Narrative:

Clearance was given to hold at ramah intersection with an 'expedite' to FL270. Captain was flying the aircraft. I read back holding instructions to ATC and the cleared descent altitude to FL270. Normal SOP would dictate that the PNF set the cleared altitude on the altitude window of the MCP. Since we were close to ramah intersection with an expedited descent, captain instructed me to set the hold for lateral navigation in the flight mgt computer. I did as instructed with the captain setting the altitude in the MCP altitude window. Upon completing the holding entry I scanned the flight instruments and noticed our descent thru FL258. I immediately notified the captain and saw the MCP altitude set at FL170. I promptly asked center to verify our descent altitude. I received a clearance to FL250, but due to the rapid descent (descending 4000+) the captain notified ATC that we passed FL250. We were reclred to FL240. At that time the captain notified ATC of his belief of being cleared to FL170. Supplemental information from acn 125277: center informed all aircraft that den airport was closed because of an aircraft that had a problem and closed the runways. Passing thru FL260, the controller asked us what altitude we were passing thru, replied FL260. I had the first officer ask the controller to confirm we were cleared to 17,000'. The controller told us to level off at FL250. Because of our rate of descent, I leveled off at FL243. The controller then told us level off at FL240. We had been observing an aircraft moving from our 2 O'clock to 12 O'clock position, slightly below us and we'd be passing thru its altitude. I figured this was the reason for the expedited descent request. I called the den ARTCC. He stated he knew very little about the incident other than the clearance and reply both indicated a clearance to FL270. He further stated the tape showed we were within 800' vertically and 4.3 mi from the other aircraft and the 'snitch' alarm had sounded. Because of the alarm, either the controller or pilot had to be assessed the fault. Callback conversation with reporter revealed the following: first officer called in to say the FAA has decided it was a flight crew error not an ARTCC error. Other aircraft was separated by 800' and 3 NM. Call was made to get ASRS to expedite return of identification strip.

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Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION OVERSHOT DURING DESCENT CAUSES LESS THAN STANDARD SEPARATION AT FL250.

Narrative: CLRNC WAS GIVEN TO HOLD AT RAMAH INTXN WITH AN 'EXPEDITE' TO FL270. CAPT WAS FLYING THE ACFT. I READ BACK HOLDING INSTRUCTIONS TO ATC AND THE CLRED DSCNT ALT TO FL270. NORMAL SOP WOULD DICTATE THAT THE PNF SET THE CLRED ALT ON THE ALT WINDOW OF THE MCP. SINCE WE WERE CLOSE TO RAMAH INTXN WITH AN EXPEDITED DSCNT, CAPT INSTRUCTED ME TO SET THE HOLD FOR LATERAL NAVIGATION IN THE FLT MGT COMPUTER. I DID AS INSTRUCTED WITH THE CAPT SETTING THE ALT IN THE MCP ALT WINDOW. UPON COMPLETING THE HOLDING ENTRY I SCANNED THE FLT INSTRUMENTS AND NOTICED OUR DSCNT THRU FL258. I IMMEDIATELY NOTIFIED THE CAPT AND SAW THE MCP ALT SET AT FL170. I PROMPTLY ASKED CENTER TO VERIFY OUR DSCNT ALT. I RECEIVED A CLRNC TO FL250, BUT DUE TO THE RAPID DSCNT (DESCENDING 4000+) THE CAPT NOTIFIED ATC THAT WE PASSED FL250. WE WERE RECLRED TO FL240. AT THAT TIME THE CAPT NOTIFIED ATC OF HIS BELIEF OF BEING CLRED TO FL170. SUPPLEMENTAL INFORMATION FROM ACN 125277: CENTER INFORMED ALL ACFT THAT DEN ARPT WAS CLOSED BECAUSE OF AN ACFT THAT HAD A PROBLEM AND CLOSED THE RWYS. PASSING THRU FL260, THE CTLR ASKED US WHAT ALT WE WERE PASSING THRU, REPLIED FL260. I HAD THE F/O ASK THE CTLR TO CONFIRM WE WERE CLRED TO 17,000'. THE CTLR TOLD US TO LEVEL OFF AT FL250. BECAUSE OF OUR RATE OF DSCNT, I LEVELED OFF AT FL243. THE CTLR THEN TOLD US LEVEL OFF AT FL240. WE HAD BEEN OBSERVING AN ACFT MOVING FROM OUR 2 O'CLOCK TO 12 O'CLOCK POSITION, SLIGHTLY BELOW US AND WE'D BE PASSING THRU ITS ALT. I FIGURED THIS WAS THE REASON FOR THE EXPEDITED DSCNT REQUEST. I CALLED THE DEN ARTCC. HE STATED HE KNEW VERY LITTLE ABOUT THE INCIDENT OTHER THAN THE CLRNC AND REPLY BOTH INDICATED A CLRNC TO FL270. HE FURTHER STATED THE TAPE SHOWED WE WERE WITHIN 800' VERTICALLY AND 4.3 MI FROM THE OTHER ACFT AND THE 'SNITCH' ALARM HAD SOUNDED. BECAUSE OF THE ALARM, EITHER THE CTLR OR PLT HAD TO BE ASSESSED THE FAULT. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: F/O CALLED IN TO SAY THE FAA HAS DECIDED IT WAS A FLT CREW ERROR NOT AN ARTCC ERROR. OTHER ACFT WAS SEPARATED BY 800' AND 3 NM. CALL WAS MADE TO GET ASRS TO EXPEDITE RETURN OF ID STRIP.

Data retrieved from NASA's ASRS site as of August 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.