Cessna pilot reported ATC advised him of several altitude deviations that resulted from anomalies wi...

Attributes
ACN	1408209
Time
Date	201612
Local Time Of Day	0601-1200
Place
Locale Reference	PAO.Airport
State Reference	CA
Environment
Flight Conditions	Marginal
Light	Daylight
Aircraft 1
Make Model Name	Cessna Single Piston Undifferentiated or Other Model
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach
Route In Use	Other GPS Runway 31
Flight Plan	IFR
Component
Aircraft Component	Vertical Speed System
Person 1
Function	Single Pilot
Qualification	Flight Crew Instrument Flight Crew Commercial
Experience	Flight Crew Last 90 Days 14 Flight Crew Total 1260 Flight Crew Type 830
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude

Narrative:

While on instrument approach GPS 31 to pao I started to receive unreliable indications of vertical speed from G1000 avionics; with vs varying momentarily to +/- 1000 ft/min. Since I was flying a coupled approach the autopilot started to correct. This resulted in the airplane descending below a segment altitude of 2000 ft. This in turn triggered [an] altitude alert from pao tower. I corrected by switching off the autopilot and flying by hand. I continued the approach and realized the indications of the G1000 were still unreliable. While in VMC and with airport and runway environment in sight I positioned the aircraft for circle to land approach and descended below circling altitude which was noted by the tower. I continued to successful landing using the standby altimeter only.in retrospect after the first indication of widely changing vertical speed from G1000 I should have continued flying using the standby instruments only. The G1000 is going to be reviewed by the authorized avionics shop. I want to stress that the event happened in VMC and I had visual contact with the ground at all times.

Original NASA ASRS Text

Title: Cessna pilot reported ATC advised him of several altitude deviations that resulted from anomalies within the G1000 vertical speed display.

Narrative: While on instrument approach GPS 31 to PAO I started to receive unreliable indications of vertical speed from G1000 avionics; with VS varying momentarily to +/- 1000 ft/min. Since I was flying a coupled approach the autopilot started to correct. This resulted in the airplane descending below a segment altitude of 2000 ft. This in turn triggered [an] altitude alert from PAO Tower. I corrected by switching off the autopilot and flying by hand. I continued the approach and realized the indications of the G1000 were still unreliable. While in VMC and with airport and runway environment in sight I positioned the aircraft for circle to land approach and descended below circling altitude which was noted by the tower. I continued to successful landing using the standby altimeter only.In retrospect after the first indication of widely changing vertical speed from G1000 I should have continued flying using the standby instruments only. The G1000 is going to be reviewed by the authorized avionics shop. I want to stress that the event happened in VMC and I had visual contact with the ground at all times.

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.