FALCON 20 FLC INADVERTENTLY TAXIED OFF RWY INTO MUD AND BECAME STRANDED DURING TAXI OUT. CREW THOUGHT THAT THE SHORT TXWY FROM THE TERMINAL CONTINUED ON THE FAR SIDE OF THE RWY.

Narrative

WE WERE TAXIING N FROM THE RAMP AT PHN; AN UNCTLED FIELD; FOR DEP ON RWY 22. IT WAS NIGHT; THE SKY WAS OVCST; AND VISIBILITY THROUGH THE WINDSCREEN WAS RESTR BY LIGHT RAIN AND MIST. AS WE APCHED RWY 28; I SAW TXWY LIGHTS AHEAD AND BEYOND RWY 28 WHICH APPEARED TO BE THE CONTINUATION OF THE TXWY WE WERE ON. I PROCEEDED ACROSS RWY 28 TO CONTINUE ON THE SAME TXWY TOWARD RWY 22. AS WE GOT TO THE N EDGE OF RWY 28; THE SURFACE BEYOND BECAME ILLUMINATED BY OUR TAXI AND LNDG LIGHTS. IT FIRST APPEARED TO BE CONCRETE. I DID NOT SEE THAT IT WAS DEAD GRASS UNTIL WE HAD TAXIED ONTO IT. THE PLANE'S WHEELS SANK THROUGH THE GRASS INTO THE RAIN SOAKED GND; AND WE WERE UNABLE TO TAXI BACK ONTO THE PAVEMENT. THE TXWY LIGHTS THAT HAD APPEARED TO CONTINUE N OF RWY 28 DIRECTLY IN FRONT OF US WERE ACTUALLY AN ILLUSION CREATED BY THE PARALLEL TXWY LIGHTS FOR RWY 22 AHEAD. THERE WAS NO DAMAGE TO THE AIRPLANE OR TO ARPT PROPERTY. NOBODY WAS HURT; AND SAFETY WAS NOT COMPROMISED. WE QUICKLY HAD RWY 28 CLOSED AND NOTAMED OTS. WE HAD REVIEWED THE ARPT DIAGRAM BEFORE LNDG; BUT IN RETROSPECT; WE SHOULD HAVE CHKED IT AGAIN BEFORE TAXIING OUT.