An SA227 pilot suffered an altitude excursion when the autopilot failed to maintain his assigned alt...

Narrative:

In cruise with the autopilot set to follow a direct course to bqn and maintain my assigned altitude of FL180 the autopilot malfunctioned and entered a ~100fpm descent resulting in an altitude deviation. On the previous 2 legs; as well as the previous hour of flight on the current leg; the autopilot had been observed to be maintaining altitude and track without any anomalies noted. As it was a long flight I had pulled out a book to read during cruise but maintained a watch for traffic as well as observance of engine instruments and verifying the autopilot was maintaining course. ATC called and queried me as to my altitude; looking at the altimeter I realized that the aircraft was no longer at FL180 but now at FL165. I responded with 'correcting' and the controller simply stated; 'actually; maintain FL160 now'. Upon reflection I realized that; while I was maintaining vigilance for traffic and the overall condition of the aircraft; I had failed to include the altimeter in my scan for approximately 10-15 minutes. The very slow descent that the autopilot had entered; and the dusk condition masking the horizon had failed to alert me to the descent as there was very little airspeed increase and no other subtle clues. This combined with the inherent fatigue of a 13 hour duty day and 5.5 hours of single pilot international flying in a very busy aircraft; as well as the natural boredom that comes with flying 2-3 hour flights over water alone; led to my distraction and subsequent failure to realize the deviation. I fully understand the gravity of the situation. Had there been traffic or any other number of things the results could have been bad. Going forward I have realized the need to eliminate distractions in the cockpit. I had only recently started to read in the cockpit in an effort to help keep my brain active so as to avoid being overly drowsy or tired during the long cruise; something that naturally occurs with such long overwater flights with little external stimuli.

Original NASA ASRS Text

Title: An SA227 Pilot suffered an altitude excursion when the autopilot failed to maintain his assigned altitude. The Pilot's immersion in a book prevented earlier detection of the autopilot's failure to do his job.

Narrative: In cruise with the autopilot set to follow a direct course to BQN and maintain my assigned altitude of FL180 the autopilot malfunctioned and entered a ~100fpm descent resulting in an altitude deviation. On the previous 2 legs; as well as the previous hour of flight on the current leg; the autopilot had been observed to be maintaining altitude and track without any anomalies noted. As it was a long flight I had pulled out a book to read during cruise but maintained a watch for traffic as well as observance of engine instruments and verifying the autopilot was maintaining course. ATC called and queried me as to my altitude; looking at the altimeter I realized that the aircraft was no longer at FL180 but now at FL165. I responded with 'correcting' and the controller simply stated; 'actually; maintain FL160 now'. Upon reflection I realized that; while I was maintaining vigilance for traffic and the overall condition of the aircraft; I had failed to include the altimeter in my scan for approximately 10-15 minutes. The very slow descent that the autopilot had entered; and the dusk condition masking the horizon had failed to alert me to the descent as there was very little airspeed increase and no other subtle clues. This combined with the inherent fatigue of a 13 hour duty day and 5.5 hours of single pilot international flying in a very busy aircraft; as well as the natural boredom that comes with flying 2-3 hour flights over water alone; led to my distraction and subsequent failure to realize the deviation. I fully understand the gravity of the situation. Had there been traffic or any other number of things the results could have been bad. Going forward I have realized the need to eliminate distractions in the cockpit. I had only recently started to read in the cockpit in an effort to help keep my brain active so as to avoid being overly drowsy or tired during the long cruise; something that naturally occurs with such long overwater flights with little external stimuli.

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	976008
Time
Date	201110
Local Time Of Day	1801-2400
Place
Locale Reference	ZSU.ARTCC
State Reference	PR
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Make Model Name	SA-227 AC Metro III
Operating Under FAR Part	Part 135
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	Altitude Hold/Capture
Person 1
Function	Single Pilot
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Flight Instructor Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 600 Flight Crew Total 3000 Flight Crew Type 600
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

An SA227 pilot suffered an altitude excursion when the autopilot failed to maintain his assigned altitude. The pilot's immersion in a book prevented earlier detection of the autopilot's failure to do his job.