Date: 201812
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: Airliner 99
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Training
Flight phase: Taxi
Reporter organization: Personal
Reporter function: First Officer
Reporter qualification: Flight Crew Commercial; Flight Crew Instrument; Flight Crew Multiengine
Human factors: Situational Awareness; Training / Qualification; Time Pressure
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Other / Unknown; Ground Event / Encounter Loss Of Aircraft Control; Ground Excursion Taxiway
Detector: Person Flight Crew
When detected: Taxi
Result: Flight Crew Regained Aircraft Control
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Human Factors

Narrative: A loss of control during taxi occurred causing the aircraft to leave the taxiway and come to rest on an embankment to the right of taxiway B. The flight was a post maintenance and familiarization flight.After returning from lunch at a local restaurant; we preflighted the aircraft and filed and instrument flight plan in preparation for returning to our base of operations. The Captain; myself and another First Officer (FO) boarded the aircraft and completed cockpit procedures and started the aircraft. I was in the right seat as FO. We received our taxi clearance from the tower and followed the lineman's direction as we taxied from the ramp to taxiway B. The Captain set the power for taxi and began to taxi the aircraft southwest bound on taxiway B. The Captain asked me to take over the movement of the aircraft during the taxi phase; approximately adjacent taxiway B4 on taxiway B. While I was involved in the taxi; the Captain was 'head down' in the aircraft completing other duties. Upon receiving positive control of the aircraft I perceived that the aircraft was moving at a rate of speed that was faster than other aircraft I have operated. I considered the speed to be fast but normal for this aircraft due to my level of familiarity with the aircraft. However; I was applying intermittent braking to manage the speed. As we approached a southerly turn in taxiway B; I brought the power back to the reverse region and to slow the aircraft down to negotiate the left turn. The aircraft did not slow down so I and the Captain applied more braking pressure to retard the speed. The aircraft did not turn left and follow the taxiway centerline but continued in a southwesterly direction and left taxiway B. As the aircraft continued through the grass off taxiway B it began a left turn and slid down the embankment. The aircraft came to rest parallel to the southerly heading portion of taxiway B when the right main gear became mired in the soft ground at the bottom of the embankment. The Captain immediately brought the condition levers the 'Idle Cutoff' position and once the aircraft was completely powered down; called our base to explain the situation and get direction on how to proceed. Our base advised us to leave the airplane as it sat and began to coordinate with ground crews from the airport authority to make arrangements for a local company with which they were familiar; to move the aircraft back to the taxiway. That operation was successful and the following day another crew was dispatched to fly the aircraft back to our base.   I think the problem arose from my lack of familiarity with the aircraft's handling characteristics; my lack of experience in operating an aircraft with a 'beta' range and a reversible pitch propeller. I also feel that the Captain should have paid more attention to my movements and actions due to my inexperience. Additionally; during a prop governor test earlier in the day; I noticed that full braking by the Captain and the other FO; would barely hold the aircraft steady during the test while testing only one engine. During the test; the aircraft yawed the right moving the nose approximately two feet. I can't be certain that a prop governor issue or an asymmetric thrust issue contributed to the incident but I suppose those could be possibilities. Later in the evening both I and the additional FO; who flew the leg from our base to ZZZ; discussed checklist usage. In the course of that discussion we discovered that we both felt that checklist usage was being rushed and that we felt behind the aircraft due to how fast the Captain wanted the checklists completed. He indicated to me that he also thought the aircraft taxied at an elevated rate of speed. The next day I indicated to the Captain that on the next flight I would like to slow the checklist process down and build some familiarity. I have had a discussion with our operations department and have given feedback as to how I would to proceed with training in order to prevent another incident like this one. I have also re-examined my personal minimums in order to direct how I should operate unfamiliar aircraft and interact with other flight officers when I feel their actions may put flight safety in jeopardy.




Synopsis: Beechcraft 99 First Officer reported a loss of control and taxiway excursion during taxi.