Descended below minimum when altitude hold feature of automatic pilot disengaged.

Attributes
ACN	92324
Time
Date	198808
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : see
State Reference	CA
Altitude	agl bound lower : 900 agl bound upper : 1520
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : san tower : see
Operator	general aviation : personal
Make Model Name	Small Transport, Low Wing, 2 Recip Eng
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : private pilot : instrument
Experience	flight time last 90 days : 110 flight time total : 500 flight time type : 120
ASRS Report	92324
Person 2
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Events
Anomaly	non adherence : published procedure non adherence : far other anomaly other
Independent Detector	other flight crewa
Resolutory Action	none taken : anomaly accepted
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

I was flying my privately owned small transport from carlsbad (crq) to gillespie (see) to deliver aircraft to maintenance facility. See was reporting 600-2 in fog. The localizer-D approach was an 1135'-one and one-quarter min. I flew an autoplt coupled approach. I requested approach to call the outer marker and tower to call the MM for me, since I had had some difficulty with marker beacon receiver. Tower confirmed a 3-MI final and I descended to the MDA 1520' (1135'), where I activated altitude hold. Shortly thereafter, before reaching maf, autoplt altitude hold failed and aircraft descended to 900' AGL. I disabled autoplt and landed normally. I observed ceiling at 900-1000'. Autoplt tested and found to be working normally. Callback conversation with reporter revealed the following: tower did not question the reporter as to his descent below min. When he broke out, the aircraft was near the airport and a circle to land was made. Has received no inquiry reference the incident from the FAA. Has electric on the aircraft that could disable the altitude hold feature, but because of the rapid trim rate, does not use. In order to trim manually, it is necessary to disengage the autoplt. Lengthy discussion on TCA problem and awareness of many pilots as far as ASRS.

Original NASA ASRS Text

Title: DESCENDED BELOW MINIMUM WHEN ALT HOLD FEATURE OF AUTO PLT DISENGAGED.

Narrative: I WAS FLYING MY PRIVATELY OWNED SMT FROM CARLSBAD (CRQ) TO GILLESPIE (SEE) TO DELIVER ACFT TO MAINT FAC. SEE WAS RPTING 600-2 IN FOG. THE LOC-D APCH WAS AN 1135'-ONE AND ONE-QUARTER MIN. I FLEW AN AUTOPLT COUPLED APCH. I REQUESTED APCH TO CALL THE OUTER MARKER AND TWR TO CALL THE MM FOR ME, SINCE I HAD HAD SOME DIFFICULTY WITH MARKER BEACON RECEIVER. TWR CONFIRMED A 3-MI FINAL AND I DSNDED TO THE MDA 1520' (1135'), WHERE I ACTIVATED ALT HOLD. SHORTLY THEREAFTER, BEFORE REACHING MAF, AUTOPLT ALT HOLD FAILED AND ACFT DSNDED TO 900' AGL. I DISABLED AUTOPLT AND LANDED NORMALLY. I OBSERVED CEILING AT 900-1000'. AUTOPLT TESTED AND FOUND TO BE WORKING NORMALLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: TWR DID NOT QUESTION THE RPTR AS TO HIS DSCNT BELOW MIN. WHEN HE BROKE OUT, THE ACFT WAS NEAR THE ARPT AND A CIRCLE TO LAND WAS MADE. HAS RECEIVED NO INQUIRY REF THE INCIDENT FROM THE FAA. HAS ELECTRIC ON THE ACFT THAT COULD DISABLE THE ALT HOLD FEATURE, BUT BECAUSE OF THE RAPID TRIM RATE, DOES NOT USE. IN ORDER TO TRIM MANUALLY, IT IS NECESSARY TO DISENGAGE THE AUTOPLT. LENGTHY DISCUSSION ON TCA PROB AND AWARENESS OF MANY PLTS AS FAR AS ASRS.

Data retrieved from NASA's ASRS site as of August 2007 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.