Narrative:

On descent into slc we were told to cross jammn at 17;000 ft MSL. I initiated the VNAV to descend 17;000 ft MSL and the VNAV captured jammn at 17;000 ft MSL. We were then given the delta 3 arrival and to cross drapr at 14;000 ft MSL then descend via the delta 3. The arrival was programmed into the FMS and the VNAV was activated. The descent was started. The procedure on the FMS VNAV is to set the altitude alert [and] capture at the lowest arrival altitude on the arrival and the VNAV will capture each altitude and descend automatically. The co-pilot set the altitude to 11;000 ft MSL as required and we confirmed that all altitudes were entered correctly. We were descending to cross drapr at 14;000 ft MSL and upon reaching 14;000 ft MSL at drapr the VNAV captured and the plane leveled out at 14;000 ft MSL. At this time for some unknown reason the VNAV did not continue to descend to the next fix spiek at 13;000 ft MSL. I noticed that it was not responding as expected so I questioned my co-pilot [and] asked for him to check on it. We both noticed that in the window of the FMS where the [vertical speed] required to [make the next restriction] is shown; that the [vertical speed] required to descent correctly was 500 FPM and that it started to increase rapidly up to 1;000 ft MSL then to 2;000 ft up to as high as 3;000 ft per minute. The co-pilot was trying to put in a descent altitude and I would hit the VNAV capture each time but the [vertical speed] showing what was needed to descend was increasing faster than he could put it in. I decided to manually control the VNAV and selected 500 ft per min descent and started to descend. We both were focused on trying to get the VNAV to work but continuing to program altitudes and then selecting the VNAV button to try to get it to work. During one of the times I was pushing the VNAV button; the VNAV captured and started a rapid descent at 2;500 ft per minute. I didn't realize that it had captured at first and by the time I did; we had descended through the spiek crossing altitude of 13;000 ft MSL. It had captured the restriction at drapr of 14;000 ft MSL because we had pre selected 14;000 ft MSL in the preselect altitude window. It did not capture the VNAV altitude at 13;000 ft MSL at spiek for unknown reasons but it would have stopped at 11;000 ft MSL because that is what we had selected in the preselect altitude window. We had descended down through 12;000 ft MSL when I noticed the error and just about the same time as the controller also noticed. He gave us an altitude alert and instructions to climb back to 13;000 ft MSL. I turned off the autopilot and I immediately started a climb back to 13;000 ft MSL and leveled out. The conditions at the time were VFR and weather was very clear. My attention was both inside and outside of the cockpit and I knew where we were. At no time did we get a TCAS alert; terrain alert; or radar altitude alert. There was no evasive action needed either; however I did initiate an immediate climb to 13;000 ft MSL. After leveling at 13;000 ft MSL I assume during multiple attempts to push and arm the VNAV that I inadvertently selected heading mode during one of those many attempts. I thought that I had activated the autopilot but must have inadvertently pushed the heading button instead of navigation. Hitting the heading button instead of navigation caused me to drift off course a little [and] I was asked by the controller to fly a heading [and] intercept the arrival. I intercepted the arrival [and] continued on the approach and landed without further problems. We have flown this arrival many times and have never had any problem with VNAV not capturing the profile.other factors I feel I need to comment on are that we had just completed a major inspection at our maintenance facility and this was our first flight after this major maintenance inspection. One of the squawks for maintenance was that we were having problems with our GPS approaches where when crossing the initial approach fix all of our waypoints would disappear. During one of the FMS test [and] diagnostic sessions I was able to sit in the cockpit with the avionics technician to duplicate the problem. We were able to duplicate the problem but not able to determine what the problem was. The avionics technician was reprogramming and configuring the FMS and the vertical navigation VNAV system trying to see if the system was set up correctly. Being that he was in the configuration part of the FMS; VNAV part; I don't know if something was accidentally changed or that he forgot to change it back out of some configure mode he had input. That being said he was not sure what was causing the problem. He has contacted the avionics rep technician to help fix the problem. They have told him the FMS [and] VNAV are capable of doing the GPS approaches coupled with VNAV without dropping the waypoints. Both our avionics technician [and] avionics rep technician are continuing to troubleshoot the problem and fix it. I will say that the waypoints will not drop off if I program in each waypoint individually.because of this event we have changed company policy to set each altitude on the approach to the waypoints individually instead of setting it to the lowest step down altitude. This will give us more control over capturing the altitude. We will further monitor the VNAV on all upcoming flights to determine if this is a problem with the VNAV; configuration of the FMS/VNAV. I have always taken command of the aircraft when there is an issue like this in the cockpit and flown the aircraft first as the main priority and have always given my co-pilot the job of figuring out the problem. For some reason; perhaps because it was such a clear beautiful day and we were repositioning the airplane without any passengers I let myself become involved trying to fix the problem. I will also keep myself focused on flying the airplane and not try [and] troubleshoot a problem at the same time. My co-pilot is very capable of evaluating [and] fixing the problem and I should have not tried to help visually just verbally. In my 42 years of flying I have never deviated an altitude. I take my job flying very seriously and I read and study these NASA reports and other articles to try and make my flying the safest as I can possibly do; still I see more ways in which I can improve.

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Original NASA ASRS Text

Title: IAI1125 flight crew experiences an altitude and track deviation while attempting to comply with the DELTA 3 RNAV arrival to SLC. Software issues and pilot error may have been factors in the deviations.

