I'm planning to fly from Spokane, WA to Flathead Lake, MT (destination is Polson 8S1) next week (mid-May). I'll be flying a stock Cessna 150, so am looking for the "best" route for my low-powered, strictly VFR-only airplane. In my case, "best" is defined as safest: lowest terrain, widest path between peaks, minimizing hazards. I'll only do this crossing if weather looks good, no scud-running for me.

There seem to be two fairly obvious choices:

1. Follow I-90 through Lookout Pass/Mullan Pass. I've flown this route once before (years ago, in an Ercoupe - not exactly a backcountry-optimized plane either, with no rudder pedals but at least it had an 0200 so a slightly better-than-stock engine). I have vivid memories of landing and taking off at Missoula in a screaming crosswind (one of my first landings in that Ercoupe...). This route has some obvious advantages: it's right over an interstate highway, so if something goes very wrong, there's that. There's also a convenient ASOS at Mullan Pass which is reassuring. But that route puts me at Missoula, so I'd have to turn north and follow US 93 to Flathead Lake (so not quite direct).

2. Follow the route just north of there: along Highway 200 and the Clark Fork, via Thompson Falls and Plains. At Plains, I'd turn ENE and follow Highway 28 towards Hot Springs (S09), then onward to Polson. This route looks to me to be a bit "better": although there are definitely towering peaks/ridges on both sides (the Bitterroots and Cabinets), the path between them looks to my eye (judging from the charts) to be substantially wider in most places. There's no ASOS along the route, and I'm sure Highway 28 makes a less-inviting road to crash onto than a nice wide interstate highway, but there are a couple of decent-looking runways along the way (Thompson Falls and Plains). The valley floor looks to be below 3000' MSL the whole way, at least 1000' lower than following I-90.

Should go without saying, but will state it because some people do dumb things: I'm a pretty cautious pilot. Though I have plenty of experience in my old C-150, I have not taken it over the biggest, most remote terrain around. I've crossed mountain passes in western WA state in my 150 many times but never around this part of Idaho/Montana. I only fly when the weather is good. I prefer to cross mountains early in the day (typically less wind, less bumpy) and I'll be doing this in mid-May, when I figure some potential summer issues shouldn't be factors (cooler weather so less DA issues, it's before peak fire season, so less smoke/fire TFR complications). I will stick to wide valleys with roads through them ("IFR") and will always have plenty of room for a 180, no entering box canyons with rising terrain, etc. I'll begin this crossing eastbound from Coeur D'Alene, starting with full fuel.

Thoughts? Have I missed anything important here? I'd welcome your input on these two options, particularly from those who have flown both these routes (especially in modest little planes) and know them well.

Thanks in advance for your insights.

Your I-90 route is the easiest. Just do it early in the morning if you want a smooth ride

I fly both routes a couple times per year. Both are nice and easy routes that your 150 and skills can easily handle. Utilize DOT road cameras and other web cams to help supplement the official aviation wx sources. I would fly both on the trip and see what one you like better. The Thompson River route is often open and safe, when weather starts to build over the top of the pass. The Washington, Cascades Passes are a more challenging crossing in my opinion. Feel free to call me if you want to discuss in more detail eight5zero-7twothree-20seven2

No passenger would help engine alone doing what you want to do. I understand you don't want to be close enough for orographic lift, but as the sun rises thermal lift is free. It causes both up and down air. Take the up with pitch up and stay in the down less with a bit more airspeed. Giving up altitude in downdrafts, going down with it, actually increases altitude over time. This is especially true in low powered airplanes like the 150. Around ten AM thermal lift begins to exceed 100 hp engine thrust lift by quite a bit on sunny days.

Good luck.

I fly the second route you stated several times a year from Seattle to St. Ignatius. You can avoid most weather that way. Pleanty of airports to land at if you have a problem. You can follow the Flathead River to the Clark fork to Sandpoint by Lake Pend Oreille. Many short private strips in case you need them. The owner may still be alive and does not mind landing in emergencies. I know because I have used them a couple of times.

I learned to fly in Missoula in a C-150, way back when, and have flown both those routes several times. Either one should be no problem. Flying "I Follow Roads" is always a safe bet in mountains.

When I fly from the Puget Sound to Flathead Lake I always take the Clark Fork - Thompson Falls - Plains - Hot Springs route.
From Hot Spring to Polson the terrain is not to foreboding. Rolling hills, not a lot of trees and easy to keep a road within gliding distance under you.

I’d follow the route via Plains, etc. good options, more direct, and beautiful country.

MTV

Thank you to all for the insights. Exactly the kind of info I was hoping for.

One specific question: Mullan Pass has an ASOS, but there's not much else in that region for weather stations (at least not that I see). I can kind of triangulate between Sandpoint, Kalispell, and Missoula, but all of those are really outside the critical mountain crossing points, and not good indicators of pass conditions.

