C172 pilot is unable to command his auto pilot to descend too step down fix altitude nor to minimums...

Narrative:

I was in contact with norcal TRACON requesting a practice GPS-B approach to hwd. Norcal cleared me for the approach and upon established; descended to 2;300 per the chart (between imply and decot intersections). At decot intersection; I commanded the autopilot to descend to 1;500 ft. The autopilot descended to 2;000 ft and did not descend any further. I then was looking at my other instruments and then asked to contact hwd tower. At 2.0 NM from mapvv; I commanded the autopilot to descend to the MDA and noticed that the altitude was still 2;000 ft. I reattempted to command the autopilot to descend but it did not respond and the autopilot continued to hold 2;000 ft over the field. After approaching the mapvv; hwd tower issued a descend to 1;500 ft and continue southbound then to re-establish contact with norcal TRACON. I believe that there was too much reliance on the autopilot and not recognizing that it may not have been in the right mode (vertical speed) when issuing the descend command. Situational awareness was lost when I was trying to figure out what was wrong with the autopilot. To correct this situation; I should have disengaged the autopilot and hand flown the approach into hwd upon realizing that I was too high at the 2.0 NM to mapvv.

Original NASA ASRS Text

Title: C172 pilot is unable to command his auto pilot to descend too step down fix altitude nor to minimums during practice GPS-B approach to HWD. Reporter passes over airport at 2000 FT.

Narrative: I was in contact with NORCAL TRACON requesting a practice GPS-B approach to HWD. NORCAL cleared me for the approach and upon established; descended to 2;300 per the chart (between imply and DECOT intersections). At DECOT Intersection; I commanded the autopilot to descend to 1;500 FT. The autopilot descended to 2;000 FT and did not descend any further. I then was looking at my other instruments and then asked to contact HWD Tower. At 2.0 NM from MAPVV; I commanded the autopilot to descend to the MDA and noticed that the altitude was still 2;000 FT. I reattempted to command the autopilot to descend but it did not respond and the autopilot continued to hold 2;000 FT over the field. After approaching the MAPVV; HWD Tower issued a descend to 1;500 FT and continue southbound then to re-establish contact with NORCAL TRACON. I believe that there was too much reliance on the autopilot and not recognizing that it may not have been in the right mode (vertical speed) when issuing the descend command. Situational awareness was lost when I was trying to figure out what was wrong with the autopilot. To correct this situation; I should have disengaged the autopilot and hand flown the approach into HWD upon realizing that I was too high at the 2.0 NM to MAPVV.

Data retrieved from NASA's ASRS site as of April 2012 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.

Attributes
ACN	829844
Time
Date	200904
Local Time Of Day	1201-1800
Place
Locale Reference	HWD.Airport
State Reference	CA
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach Landing
Route In Use	Other Traffic Pattern
Flight Plan	None
Component
Aircraft Component	Autopilot
Person 1
Function	Pilot Flying Single Pilot
Qualification	Flight Crew Private
Experience	Flight Crew Last 90 Days 8 Flight Crew Total 200 Flight Crew Type 170
Events
Anomaly	Deviation - Procedural Published Material / Policy

C172 pilot is unable to command his auto pilot to descend too step down fix altitude nor to minimums during practice GPS-B approach to hwd. Reporter passes over airport at 2000 ft.