Date: 201307
Local time of day: 1201-1800
Location: IAH.Airport, TX
Flight conditions: IMC
Weather elements / visibility: Turbulence
Light: Daylight
Aircraft: EMB ERJ 145 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class B IAH
ATC / advisory: TRACON I90
Second aircraft: Challenger Jet Undifferentiated or Other Model
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action
Contributing factors: Human Factors; Procedure
Primary problem: Ambiguous

Narrative: We were flying the HAMMU Arrival into IAH and were presently just south of the runway heading east on the downwind for Runway 27 at 6;000 FT. We were in and out of buildups pretty much the entire time we were below 10;000 and the lower we got the more continuous the IMC (I think the ATIS was calling broken at 5;000 FT). At a point just about abeam the numbers for 27 our TCAS went off alerting us to another aircraft. We saw the aircraft on our TCAS but since we couldn't see more than 50 FT in front of the aircraft due to dense clouds we were never able to get a visual contact. The other airplane (we were later told it was a Challenger) was descending quickly towards us and our separation was quickly diminishing. I asked Approach if they had any traffic near us and the radio got quiet. I'm guessing they were looking for the traffic that concerned us; but in that time the other aircraft was now rapidly descending and within 300 FT of our altitude and according to the TCAS within 1 SM of us at our 12:30 and still coming towards us. At this point I initiated a left turn approximately 20 degrees left of course and a descent that ended about 250 FT under the assigned altitude. I was trying to create some separation between us and this other aircraft. I was then told to get back on altitude for the departing traffic in which I quickly did after the other threat was no longer an issue. When we were on base; Approach informed us it was a Challenger and they said the other pilots had us in sight the entire time. But as I stated before this entire event happened while we were in hard IMC and there was absolutely no way that the other aircraft had us visually at any time. They might have had us on TCAS but not in sight. The Challenger accepted a 'maintain visual during descent' clearance while not actually having us in sight. The first time we knew what was happening was when our automation started to alert us; and not a heads up by ATC. I maneuvered the aircraft into an undesired state (off assigned altitude and heading) in order to avoid the immediate threat. It would have been great if I would have been given a heads up that another airplane is going to buzz my right wing during his descent. My actions were taken with the limited amount of information I had available. I'm sure there was more to the 'big picture' that I could not see; and given that information I might have done things differently. But that information was not shared earlier; so I had to make a decision in which I thought what would be the safest option and that was basically try to get the heck out of the way of the other aircraft. But if there is a plan that involves three separate parties; I think all three should know about it and be on the same page so this doesn't happen again.




Synopsis: EMB-145 Captain reports a TCAS event at 6;000 FT while IMC; with a Challenger jet that was descending visually and claimed to have the reporters aircraft insight. The reporter uses the TCAS display to turn away and descend without an RA command.