Narrative: On descent into SLC we were told to cross JAMMN at 17;000 FT MSL. I initiated the VNAV to descend 17;000 FT MSL and the VNAV captured JAMMN at 17;000 FT MSL. We were then given the DELTA 3 arrival and to cross DRAPR at 14;000 FT MSL then descend via the DELTA 3. The arrival was programmed into the FMS and the VNAV was activated. The descent was started. The procedure on the FMS VNAV is to set the Altitude Alert [and] capture at the lowest arrival altitude on the arrival and the VNAV will capture each altitude and descend automatically. The co-pilot set the altitude to 11;000 FT MSL as required and we confirmed that all altitudes were entered correctly. We were descending to cross DRAPR at 14;000 FT MSL and upon reaching 14;000 FT MSL at DRAPR the VNAV captured and the plane leveled out at 14;000 FT MSL. At this time for some unknown reason the VNAV did not continue to descend to the next fix SPIEK at 13;000 FT MSL. I noticed that it was not responding as expected so I questioned my co-pilot [and] asked for him to check on it. We both noticed that in the window of the FMS where the [vertical speed] required to [make the next restriction] is shown; that the [vertical speed] required to descent correctly was 500 FPM and that it started to increase rapidly up to 1;000 FT MSL then to 2;000 FT up to as high as 3;000 FT per minute. The co-pilot was trying to put in a descent altitude and I would hit the VNAV capture each time but the [vertical speed] showing what was needed to descend was increasing faster than he could put it in. I decided to manually control the VNAV and selected 500 FT per min descent and started to descend. We both were focused on trying to get the VNAV to work but continuing to program altitudes and then selecting the VNAV button to try to get it to work. During one of the times I was pushing the VNAV button; the VNAV captured and started a rapid descent at 2;500 FT per minute. I didn't realize that it had captured at first and by the time I did; we had descended through the SPIEK crossing altitude of 13;000 FT MSL. It had captured the restriction at DRAPR of 14;000 FT MSL because we had pre selected 14;000 FT MSL in the preselect altitude window. It did not capture the VNAV altitude at 13;000 FT MSL at SPIEK for unknown reasons but it would have stopped at 11;000 FT MSL because that is what we had selected in the preselect altitude window. We had descended down through 12;000 FT MSL when I noticed the error and just about the same time as the Controller also noticed. He gave us an altitude alert and instructions to climb back to 13;000 FT MSL. I turned off the autopilot and I immediately started a climb back to 13;000 FT MSL and leveled out. The conditions at the time were VFR and weather was very clear. My attention was both inside and outside of the cockpit and I knew where we were. At no time did we get a TCAS alert; Terrain alert; or Radar Altitude alert. There was no evasive action needed either; however I did initiate an immediate climb to 13;000 FT MSL. After leveling at 13;000 FT MSL I assume during multiple attempts to push and arm the VNAV that I inadvertently selected HDG mode during one of those many attempts. I thought that I had activated the autopilot but must have inadvertently pushed the heading button instead of NAV. Hitting the heading button instead of NAV caused me to drift off course a little [and] I was asked by the Controller to fly a heading [and] intercept the arrival. I intercepted the arrival [and] continued on the approach and landed without further problems. We have flown this arrival many times and have never had any problem with VNAV not capturing the profile.Other factors I feel I need to comment on are that we had just completed a major inspection at our maintenance facility and this was our first flight after this major maintenance inspection. One of the squawks for Maintenance was that we were having problems with our GPS approaches where when crossing the initial approach fix all of our waypoints would disappear. During one of the FMS test [and] diagnostic sessions I was able to sit in the cockpit with the Avionics Technician to duplicate the problem. We were able to duplicate the problem but not able to determine what the problem was. The Avionics Technician was reprogramming and configuring the FMS and the vertical navigation VNAV system trying to see if the system was set up correctly. Being that he was in the configuration part of the FMS; VNAV part; I don't know if something was accidentally changed or that he forgot to change it back out of some configure mode he had input. That being said he was not sure what was causing the problem. He has contacted the Avionics Rep Technician to help fix the problem. They have told him the FMS [and] VNAV are capable of doing the GPS approaches coupled with VNAV without dropping the waypoints. Both our Avionics Technician [and] Avionics Rep Technician are continuing to troubleshoot the problem and fix it. I will say that the waypoints will not drop off if I program in each waypoint individually.Because of this event we have changed company policy to set each altitude on the approach to the waypoints individually instead of setting it to the lowest step down altitude. This will give us more control over capturing the altitude. We will further monitor the VNAV on all upcoming flights to determine if this is a problem with the VNAV; configuration of the FMS/VNAV. I have always taken command of the aircraft when there is an issue like this in the cockpit and flown the aircraft first as the main priority and have always given my co-pilot the job of figuring out the problem. For some reason; perhaps because it was such a clear beautiful day and we were repositioning the airplane without any passengers I let myself become involved trying to fix the problem. I will also keep myself focused on flying the airplane and not try [and] troubleshoot a problem at the same time. My co-pilot is very capable of evaluating [and] fixing the problem and I should have not tried to help visually just verbally. In my 42 years of flying I have never deviated an altitude. I take my job flying very seriously and I read and study these NASA reports and other articles to try and make my flying the safest as I can possibly do; still I see more ways in which I can improve.

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.