Prime Time For Icing

Many parts of the country can still experience snow and ice storms until relatively late in the spring. Even frosty early-morning departures can be a nuisance. Dealing with ground icing conditions or snow accumulation is a relatively obvious matter. Simply stated, if there is anything sticking to the airplane (including mud) that doesn't belong there, don't take off until it's removed. If you can't remove it or it accumulates faster than you can remove it prior to takeoff, don't go flying until conditions improve.

Perhaps, the first thing we should look at is how to detect the contamination in the first place. Ice contamination can take on some pretty subtle and hard-to-detect forms. Potentially, the most dangerous of these is frost. It can take on an appearance as subtle as dulling paint and it's virtually undetectable to the eye. Sometimes, the only way to reliably detect it is to get out and touch the airframe. This is referred to in the FAA circles as the "tactile test." It usually will feel like 80-grit sandpaper. However, it also can be as smooth as glass, which is called clear ice. In any case, the best way to really be sure that the aircraft is truly free from contamination is to get out and closely inspect it with your eyes and hands.

The second part of dealing with ground icing conditions is, once you've determined there is contamination on the aircraft, how to get it off. The answer can be as simple as going home and waiting for things to warm up or as complicated as calling over the de-ice crew with all their fancy (and expensive) equipment. Most general-aviation pilots don't consider de-icing equipment as a viable option---and that's not all bad.

A rather infamous cargo operator spent over $35,000 one night de-icing his DC-8 in the Midwest when freezing rain came along and sent the crew back to the hotel to wait out the storm. They departed eight hours later in gorgeous conditions.

Prior to any takeoff, it's also important to be aware of the contamination on the taxiways and runways. Snow, slush, mud and water that get flung up on the aircraft and into the landing gear and brakes actually can prevent the gear from retracting properly or coming back down when you'd really like to land.

Wet and icy runway conditions are, of course, not limited to takeoffs. Arrivals can be just as hair-raising. Runway braking conditions may make it difficult, if not impossible, to land the aircraft safely. There are only a few things that can make that pit in your stomach do cartwheels, and battling with the weather while dodging all that mother nature has to throw at you, only to find yourself on the ground and traveling down the runway sideways, is one of them.

We've all read lots about airborne icing. Most general-aviation aircraft that are equipped and certified to fly into known icing conditions as a practical matter should only be flown into light forecast conditions. There are no U.S.-certified aircraft that are approved for flight into severe icing (not even the DC-8). Flight into forecast moderate icing is just asking for trouble. So remember: Exercise extreme caution. Do you really trust a weather report prepared by someone who works in a building with no windows?

The FAA says that airframe ice can occur in visible moisture between plus-five degrees C and minus-10 degrees C. Ice can even form when the airframe is cooled below freezing and flown through very humid conditions, even in VFR conditions. This type of risk is called sublimation of water vapor.

Basically, there are three kinds of icing: clear, rime and mixed. Clear ice is formed when large, super-cooled droplets hit the airframe, freezing as they spread along the surface, and it's usually the kind that is most likely encountered in IFR conditions. It's the most difficult to remove and it may form beyond the area that is covered by the de-icing systems. It's almost exclusively found in cumulus clouds. Rime ice is formed when smaller, fast-moving, super-cooled droplets hit the airframe and freeze instantly where they hit. Mixed ice is just what it says: A mix of clear ice and rime ice is formed when droplets vary in size or when snow, various-sized droplets and ice pellets make up the mix that is hitting the plane. Mixed ice is the most dangerous kind of airframe icing due to its weight and disruption to the airflow. It also tends to be the fastest accumulating.

There are other types of icing that are of concern. Induction or carburetor ice is formed when moisture is pulled into the carburetor and when fuel is vaporized, causing the temperature to drop (refrigeration cooling and adiabatic cooling). The moisture freezes, causing the induction to choke. Impact ice can also blank off the air filter under some conditions. Instrument ice is when the pitot system or the static system becomes blocked with ice. Some aircraft have pitot and static-system drains that should be checked during preflight. Pitot and static heaters are anti-ice systems and should be on when the wheels are off the ground. "Saving" the anti-ice systems is unwise, especially if you "save" it and discover that it doesn't work when you really need it. All aircraft over 12,500 pounds are supposed to have its pitot/static heaters on when the wheels are off the ground---you should, too. The tables to the left show what happens when the given side of the pitot/static system becomes blocked by ice.

So, when the first signs of spring come along, and before hopping into your airplane, be mindful of unforeseen icing, rime and snow conditions that may get you in trouble. Doing so won't only save your life, but it will also allow you to enjoy more of the wonderful flying that spring is known for.

Spring Ice

1. Get a thorough weather briefing before you fly. You can't make informed, intelligent decisions without good, sound and reliable information. By the way, watching The Weather Channel alone doesn't count.

2. Be informed about de-icing procedures for your aircraft. Refer to your manufacturer's aircraft flight manual (AFM). Some aircraft ice procedures are actually in the aircraft maintenance manual or service supplements.

3. Never attempt a takeoff with any contamination on the aircraft (the clean wing concept).

4. Never attempt to take off in freezing drizzle or freezing rain. Bad things will happen!

5. Never take off in moderate icing or continue flight in severe icing conditions.

6. Make sure that anti-ice systems are turned on prior to encountering icing conditions. The pitot/static heaters should be on from takeoff to touchdown as a matter of procedure.

7. Use the de-ice systems in accordance with the manufacturer's instructions in the AFM. Make certain that they're in working order as part of the preflight inspection.

8. Listen attentively for pilot reports on flight watch or local flight-service-station frequencies.

9. Report icing conditions to flight service when you encounter them so that others can simultaneously benefit and have sympathy for your predicament.

10. Don't hesitate to take action to alter your course, change altitude or get on the ground should you encounter worsening conditions beyond your limitations and skills.

11. Know before you go what the limitations of your aircraft and your own abilities are (be conservative in that estimation).

12. Plan on extra reserve fuel for unexpected conditions. Extra fuel sometimes compensates for stupidity or wrong choices.