Flight Crew Commercial; Flight Crew Flight Instructor; Flight Crew Instrument
Flight Crew Last 90 Days 25; Flight Crew Total 2360; Flight Crew Type 300
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew FLC Overrode Automation; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft; MEL; Procedure; Chart Or Publication
We have an old PFD/MFD system made by a brand named Chelton. We have had numerous minor failures or malfunctions with this system; mainly related to GPS navigation. Some of the other pilots at my base along with myself have brought this system up as a safety concern in the past to others in our company. Repairs have been attempted at least once; but these problems with the Cheltons persist. However; this flight was the first time I had seen the Attitude Indicator fail on the system.There is no analog standby Attitude Indicator in our aircraft. Rather; the standby Attitude Indicator is displayed on the MFD screen directly below the PFD. This standby AI (Attitude Indicator) is displayed on the MFD by simply pressing a knob on the lower right-hand corner of that display. This standby Attitude Indicator was still functioning normally when the Primary one failed.For part of our patient-loaded leg from ZZZ to ZZZ1; the AI on the PFD gave erroneous readings; showing the aircraft in approximately a 35 degree bank to the right when the aircraft was actually in straight and level flight; and giving an audible; 'Attitude failure; attitude failure' warning. On our return leg after dropping off the patient; the PFD continuously and erratically cycled through 3 or 4 different displays; including: a correct indication of our attitude and upcoming terrain; the same right-hand bank attitude; a red 'X' in the middle of the screen; and a display where the half of the screen above the horizon was brown instead of blue. Each display would show for 2 or 3 seconds; then go to a different display. This continued for the entire flight back. I did not find a way to stop the display from doing this. Along with the visual abnormality; the audible female voice saying; 'Attitude failure; attitude failure' repeated continuously the whole flight back.I did not put the item on MEL until the following day after ferrying the aircraft to ZZZ2 to be repaired. This was to prevent the aircraft from being grounded in ZZZ; away from ZZZ2 where repairs could be more readily carried out. Honestly this was partly because of knowing that the company would probably not like for me to have written it up then (I did inform the Chief Pilot of the situation upon return to ZZZ; however). It was also partly because; while I pretty much knew this item needed to be written up; I had misinterpreted the MEL when I had read it that night; not reading it carefully enough.
Pilot reported misinterpreting MEL and applied the incorrect MEL after failures of the Attitude Indicator on the Primary Flight Display.