Narrative:

On descent toward 14000' MSL at 17000' MSL, when approaching dca from bos, center advised unverified traffic at about our 2 O'clock position indicating 16600' and another indicating a little lower. The copilot was flying, but when I heard the traffic call I rolled his autoplt knob out of the descent and into a climb. The controller said, 'we will have to hold your altitude for a minute and then continue the descent.' we were able to arrest the descent by 16800' and returned to 17000'. A twin-tailed fgt passed beneath us at the reported 16600'. He was not in communication with ATC and was reportedly VFR. Another fgt passed clear behind us. Although no illegality was observed, continued descent in accordance with our clearance may have been catastrophic. ATC said that they did not see him on the scope until the late warning. I believe there are the following factors in need of consideration. 1) the flight plan routing for this particular segment was not on an arwy (from zizzi intersection on J174 direct ATR via the 065 degree ATR right. VFR traffic may view this as an area relatively free of IFR traffic, when in fact it is a common and congested route used on bos direct dca shuttles. It also cuts perpendicularly across (east and west) several n-s routes. 2) ATC radar and computer gear needs to be able to project C/a's between an IFR aircraft and any other aircraft with mode C, and they must do so even if it involves climbing/descending aircraft and with enough time to rectify the conflict. Only because of the alert by the controller and rapid reaction to level off was a more narrow near miss avoided. 3) while we have mandatory communications for TCA's and TRSA's and other obviously busy areas, there is no requirement for aircraft in these congested corridors to be in touch with ATC. 4) see and be seen doesn't work at these speeds, despite good visibility, especially with military aircraft that may be more difficult to perceive. Also, the relative altitude of an approaching aircraft at higher altitudes from the cockpit of a jet liner that positions the pilots vantage at varying deck angles west/O longitudinal reference for sighting ahead is extremely inaccurate. 5) I favor mandatory radar sep from all mode C aircraft, IFR or VFR, if not by altitude (due to unverified mode C reports) then by radar vector if the indicated altitude is anywhere close to mine.

Related reports:

Google
 

Original NASA ASRS Text

Title: NMAC BETWEEN MLG IFR IN VMC AND FLT OF 2 MIL FGT VFR.

Narrative: ON DSCNT TOWARD 14000' MSL AT 17000' MSL, WHEN APCHING DCA FROM BOS, CENTER ADVISED UNVERIFIED TFC AT ABOUT OUR 2 O'CLOCK POS INDICATING 16600' AND ANOTHER INDICATING A LITTLE LOWER. THE COPLT WAS FLYING, BUT WHEN I HEARD THE TFC CALL I ROLLED HIS AUTOPLT KNOB OUT OF THE DSCNT AND INTO A CLB. THE CTLR SAID, 'WE WILL HAVE TO HOLD YOUR ALT FOR A MINUTE AND THEN CONTINUE THE DSCNT.' WE WERE ABLE TO ARREST THE DSCNT BY 16800' AND RETURNED TO 17000'. A TWIN-TAILED FGT PASSED BENEATH US AT THE RPTED 16600'. HE WAS NOT IN COM WITH ATC AND WAS REPORTEDLY VFR. ANOTHER FGT PASSED CLR BEHIND US. ALTHOUGH NO ILLEGALITY WAS OBSERVED, CONTINUED DSCNT IN ACCORDANCE WITH OUR CLRNC MAY HAVE BEEN CATASTROPHIC. ATC SAID THAT THEY DID NOT SEE HIM ON THE SCOPE UNTIL THE LATE WARNING. I BELIEVE THERE ARE THE FOLLOWING FACTORS IN NEED OF CONSIDERATION. 1) THE FLT PLAN ROUTING FOR THIS PARTICULAR SEGMENT WAS NOT ON AN ARWY (FROM ZIZZI INTXN ON J174 DIRECT ATR VIA THE 065 DEG ATR R. VFR TFC MAY VIEW THIS AS AN AREA RELATIVELY FREE OF IFR TFC, WHEN IN FACT IT IS A COMMON AND CONGESTED ROUTE USED ON BOS DIRECT DCA SHUTTLES. IT ALSO CUTS PERPENDICULARLY ACROSS (E AND W) SEVERAL N-S ROUTES. 2) ATC RADAR AND COMPUTER GEAR NEEDS TO BE ABLE TO PROJECT C/A'S BTWN AN IFR ACFT AND ANY OTHER ACFT WITH MODE C, AND THEY MUST DO SO EVEN IF IT INVOLVES CLBING/DSNDING ACFT AND WITH ENOUGH TIME TO RECTIFY THE CONFLICT. ONLY BECAUSE OF THE ALERT BY THE CTLR AND RAPID REACTION TO LEVEL OFF WAS A MORE NARROW NEAR MISS AVOIDED. 3) WHILE WE HAVE MANDATORY COMS FOR TCA'S AND TRSA'S AND OTHER OBVIOUSLY BUSY AREAS, THERE IS NO REQUIREMENT FOR ACFT IN THESE CONGESTED CORRIDORS TO BE IN TOUCH WITH ATC. 4) SEE AND BE SEEN DOESN'T WORK AT THESE SPDS, DESPITE GOOD VISIBILITY, ESPECIALLY WITH MIL ACFT THAT MAY BE MORE DIFFICULT TO PERCEIVE. ALSO, THE RELATIVE ALT OF AN APCHING ACFT AT HIGHER ALTS FROM THE COCKPIT OF A JET LINER THAT POSITIONS THE PLTS VANTAGE AT VARYING DECK ANGLES W/O LONGITUDINAL REF FOR SIGHTING AHEAD IS EXTREMELY INACCURATE. 5) I FAVOR MANDATORY RADAR SEP FROM ALL MODE C ACFT, IFR OR VFR, IF NOT BY ALT (DUE TO UNVERIFIED MODE C RPTS) THEN BY RADAR VECTOR IF THE INDICATED ALT IS ANYWHERE CLOSE TO MINE.

Data retrieved from NASA's ASRS site as of August 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.