Date: 201701
Local time of day: 0601-1200
Location: BTP.Airport, PA
Weather elements / visibility: 6
Light: Daylight
Aircraft: Citation V/Ultra/Encore (C560)
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
Reporter organization: Corporate
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Communication Breakdown; Confusion; Situational Awareness
Anomaly: Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown; Ground Excursion Taxiway
Detector: Person Flight Crew
When detected: Taxi
Result: Flight Crew Returned To Gate; General Flight Cancelled / Delayed
Contributing factors: Company Policy; Human Factors
Primary problem: Human Factors

Narrative: The First Officer (FO) Pilot not Flying (PNF) (an experienced Citation contract pilot) and I Pilot Flying (PF) departed the ramp part 91 after completing the approved after start checklist. The thrust reverser checks were completed per company SOP with the PF remaining head outside and the PNF calling for '(L/R) paddle up; 3 lights; emer. stow; arm; paddle down; no changes; complete.' This completed the final after start checks.As we were taxing on the parallel to runway 26 we needed to add a fix to the FMS. I elected to add the fix as I am more familiar with the FMS and transferred aircraft control to FO. I stated 'Your controls' and the FO responded 'my controls.' I asked FO to repeat the spelling of the fix and as I entered the fix I stated 'I got it'. At this time FO believed I was again taxiing and looked down to confirm my entry. The wind was from the right at approximately 90 degrees and 15 knots and yawed the aircraft enough to cause the right main to drop slightly off the taxiway. The edge was wet and the right aircraft main settled in the mud. Taxi speed was a walking pace. We shut down and coordinated with Airport Operations for Passenger transportation back to the FBO.I believe this incident could have been avoided completely by electing to stop and set parking brake before attending to any cockpit chores after the after start check is completed. In the event that it is not possible to stop in a taxi line of aircraft it should have been made clear that that the proper command for aircraft transfer should have been 'My controls;' 'Your controls' as well as always announcing and verifying when one crew member's eyes are not outside the aircraft during ground movement. My use of a statement of 'I got it' is also vague and should not have been used.Our after start checklist is designed to have all checks except thrust reversers completed before taxi for the sole purpose of freeing both pilots attention during ground movement. However; routing changes or taxi chart verifications sometimes demand one pilot be heads down. We will amend our SOP's to include concise verbiage-and distribute to all pilots- concerning these issues.




Synopsis: A CE-560 crew was taxiing while the Captain went heads down to enter a fix and passed off the controls to the First Officer Once he found the fix the Captain said 'I got it.' The First Officer thought this meant he was taking control of the airplane back and then they rolled off the right side of the taxiway because no one was in control.