Narrative:

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.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site 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.