Narrative:

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]

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Original NASA ASRS Text

Title: 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.

Narrative: 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]

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.