Are there any other good sources for ceilings and visibility, especially along that Clark Fork - Thompson Falls - Plains route (besides my Mark I eyeballs if I go poking around for a look)? I assume that the automated weather that's reported in Mullan Pass may be quite different from the conditions just over that high ridge to the north, with the Clark Fork - Thompson Falls route experiencing different weather, no?

(BTW, what's the name for the pass there - I don't see any designated on charts or maps, but I assume it's got a name that locals use?)

Thanks.

I've made the crossing being discussed here dozens of times in my poor, old, long-suffering 172; and drove it literally hundreds of times on "business" in vehicles. The Sandpoint-Thompson Falls route is a long valley, not a pass. What I do is old fashoned, but I scope out NOAA weather radio and Weather Underground for the surface conditions , then call 800 WX-Brief. (see.. I said old fashioned... ) the evening before the proposed flight.

If it's not forecasted VFR- they will gladly tell you. If it is VFR many times they will brief you on "mountain obscuration". This is not unusual, the Cabinet mountain peaks are high and even scattered clouds can obscure the peaks. Be sure to note the Barometric pressures between COE and Polson. A large difference is not a good sign. Check winds aloft.

Your plan to fly early is great. As you climb over COE, you may not be able to see the actual cloud cover, but what I do is climb as high as I can, as soon as I can an "take a look". 8,500 feet will clear all but the highest peaks. If it looks good; I will fiy GPS direct and enjoy the view... and the view is GREAT.

If it's dicey, I stay as high as I can and "cut corners": The drainages run diagonally Northwest/ Southeast and you want to go pretty much East.I steer up towards Sandpoint (SZT) and look for a chance to cross into the Clark Fork River drainage and stay near it until I can see the conditions towards Flathead Lake. Nothing wrong with staying in the Drainage, but I usually get tempted to cross the Cabinets at "Chain Lakes" or another convenient spot. Always have a "way out". Look behind you as much as possible. I consider myself a cautious pilot, but I have had pilot friends question my choices. After I explaned the process I used, there were no complaints. There is no substitute for experience, but this is stuff no flight instructor taught me. I was mentored by others.

I personally don't care for the Mullan pass route. It is high and I-90 has few inviting stretches that look easily landable. If you do go that way, you can cut northeast at St. Regis( the Clark Fork River again) and end up near Plains, and then work your way to Polson.

Yes I have been criticized for flying in places where an engine failure would lead to a difficult forced landing situation. This is true. It has been my experience in the Mountain West that it is almost impossible to fly almost anywhere without being out of gliding distance of a "safe" spot to put the plane down. This is the kind of flying most of us on this site do: I don't know how to avoid this particular risk. But it is a LOT of fun......

One more for the Plains route. The problem with Mullan pass is the freeway is your only option for surprise landings while the Thompson Falls route has quite a bit more flat spots along the way. I'm in Missoula and fly at least a few times a week so shoot me a message for a PIREP if you want.

I'm based in Spokane and fly the area a fair bit - though not as much as some here.
I'd pick based mostly on weather, good points here about barometer at COE/MLP/MSO for the I90 route.
Perhaps a different perspective, for my day job once a year I'm a passenger in a helicopter patrolling the power lines in the region for a few weeks. Since I'm familiar with the route I'd be tempted to take I90 to Wallace and kick left up Burke canyon. Follow the lines from there to Thompson Falls then Plains then straight to Polson.
Typically we're taught to steer clear of lines - which is good. I don't want to have to replace structures for airplane hit poles/wires please. But, most high line corridors are wide enough you could sneak a small plane around the wires/structures without smacking headfirst into a tree so possibly a decent place to wreck the plane in an attempt to walk away. Also of note, most lines have access roads built for line trucks, easy(er) walking on the way out.
That said, they can be pretty remote so you might be walking a while. Carry a good emergency kit and evaluate your own risk/reward balance appropriately.

Looks like the weather is cooperating: we've got big high pressure moving in, we should be looking at clear sky and warm (for Springtime) temps across the region starting today (Monday). The Cascade Passes are still socked-in with some low crud (I'm starting from Seattle) but that is forecast to clear up by midday. As long as that happens, I should have a clear shot across the state for an easy flight this afternoon. I'm planning to RON around Coeur D'Alene, then get an early start Tuesday morning for the hop over the hills to western Montana. Forecast looks good all the way, Mullan Pass is forecast to be clear and calm through Tuesday evening (low clouds in the pass forecast after 5 pm Tuesday, but I should be long gone by then). So as long as the forecast holds true, looks like a piece of cake and a fun trip. I might have additional questions tonight/early Tuesday morning before heading over (I'm planning to take the northern route, Sandpoint, Thompson Falls, Plains) but with good weather should be fun and low-stress.

Thanks again, I appreciate the useful info and tips you all have shared.

