Date: 201811
Local time of day: 1801-2400
Location: SMF.Airport, CA
Flight conditions: VMC
Light: Night
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Taxi
ATC / advisory: Ground SMF
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Situational Awareness; Distraction; Confusion
Anomaly: Deviation / Discrepancy - Procedural Clearance; Ground Excursion Taxiway; Ground Incursion Taxiway
Detector: Person Flight Crew; Person Air Traffic Control
When detected: Taxi
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Gate; General Flight Cancelled / Delayed
Contributing factors: Human Factors; Airport; Environment - Non Weather Related; ATC Equipment / Nav Facility / Buildings
Primary problem: Airport

Narrative: While taxiing from the ramp to Runway 34L; I accidentally passed Taxiway A13; the turn off to the runway. Taxiway A turns into Taxiway G1; but it's still a straight taxiway. We'd already switched and talked to Tower at that time. We realized what had happened about the same time Tower called us. He said we were on G1; that it may not be strong enough for our weight and that we would need to do a 180. The First Officer asked if that could be accomplished on the General Aviation ramp; which he said we could. During the turn toward the General Aviation ramp; our left main gear departed the taxiway briefly; but at the time; we didn't know. After turning around on that ramp; we continued up Taxiway G2 to the regular taxiway; and followed instructions to Taxiway A13. We were cleared for takeoff; but due to a fuel system alert; and probably better decision making; by then; we taxied back to the gate; where maintenance personnel inspected the gear; and worked through the alert. Unfortunately; this was human error. I was looking straight down the taxiway; which continues past the turnoff to the runway. It's pretty dark on that taxiway; and the only sign for Taxiway A13 is on the left; which is opposite the turn direction; and is one of 3 small signs all together; and I accidentally missed it. My first thought; once we realized where we were was to stop and get towed; but it was about then that the controller spoke to us and said we needed to do the 180. To prevent another event like this; I would suggest a couple things. First; for me personally; is to obviously pay even closer attention; both to the chart and taxiways. That sign for the A13 turnoff is there; although it's not very prominent. Like the First Officer and I discussed; we have to be 100% on; 100% of the time. I know we're all human and that's not realistic; but that's the aviation industry. I have to absolutely be certain at all times where I am. Having said that; I would also suggest some improvement to that area in the way of better signs. I feel there should be something pretty distinct at that intersection indicating 'You're at the end of the runway.' Hindsight being 20/20; I would have told the controller that we were going to stop; even though we were told otherwise.




Synopsis: Air Carrier Captain reported experiencing a taxiway incursion at SMF airport. Then while turning the aircraft around; the left main gear went off the taxiway.