by Bruce Chien
Flying in snow is a wintertime challenge many pilots take far too lightly. The challenges involve every aspect, from departure planning and takeoff to in-flight decision-making and landing.
Fortunately, the dynamics of most snow systems are fairly consistent and easy to understand. The usual weathermaker that creates snow is the wintertime low pressure center. There are two sorts of snow: dangerous high-moisture snow – usually found on the warm side of the low – and drier, lower moisture content snow – found on the backside of the low. Everyone in the Midwest knows about the two types. The former clogs the snow blower; the latter is easily moved with a shovel. The former is like glue; the latter like talcum powder.
Consider a typical winter front in the mid-continent in winter. The low is centered over Jacksonville, Ill. A warm front extends to the east as far as Cincinnati. A cold front extends to the southwest to Albuquerque.
Recall from basic PVT-ASEL weather training that the warm front is depicted at the surface and actually may climb for 150 miles toward the northeast, while the cold front is relatively vertical.
Now add in the lifting action of a low. The airflow is counterclockwise in North America, into the low. The convergent flow rises as it turns in the funnel. Now imagine if you will, the mixing action of a bowl of still loose gelatin, being lifted and mixed by a counterclockwise paint-mixing screw. The warm air is mixed with the cold as it is lifted. Its precisely this lifting of the moisture-laden warm air into the upper altitudes, and its mixing with the cold air, that creates the frozen precipitation we call snow.
Note that cold air dominates on the left side of the low weve constructed. As the warm air is lifted, the high moisture content of the air produces icing and heavy, wet snow. The red zone is where the wet stuff is.
As the air mass rotates to the northwest, more and more of the moisture is rained out, leaving lower moisture content precipitation to the northwest. This is the dry fluffy stuff that we get on our driveways after the brunt of the storm has passed.
Where are the super cooled drizzle drops located? Right over Roselawn, Ind., in this example. Thats not the place to be. Meanwhile, in eastern Nebraska theres blowing snow on the runway – the light stuff – and Rochester, Minn., is reporting SIRBRAP from mix of the heavy and light stuff.
Moral of the lesson: The awful stuff is located from 4 oclock to the low back around to about 11 oclock. This red zone is the danger segment Mere mortals dont go there except on top and in the clear.
That having been said, I will admit to having flown turbocharged known-ice aircraft from the danger segment into the cold side, but never toward the danger segment and never to the northeast. The trick involved is surviving long enough with your deice gear to find a warm layer or to make it out on top.
But recognize that the tops of these systems are usually above 20,000 feet and icing can be worst just where vertical lift runs out … just shy of the tops. It can overwhelm known-ice systems. If ice becomes unmanageable the out is to turn back or fly away from the low to the south or west. Get the heck out!
So the prime rule for planning a departure is to not fly into the red zone. If youre located in northeastern Illinois and want to fly west, you might be all right if surface temperatures are enough above freezing to give you an out. Otherwise even the skipper of a known ice ship will take pause because the tops are 25 minutes away. No way should you fly east or northeast in this scenario. South might or might not be all right.
Good to Go?
What about departure itself? The only kind of snow that really comes off with a brush is that light fluffy kind. At the other end of the spectrum, heavy wet snow invariably leaves a trail of frozen snow firmly adherent to the wing. Yet pilots apparently are unable to make the distinction, because winter after winter pilots succumb to the delusion their snow-laden wings will blow clear during the takeoff roll.
So you tug your bird into a warm hangar and thaw it out. Good. But in solving that problem, youve created another: The melted moisture can run back into the flaps and ailerons, trim tabs and jackscrews and refreeze in the cold air.
Even if you get them good and dry, every last spot, when you pull the aircraft out into the snow, the falling snow will melt on your warm fuel tanks and the wet will run back – and you can figure out the rest when the airframe subsequently gets cold.
The ideal snow departure situation therefore contains dry snow on the back of the cold front, a clean and cold airframe, westbound departure, and decent ceilings for the emergency out. Such conditions might be present in Fargo.
So, even if you have the luxury of a heated hangar and you suspect a snow departure in the morning, plug in your engine heater and cabin preheater. Make sure your pitot heat works and your static system is drained of moisture. If the departure prediction is for surface temps above 32 degrees, set the thermostat in the hangar to 40 degrees. If the prediction is for temps below freezing, set the thermostat to 30 degrees. Turning the heat off and allowing the airplane to be at ambient temperature is OK, too. And, never mind the cold engine. Youve got to warm that puppy separately. The bottom line is that no runback ice is allowed.
Departure with wet snow with temps below freezing requires liquid deice. You are likely in the red zone. Hot aviation deice fluid is approved for aircraft because it has adhesion properties that protect the aircraft for sometimes as much as 40 minutes after application.
