A HS125 captain reported difficulty with the mach trim system that led to an altitude deviation and ...

Attributes
ACN	971713
Time
Date	201109
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Make Model Name	HS 125 Series
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	Mach Trim
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Flight Instructor Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 60 Flight Crew Total 15000 Flight Crew Type 3000
Events
Anomaly	Aircraft Equipment Problem Critical Conflict Airborne Conflict Deviation - Altitude Excursion From Assigned Altitude
Miss Distance	Horizontal 2000 Vertical 600

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.

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.

A HS125 captain reported difficulty with the mach trim system that led to an altitude deviation and TCAS RA.