Small Aircraft
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 6.2
Other / Unknown
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Aircraft; Environment - Non Weather Related; Human Factors; Weather
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.
BJC Tower Controller reports of an aircraft that was inbound to BJC and had two low altitude alerts while on final.