Narrative:

I was working the radar associate position. When the CRJ2 initially checked on frequency; I noticed that their radio was very scratchy and full of static. When asked which approach they wanted; the pilot asked for the ILS runway 03 approach. The pilot asked to begin the approach at the fix. The pilot was issued a clearance; direct to the fix; and also was issued a descent clearance to 170 (17;000 ft MSL). The read back was scratchy; and the pilot stumbled over their wording; but I heard 'seventeen' as the altitude read back. The aircraft descended below 170. The person conducting training on the radar position issued a low altitude alert as the aircraft was descending through 160. The aircraft leveled at 150 before climbing back up to 170. The aircraft was then established on the approach routing and subsequently cleared for approach. As they got to a lower altitude; they called the airport in sight; and were given a visual approach clearance. When the aircraft was cleared to change to the airport advisory frequency; there was some confusion as to the transmission from the pilot. They were advised that they were cleared for a visual approach and were to change to advisory frequency. A different voice then called from the CRJ2; asking us what had happened with the altitudes 'back there'. This pilot seemed to want to argue the issue; so the radar training instructor keyed up and told him that there was some apparent miscommunication; that they assigned altitude was 170; and the alert was issued because the went below our minimum assignable IFR altitude of 16;100 ft. The second pilot said 'he/she read back 150; and that is what I heard. All 3 of us manning the sector were just as sure he/she had read back 170. A review of the audio recording was inconclusive; as there was too much static picked up by the recorder to be absolutely sure. It still sounded like seventeen to all 3 of us from the sector. Recommendation; have new communications equipment installed. Have the CRJ2 get their radio equipment checked out. They were the only aircraft that we had any difficulty with in communications. They have been advised previously that this aircraft's radios were unintelligible at times. Establish a way to ensure aircraft with faulty radios can be reported and checked for operational clarity. This can become a severe safety issue for aircraft flying in IFR conditions in mountainous terrain.

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Original NASA ASRS Text

Title: Enroute Controller described a Low Altitude Alert event when traffic inbound for an approach apparently misunderstood the issued altitude. Both aircraft and ATC equipment issues were listed as causal factors by the reporter.

Narrative: I was working the RADAR Associate position. When the CRJ2 initially checked on frequency; I noticed that their radio was very scratchy and full of static. When asked which approach they wanted; the pilot asked for the ILS Runway 03 Approach. The pilot asked to begin the approach at the fix. The pilot was issued a clearance; direct to the fix; and also was issued a descent clearance to 170 (17;000 FT MSL). The read back was scratchy; and the pilot stumbled over their wording; but I heard 'seventeen' as the altitude read back. The aircraft descended below 170. The person conducting training on the RADAR position issued a Low Altitude Alert as the aircraft was descending through 160. The aircraft leveled at 150 before climbing back up to 170. The aircraft was then established on the approach routing and subsequently cleared for approach. As they got to a lower altitude; they called the airport in sight; and were given a Visual Approach Clearance. When the aircraft was cleared to change to the airport advisory frequency; there was some confusion as to the transmission from the pilot. They were advised that they were cleared for a Visual Approach and were to change to advisory frequency. A different voice then called from the CRJ2; asking us what had happened with the altitudes 'back there'. This pilot seemed to want to argue the issue; so the RADAR training instructor keyed up and told him that there was some apparent miscommunication; that they assigned altitude was 170; and the alert was issued because the went below our minimum assignable IFR altitude of 16;100 FT. The second pilot said 'he/she read back 150; and that is what I heard. All 3 of us manning the sector were just as sure he/she had read back 170. A review of the audio recording was inconclusive; as there was too much static picked up by the recorder to be absolutely sure. It still sounded like seventeen to all 3 of us from the sector. Recommendation; have new communications equipment installed. Have the CRJ2 get their radio equipment checked out. They were the only aircraft that we had any difficulty with in communications. They have been advised previously that this aircraft's radios were unintelligible at times. Establish a way to ensure aircraft with faulty radios can be reported and checked for operational clarity. This can become a severe safety issue for aircraft flying in IFR conditions in mountainous terrain.

Data retrieved from NASA's ASRS site as of April 2012 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.