Date: 201204
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: Marginal
Aircraft: PA-31 Navajo/Chieftan/Mojave/T1040
Flight plan: IFR
Flight phase: Descent
Airspace: Class D ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Government
Reporter function: Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Situational Awareness
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
Result: Air Traffic Control Issued Advisory / Alert
Contributing factors: Human Factors; Weather; Procedure
Primary problem: Weather

Narrative: A few minutes prior to the event; I observed a PA31 tagged inbound from the northeast; shown to be receiving VFR flight following; but also tagged for ILS approach. I coordinated with TRACON to confirm the aircraft was indeed IFR; since our latest weather observation put us below basic VFR minimums; and the TRACON Controller confirmed he was; and that he was trying to figure out how to remove the VFR entries from the Data Block. When the PA31 checked on frequency with 'on approach' a few minutes later; I observed him descending out of 070 approaching the FAF and the Low Altitude Alert immediately sounded. As TRACON called over the shout line for the Low Altitude Alert; I immediate issued a Low Altitude Alert to the PA31 while another Controller was on the shout line to TRACON. I issued the pilot the decision height of 5;750 FT for the 29R ILS Approach with the alert. The pilot acknowledged the safety alert and shortly thereafter I realized he was continuing to descend further. I suddenly realized; that based on the decision height I issued in my Low Altitude Alert; the pilot might not understand he was supposed to be established at/above 070 prior to the FAF; and when I realized this I suggested that the pilot immediately climb to 070; advising that's where he should be for the ILS approach. I am not certain; but his altitude may have shown as low as 065 or 063 prior to the pilot starting a climb. Once I saw that the pilot was back at a safe altitude just inside the FAF; I requested he 'say intentions'; to which he replied he wanted to land. At this point; I responded by asking him if he was established on the ILS approach; to which replied that he was not. I was about to start coordinating missed approach instructions with TRACON; but then I heard someone in the cab suggest that I might see if he had the airport in sight; there was a low layer of clouds estimated around 800 FT AGL east of the airport; but otherwise visibility was unrestricted. When asked; the pilot reported that he did have the airport in sight and I decided that based on that information; the safest course of action was to clear the pilot to land 29R. Because of the pilot's response of 'airport in sight' at that position; I believe the airport was back above basic VFR minimums at that time. From this point on; I had the PA31 in sight; and the rest of the approach and landing were uneventful. I was thankful that other team members assisted with lower priority items (coordination; watching for changes in the weather; etc.) during the event while I focused on the PA31. Once I was relieved from position; I later on began to wonder what led the PA31 to descend out of 070 and why he didn't seem overly concerned of his altitude or not being established on the ILS or advising ATC of any difficulties/intentions to do something other than an ILS approach. Based on the weather at the time; I suspect he might have had ground contact and was perhaps attempting to remain in VFR conditions below the clouds for a Contact Approach/SVFR-type operation. However; no request of this nature was ever received from the pilot; or at least it was never relayed to me. Now that I think about it; though the Data Block and TRACON Controller both confirmed the aircraft was IFR; perhaps the pilot was still attempting to remain in VMC based on the VFR advisories he was receiving on the rest of his trip inbound. As I replay the event in my head; the one thing that I am incredibly thankful for is that it was made clear to me during my OJT of how important it is to issue safety alerts immediately and utilizing correct phraseology. Sometimes we go for long periods of time without going IMC in this area; so when IMC weather does come into play; I make it a point to review the approach plates under the plexiglass at the Local Control 1 position; especially reviewing DH/MDA numbers in case a safety alert is needed. However; this event illustrated that though issuing decision height might be of thehighest priority when the pilot is VERY close to the ground; it's also important to be ready to immediately issue the FAF crossing altitude in situations when that applies. Thinking back; I remember that I started getting a 'gut feeling' that something didn't seem right; and that's when I went with my instinct and told the pilot to immediately climb to 7;000. This event reminds me of something one of my OJTIs told me once: when you start getting a 'gut feeling' that some thing's wrong or not going to work in ATC; don't fight those feelings but rather investigate so you can take action to fix things.




Synopsis: Tower Controller described a 'Low Altitude' alert event when traffic on the ILS descended below the published mandatory altitudes; the reporter listing possible confusion regarding flight plan status and weather conditions as contributing factors.