Trace (Icing) is the Sum of the Elements

The Insidious Nature of Aircraft Icing

In the mid 1950鈥檚, a pilot learning to fly in Eastern Washington state seldom worried about ice. He or she only flew when the weather was reasonable and the sun warm on ones back. If an ignorant soul violated that basic canon, a search team would be sent into the hills or mountains to look for the remains.  Instrument flying in the clouds was for the airlines and even in that venue the results in winter were occasionally tragic. The four-course radio range, called the Adcock range, was still a mainstay for cross country navigation in most areas of North America. VOR ranges were being slowly installed in the USA, but the British were plotting to make the World-wide standard a convoluted system called Decca. This caused enough furor at the ICAO that the CAA (1950s remember?) was in no hurry to speed up installations. The tried-and-true NDB was the primary approach facility to the majority of the nation鈥檚 airports unless the city was large enough to warrant an ILS.

In 1956 the former Army Air Base at Walla Walla was a rather busy general aviation (GA) airport.  There were many pilots; yet I knew of only two that possessed an instrument rating. Neither of them was current, as the only method to maintain currency was in an airplane flying in the clouds. Currency was expensive and they weren鈥檛 going to fly IFR anyway as the majority of aircraft didn鈥檛 have the electronics, instruments or a method of combatting ice. A new airplane out from the factory with a heated pitot was a big deal, and the instrument six-pack was a few years down the road.

It was a cold, grey, overcast day, the ceiling about 400 feet with a light drizzle falling. It was a Saturday in early February.  I was working at the airport after school and weekends sweeping out one of the hangers that always seemed to fill with dust from the local wheat fields every time the large doors were opened. For the last hour, the only sound I heard was that of the broom scratching across the concrete.

There were only two airline flights into Walla Walla on a Saturday, and nothing was due for several hours. Yet, far off, I heard the sound of an approaching light aircraft engine turning at very high RPM. I walked out of the hanger to look toward the sound somewhere to the south. There, clinging to the bottoms of the clouds, low over the buildings was a Piper Tri-Pacer on a tight base turn to the end of the runway 20, its engine at full throttle. The pilot lined up with the runway and at the last minute cut the engine, the plane dropping the last 6 feet like a rock, tires slamming into the concrete, here and there shards of ice knocked loose by the abrupt landing fell from the airframe. The Tri-Pacer slowed, then taxied off the runway toward the FBO, pulling up to a stop where I stood. No sooner than the propeller stopped when the cockpit door slammed open, the pilot jumped out of the plane, running past me onto the wet grass lawn and then behind our little office shack that adjoined the hanger. There, he answered the call of nature, his back heaving with each breath he took.

Margret, our office manager, took dim view of what was now going on outside her office window. She stormed out the side door and yelled at the pilot. He turned toward her, his face pallid and body shaking, revealing the aftereffects of a terrifying experience. Uncharacteristically Margret took pity, ushering him into the office shack for some coffee and a cigarette.  The Tri-Pacer was covered with about 1/4 to 3/4鈥 of clear and mixed ice on every leading edge. On the wings the ice went back a good 8鈥 over the top and 3鈥 on the bottom. The leading wing struts were not visible beneath the heavy mixed ice that completely enclosed them. I had never seen aircraft icing before and looked on in awe.  The large prop spinner was covered with thick clear ice. I was able to pull off a complete ice mold of the spinner and I took it into the office.

Upon seeing the ice in my hands, they paused their conversation for a moment and then the pilot lowered his head and spoke. 鈥淚 flew 28 missions over Europe, I think I鈥檓 done with this. Done.鈥 He paused, then spoke again. 鈥淒oes West Coast [1] fly out of here to Boise?鈥

