19950131005069I

Narrative: as daytime duty pilot for lifeflite air ambulance in window rock, AZ, I received a call from our dispatch office at approximately 1450 on Tuesday, 31 January, 1995 informing me of a Chinle flight. I was asked by dispatch how I felt about going into the dirt runway at Chinle. I responded that I didn't think I should go because of the recent melting snow and possible wet or muddy runway at Chinle. Dispatch relayed to me that the Chinle police had reported to them that the runway was dry. Knowing that the weather was cavu and with this added runway information, I elected to accept the flight. Prior to leaving the crew quarters for the aircraft, I received a telephone call from the president of lifeflite advising me to land on center line to the north and take off to the south. Preflight, aircraft-runup, take-off and enroute was normal. I departed window rock at 1515 with an estimated land time in Chinle of 1545. This information was relayed to my dispatch office. The flight was conducted under FAR part 91. Final approach was on-speed (105KTS) touchdown was at approximately 85 to 90 knots and on the runway center line. On touch-down, I noticed the runway felt softer and spongier than I had anticipated but everything else appeared normal. Approximately 400 to 500 feet after touch-down, I saw a small dip across the runway that was perpendicular to the center line. The aircraft went through this dip and traded to vault or go slightly airborne upon coming up out of the backside of this dip. When the aircraft settled back down to the runway, airspeed had dropped to a point that the rudder was becoming ineffective. The aircraft started to yaw right, which I corrected back to center line. The aircraft then started to yaw to the left. I was unable to control or stop the left yaw and the aircraft continued left and departed the runway. Nose steering through the rudder pedals was not effective at this point. For one brief moment, I thought about adding take-off power and attempting a go-around. Considering my runway remaining, my low airspeed at this point and the extra drag/resistance I felt on touchdown, I elected not to go-around. I estimate the airspeed to have been 30 to 40 knots when the aircraft departed the runway. The aircraft ended up perpendicular to the runway center line with the nose pointing away from the centerline, off the runway and half way through a barbed wire fence which surrounded the runway. I secured electrical and fuel to the aircraft and ordered the medical crew to exit the aircraft through the main cabin door. There were no injuries to any person in or out of the aircraft. Upon examining the runway condition it was found that the runway did look dry on the surface. There was dry dirt/powder about 1" to 2" deep on the surface. However, unseen and under this dry layer was a soft (not muddy), mushy base. This closes this incident.