Narrative:

I was working aircraft X. He departed on the SID and was climbing via the SID. On initial contact; I instructed the pilot to climb and maintain his filed altitude. I then talked to 2 to 3 other contacts before returning to aircraft X. The next instruction I gave aircraft X was 'reaching 13;000 cleared direct destination.' the pilot read back; '13;000; direct destination.' I was fairly busy; and did not catch the fact that this was a somewhat incomplete read back. I again moved on to other duties. Working 2 frequencies and 6 transmitters; aircraft often step on each other; this happened many times during this session. The pilot of aircraft X had 'a request'; but was covered by other aircraft. I told him to standby because I thought that he was climbing up and did not thing it was important; and moved on to other aircraft. He asked at least once more; and I when finally was able to respond and fully look at aircraft X I realized that he was still at 13;000 feet in a 13;700 mia(minimum IFR altitude). I gave him the instruction to climb; and told the pilot no delay through 13;700 feet to reach my mia. I was worried that I gave the pilot a bad instruction; so immediately went and reviewed the playback. After listening to the audio; I realized that the pilot did not read back the instruction as precisely as he should have and I should have caught this inaccuracy. In the future I will change the way I give the instruction for an on course turn. Instead of 'reaching 13;000; cleared direct' I will try to say 'leaving 13;000; cleared direct'. I will also be very vigilant to watch aircraft climbing on the SID after that instruction is issued.

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Original NASA ASRS Text

Title: Center Controller reported they missed an incomplete readback of a clearance and an aircraft flew below the Minimum IFR Altitude.

Narrative: I was working Aircraft X. He departed on the SID and was climbing via the SID. On initial contact; I instructed the pilot to climb and maintain his filed altitude. I then talked to 2 to 3 other contacts before returning to Aircraft X. The next instruction I gave Aircraft X was 'reaching 13;000 cleared direct destination.' The pilot read back; '13;000; direct destination.' I was fairly busy; and did not catch the fact that this was a somewhat incomplete read back. I again moved on to other duties. Working 2 frequencies and 6 transmitters; aircraft often step on each other; this happened many times during this session. The pilot of Aircraft X had 'a request'; but was covered by other aircraft. I told him to standby because I thought that he was climbing up and did not thing it was important; and moved on to other aircraft. He asked at least once more; and I when finally was able to respond and fully look at Aircraft X I realized that he was still at 13;000 feet in a 13;700 MIA(Minimum IFR Altitude). I gave him the instruction to climb; and told the pilot no delay through 13;700 feet to reach my MIA. I was worried that I gave the pilot a bad instruction; so immediately went and reviewed the playback. After listening to the audio; I realized that the pilot did not read back the instruction as precisely as he should have and I should have caught this inaccuracy. In the future I will change the way I give the instruction for an on course turn. Instead of 'reaching 13;000; cleared direct' I will try to say 'leaving 13;000; cleared direct'. I will also be very vigilant to watch aircraft climbing on the SID after that instruction is issued.

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.