Narrative:

Flight was en route from billings, mt, to spokane, wa. WX forecast from billings for the route was lowest 6000 ft scattered with 5 mi in light rain and fog and no airmets. At missoula I checked WX in flight with great falls FSS. New WX airmet for our route partial mountain obscuration and lowest ceiling and visibility reported and forecast was 6000 ft broken and 5 mi due to light rain. The WX deteriorated close to mullan pass but I decided I had ample WX to fly over the pass. 5 mi west of mullan, identification, the clouds had obscured the valley to a point that I felt it was unsafe to proceed wbound to spokane. I turned around to head back toward missoula to land only to find that mullan pass was now completely obscured by fog. I was trapped in the valley and circled mullan for about 20 mins and decided to land on the freeway because the ceiling and visibility was lowering. I landed on interstate 90 without damage or harm to the plane, road, vehicles or passenger. Hindsight, yes I should have turned around before mullan pass and returned to missoula to wait out the WX. Now, I see that that was the most reasonable choice for safety especially in unfamiliar terrain. The reason for continuing was the push to complete the trip. I guess my judgement was impaired due to exhaustion from the length of the trip (since it had originated from enid, ok). I had 2 fuel stops in route from enid and had about 6 hours flying time to the point of landing on the freeway.

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Original NASA ASRS Text

Title: ACFT WITH NO FLT PLAN ENTERED MOUNTAIN PASS, WX DETERIORATED, BECAME BOXED IN AND LANDED ON INTERSTATE HWY.

Narrative: FLT WAS ENRTE FROM BILLINGS, MT, TO SPOKANE, WA. WX FORECAST FROM BILLINGS FOR THE RTE WAS LOWEST 6000 FT SCATTERED WITH 5 MI IN LIGHT RAIN AND FOG AND NO AIRMETS. AT MISSOULA I CHKED WX IN FLT WITH GREAT FALLS FSS. NEW WX AIRMET FOR OUR RTE PARTIAL MOUNTAIN OBSCURATION AND LOWEST CEILING AND VISIBILITY RPTED AND FORECAST WAS 6000 FT BROKEN AND 5 MI DUE TO LIGHT RAIN. THE WX DETERIORATED CLOSE TO MULLAN PASS BUT I DECIDED I HAD AMPLE WX TO FLY OVER THE PASS. 5 MI W OF MULLAN, ID, THE CLOUDS HAD OBSCURED THE VALLEY TO A POINT THAT I FELT IT WAS UNSAFE TO PROCEED WBOUND TO SPOKANE. I TURNED AROUND TO HEAD BACK TOWARD MISSOULA TO LAND ONLY TO FIND THAT MULLAN PASS WAS NOW COMPLETELY OBSCURED BY FOG. I WAS TRAPPED IN THE VALLEY AND CIRCLED MULLAN FOR ABOUT 20 MINS AND DECIDED TO LAND ON THE FREEWAY BECAUSE THE CEILING AND VISIBILITY WAS LOWERING. I LANDED ON INTERSTATE 90 WITHOUT DAMAGE OR HARM TO THE PLANE, ROAD, VEHICLES OR PAX. HINDSIGHT, YES I SHOULD HAVE TURNED AROUND BEFORE MULLAN PASS AND RETURNED TO MISSOULA TO WAIT OUT THE WX. NOW, I SEE THAT THAT WAS THE MOST REASONABLE CHOICE FOR SAFETY ESPECIALLY IN UNFAMILIAR TERRAIN. THE REASON FOR CONTINUING WAS THE PUSH TO COMPLETE THE TRIP. I GUESS MY JUDGEMENT WAS IMPAIRED DUE TO EXHAUSTION FROM THE LENGTH OF THE TRIP (SINCE IT HAD ORIGINATED FROM ENID, OK). I HAD 2 FUEL STOPS IN RTE FROM ENID AND HAD ABOUT 6 HRS FLYING TIME TO THE POINT OF LNDG ON THE FREEWAY.

Data retrieved from NASA's ASRS site as of July 2007 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.