Light Transport
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 25; Flight Crew Total 13600; Flight Crew Type 1650
Ground Excursion Taxiway
Flight Crew Became Reoriented
Human Factors
After landing at Frederick Airport (KFDK) during darkness; I exited runway 23 and started taxiing north on Taxiway 'A'. I asked the co-pilot; to help give me taxi guidance; since the Tower was not yet in operation. She told me to taxi left onto 'G' taxiway. That led us incorrectly; to the AOPA Gangar. Unable to go further; I asked her about the taxiway on her side; the right side of the aircraft. It was too dark to see where it led and she said 'it looks like it ends and goes into a grass area.' I looked left and saw a taxiway that I assumed would take us back to 'A' and then the approach end of Runway 5. These taxiways ½ way down 'G' taxiway are unmarked and unlit at any time. The taxiway did not go very far left; so I followed it around to the right making a 90 degree right turn. As I turned; we were facing a set of 'T' hangars. I stopped and asked again; what she saw; which was hard for her to see. It appeared to me that we were about to cross a small section of turf and I could see the AOPA concrete and taxiway just ahead. I slowly taxied and the aircraft slid down a grass embankment that neither of us had seen; because of the cockpit being so high. All the landing lights didn't illuminate a thing. I attempted forward and reverse thrust to free us. But to no avail.My recommendations are:1.Both of us should have watched our iPads and noticed unmarked taxiways ahead. 2.Taxiways should all be illuminated.3.On 'G' taxiway facing AOPA I should have shut down and called for a tow.4.The irrigation depression that the aircraft slid down should be marked on the taxi chart.5.Not flying with only 6 hours of sleep.6.Having copilots be type rated as PIC in [the aircraft being flown]
Captain reported getting lost on unlighted taxiways after landing before dawn with the Tower closed at FDK. A taxiway excursion occurs and the aircraft becomes stuck in soft soil.