Narrative:

We were flying inbound on the westminster five arrival into bwi. There was a mature thunderstorm with anvil visually identified as well as depicted on the weather radar just north of martinsburg (mrb) VOR with an additional cell just northeast of westminster (EMI) VOR. The hazardous area was tracking just north of the arrival along ruane. Between lundy and yaark we requested deviation south of the arrival to avoid the cells. Washington center kept us on the arrival and said potomac approach would vector us around the weather on the arrival. The first officer (PF) checked in with potomac approach as the captain was making a PA to seat the passengers and flight attendants. On initial check-in with potomac; the first officer requested a deviation to the south to avoid the cell near mrb. The controller approved deviations to the right of course but to let ATC know if we would need to deviate more than 30 degrees right. We turned 10 degrees right of course initially and up to 5 more degrees right during deviation. Approximately south of mrb; I believe the ATC controller said direct ruane when able to avoid converging traffic departing from the south climbing north to 17;000 feet. At this time we were at 15;000. The controller stated a few minutes later they need us on a 040 heading; direct ruane when able. Direct ruane would have been a 040 heading or so and the best we could do to avoid flying under the anvil; avoid a visible rain and/or a potential hail shaft; and directly towards the cell over EMI was about a 090 heading. The captain told potomac approach we were unable direct ruane for weather. The controller responded by stating; if unable direct ruane; I cannot be responsible for separation from the traffic climbing to 17;000. The controller also stated everyone else was flying the arrival and he/she did not depict any precipitation on their scope. We did not see any other aircraft on the arrival on TCAS. We did see one aircraft; a regional jet flying in the opposite direction that was entirely too close to the mature cell in our opinion. Thankfully; we were in VMC at the time and the first officer spotted a heavy jet climbing northbound but there was some confusion among the flight deck as to where the aircraft was located that ATC told us they could not be responsible for separating us. We did not have any traffic conflicts noted on TCAS but that doesn't mean that they may not have existed (TCAS deferred or in stdby). This was an eye opening experience for the flight deck because if ATC does not have responsibility for separating traffic in congested airspace; who does? The flight was then handed over to another controller that assisted us in vectors around the weather and an uneventful landing at bwi. I believe this situation could have led to a near mid-air collision and creates a safety hazard if the controller is unable to maintain separation of metal to commercial aircraft.

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Original NASA ASRS Text

Title: An air carrier pilot on descent into BWI voiced concern regarding an ATC handling and separation disclaimer during weather deviations.

Narrative: We were flying inbound on the Westminster Five Arrival into BWI. There was a mature thunderstorm with anvil visually identified as well as depicted on the weather radar just north of Martinsburg (MRB) VOR with an additional cell just northeast of Westminster (EMI) VOR. The hazardous area was tracking just north of the arrival along RUANE. Between LUNDY and YAARK we requested deviation south of the arrival to avoid the cells. Washington Center kept us on the arrival and said Potomac Approach would vector us around the weather on the arrival. The First Officer (PF) checked in with Potomac Approach as the Captain was making a PA to seat the passengers and flight attendants. On initial check-in with Potomac; the First Officer requested a deviation to the south to avoid the cell near MRB. The controller approved deviations to the right of course but to let ATC know if we would need to deviate more than 30 degrees right. We turned 10 degrees right of course initially and up to 5 more degrees right during deviation. Approximately south of MRB; I believe the ATC controller said direct RUANE when able to avoid converging traffic departing from the south climbing north to 17;000 feet. At this time we were at 15;000. The controller stated a few minutes later they need us on a 040 heading; direct RUANE when able. Direct RUANE would have been a 040 heading or so and the best we could do to avoid flying under the anvil; avoid a visible rain and/or a potential hail shaft; and directly towards the cell over EMI was about a 090 heading. The Captain told Potomac Approach we were unable direct RUANE for weather. The controller responded by stating; if unable direct RUANE; I cannot be responsible for separation from the traffic climbing to 17;000. The controller also stated everyone else was flying the arrival and he/she did not depict any precipitation on their scope. We did not see any other aircraft on the arrival on TCAS. We did see one aircraft; a regional jet flying in the opposite direction that was entirely too close to the mature cell in our opinion. Thankfully; we were in VMC at the time and the First Officer spotted a heavy jet climbing northbound but there was some confusion among the flight deck as to where the aircraft was located that ATC told us they could not be responsible for separating us. We did not have any traffic conflicts noted on TCAS but that doesn't mean that they may not have existed (TCAS deferred or in STDBY). This was an eye opening experience for the flight deck because if ATC does not have responsibility for separating traffic in congested airspace; who does? The flight was then handed over to another controller that assisted us in vectors around the weather and an uneventful landing at BWI. I believe this situation could have led to a near mid-air collision and creates a safety hazard if the controller is unable to maintain separation of metal to commercial aircraft.

Data retrieved from NASA's ASRS site as of April 2012 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.