Narrative:

Aircraft X called and reported northeast of bjc class D airspace. The pilot requested to proceed inbound to land; but the field was below basic VFR minimums. I advised the pilot and he requested radar vectors. I responded that I was unable to provide radar vectors; and provided him with the frequency for denver approach. The pilot said he simply wanted to fly the ILS 'on his own' before turning inbound; to which I once again replied that the field was below basic VFR minimums. At that point; he advised he was changing frequencies over to denver approach.I then observed the aircraft acquire an IFR data block and listened in on the approach frequency as they established radar contact and began providing him radar service for an ILS approach. I observed aircraft X climbing to 7000 ft as he turned onto the final.aircraft X checked on outside of the final; level at 7000 ft for the ILS approach. I provided the pilot with the latest pilot report and weather conditions and cleared him to land. Before the aircraft passed the FAF; I observed it below 7000 ft and the low altitude alert sounded. I immediately issued the low altitude alert to the pilot and he acknowledged. The approach controller was simultaneously calling so I could not hear the pilot's response. On the second try; the pilot confirmed he was 'correcting back to 7000 ft' for the approach. I did not see him reach 7000 ft again; but I did see that he stopped his descent on the final and did not need to issue any additional alerts. The pilot continued inbound and landed without further incident.continuous pilot and controller training on these kinds of situations can help prevent re-occurrences of this event. What are your options as an airman if you're operating VFR but trying to land at a controlled airport reporting conditions below basic VFR? I suspect the pilot may have desired a SVFR (special VFR) clearance; but did not seem to understand what steps he would need to take to initiate the request. After picking up an IFR clearance; the pilot may have also initiated a request for a contact approach if he wanted to remain clear of the clouds as he turned inbound. However; without exercising one of those options; I was not expecting him to descend like he did prior to the FAF.as always; pilots should always keep in mind if they are encountering an emergency situation; controllers may issue a clearance into the airspace for landing; after confirming that an emergency exists.

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Original NASA ASRS Text

Title: BJC Tower Controller reported issuing an aircraft a low altitude alert after it descended below the FAF crossing altitude.

Narrative: Aircraft X called and reported northeast of BJC Class D airspace. The pilot requested to proceed inbound to land; but the field was below basic VFR minimums. I advised the pilot and he requested radar vectors. I responded that I was unable to provide radar vectors; and provided him with the frequency for Denver Approach. The pilot said he simply wanted to fly the ILS 'on his own' before turning inbound; to which I once again replied that the field was below basic VFR minimums. At that point; he advised he was changing frequencies over to Denver Approach.I then observed the aircraft acquire an IFR data block and listened in on the approach frequency as they established radar contact and began providing him radar service for an ILS approach. I observed Aircraft X climbing to 7000 ft as he turned onto the final.Aircraft X checked on outside of the final; level at 7000 ft for the ILS approach. I provided the pilot with the latest pilot report and weather conditions and cleared him to land. Before the aircraft passed the FAF; I observed it below 7000 ft and the low altitude alert sounded. I immediately issued the low altitude alert to the pilot and he acknowledged. The approach controller was simultaneously calling so I could not hear the pilot's response. On the second try; the pilot confirmed he was 'correcting back to 7000 ft' for the approach. I did not see him reach 7000 ft again; but I did see that he stopped his descent on the final and did not need to issue any additional alerts. The pilot continued inbound and landed without further incident.Continuous pilot and controller training on these kinds of situations can help prevent re-occurrences of this event. What are your options as an airman if you're operating VFR but trying to land at a controlled airport reporting conditions below Basic VFR? I suspect the pilot may have desired a SVFR (Special VFR) clearance; but did not seem to understand what steps he would need to take to initiate the request. After picking up an IFR clearance; the pilot may have also initiated a request for a contact approach if he wanted to remain clear of the clouds as he turned inbound. However; without exercising one of those options; I was not expecting him to descend like he did prior to the FAF.As always; pilots should always keep in mind if they are encountering an emergency situation; controllers may issue a clearance into the airspace for landing; after confirming that an emergency exists.

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.