Single pilot of a general aviation light aircraft reported a problem with the autopilot during an ap...

Attributes
ACN	1511527
Time
Date	201801
Local Time Of Day	0001-0600
Place
Locale Reference	SAC.Airport
State Reference	CA
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	Small Aircraft Low Wing 1 Eng Retractable Gear
Operating Under FAR Part	Part 91
Flight Phase	Final Approach
Route In Use	Other RNAV GPS Rwy 2
Flight Plan	IFR
Person 1
Function	Pilot Flying Single Pilot
Qualification	Flight Crew Commercial Flight Crew Instrument
Experience	Flight Crew Last 90 Days 25 Flight Crew Total 2050 Flight Crew Type 300
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Overshoot Deviation - Procedural Published Material / Policy Inflight Event / Encounter CFTT / CFIT

Narrative:

I inadvertently descended below approach segment altitudes while on the RNAV GPS RWY2 approach into sac. The ceilings were at minimum when I started the approach. I was going to the airport to have an intermittently malfunctioning auto pilot repaired. The auto pilot malfunctioned as I started the approach and I failed to notice that the garmin 430W indicated LNAV instead of lpv and there was no glide slope. While dealing with the malfunctioning auto pilot I descended below the published segment altitudes and received low altitude alerts from the controllers. I climbed and then continued the approach and successfully landed. The problem was that the malfunctioning auto pilot distracted me and I didn't notice the LNAV indication before beginning the approach. I wasn't properly prepared for an approach without the glide slope or auto pilot.I learned that when something unexpected happens on an approach; go missed approach and sort things out away from the airport. I would not have continued an LNAV approach down to minimums with no glide slope and a malfunctioning autopilot if I hadn't been distracted and had realized that it was going to be a LNAV approach instead of a lpv approach. Fortunately; the approach ended well.

Original NASA ASRS Text

Title: Single Pilot of a general aviation light aircraft reported a problem with the autopilot during an approach resulting in a low altitude alert from ATC.

Narrative: I inadvertently descended below approach segment altitudes while on the RNAV GPS RWY2 approach into SAC. The ceilings were at minimum when I started the approach. I was going to the airport to have an intermittently malfunctioning auto pilot repaired. The auto pilot malfunctioned as I started the approach and I failed to notice that the Garmin 430W indicated LNAV instead of LPV and there was no glide slope. While dealing with the malfunctioning auto pilot I descended below the published segment altitudes and received low altitude alerts from the controllers. I climbed and then continued the approach and successfully landed. The problem was that the malfunctioning auto pilot distracted me and I didn't notice the LNAV indication before beginning the approach. I wasn't properly prepared for an approach without the glide slope or auto pilot.I learned that when something unexpected happens on an approach; go missed approach and sort things out away from the airport. I would not have continued an LNAV approach down to minimums with no glide slope and a malfunctioning autopilot if I hadn't been distracted and had realized that it was going to be a LNAV approach instead of a LPV approach. Fortunately; the approach ended well.

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.

Single pilot of a general aviation light aircraft reported a problem with the autopilot during an approach resulting in a low altitude alert from ATC.