Narrative:

I was working a low altitude arrival sector for denver stapelton airport. There were numerous thunderstorms in the area and we were on a meter time program for the arrs, with delays of about 10 mins. Air carrier X and air carrier Y were denver arrs that were proceeding to the meter fix with about 7 mi in trail. Both aircraft were cleared to cross the meter fix at 17000 MSL. Y asked for a deviation to the right which was approved and deviated to the area where most previous flts had gone to avoid the WX. About 30 seconds later X asked for a 'slight deviation' to the right which was approved. I observed the aircraft turn toward the same area as the other aircraft and then turned my attention to other aircraft that were entering the holding pattern. I looked back at X after about 20-30 seconds and observed he had turned at least 60 degrees and appeared to be continuing his turn. I asked when he could turn back and was told it would be several more mi. X was level at 17000 ft and X was at FL182. I instructed Y to maintain FL180. I immediately instructed X to descend and maintain 16000 ft. Due to Y's rate of descent, the aircraft descended to 17600 ft and then climbed back up to FL180. The closest proximity was 4.6 mi and 800 ft. Also adding to the difficulty was that the adjacent departure gate was deviating into our arrival gate reducing the amount of room we had to maneuver. I should have placed a restr on X to limit his deviation or stopped Y at FL180 when X asked for a deviation. Either of these actions would have prevented the incident. While I am not trying to reduce my responsibility in this situation, there are a couple of contributing factors that I would like to emphasize. First, the pilot of X asked for a 'slight deviation.' while there is no legal definition of a slight deviation, I think that most people would agree that a 60-90 degree turn for 2 mins is not a slight deviation. Second I disagree with using meter time programs in these type of WX conditions. The meter time interval between X and Y was 1 min. The time between Y and the next aircraft was 2 mins, followed by another aircraft 1 min behind. With the ground speeds these aircraft are doing, to keep airplanes on time there would be only about 20 mi from the front aircraft to the back aircraft. In this situation with thunderstorms on the left and departing aircraft on the right, I had less than 10 mi at the arrival gate to maneuver. If we were in a mi in trail program at least the airplanes would be spaced out more evenly, and we would have the option of running aircraft farther apart to allow for these kinds of deviations.

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Original NASA ASRS Text

Title: ACR X WX DEV HAD LTSS FROM ACR Y. SYS ERROR.

Narrative: I WAS WORKING A LOW ALT ARR SECTOR FOR DENVER STAPELTON ARPT. THERE WERE NUMEROUS TSTMS IN THE AREA AND WE WERE ON A METER TIME PROGRAM FOR THE ARRS, WITH DELAYS OF ABOUT 10 MINS. ACR X AND ACR Y WERE DENVER ARRS THAT WERE PROCEEDING TO THE METER FIX WITH ABOUT 7 MI IN TRAIL. BOTH ACFT WERE CLRED TO CROSS THE METER FIX AT 17000 MSL. Y ASKED FOR A DEV TO THE R WHICH WAS APPROVED AND DEVIATED TO THE AREA WHERE MOST PREVIOUS FLTS HAD GONE TO AVOID THE WX. ABOUT 30 SECONDS LATER X ASKED FOR A 'SLIGHT DEV' TO THE R WHICH WAS APPROVED. I OBSERVED THE ACFT TURN TOWARD THE SAME AREA AS THE OTHER ACFT AND THEN TURNED MY ATTN TO OTHER ACFT THAT WERE ENTERING THE HOLDING PATTERN. I LOOKED BACK AT X AFTER ABOUT 20-30 SECONDS AND OBSERVED HE HAD TURNED AT LEAST 60 DEGS AND APPEARED TO BE CONTINUING HIS TURN. I ASKED WHEN HE COULD TURN BACK AND WAS TOLD IT WOULD BE SEVERAL MORE MI. X WAS LEVEL AT 17000 FT AND X WAS AT FL182. I INSTRUCTED Y TO MAINTAIN FL180. I IMMEDIATELY INSTRUCTED X TO DSND AND MAINTAIN 16000 FT. DUE TO Y'S RATE OF DSCNT, THE ACFT DSNDED TO 17600 FT AND THEN CLBED BACK UP TO FL180. THE CLOSEST PROX WAS 4.6 MI AND 800 FT. ALSO ADDING TO THE DIFFICULTY WAS THAT THE ADJACENT DEP GATE WAS DEVIATING INTO OUR ARR GATE REDUCING THE AMOUNT OF ROOM WE HAD TO MANEUVER. I SHOULD HAVE PLACED A RESTR ON X TO LIMIT HIS DEV OR STOPPED Y AT FL180 WHEN X ASKED FOR A DEV. EITHER OF THESE ACTIONS WOULD HAVE PREVENTED THE INCIDENT. WHILE I AM NOT TRYING TO REDUCE MY RESPONSIBILITY IN THIS SIT, THERE ARE A COUPLE OF CONTRIBUTING FACTORS THAT I WOULD LIKE TO EMPHASIZE. FIRST, THE PLT OF X ASKED FOR A 'SLIGHT DEV.' WHILE THERE IS NO LEGAL DEFINITION OF A SLIGHT DEV, I THINK THAT MOST PEOPLE WOULD AGREE THAT A 60-90 DEG TURN FOR 2 MINS IS NOT A SLIGHT DEV. SECOND I DISAGREE WITH USING METER TIME PROGRAMS IN THESE TYPE OF WX CONDITIONS. THE METER TIME INTERVAL BTWN X AND Y WAS 1 MIN. THE TIME BTWN Y AND THE NEXT ACFT WAS 2 MINS, FOLLOWED BY ANOTHER ACFT 1 MIN BEHIND. WITH THE GND SPDS THESE ACFT ARE DOING, TO KEEP AIRPLANES ON TIME THERE WOULD BE ONLY ABOUT 20 MI FROM THE FRONT ACFT TO THE BACK ACFT. IN THIS SIT WITH TSTMS ON THE L AND DEPARTING ACFT ON THE R, I HAD LESS THAN 10 MI AT THE ARR GATE TO MANEUVER. IF WE WERE IN A MI IN TRAIL PROGRAM AT LEAST THE AIRPLANES WOULD BE SPACED OUT MORE EVENLY, AND WE WOULD HAVE THE OPTION OF RUNNING ACFT FARTHER APART TO ALLOW FOR THESE KINDS OF DEVS.

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.