B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 216; Flight Crew Total 17834; Flight Crew Type 14145
Communication Breakdown; Situational Awareness
Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Human Factors
The flight was originally scheduled to arrive to ORD on the WATSN 2 RNAV arrival. The descent; expected approach to Rwy 27R and taxi plan were thoroughly briefed prior to Top of Descent. Shortly after the DAIFE intersection on the arrival; we were told to expect to hold due to runway changes in progress at O'Hare. We were initially given a 360 degree right hand turn just prior to the WATSN intersection; and after completing the turn were told to expect the ESSPO 1 RNAV Arrival to Rwy 28C. The ATC controller gave us clearance direct ESSPO. As we were less than 30 miles from the intersection; FMC changes were made in a rapid fashion. Shortly after making the changes and attempting to brief as much as possible prior to descent; the ATC controller cleared the flight 'via the ESSPO 1 RNAV arrival'. The crew misinterpreted the clearance as 'descend via the ESSPO 1 RNAV arrival;' and the pilot flying consulted with the Monitoring Pilot to confirm the descent to cross ESSPO at 12;000 FT. The error by the crew was that the 12;000 FT altitude was only an 'expect' clearance and ATC had not cleared the flight to that altitude. The crew initiated a shallow descent to 12;000 FT and almost immediately noticed converging traffic from the west. The pilot flying pointed out the traffic to the monitoring pilot as a potential threat. The crew picked up the traffic visually as well and attempted to query ATC about it; but was unable to due to radio congestion. Shortly thereafter; ATC instructed the flight to level off at 16;000 ft. The crew received a momentary TCAS RA during level off at 16;000 ft; and remained approximately 1000 ft. altitude above the converging traffic. The remainder of the arrival and landing was uneventful.
[Report narrative contained no additional information].
B737 flight crew reported an excursion from their cleared altitude when they misinterpreted a clearance to 'proceed via' the arrival as a clearance to 'descend via' the arrival.