Date: 201601
Local time of day: 1201-1800
Location: BHM.Airport, AL
Flight conditions: Mixed
Weather elements / visibility: Rain; 2
Light: Daylight
Aircraft: Citation I/SP (C501)
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class C BHM
ATC / advisory: TRACON BHM
Reporter organization: Corporate
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Human factors: Situational Awareness; Training / Qualification
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: The crew was flying into BHM from the east. A cold front had just passed and was producing some light to moderate rain. The crew was flying in a CE501 which was on loan to them through a lease agreement. The crew was legal and current to be flying in this aircraft. However; it is not the aircraft that they fly on a daily basis.  Approach control gave vectors onto final to intercept the LOC of the ILS Runway 24 approach. The aircraft was approaching the LOC from the east. Approach control gave a final vector and told the crew to maintain 3000 feet until established and cleared them for the ILS Runway 24 approach. PIC believes they intercepted the LOC in proximity to SPATT intersection. Neither crewmember remembers whether the controller gave a distance referenced to any of the fixes along the LOC when cleared for the ILS Runway 24 approach. After about a minute of intersecting the LOC; the PIC decided to descend to GS intercept attitude of 2200 feet. Soon after descending; the controller issued a low altitude alert. The aircraft was between layers and had some visual contact with the ground; based on this the crew was not alarmed and verbally acknowledge the controller. Just prior to intercepting the GS at 2200 feet; the controller asked the crew for their altitude and the SIC responded with 2200 feet. The controller responded back with 'because the low altitude alert goes off below minimum vectoring altitude of 2600 feet'. The aircraft intercepted the GS and landed uneventful.The crew discussed the approach after the flight. The crew is uncertain what caused the low altitude alert; but theorized two possibilities. The PIC could have descended below the stepdown altitude of 2700 feet prior to reaching HUKEV stepdown fix. Another possibility could be a descent rate towards GS intercept altitude that triggered the alert.The aircraft flown was a CE501 on loan; instead of the crew's daily aircraft of CE525A and CE560. The FMS used as long range navigation was a Garmin 400; which the PIC is moderately proficient with. Since the crew was vectored onto final; the SIC programed the Garmin 400 using the Vectors to Final feature to load the ILS Runway 24 approach for situational awareness. One consideration of 'Vectors to Final' feature is that it removes from view any fixes prior to the FAF. In this instance; it removed from view stepdown fixes SPATT and HUKEV prior to FAF LOWGA. Had the stepdown fixes been in view; the crew might have remembered to consider the stepdown altitude at HUKEV.Also; the autopilot is a bit quirky especially regarding climbs and descents. Therefore; it was suggested to the PIC to fly autopilot in pitch mode. Autopilot responds smoother; but requires more attention to descend at a constant VS. With the high workload of an approach especially considering the weather in the vicinity; the PIC may have allowed the descent rate to fluctuate more than intended. This may have triggered the low altitude alert.Contributing factors:Proficiency with aircraft especially avionics and autopilot. Weather distractions.Corrective actions:Thoroughly brief approaches to include step down fixes and altitudes prior to FAF or FAP; even if they aren't anticipated to be used. Always load approaches into FMS to include step down fixes and altitudes prior to FAF or FAP to aid in situation awareness. Constantly maintain situation awareness referenced to your altitude.




Synopsis: CE501 Captain reported receiving a low altitude alert from ATC on approach to BHM.