Bonanza 35
Flight Crew Instrument; Flight Crew Private
Flight Crew Last 90 Days 21; Flight Crew Total 680; Flight Crew Type 225
Distraction; Human-Machine Interface; Training / Qualification; Workload
Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Aircraft; Human Factors; Weather
Had been stepped down from 9;000 FT to 3;000 FT and was number 3 or 4 aircraft for the approach. Approach vectored me to intercept ILS 33 at the IAF. Instructed to 'turn to 360 degrees and maintain 2;500 FT until established; cleared for ILS 33; contact Tower.' I was using autopilot heading mode (steering with heading bug) turning to 360 with HSI pointer to 332 degrees (final approach course) and engaged vertical speed down and put landing gear down to get from 3;000 FT to 2;500 FT MSL. I then engaged NAV on S-Tec 60-2 which showed capture and completed the intercept of the localizer. My practice from training is to keep hand on landing gear until gear down and locked confirmation (single green and barber pole). [I] confirmed autopilot capture and control of localizer course; thinking that the autopilot would capture glide slope. Called Tower to report inbound on ILS 33; and then noticed aircraft still descending rapidly and below target altitude. Simultaneously; Tower called Altitude Alert at 1;500 FT and I should be at 2;500 FT. I immediately disengaged autopilot and climbed. At this point; was below ceiling with runway in sight with VASI guidance and adjusted altitude to 2 white/2 red. No issue landing. In reviewing post-flight; I should have controlled the aircraft by hand to the intercept altitude of 2;500 FT rather than using vertical speed and confusing NAV capture of Localizer with also engaging glide slope capture. I will fly with my instructor to practice autopilot scenarios as most of my approaches are hand flown.
A BE35 pilot descended to 1;500 FT before crossing the initial approach fix 2;500 FT constraint because he was using the autopilot Vertical Speed mode while managing the landing gear and monitoring LOC capture; but with glideslope capture not armed.