Narrative:

Aircraft X was inbound ILS to 29R. I observed that the aircraft appeared to be making abnormal turns outside the FAF while they were inbound with approach. Aircraft X checked in; and I believe I advised that traffic would depart ahead (king air). After rolling the king air; I believe I heard the first low altitude alert (la) and immediately issued the phraseology 'low altitude alert; check your altitude immediately.' while doing this; the flm handled the shout line; acknowledging we issued the la alert. Pilot acknowledged and I saw he was around 065 near the outer marker (indicating 500 ft low).as aircraft X continued inbound; it looked like he was south of the final and another la alert sounded. Approach called again; and again the flm handled the line while I issued the alert. I believe the flm asked me to relay to pilot that it looked like he was south of course as well. I advised the pilot that it appeared he was south of final and below the glide slope; and pilot reported he was aware and correcting back. It then looked like the pilot was relatively high as he approached the threshold (maybe about 300 ft higher than normal); and I finally saw him break out; high and further down the runway than expected. The pilot got the aircraft down; and exited approximately 2/3 of the way down the runway. Pilot exited and went to fuel. Later on; after fueling; the approach supervisor had called our cab and asked out flm to request that the pilot call them on the phone to discuss their approach earlier. I heard the flm provide the approach phone number to the pilot as he taxied to the hangar.I am not certain about the reasons behind the pilot's abnormal approach; but based on the communications from the approach controllers; it appeared they had difficulty getting him onto the ILS. Some possible reasons I suspect for the strange approach might be if the pilot was having equipment difficulties; possibly some training was taking place; or maybe the pilot was simply 'behind the airplane.' regardless; I became very concerned as the pilot was on final in IMC and it did not seem there was a very stable approach. In these scenarios I felt it was still best to provide maximum timely information to the pilot (low altitude alerts; advisory about being left/right of final; etc.) and allow them to fly the place and decide their safest course of action. Frankly; as erratically as the pilot was flying the ILS; I was very surprised that a go-around/missed approach did not occur. This is another vivid reminder to be prepared for anything in ATC; especially during low IMC conditions. I was glad I had previous opportunities to issue low altitude alerts in similar situations; so the phraseology spilled out without hesitation as soon as the alarm sounded. While la [low altitude] alarms might seem a nuisance at times (like when an aircraft on visual approach; in sight and in good VMC; rapidly descends toward the airport setting off an alarm); these alerts are vital in cases like this to serve as a 'last line of defense' for ATC to warn astray pilots on instrument approaches.

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Original NASA ASRS Text

Title: BJC Tower Controller reports of an aircraft that was inbound to BJC and had two low altitude alerts while on final.

Narrative: Aircraft X was inbound ILS to 29R. I observed that the aircraft appeared to be making abnormal turns outside the FAF while they were inbound with approach. Aircraft X checked in; and I believe I advised that traffic would depart ahead (King Air). After rolling the King Air; I believe I heard the first low altitude alert (LA) and immediately issued the phraseology 'low altitude alert; check your altitude immediately.' While doing this; the FLM handled the shout line; acknowledging we issued the LA alert. Pilot acknowledged and I saw he was around 065 near the outer marker (indicating 500 FT low).As Aircraft X continued inbound; it looked like he was south of the final and another LA alert sounded. Approach called again; and again the FLM handled the line while I issued the alert. I believe the FLM asked me to relay to pilot that it looked like he was south of course as well. I advised the pilot that it appeared he was south of final and below the glide slope; and pilot reported he was aware and correcting back. It then looked like the pilot was relatively high as he approached the threshold (maybe about 300 FT higher than normal); and I finally saw him break out; high and further down the runway than expected. The pilot got the aircraft down; and exited approximately 2/3 of the way down the runway. Pilot exited and went to fuel. Later on; after fueling; the Approach Supervisor had called our Cab and asked out FLM to request that the pilot call them on the phone to discuss their approach earlier. I heard the FLM provide the Approach phone number to the pilot as he taxied to the hangar.I am not certain about the reasons behind the pilot's abnormal approach; but based on the communications from the Approach controllers; it appeared they had difficulty getting him onto the ILS. Some possible reasons I suspect for the strange approach might be if the pilot was having equipment difficulties; possibly some training was taking place; or maybe the pilot was simply 'behind the airplane.' Regardless; I became very concerned as the pilot was on final in IMC and it did not seem there was a very stable approach. In these scenarios I felt it was still best to provide maximum timely information to the pilot (low altitude alerts; advisory about being left/right of final; etc.) and allow them to fly the place and decide their safest course of action. Frankly; as erratically as the pilot was flying the ILS; I was very surprised that a go-around/missed approach did not occur. This is another vivid reminder to be prepared for anything in ATC; especially during low IMC conditions. I was glad I had previous opportunities to issue low altitude alerts in similar situations; so the phraseology spilled out without hesitation as soon as the alarm sounded. While LA [Low Altitude] alarms might seem a nuisance at times (like when an aircraft on visual approach; in sight and in good VMC; rapidly descends toward the airport setting off an alarm); these alerts are vital in cases like this to serve as a 'last line of defense' for ATC to warn astray pilots on instrument approaches.

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.