Narrative:

The first officer was PF and was cleared to descend to FL180. The first officer initiated the descent, the first officer left the cockpit to use the lavatory. The captain assumed control of the aircraft. While the first officer was absent, ATC advised the flight to expect to cross mumsy at 15000 ft. The captain read back the clearance and entered 15000 ft at mumsy in the FMS. He also entered 15000 ft in the altitude selector, thinking this was the clearance. When the first officer returned, the flight was at approximately FL200. The captain xferred control of the aircraft to the first officer, and proceeded to obtain ATIS. Just below FL180, ATC asked the flight to verify leveling at FL180. The first officer told ATC we were given 15000 ft at mumsy. ATC instructed the flight to hold altitude. The first officer selected altitude hold, and asked the captain if we had been cleared to 15000 ft. The captain told the first officer we had been told to expect mumsy at 15000 ft. Shortly after, ATC cleared the flight to 15000 ft and advised the flight that we had been told to expect 15000 ft. The captain contacted ATC upon arrival. The controller later contacted the captain and advised him that the flight had a loss of separation with another aircraft. Several factors contributed to this event. The first officer was absent during a busy time. His absence was unavoidable, but a major factor. The absence of the first officer did not allow for the standard protocol regarding altitude readback between crew members. The captain should not have set the expected altitude in the altitude selector. The captain did not brief the first officer when the first officer returned. The first officer did not immediately respond when ATC asked if he was leveling at FL180. Instead he told ATC something he was not sure of, that we were cleared to 15000 ft. After saying this, he asked the captain. Finally, although common and understood, the use of an expected clearance by ATC contributed to the workload by requiring a readback, but did not contribute useful information. We do not need to know what we can expect to receive later as a clearance. Supplemental information from acn 615952: when I re-entered the flight deck and buckled in, the plane was in a descent. I stated that I saw 15000 ft in the altitude preselect and the crossing restr of 15000 ft at mumsy in the FMS. The captain agreed with my readback and xferred controls to my side again. Through FL180, ZDC asked if we were leveling at FL180. Asked the captain if we were given expect mumsy at 15000 ft. At that point he realized his mistake. Supplemental information from acn 615412: aircraft #2 was in level flight at 17000 ft. Aircraft #1 was an arrival cleared to FL180 to cross over the aircraft #1. Aircraft #1 was told to expect mumsy intersection at 15000 ft. Pilot read back FL180 as the clearance, but questioned to expect clearance. The full clearance was restated as 'descend to and maintain FL180, expect mumsy intersection at 15000 ft.' aircraft #1 may have gotten confused on the assigned altitude because they descended to 15000 ft. Separation was lost between the 2 aircraft.

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Original NASA ASRS Text

Title: CL65 FLT CREW WITH ZDC DSNDED BELOW ASSIGNED ALT DURING ARR TO BWI, CONFLICTING WITH A PAY3 AT 17000 FT.

Narrative: THE FO WAS PF AND WAS CLRED TO DSND TO FL180. THE FO INITIATED THE DSCNT, THE FO LEFT THE COCKPIT TO USE THE LAVATORY. THE CAPT ASSUMED CTL OF THE ACFT. WHILE THE FO WAS ABSENT, ATC ADVISED THE FLT TO EXPECT TO CROSS MUMSY AT 15000 FT. THE CAPT READ BACK THE CLRNC AND ENTERED 15000 FT AT MUMSY IN THE FMS. HE ALSO ENTERED 15000 FT IN THE ALT SELECTOR, THINKING THIS WAS THE CLRNC. WHEN THE FO RETURNED, THE FLT WAS AT APPROX FL200. THE CAPT XFERRED CTL OF THE ACFT TO THE FO, AND PROCEEDED TO OBTAIN ATIS. JUST BELOW FL180, ATC ASKED THE FLT TO VERIFY LEVELING AT FL180. THE FO TOLD ATC WE WERE GIVEN 15000 FT AT MUMSY. ATC INSTRUCTED THE FLT TO HOLD ALT. THE FO SELECTED ALT HOLD, AND ASKED THE CAPT IF WE HAD BEEN CLRED TO 15000 FT. THE CAPT TOLD THE FO WE HAD BEEN TOLD TO EXPECT MUMSY AT 15000 FT. SHORTLY AFTER, ATC CLRED THE FLT TO 15000 FT AND ADVISED THE FLT THAT WE HAD BEEN TOLD TO EXPECT 15000 FT. THE CAPT CONTACTED ATC UPON ARR. THE CTLR LATER CONTACTED THE CAPT AND ADVISED HIM THAT THE FLT HAD A LOSS OF SEPARATION WITH ANOTHER ACFT. SEVERAL FACTORS CONTRIBUTED TO THIS EVENT. THE FO WAS ABSENT DURING A BUSY TIME. HIS ABSENCE WAS UNAVOIDABLE, BUT A MAJOR FACTOR. THE ABSENCE OF THE FO DID NOT ALLOW FOR THE STANDARD PROTOCOL REGARDING ALT READBACK BTWN CREW MEMBERS. THE CAPT SHOULD NOT HAVE SET THE EXPECTED ALT IN THE ALT SELECTOR. THE CAPT DID NOT BRIEF THE FO WHEN THE FO RETURNED. THE FO DID NOT IMMEDIATELY RESPOND WHEN ATC ASKED IF HE WAS LEVELING AT FL180. INSTEAD HE TOLD ATC SOMETHING HE WAS NOT SURE OF, THAT WE WERE CLRED TO 15000 FT. AFTER SAYING THIS, HE ASKED THE CAPT. FINALLY, ALTHOUGH COMMON AND UNDERSTOOD, THE USE OF AN EXPECTED CLRNC BY ATC CONTRIBUTED TO THE WORKLOAD BY REQUIRING A READBACK, BUT DID NOT CONTRIBUTE USEFUL INFO. WE DO NOT NEED TO KNOW WHAT WE CAN EXPECT TO RECEIVE LATER AS A CLRNC. SUPPLEMENTAL INFO FROM ACN 615952: WHEN I RE-ENTERED THE FLT DECK AND BUCKLED IN, THE PLANE WAS IN A DSCNT. I STATED THAT I SAW 15000 FT IN THE ALT PRESELECT AND THE XING RESTR OF 15000 FT AT MUMSY IN THE FMS. THE CAPT AGREED WITH MY READBACK AND XFERRED CTLS TO MY SIDE AGAIN. THROUGH FL180, ZDC ASKED IF WE WERE LEVELING AT FL180. ASKED THE CAPT IF WE WERE GIVEN EXPECT MUMSY AT 15000 FT. AT THAT POINT HE REALIZED HIS MISTAKE. SUPPLEMENTAL INFO FROM ACN 615412: ACFT #2 WAS IN LEVEL FLT AT 17000 FT. ACFT #1 WAS AN ARR CLRED TO FL180 TO CROSS OVER THE ACFT #1. ACFT #1 WAS TOLD TO EXPECT MUMSY INTXN AT 15000 FT. PLT READ BACK FL180 AS THE CLRNC, BUT QUESTIONED TO EXPECT CLRNC. THE FULL CLRNC WAS RESTATED AS 'DSND TO AND MAINTAIN FL180, EXPECT MUMSY INTXN AT 15000 FT.' ACFT #1 MAY HAVE GOTTEN CONFUSED ON THE ASSIGNED ALT BECAUSE THEY DSNDED TO 15000 FT. SEPARATION WAS LOST BTWN THE 2 ACFT.

Data retrieved from NASA's ASRS site as of July 2007 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.