF COURSE MAY HELP TO GIVE RELATIVE MOVEMENT AND MAKE IT EASIER TO SEE AND AVOID OTHER TFC. ALTHOUGH THE MECH AND I SAW THE TFC AT ABOUT THE SAME TIME; HAVING ANOTHER PERSON ON BOARD ALSO LOOKING FOR TFC IS A HELP. I NORMALLY ASK A PAX (OR IN THIS CASE A MECH) IN THE R SEAT TO HELP ME WATCH FOR OTHER ACFT.