Before leaving our office, the pilot recounted how he was on a business trip from Boise to the Tri-Cities. The weather in Boise was just scattered clouds, but after he passed Nampa and closer to the Blue Mountains the clouds began to close up and the ceiling lowered with intermittent patches of dry snow falling. He could still see the ground and the mountain tops ahead, so he pressed on for La Grande. There he found that he no longer had a good view of the hill tops, as they were obscured by low clouds or wetter snow fall. La Grande, OR was below him yet he chose to climb up into the clouds to an altitude higher than the mountain tops and continue on to Pasco. If my memory serves, his airplane had an externally mounted venturi that drove the artificial horizon and directional gyro. His VHF radio was a 鈥淣arco Omnigator鈥, the ADF a Mitchell 鈥淎irboy鈥; both considered relics even 10 years later. Without a heated pitot he climbed into the clouds and into the freezing precipitation that was plainly in view ahead and around him. Somehow, he managed to keep it in the air over the pass across the Blue Mountains, but on the upslope side he started icing up so bad he could no longer keep the wings level or hold altitude. Luckily, just east of Pendleton he found a hole and dove down leveling below the thickest clouds around five hundred feet AGL,following a two-lane highway north and west. Yet, here and there light rain and drizzle was falling, and his airplane skin was very cold from flight in the higher altitudes causing the moisture to freeze to the airframe. If that wasn鈥檛 enough, in a few miles, the light rain and drizzle changed to consistently freezing rain and drizzle. Now he was icing at a faster rate with clear ice coating every exposed edge. Each time he attempted to turn toward the lower terrain and warmer air to the west, the clouds blocked his path. He continued to feel his way to the airport at Walla Walla. During the last 20 minutes of flight the Tri-Pacers engine was at full power; the plane often shuddering on the edge of a stall as he approached to land. 

He left the airplane there until late March when a salesman arrived to fly it to a new owner.

In 1956 here was a formula for catastrophe. A businessman on a mission. Military trained, but assuredly not current for IFR flight. Aircraft minimally equipped for IFR flight even in that day鈥檚 lax standard. The known weather forecast that day for icing and low ceilings from Stampede pass to La Grande. Even I, a green not yet soloed student pilot, knew that. Almost every moderate sized airport in the Northwest had a Flight Service Station or the Weather Bureau based on or close to the field and a rather good weather report with forecasts and graphic charts were very available along with full explanations. Yet, he went.

Now today ask yourself. What in 60+ years has changed? What has not?

Unless completely ignorant, every pilot has read the usual training materials about icing and how it affects an airplane. Maybe they have experienced a bit of trace that quickly wore off. The occurrence only becoming something to talk about over drinks, nothing more. The problem is that most GA pilots have yet to find out just how quickly and completely an airplane can be overwhelmed by icing. Airframe icing is insidious. You may notice a trace on the leading edge or around the windscreen, not really anything to be concerned about you think. You continue on, until in a blink of an eye you are packing it on to the point the deicing and anti-ice systems are close to being overwhelmed. 

Ice has been explained in many ways and NOAA has defined it into just four types. You know of them; Clear, rime, mixed and frost. Indeed, each type of ice (they say) is formed from specific types of clouds associated with a front or condition. Yet, the local geographical layout, air mass types, combined with a suitable feeder of moisture often produce the larger, more sustained, and subtle continuing dangerous changes. Once ice is attached to your airframe, it is difficult to remove. Let us consider some of these dangerous areas across North America.

The Cascades

This range of mountains start in Canada and continue south into Northern California. They are considered without parallel for creating some of the most intense icing conditions in North America. Cool, moisture laden air is quickly lifted up the mountain sides from six to fourteen thousand feet. The average wind at the 500 mb level (18,000鈥) is seldom less than 50 knots out of two hundred and twenty degrees (220掳) from October to April. As low as 14,000鈥 the prevailing winds can obtain speeds greater than 120 knots with frontal passages. Concentrated Pacific Ocean moisture is quickly lifted and cooled, being thrust like a fast-flowing river tumbling over huge boulders that impede the flow. The turbulence in the moisture can harbor pockets of differing types or intensity levels of ice. The build rate is greater than what might be found over the Alleghanies, or even the foulest accretion over the Rocky Mountains. 

Icing conditions on the west side of the Cascades are commonly at altitudes from 4000鈥 up to 22,0000鈥. On the east side of the mountains the freezing level and possibility for icing is from the ground up. If you are in route from east to west and start picking up icing as far east as Moses Lake or the Tri-Cities expect moderate or greater (severe) levels of mixed rime past Ellensburg or Wenatchee and on the descent into the Seattle area.

The Cascade passes

In the winter the cold Canadian Polar and Plains air masses will sometimes settle in over eastern Washington and Oregon with the resultant temperatures to be much colder on the east side of the state. This polar high builds and creates a steep pressure gradient from east to west often producing very cold VFR conditions in central Washington. Because of the lower pressure on the westside the colder high-pressure air starts out-flowing down any available pathway to fill the lower pressure western Washington and Oregon.

