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I first flew over the Montana Rockies as a junior flight surgeon with the Air Force, out of Great Falls, Montana (Malmstrom AFB). In circa 1992, a friend made a great pitch to get my private pilot’s license with him. With three years of intensive medical training and a growing family still ahead, I deferred until I had enough time and money to stay proficient. Instead, I built a cabin for family and friends to enjoy.

Twenty-one years later, my 17-year-old son and I received our private pilot licenses together and launched right into our instrument training. He received his instrument rating before age 18. Me, I detoured to receive mountain flying instruction and keep proficient in my day job. In 2015, I punched my instrument ticket. In 2016, I flew from Omaha to Washington State, to scout job opportunities in Montana and visit friends in the Puget Sound region. The mountains of Montana and western Washington have plenty of IMC and icing considerations to consider in May.

Continental Divide

The terrain in Montana can create its own weather.

I fly cross country in an aero club 1981 Cessna 182R, purchased from the Civil Air Patrol. Nearly 500 of my 800 hours of flying have been in this aircraft. This includes flying from Charleston, South Carolina, to Hoquiam, Washington (Pacific Coast), to visit family. Ironically, Seattle Center held me at a fix named “SOUPY” in IMC, awaiting a Coast Guard helicopter to finish an IMC approach into Hoquiam (KHQM). I also fly this plane with supplemental oxygen, as appropriate. This includes a couple of times being cleared for Class A airspace going over weather. However, in 2016, I did not know the full capabilities or all of the intricacies of this aircraft.

Not fully satisfied in my 2016 day job in Omaha, I explored Montana job opportunities. From 1991-2015, we had maintained a “home of record” in Montana, while moving with the US Air Force (USAF). Montana’s Bob Marshall Wilderness (the Bob) is where I had flown as an aerial observer-medic in my first USAF job as a flight surgeon. The USAF UH-1Ns at Malmstrom provide frequent search and rescue (SAR) assistance to civil authorities, most commonly for a lost hunter or hiker. The Bob provided my first “save” in 1991 as an aerial observer, spotting a lost hiker on Renshaw Peak, a few miles from Benchmark (3U7); a 6,000 ft x 100 ft asphalt forest service runway. Benchmark is the only paved runway in the Northern Rockies. Flying and landing back country had been on my bucket list of future flying for many years. “Bucket” nearly became a reality.

In 2016, I filed for my first flight over the Continental Divide; KHLN (Helena, Montana) to KGPI (Glacier Park International, Kalispell, Montana). Weather conditions called for a broken ceiling over KHLN, Airmet Zulu for potential icing, and Airmet Sierra for mountain obscuration. Lower level of freezing had been forecast for 9,000 ft. I filed WOKEN-V365-CHOTE-V536-FCA to GPI at 12,000 feet. KHLN and KGPI both forecasted broken ceilings with a current field temperature of around 45 degrees. KHLN has no radar coverage, hence I filed and flew the Divide One obstacle departure (I had briefed this departure for my FAA instrument check ride).


In between layers should be fine, right?

Victor 365 from CHOTE to FCA is essentially completely over the Northern Rockies/Bob Marshall Wilderness, south of Glacier National Park. It has a MOCA of 11,400. Upon reaching CHOTE, I realized the clouds had a couple of layers. A broken layer from the mountain peaks to 11,000 and another layer at approximately 13-15,000 ft. I had a “nice” clear passage through at 12,000. I thought, “no problem” as it is just a few minutes… not.

Flying between the layers, I realized I had few options if I lost an engine, or, if my “window between layers” happened to close. Thankfully, all went well, until switching frequencies for my ILS 02 into KGPI. Switching Salt Lake frequencies as instructed, I could no longer reach Salt Lake Center. I was on the initial glide path for the ILS 02, hence switched to tower, suspecting the mountains obscured my transmission. Tower cleared me to land and I descended about 1,000 feet when tower instructed me to return to a Salt Lake Center frequency and break-off the approach (I still had ~8,000 feet to descend).

Salt Lake desired to bring in a couple of Hawkers. My “break-off-leveling” at ~10,000 feet in broken IMC, without a horizon, led to a visual illusion of tumbling forward… and the radar vector led me into IMC and light rime icing. After the Hawkers passed, I resumed my descent without event – other than being a little nervous about my first trip over the divide. The Instrument Handbook talks about “tumbling backward” in IMC if leveling off too quickly on ascent. I experienced the same in reverse, tumbling forward, when breaking off my descent without a horizon. To me, I felt like I was skiing down the tops of the clouds; however, reference to my altimeter had me in a slight climb.

