Narrative:

When told by ATC to cross [the] VOR at and maintain FL240 I programmed the VNAV to descend. The FD began the descent. The PNF called out the descent checklist starting with 'circuit breakers'. I turned to check the breakers to my right and behind the copilot seat utilizing the light wand behind the copilot seat. During this time the autopilot vertical control system failed and the aircraft started a climb. There was no aural warning and the autopilot remained engaged. At this same time; I became aware of the problem and ATC asked for our altitude which the sic responded. Additionally; we received a TCAS RA. The 'mach trim fail' light was illuminated on the master annunciator panel along with the red 'trim' light on the pfd. These lights remained illuminated for the rest of the flight; approximately 30 minutes. The autopilot had to be disengaged two more times to prevent other incursions; including on the ILS approach. The system on this aircraft is the collins Proline21. The aircraft just came out of an inspection which included the area where the affected components are housed in the aft equipment area; (hell hole). The aircraft was easily manageable but required extreme vigilance after the event. Our maintenance person was on board at the time and is presently investigating the cause.

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Original NASA ASRS Text

Title: A HS125 Captain reported difficulty with the Mach trim system that led to an altitude deviation and TCAS RA.

Narrative: When told by ATC to cross [the] VOR at and maintain FL240 I programmed the VNAV to descend. The FD began the descent. The PNF called out the descent checklist starting with 'Circuit Breakers'. I turned to check the breakers to my right and behind the copilot seat utilizing the light wand behind the copilot seat. During this time the autopilot vertical control system failed and the aircraft started a climb. There was no aural warning and the autopilot remained engaged. At this same time; I became aware of the problem and ATC asked for our altitude which the SIC responded. Additionally; we received a TCAS RA. The 'MACH TRIM FAIL' light was illuminated on the master annunciator panel along with the red 'TRIM' light on the PFD. These lights remained illuminated for the rest of the flight; approximately 30 minutes. The autopilot had to be disengaged two more times to prevent other incursions; including on the ILS approach. The system on this aircraft is the Collins Proline21. The aircraft just came out of an inspection which included the area where the affected components are housed in the aft equipment area; (hell hole). The aircraft was easily manageable but required extreme vigilance after the event. Our maintenance person was on board at the time and is presently investigating the cause.

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.