Narrative:

Our flight with our automatic flight control computer causing problems once we leveled off at 9;000 ft. To explain the gripe it would be that without good warning the autopilot features would stop functioning. Some of those 20 functions include heading control; airspeed control; attitude control; altitude control; and stability augmentation. On our first leg from our base to a military airport for training we noted the problem and disabled the system flying without its assistance. In general system frequently has trouble when it is overheated and the ramp temperature at our departure airfield was 108. We activated our return leg while in the pattern at our training base. After flying and working in their PAR pattern we determined flight control computer might have cooled down enough and re-engaged the system for the return leg to our base. We requested vectors back to the training base and refueled. When we departed the second time it was on a different flight plan using a different victor airway. ATC requested us to climb to 10;000 ft and we were able to comply. Enroute we had the first indications that our automatic flight control system (afcs) (autopilot was degraded) the digital stability augmentation had stopped functioning; and was causing aircraft induced oscillations in the yaw plane. After securing the digital side or (stability augmentation system) sas 2 and using our sas 1 analog stability control the oscillations stopped. A majority of the autopilot features are tied to the sas 2 including altitude control. In hindsight I should have noted this new degradation; reported it to ATC; and watched my gauges more carefully but I continued with the same routine scan pattern. At 10;000 ft we received a call from center to look out for traffic off the nose 20 miles at 11;000 ft headed the opposite direction. We reported searching to center. My copilot noticed we were losing TACAN reception and began switching me to the next NAVAID but before we received reception he gave me a navigation solution and began amending our flight plan which gives a similar HSI display information. In the process my horizontal video display needed to be changed to view the navigation solution. When I diverted my attention to change my hsvd (horizontal situation video display) display I must [have] allowed the aircraft to climb unknowingly because the next thing I heard was a call from ATC say in a very emphatic voice 'why are you climbing.' I saw 10;500 ft on my baro and descended quickly back to 10;000 ft. To further add to ATC's frustration; the radios are adequate but frequently have trouble going out properly. We sound scratchy and there is likely a lot of aircraft noise that gets fed into the channel since we are an unpressurized cockpit. ATC asked why we were climbing several times and each time we responded he either did not hear us or was unable to understand us. After four unsuccessful attempts he directed us to call. I did so immediately on the ramp after landing and tried to explain that we were having afcs problems. Human performance considerations were that I became distracted and my instrument scan broke down. While the afcs degradation did contribute to my workload that would be classified as a precondition combined with my routine habit pattern to trust the afcs system since it does a great job normally. The unsafe act of an altitude deviation was caused by a skill based error of in attention and a decision error to adjust my hsvd when we were searching for traffic.

Related reports:

Google
 

Original NASA ASRS Text

Title: A SH-60F Pilot distracted by a malfunctioning Automatic Flight Guidance Computer climbed 500 FT and was urgently told by ATC to descend for opposite direction traffic.

Narrative: Our flight with our automatic flight control computer causing problems once we leveled off at 9;000 FT. To explain the gripe it would be that without good warning the autopilot features would stop functioning. Some of those 20 functions include heading control; airspeed control; attitude control; altitude control; and stability augmentation. On our first leg from our base to a military airport for training we noted the problem and disabled the system flying without its assistance. In general system frequently has trouble when it is overheated and the ramp temperature at our departure airfield was 108. We activated our return leg while in the pattern at our training base. After flying and working in their PAR pattern we determined flight control computer might have cooled down enough and re-engaged the system for the return leg to our base. We requested vectors back to the training base and refueled. When we departed the second time it was on a different flight plan using a different Victor airway. ATC requested us to climb to 10;000 FT and we were able to comply. Enroute we had the first indications that our Automatic Flight Control System (AFCS) (autopilot was degraded) the digital stability augmentation had stopped functioning; and was causing aircraft induced oscillations in the yaw plane. After securing the digital side or (Stability Augmentation System) SAS 2 and using our SAS 1 analog stability control the oscillations stopped. A majority of the autopilot features are tied to the SAS 2 including altitude control. In hindsight I should have noted this new degradation; reported it to ATC; and watched my gauges more carefully but I continued with the same routine scan pattern. At 10;000 FT we received a call from Center to look out for traffic off the nose 20 miles at 11;000 FT headed the opposite direction. We reported searching to Center. My copilot noticed we were losing TACAN reception and began switching me to the next NAVAID but before we received reception he gave me a NAV solution and began amending our flight plan which gives a similar HSI display information. In the process my Horizontal Video Display needed to be changed to view the NAV solution. When I diverted my attention to change my HSVD (Horizontal Situation Video Display) display I must [have] allowed the aircraft to climb unknowingly because the next thing I heard was a call from ATC say in a very emphatic voice 'why are you climbing.' I saw 10;500 FT on my BARO and descended quickly back to 10;000 FT. To further add to ATC's frustration; the radios are adequate but frequently have trouble going out properly. We sound scratchy and there is likely a lot of aircraft noise that gets fed into the channel since we are an unpressurized cockpit. ATC asked why we were climbing several times and each time we responded he either did not hear us or was unable to understand us. After four unsuccessful attempts he directed us to call. I did so immediately on the ramp after landing and tried to explain that we were having AFCS problems. Human performance considerations were that I became distracted and my instrument scan broke down. While the AFCS degradation did contribute to my workload that would be classified as a precondition combined with my routine habit pattern to trust the AFCS system since it does a great job normally. The unsafe act of an altitude deviation was caused by a skill based error of in attention and a decision error to adjust my HSVD when we were searching for traffic.

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.