Narrative:

19 mi southeast ont airport, cleared descent to cross petis waypoint above 4000 ft. Cleared ILS runway 26L ont. Approach called out traffic 2 O'clock position, 10 mi, C-17. C-17 was in sight and was on a collision course. I asked approach where the C-17 was going. Response was runway 14 at march AFB. He added he should pass below us. Both aircraft matched rates of descent. In my judgement, we would have merged prior to petis and above 4000 ft. I notified approach we were stopping our descent and approach assigned 6000 ft as we descended through 6500 ft. Pass was still close and may have generated an RA. So directed first officer to climb. We climbed back to 6500 ft. As I reported climb to approach, C-17 pitched down -- greatly increasing rate of descent. C-17 was not on approach frequency and our controller did not simulcast with UHF, so we were not aware of the C-17's clearance and communications. Aircraft passed 2000 ft vertically. Due to visual conditions, we were able to react prior to RA. Target had upgraded to priority target. We continued approach and landing without further incident.

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Original NASA ASRS Text

Title: ON SCT VECTOR TO RWY 26L ONT, MD80 FLT CREW INITIATES EVASIVE MANEUVER FROM A C-17 ON VECTOR TO RIV RWY 14 ON ANOTHER SCT APCH FREQ, APPARENTLY BEING PROVIDED VISUAL SEPARATION.

Narrative: 19 MI SE ONT ARPT, CLRED DSCNT TO CROSS PETIS WAYPOINT ABOVE 4000 FT. CLRED ILS RWY 26L ONT. APCH CALLED OUT TFC 2 O'CLOCK POS, 10 MI, C-17. C-17 WAS IN SIGHT AND WAS ON A COLLISION COURSE. I ASKED APCH WHERE THE C-17 WAS GOING. RESPONSE WAS RWY 14 AT MARCH AFB. HE ADDED HE SHOULD PASS BELOW US. BOTH ACFT MATCHED RATES OF DSCNT. IN MY JUDGEMENT, WE WOULD HAVE MERGED PRIOR TO PETIS AND ABOVE 4000 FT. I NOTIFIED APCH WE WERE STOPPING OUR DSCNT AND APCH ASSIGNED 6000 FT AS WE DSNDED THROUGH 6500 FT. PASS WAS STILL CLOSE AND MAY HAVE GENERATED AN RA. SO DIRECTED FO TO CLB. WE CLBED BACK TO 6500 FT. AS I RPTED CLB TO APCH, C-17 PITCHED DOWN -- GREATLY INCREASING RATE OF DSCNT. C-17 WAS NOT ON APCH FREQ AND OUR CTLR DID NOT SIMULCAST WITH UHF, SO WE WERE NOT AWARE OF THE C-17'S CLRNC AND COMS. ACFT PASSED 2000 FT VERTICALLY. DUE TO VISUAL CONDITIONS, WE WERE ABLE TO REACT PRIOR TO RA. TARGET HAD UPGRADED TO PRIORITY TARGET. WE CONTINUED APCH AND LNDG WITHOUT FURTHER INCIDENT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.