Narrative:

Aircraft X was a las departure heading to ZZZ. When aircraft X was out of FL270 and approximately 8 minutes from traffic I asked aircraft X if he could be level in 4 minutes or less? Aircraft X responded that he could. I issued aircraft X a restriction to be climb and maintain FL330 in 4 minutes or less for crossing traffic at FL320. This was not unreasonable a 1200 ft per minute climb would have been more than enough. In this clearance I gave myself a good 2 or more minutes of buffer incase aircraft X could not make the clearance. When aircraft X was at FL320 and climbing and was approximately 3 minutes from loss of separation with crossing level traffic aircraft Y. I asked aircraft X if he was 'losing steam' attempting to get aircraft X to give me a reason to doubt their ability to level beings they had already blown the time; aircraft X responded that he could do it. I continued to monitor the situation as I only need 800 more feet for round off. When aircraft X was at FL327 and they were about 20 miles from aircraft Y (ATC window of opportunity is close) in this time frame aircraft X was level for 2 hits at FL327 and on the 3rd hit showed FL326. I immediately descended aircraft Y to FL310 when I saw the 100 ft loss as the counter measure and told him to 'push it down now'. Aircraft Y read back the clearance and then aircraft X keyed up and said he is not making his time or something that did not make any sense. I then told aircraft X to turn 50 degrees left immediately; I did not specify a new altitude because now aircraft X altitude read FL316; and I was in the impression he is in an emergency loss of power or flight altitude control or something. In the same breath I cleared aircraft Y 50 degrees right immediately. Aircraft Y read back the clearance 15 degrees and questioned me about descending to FL310; I repeated the clearance to 'turn 50 degrees right immediately and maintain FL310.' I know why aircraft Y questioned because aircraft Y could see aircraft X 2-3 miles off the left now level at FL310. I call traffic for aircraft X and tell them 'verify climbing to FL330 and say intentions?' they responded that they couldn't make time and they are climbing immediately to FL330; 50 left and have traffic in sight. By this time the aircraft are gaining separation via the 100 degree accumulative vectors and. I told aircraft X cleared ZZZ reaching FL330. I was relieved from the sector about 4 minutes after this clearance and aircraft X was again stuck at FL327 for 4-5 hits. I'm not sure if he ever made his restriction even after 6 or more minutes from aircraft X's original be level time. I don't know if aircraft X's flight computer halted control due to weight or temp; or a TCAS alert and they responded or what happened to his loss of altitude but it was in the absolute worst time other than 2 ft off the runway. Aircraft X descended or lost 1700 ft in 3 hits or 36 seconds and did this in a climb configuration. Very uncharacteristic. If this was a TCAS event it would have been nice for aircraft X to tell me so I could have used a different method of separation or countermeasure. If this was a stall or loss of flight control; pilots need to make sure that clearances made 8 minutes in advance were valid for their aircraft and conditions. If this was a stall it would have been nice to know in the same regard to the TCAS alert so that I could make appropriate countermeasure.

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Original NASA ASRS Text

Title: ZLA controllers report of asking a pilot if he could make a new altitude restriction. The pilot advised that he could and then Controller issued the instruction. Controller was concerned that the aircraft was 'losing steam' as it approached it assigned altitude. Controller vectored traffic away from climbing aircraft and has a loss of separation due to climbing aircraft not getting level as they said they could.

Narrative: Aircraft X was a LAS departure heading to ZZZ. When Aircraft X was out of FL270 and approximately 8 minutes from traffic I asked Aircraft X if he could be level in 4 minutes or less? Aircraft X responded that he could. I issued Aircraft X a restriction to be climb and maintain FL330 in 4 minutes or less for crossing traffic at FL320. This was not unreasonable a 1200 ft per minute climb would have been more than enough. In this clearance I gave myself a good 2 or more minutes of buffer incase Aircraft X could not make the clearance. When Aircraft X was at FL320 and climbing and was approximately 3 minutes from loss of separation with crossing level traffic Aircraft Y. I asked Aircraft X if he was 'losing steam' attempting to get Aircraft X to give me a reason to doubt their ability to level beings they had already blown the time; Aircraft X responded that he could do it. I continued to monitor the situation as I only need 800 more feet for round off. When Aircraft X was at FL327 and they were about 20 miles from Aircraft Y (ATC window of opportunity is close) in this time frame Aircraft X was level for 2 hits at FL327 and on the 3rd hit showed FL326. I immediately descended Aircraft Y to FL310 when I saw the 100 ft loss as the counter measure and told him to 'Push it down now'. Aircraft Y read back the clearance and then Aircraft X keyed up and said he is not making his time or something that did not make any sense. I then told Aircraft X to turn 50 degrees left immediately; I did not specify a new altitude because now Aircraft X altitude read FL316; and I was in the impression he is in an emergency loss of power or flight altitude control or something. In the same breath I cleared Aircraft Y 50 degrees right immediately. Aircraft Y read back the clearance 15 degrees and questioned me about descending to FL310; I repeated the clearance to 'Turn 50 degrees right immediately and maintain FL310.' I know why Aircraft Y questioned because Aircraft Y could see Aircraft X 2-3 miles off the left now level at FL310. I call traffic for Aircraft X and tell them 'Verify climbing to FL330 and say intentions?' They responded that they couldn't make time and they are climbing immediately to FL330; 50 left and have traffic in sight. By this time the aircraft are gaining separation via the 100 degree accumulative vectors and. I told Aircraft X cleared ZZZ reaching FL330. I was relieved from the sector about 4 minutes after this clearance and Aircraft X was again stuck at FL327 for 4-5 hits. I'm not sure if he ever made his restriction even after 6 or more minutes from Aircraft X's original be level time. I don't know if Aircraft X's flight computer halted control due to weight or temp; or a TCAS alert and they responded or what happened to his loss of altitude but it was in the absolute worst time other than 2 ft off the runway. Aircraft X descended or lost 1700 ft in 3 hits or 36 seconds and did this in a climb configuration. Very uncharacteristic. If this was a TCAS event it would have been nice for Aircraft X to tell me so I could have used a different method of separation or countermeasure. If this was a stall or loss of flight control; pilots need to make sure that clearances made 8 minutes in advance were valid for their aircraft and conditions. If this was a stall it would have been nice to know in the same regard to the TCAS alert so that I could make appropriate countermeasure.

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.