Trouble is, its generally not available at your mom and pop strip. Even if it is or youre at a major airport with hot deice, it runs upward of $8 per gallon. Ive had $500 de-ice jobs in the Rockies. Yowse!
Wing covers cost less than that, even for many twins, but without liquid deice its a race against the clock from cover removal to when the wet glop starts to accumulate. You end up violating 91.527: No pilot shall take off in any airplane that has (1) frost, ice, or snow adherent to any…
And please do not show me a spray bottle filled with automotive windshield alcohol/water mix. The alcohol evaporates, leaving your old enemy: water.
En Route Options
Managing en route in frozen precipitation is a discussion unto itself. It is not necessarily true that freezing rain means warmer temperatures on top. It may be yet colder, and the drops are super cooled. One needs always to have an out.
On top is only an out if you have turbocharged climb capability, de-ice, oxygen, and plenty of lift reserve. In the red zone even known deice ships can be overwhelmed. On the cold side, if you are far enough west, you can frequently get on top at 8,000 in a normally aspirated aircraft.
But before you launch, you must have pireps and know that the tops are reachable. En route on the cold side, its not unusual to fly in snow and accumulate nothing. The snow is dry and of low water content. (Leave the strobes off; it can be quite a disorienting light show.)
But you still have to have an out if faced with accumulation, and the surface is too cold and is therefore not an acceptable out. Flying on top in the clear far to the northwest is an out. Crossing the front to the southeast in the clear, to the warm, is an out. Leaving no out is an invitation for us to read about your final actions.
Now a word about approach: Typically on the cold side there is a layer of clouds from about 500 agl to 2,500-3,000 agl. Thats exactly where the FAA puts the initial approach fix for the ILS into most airports. There wont be much ice if youre far back at North Platte, in the weather scenario outlined earlier, but at Lincoln, Neb., you might still get shellacked.
Surface temps need to be above freezing if you are not deiced, otherwise you have no out. Got enough fuel to go further west to Kearney, where its clear? I hope so. Fuel is your friend.
Beware of the mountain destination airport: Theres usually a fly visual segment at the end. This means, after the missed approach point, when you no longer have the guarantee of a safe miss, if you lose the airport you have no options.
Remember the Gulfstream G-III crash in Aspen in March 2001? During the last 1.4 miles, the crew had the field, then they didnt. Then they did and they yanked and banked for it, because there was an 8,800-foot wall of granite ahead. Then they stalled, and died.
Look at the LDA-8 for JNU Alaska. Same thing for 3.2 miles from Coughlan Island to the runway. You have to really have the airport, and blowing snow is a poor companion when youre stuck with no options if you lose visual contact with the field.
Even if you are deiced, plan your descent to minimize time in the layer. There is no good time to operate boots on an ILS. Once the boots come up, you need to add several inches of MP just to say on the slope. When you break out and are low and slow, thats also a lousy time to deice – unless you have to due to accumulation. If you have TKS deice fluid, start pumping before entering the cloud layer.
Plan a prudent long runway and keep your airspeed up. Omit flaps, thereby minimizing the chance of a tail-plane stall. This is no time to be going into a 3,000-foot runway. And have enough fuel to go farther west into the clear.
And now a word about arrival: In the Midwest and in New England, just after a storm, the wind is howling form the northwest. The north/south runway has multiple triangle-shaped drifts extending from the western side of the runway to near the midline. The choice is to land with the left mains on the drifts, or to land on the right hand clear portion of the runway. Its a devils choice.
Should the upwind main bog down, its an instant ground loop. Should you get a gust while landing on the downwind half, you may end up in the bank. Always have a Class B or C alternative to where you want to go. The county crew may have cleaned up well at 09:00, but by 13:00 when you arrive it may well be different. Remember, fuel is your friend. Remember also that the 150 foot-wide runway at a Class C is a wonderful place to be when its blowing and drifting.
There are some other wintertime items, too: Does your windshield seal well? A little leak means snow in the cabin melting all over your hot avionics. Bummer! Do you have a pair of gloves under the seat? When the door pops open it gets really cold really quickly. How about a coat? How about your instrument currency? Can you make it to an out option partial panel? Do you have a carbon monoxide detector and have you checked the integrity of your heating systems lately?
Then also, you have to get out from wherever it is that youve gone. After getting all the crud off the flying surfaces, putting on the wing ice covers and the cabin cover, you need to be sure you can get out of there. Got a good battery? Is it well desulphated? Got a source of power for your engine heater? How about cabling? How about power for the cabin preheat system? Oh, and did I mention fuel?
-Bruce Chien is a CFII and AME who flies his de-iced Seneca to the mountains for skiing.