Here are my initial lessons learned. One, I could (should) have asked to gain altitude (circle) to get above the layers before crossing over the Bob Marshall Wilderness. I did not have to stay exactly on the Victor, if I needed to gain altitude (with ARTCC coordination) to better avoid icing conditions. Knowing the climb/altitude capability of my aircraft above 12,000 feet would have given me another “out” that I had not initially contemplated.

On top in 182

Disorientation can be a powerful sensation in IMC.

Two: I could have requested a Special VFR to stay out of the (broken) icing conditions above Kalispell. I was not too concerned with icing on the ILS approach, as it was 45 degrees on the ground with a broken ceiling. I would not have flown this flight in complete IMC over KGPI. Indeed, I deferred flying to Seattle/Everett (KPAE) the next day due to solid IMC over Puget Sound from 4,000-10,000 feet with icing predicted.

Here is my next lesson, taught to me by the Bob Marshall Wilderness. Since Puget Sound had icing conditions, I decided to fly back east over the continental divide to Great Falls, MT (KGTF), and spend the night at our cabin. I departed south out of KGPI to join V120 (MEA 13,000 ft, MOCA 12,100 ft). Initially flying at 11,000 feet, I noted a layer above the mountains, and requested 13,000 to stay on top. Upon reaching 13,000, I re-leaned my normally aspirated, carbureted engine. It should have leaned to about 10 gallons per hour. The engine leaned to 12.5 GPH. I re-leaned the engine… still 12.5 GPH. I instituted a self-taught checklist for when the engine did not perform as expected: primer in-locked, cowl flaps closed, wing flaps 0 degrees, coordinated flight, engine oil/temp normal, applied carb heat – and the plane started to shake violently.

Startled, I pushed the carb heat knob back in (no carb heat). “Wait,” I thought, “plane shaking” is what happens when there is indeed carburetor icing. I had read an NTSB report where the PIC did not re-instate carburetor heat after applying and the plane shook; he had a non-fatal engine-out crash. When the aircraft shook violently, my first thought was, “This is not how I planned to first visit the Benchmark,” the back country Forest Service runway. Fortunately, through great flight instructors and reading mishap reports, I re-applied the carburetor heat. The aircraft again shook violently, but as described, the shaking ceased within 10-15 seconds. The engine then leaned to 10 GPH.

Continuing to read, study and fly had paid off. I had flown about 200 hours cross country by that time with about 30 hours IMC. One flight from Omaha to Texas, while flying around some convective activity, I experienced difficulty maintaining 12,000 feet (near full useful load). Researching the difficulty when back at home, I came across a carburetor icing diagram.

To my surprise, I saw that carb-icing could occur with full throttle (I had initially learned carb icing occurred when the engine had been placed in idle upon approach). This surprised me, as I then realized that the altitude at which I often flew cross country, with outside air temps 0-10 degrees Celsius, could indeed have a possibility of carburetor icing. My outside air temperature over the Rockies had been near 0 degrees Celsius.

Undoubtedly, the relative humidity just above the cloud layer I topped was close to 100 percent. Knowing my aircraft well let me know when something subtle was happening. Recognizing an improper lean, combined with my checklist of what to review when engine performance did not meet expected parameters, led me to apply carburetor heat and avoid an emergency landing at Benchmark or some lonely ridge. Two years later, I did land at Benchmark, and overflew the ridge where we had spotted the lost hunter 26 years prior.

Dale Agner
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2 replies
  1. Eric Muetterties
    Eric Muetterties says:

    “Carburetor icing is caused by the temperature drop in the carburetor, as an effect of fuel vaporization, and the temperature drop associated with the pressure drop in the venturi. If the temperature drops below freezing, water vapor will freeze onto the throttle valve, and other internal surfaces of the carburetor. The venturi effect can drop the ambient air temperature by 70 absolute degrees Fahrenheit (F), or 38.89 absolute degrees Celsius (C). In other words, air at an outside temperature of 100 degree F, can drop to 30 degrees F in the carburetor. Carburetor icing most often occurs when the outside air temperature is below 70 degrees F (21 degrees C) and the relative humidity is above 80 percent.[1]”



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