Narrative:

I was on an IFR flight plan at 7000' from N88 to B06 which is an unattended, no instrument approach grass strip. At albany I was informed that the glens falls VOR was unusable below 11000', although I received it well including DME. The burlington VOR, by NOTAMS, was out of service which is the closest RNAV VOR to B06 but the alternate VOR used for rnaving to B06, montpelier, was working. They started vectoring me up V489 on 225 degree heading to krown as filed but I could receive B06 on the RNAV and asked to go direct. This was 40 to 60 mi out. I could see the ground but not straight ahead and I wanted to get down since there is no instrument approach at B06 so I asked boston center for lower and direct. Center told me to descend to 6000' and when reaching 6000' go direct B06. As I approached 6000', I pushed in the altitude hold and concentrated on lining up the CDI for an RNAV course direct B06 and at the same time I reached across the panel to put the RNAV on RNAV approach. As I rechked the altimeter it showed me to be around 5600' (400') below assigned altitude. I started to regain the lost altitude when boston center told me to maintain 6000'. As I was going back up center asked me to verify my altitude which was exactly 5700' and I so reported as I returned to 6000'. I use an ap/FD which holds heading and altitude very precisely however, there are several buttons on the yoke (including autoplt disconnect and the trim button) which if even slightly grazed will disconnect the autoplt inadvertently. This is apparently what happened and the plane continued to descend so gradually that I was unaware as I was working on the new RNAV course.

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Original NASA ASRS Text

Title: ALT DEVIATION BY PLT QUALIFIED FOR IFR WITH COM AND INSTRUMENT RATINGS.

Narrative: I WAS ON AN IFR FLT PLAN AT 7000' FROM N88 TO B06 WHICH IS AN UNATTENDED, NO INSTRUMENT APCH GRASS STRIP. AT ALBANY I WAS INFORMED THAT THE GLENS FALLS VOR WAS UNUSABLE BELOW 11000', ALTHOUGH I RECEIVED IT WELL INCLUDING DME. THE BURLINGTON VOR, BY NOTAMS, WAS OUT OF SVC WHICH IS THE CLOSEST RNAV VOR TO B06 BUT THE ALTERNATE VOR USED FOR RNAVING TO B06, MONTPELIER, WAS WORKING. THEY STARTED VECTORING ME UP V489 ON 225 DEG HDG TO KROWN AS FILED BUT I COULD RECEIVE B06 ON THE RNAV AND ASKED TO GO DIRECT. THIS WAS 40 TO 60 MI OUT. I COULD SEE THE GND BUT NOT STRAIGHT AHEAD AND I WANTED TO GET DOWN SINCE THERE IS NO INSTRUMENT APCH AT B06 SO I ASKED BOSTON CTR FOR LOWER AND DIRECT. CTR TOLD ME TO DSND TO 6000' AND WHEN REACHING 6000' GO DIRECT B06. AS I APCHED 6000', I PUSHED IN THE ALT HOLD AND CONCENTRATED ON LINING UP THE CDI FOR AN RNAV COURSE DIRECT B06 AND AT THE SAME TIME I REACHED ACROSS THE PANEL TO PUT THE RNAV ON RNAV APCH. AS I RECHKED THE ALTIMETER IT SHOWED ME TO BE AROUND 5600' (400') BELOW ASSIGNED ALT. I STARTED TO REGAIN THE LOST ALT WHEN BOSTON CTR TOLD ME TO MAINTAIN 6000'. AS I WAS GOING BACK UP CTR ASKED ME TO VERIFY MY ALT WHICH WAS EXACTLY 5700' AND I SO RPTED AS I RETURNED TO 6000'. I USE AN AP/FD WHICH HOLDS HDG AND ALT VERY PRECISELY HOWEVER, THERE ARE SEVERAL BUTTONS ON THE YOKE (INCLUDING AUTOPLT DISCONNECT AND THE TRIM BUTTON) WHICH IF EVEN SLIGHTLY GRAZED WILL DISCONNECT THE AUTOPLT INADVERTENTLY. THIS IS APPARENTLY WHAT HAPPENED AND THE PLANE CONTINUED TO DSND SO GRADUALLY THAT I WAS UNAWARE AS I WAS WORKING ON THE NEW RNAV COURSE.

Data retrieved from NASA's ASRS site as of July 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.