Just posting a follow up:

On Monday I flew from Seattle to Coeur D'Alene, ID and spent the night there. The weather was OK but not quite as good as forecast -- I did run into some scud crossing Snoqualmie Pass, had to scoot around some clouds but it was clear enough with enough space between the clouds for me to feel reasonably comfortable. Decent weather weather across the state (even a nice tailwind). Next morning (Tuesday) the weather forecast for the whole region, including Mullan Pass and the western Montana reporting stations all looked good. But I could see some weather over the mountains, though it looked like clouds were scattered and pretty high.

I launched from COE and headed up the Clark Fork route at about 6000' MSL. Cloud bases were about 6500-7000, and I could see some isolated rain among the peaks, with some across the route ahead. The further I got into the crossing, the more things started to deteriorate, but most of the clouds remained high, and although there were a few scattered across my path ahead, none were solid or low enough to completely block the route. I did fly through some rain showers several times, once or twice it looked more to be snow (hard to really tell, but nothing was sticking to the airframe). OAT gauge was pegged at right around 0° C so I was watching carefully. I only passed through precip briefly but it certainly was enough to get my attention.

The clouds and precip were minor and not really in my face for roughly the first half of the route -- from Lake Pend Oreille to Thompson Falls. Through that portion of the route, the valley is wide, with plenty of decent-looking flat, grassy spots that looked like great options in an emergency. Past Thompson Falls, the valley narrowed quite a bit, the stretch between Thompson Falls and Plains especially. That's also where the weather started going downhill a bit. Cloud bases lowered a bit and the spaces between them started getting smaller, more precip, and a bit more bumpy. This was the narrowest part of the crossing, and also where I had to drop down slightly (around 5000-5500') to stay in the clear - closer to some terrain than I would have liked. Up until this point I was checking the view behind me to be sure I could always do a 180 and get back to someplace that looked better, but at 5500' that stretch between Thompson Falls and Plains looked like a 180 in there would have to be pretty tight. Fortunately, that narrow choke-point was brief, and beyond I could see that Plains sits in a very wide, open "plain"; there was even a spot of some blue sky and sunshine there.

Unfortunately, there was also more weather just beyond. My original plan was to jog NE at Plains, following some power lines through a low saddle to Hot Springs as a shortcut, then over one last ridge to Polson. Alas, there was a squall line that sat directly across that route, with solid clouds going down almost to the surface, it didn't take me long to decide that looked way too ugly for me, and I needed a Plan B. Plan B was to continue following the same road (now along the Flathead River) to Paradise, Perma, and Dixon, before turning north to Polson. The weather ahead looked good enough, though I did have to drop down a bit and continue through more rain and bumps (the big runway at Plains gave me enough confidence to know I had multiple outs). This proved to be a good decision. By the time I reached Perma, the ceilings were lifting, and I swung NE towards Polson, crossing one last ridge before starting my descent. There was more rain and bumps as I got closer, and as I looked back over my shoulder, the sky looked black and nasty over what had been my original "shortcut" to Hot Springs.

I landed at Polson and all was well. But during the next hour or two, a series of intense cells moved through, with screaming high winds, low ceilings and heavy rain. None of which I had seen in a forecast. I was very glad to be on the ground at that point.

I'll be going back the other direction in a couple weeks. I might fly the same route again (now that I "know" it) or I might opt for Mullan Pass instead -- depends on the weather. We'll see.

Thanks to all who provided their insights on this route, my first major mountain crossing in a long time.

flyingmonkey wrote:I landed at Polson and all was well. But during the next hour or two, a series of intense cells moved through, with screaming high winds, low ceilings and heavy rain. None of which I had seen in a forecast. I was very glad to be on the ground at that point.

Yeah...that was quite a little squall! I'm just North of there about 30 miles and was working in the hangar with the door open. It came on fast, and by the time I got the door closed, it was throwing hail in the hangar and blowing like mad! We wound up with 3/8" of moisture in about half an hour.
John

If you're into geology or have heard anything about Glacial Lake Missoula and the Spokane Floods, you might find it interesting to look at the giant ripple marks in the Camas Prairie around 47.512949° -114.583795° on your way back. From the ground they don't look like much but from the air the view is great.

https://inside.mines.edu/UserFiles/File/Geology/Camas_red.pdf

https://en.wikipedia.org/wiki/Lake_Missoula

Lots more to see in Washington on your way back too like the Channeled Scablands and Dry Falls.

https://en.wikipedia.org/wiki/Missoula_floods

https://en.wikipedia.org/wiki/Dry_Falls

Flying the western US and Canadian Rockies gives us plenty of "God is bigger stuff to look down at." Uphill flooding, like tidal ebb and flow, may hurt our physics sensibilities. But God has a Navy and He goes where it pleases him. The BC mountains SW of Grande Prairie hurt my sensibilities when I realized I had no slope way to follow drainage out of the mazes. Geologically stuff happens out there.