For many years, the only way to fly from one side of Washington or Oregon to the opposite was via a few passes marked on an aeronautical chart. Flying at 500鈥 or 1000鈥 AGL, twisting and turning between the ridges to stay under the clouds, pilots crossed via Stampede or Stevens Pass or via the Columbia River Gorge. Many pilots, including myself did it safely. However, there were always issues with these routes. Just when you were about to clear the mountains there was a bend in the path and here you would find a wall of dense fog, rain, or severe icing conditions. Attempting to turn around in the narrow confines of the Gorge often resulted in disaster.

The Coastal Ranges

The Olympics, the Siskiyou鈥檚 and California Klamath are a coastal range of mountains that are the first to feel the western steering winds that come charging off of the Pacific Ocean. Here super moisture laden air is first lifted and cooled.  If it is raining at 4000鈥 and about 2掳C on the approach to Astoria or Hoquiam expect up to moderate mixed ice lurking in the clouds above 5000鈥 MSL if you head east. Flight from the coastal airports into Eugene, Medford, Klamath Falls or Northern California can produce soul searching moments with fighting icing crossing over these ranges. The ice here has a greater moisture level and heavy mixed ice should be expected.

The Alleghanies

The writer Ernest K. Gann[2] wrote of one night battling ice in a DC-3 over the Alleghanies. A similar ice battle was previously described in one of his earlier books[3] of ice鈥檚 effects upon a lone mail plane and pilot as they challenged the weather Gods to complete their route.

Across that range to coastal New England flying on a winter鈥檚 night even into mid-May you鈥檒l find thunder snow and aggressive rime greeting you as you descend from the flight levels. Indeed, a flight across the Allegany and Pocono mountains below 17,000鈥 can present clear, rime, and mixed ice with a touch of freezing drizzle in just a 250 miles segment. The northern part of the range is affected by Lake Effect Snow that can cause airport delays that mount into days. Ice accumulation can be slow and stealthy or sudden as super cooled water droplets slam into your plane, sounding like the impact of buckshot from a shotgun. Without fail it will build to the point of overwhelming GA aircraft if flight is sustained too long in the freezing air. The (ASI) recently produced an excellent video of two GA icing related crashes, occurring within two days of each other over this range of mountains. All pilots should view that video.

The Great Lakes Effect

Winters in the region of states and provinces that border the Great Lakes are known for sudden and severe ice and snowstorms. The lakes themselves do not 鈥渕oderate鈥 or warm the temperature as the Pacific Ocean or Puget Sound might affect the Seattle area. Instead, the Great Lakes add a base line of cold moisture to the colder Canadian and Polar air masses that sweep down across the Plains from the north. This then mixes with advancing fronts from the west and is often compounded when warmer Gulf air feeds in over lower Illinois, Iowa, Indiana and Ohio. Rime and mixed ice will form quickly during a crossing these fronts. If there are thunderstorms lining up expect clear ice, too. Heavy snow fall in the form of large flakes can obliterate forward visibility to zero in a heartbeat leaving one to depend upon their instrument flying skills. If you鈥檙e lucky the worst you鈥檒l experience is being stuck for days when the airport can鈥檛 keep the runways open, i.e., Buffalo and Albany, NY.  In recent times, two Regional carriers, one flying an ATR, the other a Dash 8 crashed due to icing, and the weather continues to claim lives, especially in GA aircraft.

The Rockies

This massive backbone range of the American continent faces the prevailing westerlies that sweep off of the Pacific Ocean from Canada south to the warmer climes of Southern California and Mexico. Yet this range is actually the second or third range of mountains that have been kissed by ocean washed air. The leading edge of the air mass crossing the range will develop upslope conditions similar to that across the mountains closer to the Pacific, yet thankfully unless fed by a thunderstorm, the ice usually has less intensity as droplets have less moisture. High winds, turbulence, snow, and very limited visibility against a white background trying to identify a runway are common problems on approach. Our old nemesis Rime ice will show up at the lower altitudes or around the temp line of an inversion.

Today, we have more digital and visual weather products at our eyes and fingertips than at any time in aviation. From applications on mobile devices to digital weather reports through GPS in the cockpit, there is an abundance of information available to pilots. However, while parsing through the pages of information it can be easy to gloss over important weather forecasts that could give clue to icing hazards. We as pilots should maintain a better understanding of the unique regional weather patterns that can lead to aircraft icing, the associated hazards, and develop skills to actively manage the constantly changing situation we may find while flying in the winter months.

By Mike Oswald – CFI, CFII, MEI, ATP

[1] West Coast Airlines, an early PNW airline that later was merged into Hughes AirWest.

[2] 鈥淔ate is the Hunter鈥, E. K. Gann

[3] 鈥淏laze of Noon鈥, E.K. Gann

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