Date: 201501
Local time of day: 1201-1800
Location: RDU.TRACON, NC
Flight conditions: Mixed
Weather elements / visibility: 4
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class C RDU
ATC / advisory: TRACON RDU
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown
Second reporter function: Captain; Pilot Not Flying
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Other / Unknown
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Procedure
Primary problem: Procedure

Narrative: Washington Center directed us to descend via the BLOGS1 RNAV Arrival; Runway 5 transition. After checking-in with Raleigh Approach Control; using the correct verbiage; we were informed that there may be a runway change. (Extremely busy radio). Approaching CABED fix; we were instructed to transition to Runway 23. The transition 23 altitude is 6;000 feet at TIBIA. The clearance altitude for the Runway 5 transition is 8;000 feet. We reprogrammed the FMC and selected 6;000 feet on the MCP. At five miles southeast at SULIE; the aircraft continued its descent on VNAV PATH as commanded to reach TIBIA at 6;000 feet. At SULIE waypoint at approximately 7;000 feet; Raleigh Control queried our altitude. We responded that we were at 7;000 feet descending to 6;000 feet on the arrival. We were immediately directed to climb to 8;000 feet then we were vectored for sequencing to Runway 23L ILS. ATC was extremely busy coordinating with aircraft for the runway change. Clarification on the assigned altitude to ATC would have alleviated any confusion. However; when you are cleared to descend via and there is a specific branch; both ATC and Pilots need to be aware that depending on the branch to a different runway transition the altitude (of course) may be different. What I learned is; if directed to an RNAV branch for a different runway; confirm with ATC... 'Do you want me to CONTINUE to descend via the Arrival.'


Second narrative: On descent into RDU on the BLOGS 1 Arrival; Washington Center cleared us to descend via the BLOGS 1 Arrival Runway 05 transition. We were descending in VNAV mode. Upon checking in with RDU Approach Control; we were told to descend for Runway 05R. Raleigh Approach later informed all aircraft that they were turning the airport around and would be landing Runways 23L and 23R. Approach called us and said to switch over to Runway 23L. This clearance was given with about five miles to go to waypoint CABED; which had a crossing restriction of 8;000 feet or above. We quickly changed the runway in the FMC and continued descent in VNAV mode. The Runway 05 transitions have an altitude limit of 8;000 feet while the Runway 23 transition has a lower altitude limit of 6;000 feet. We verified the altitude on our approach plates and in the FMC and the First Officer (Pilot Flying) rolled 6;000 feet into the FMC and continued descent in VNAV as previously cleared. Several minutes later while descending on the arrival through 7;000 feet; Approach Control queried us about our altitude assignment. We responded that our last clearance was to descend via the BLOGS 1 Arrival. Approach Control then directed us to climb back up to 8;000 feet and continued to vector us for the approach. An uneventful approach and landing ensued. The communication problem happened due to ATC being extremely busy handling aircraft with the runway change and inclement weather.The only issue was a lack of communication from ATC as to an altitude clearance when they switched the runway assignment. The runway change effectively changed the lowest altitude limit on the arrival from 8;000 feet to 6;000 feet. ATC could have informed us to maintain 8;000 vice the 6;000 limit on the 23 transition of the arrival. The Air Crew could have queried ATC about whether they still wanted us to descend via the arrival for the different runway. The Air Crew didn't get a chance to query ATC about the clearance due to how busy ATC was on the radio during the runway change.


Synopsis: B737-700 flight crew reports being cleared to descend via the BLOGS1 RNAV Arrival; Runway 5 transition. Prior to CABED the crew is switched to the Runway 23 transition with a bottom altitude of 6;000 feet vs. 8;000 feet for the Runway 5 transition. The crew continues to 'descend via;' which is apparently not what ATC intended.