Narrative:

Aircraft X was approaching the airport and I noticed he appeared to be north of the approach course; still outside our airspace. As he got closer; I noticed he was still north and appeared to be descending below the glide slope. I called in the blind to see if he was on frequency and as he checked on; the low altitude alert sounded. The approach controller was simultaneously calling on the shout line about the safety alert and the ground control position handled the coordination while I issued the alert. I immediately issued the low altitude alert to aircraft X; advised of his altitude; and asked for his intentions. I don't believe I received a response and immediately suggested for the pilot to climb; then more firmly issued to him to 'maintain 8000; turn right 010' (missed approach instructions). The pilot still did not comply; but then finally reported he had the airport in sight. I felt I might have little time to act; and suspected the pilot was behind the airplane and might be encountering difficulty (or even an emergency); so I instructed that if he had the airport in sight; he was cleared to land. I visually spotted the aircraft as well. The pilot acknowledged and I saw him fly a tight pattern into the runway; and he landed safely.during the event; aircraft Y was holding short. Prior to aircraft X checking on; I decided I did not want to launch the departure; as I was not sure I could ensure IFR separation if aircraft X became a missed approach. Another strong reminder to plan ahead and control conservatively during some situations in the tower environment; particularly when visual separation is not an option. After the incident; I reflected that we did have descent visibility below the bases and I wondered if perhaps the pilot had ground contact and decided to initiate a contact approach. However; if this was the case; he did not inform the approach controller or us; and thus it was quite a scare to see an aircraft descend that low a good distance from the airport during IMC conditions. A strong reminder to always be prepared to issue safety alerts. It's important for pilots to understand that if they are encountering difficulty or even finding an approach challenging; ATC needs to know about it as soon as practical. We're on duty to provide you with the maximum assistance we can; but we can't help if we don't know a problem exists or what you'd like to do. Our control instructions include assumptions that pilots will fly an instrument approach a certain way; and we need to know as soon as possible if the flight crew intends to deviate off that procedure (such as to complete a contact approach or more routinely; a visual approach). Unless invoking pilot in command authority; make sure ATC knows and has approved your request first! There may be other traffic or obstacles nearby; coordination may first be required with another facility; etc. And ATC will respond accordingly to the request.

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Original NASA ASRS Text

Title: BJC Local Controller reported an aircraft on approach descended below the glideslope and triggered a low altitude alert.

Narrative: Aircraft X was approaching the airport and I noticed he appeared to be north of the approach course; still outside our airspace. As he got closer; I noticed he was still north and appeared to be descending below the glide slope. I called in the blind to see if he was on frequency and as he checked on; the low altitude alert sounded. The approach controller was simultaneously calling on the shout line about the safety alert and the Ground Control position handled the coordination while I issued the alert. I immediately issued the low altitude alert to Aircraft X; advised of his altitude; and asked for his intentions. I don't believe I received a response and immediately suggested for the pilot to climb; then more firmly issued to him to 'Maintain 8000; turn right 010' (missed approach instructions). The pilot still did not comply; but then finally reported he had the airport in sight. I felt I might have little time to act; and suspected the pilot was behind the airplane and might be encountering difficulty (or even an emergency); so I instructed that if he had the airport in sight; he was cleared to land. I visually spotted the aircraft as well. The pilot acknowledged and I saw him fly a tight pattern into the runway; and he landed safely.During the event; Aircraft Y was holding short. Prior to Aircraft X checking on; I decided I did not want to launch the departure; as I was not sure I could ensure IFR separation if Aircraft X became a missed approach. Another strong reminder to plan ahead and control conservatively during some situations in the tower environment; particularly when visual separation is not an option. After the incident; I reflected that we did have descent visibility below the bases and I wondered if perhaps the pilot had ground contact and decided to initiate a contact approach. However; if this was the case; he did not inform the approach controller or us; and thus it was quite a scare to see an aircraft descend that low a good distance from the airport during IMC conditions. A strong reminder to always be prepared to issue safety alerts. It's important for pilots to understand that if they are encountering difficulty or even finding an approach challenging; ATC needs to know about it as soon as practical. We're on duty to provide you with the maximum assistance we can; but we can't help if we don't know a problem exists or what you'd like to do. Our control instructions include assumptions that pilots will fly an instrument approach a certain way; and we need to know as soon as possible if the flight crew intends to deviate off that procedure (such as to complete a contact approach or more routinely; a visual approach). Unless invoking pilot in command authority; make sure ATC knows and has approved your request first! There may be other traffic or obstacles nearby; coordination may first be required with another facility; etc. and ATC will respond accordingly to the request.

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.