Understanding Icing

by Crista V. Worthy

As winter bears down upon us, it’s worth reviewing some basic principles of icing. Certainly your goal should always be to quickly exit an area of icing, even in an airplane approved for flight into known ice (FIKI). Icing can occur if: (1) the temperature is below freezing, (2) you are in a cloud with liquid water, (3) the droplets are large enough, and (4) you stay in the cloud long enough for ice to build up. The rate of ice build-up may depend upon what type of cloud you’re in.

Ice can build up twice as fast in cumulus clouds with high water content, but the area of icing in cumulus clouds is normally not as widespread as in a stratus cloud, so as you move through, or climb or descend, the time spent in icing conditions could be small. Another important thing to remember about clouds is that the greatest amount of liquid water generally occurs near cloud tops, due to the cooling and condensing with height above cloud bases (this is especially true for cumulus clouds).

Similarly, the chances of icing are greater just below the freezing level; at colder temperatures water droplets are tinier, and as the temperature continues to fall, water droplets are more likely to become ice crystals. One dangerous condition can cause icing even if you are not in a cloud: freezing rain or drizzle can exist down to ground level below a cloud deck, causing ice to form during takeoff, landing, or ground ops if the surface temperature is below freezing.

Dangerous Performance Reduction

As ice accumulates, it can decrease your aircraft’s performance and alter its handling characteristics. The effects are directly related to the amount and shape of ice formation. In flight, air molecules stream above and below the wing (or airfoil). As the airfoil approaches water droplets in a cloud, there is a battle between the tendency of the droplets to follow the airstreams above and below the wing, and the inertia of the droplets to strike the airfoil head-on. Smaller droplets tend to follow the airstreams unless they hit the wing dead-on, whereas larger droplets, with more mass, may strike the wing several inches back on the upper or lower surfaces. Thus at lower temperatures with small droplets, ice tends to build more evenly. At temperatures near freezing, however, with larger drops, water often runs back and freezes slowly and irregularly, sometimes with flat or concave surfaces and protuberances facing the airstream on either side of centerline.

The rate of ice build-up can also depend upon the cross-section of the airfoil. A thick airfoil pushes more air ahead of it, and water droplets have more room to move out of the way. Because the tail surfaces have smaller leading edge radii and chord length than wings, they can collect more ice more quickly.

As icing alters the leading edges aerodynamically, lift and stall angle are decreased, while drag and stall speed are increased. Particularly as you fly at higher altitudes in a normally-aspirated aircraft, ice-induced drag can decrease your margin between cruising speed and stalling speed. Eventually you may find the only way to maintain aircraft control is to descend, possibly even with full power.

Icing can reduce total available lift by as much as 50 percent. With less lift, you need a higher angle of attack (AOA) to maintain level flight, which means you will be cruising closer to stall. Additionally, the ice causes the wing to stall at a lower AOA. Of course that means the plane will stall before your stall horn goes off, since it’s set to a different AOA. You won’t have the other usual warnings of a stall either. Normally, a wing reaches maximum lift shortly before the stall, then lift decreases, then the wing stalls, so you have time to react. By contrast, an iced wing can frequently go from max lift to almost none: no warning and you’re suddenly in uncontrolled flight. Additionally, the thinner wingtips may accumulate more ice and stall first, leaving you without aileron control.

Tail Surfaces Are Critical

If your wings are iced up, your tail is probably worse, with its thin leading edges. If the stabilizer is contaminated, there can be some airflow separation and reattachment (bubble) on the underside of the airfoil. Remember, the tailplane acts like an upside-down wing, producing downforce to counteract the heavy aircraft nose. Tailplane ice can become critical during the landing process, as flaps are added. Flaps increase wing downwash so relative wind comes more from above, increasing the AOA of the tail. Flaps also shift the center of lift back, increasing the tendency for the nose to pitch forward, requiring more downward “lift” from the tail, which further increases its AOA. Increased thrust aggravates both of these problems. As the separated air (bubble) moves aft, the stabilizer becomes less effective. Critical to recognizing an impending tailplane stall is that buffeting will be felt in the yoke, not the airframe—pitch excursions, pilot-induced oscillations, difficulty trimming, and the yoke pulsing or vibrating (an autopilot can mask this, so it should be turned off if you collect ice). If the bubble moves back past the hinge, it will now affect the movable elevator: negative pressure under the elevator will drive it down so suddenly it may actually slam the yoke full forward. Recovery is the exact opposite of a normal stall. After much study by NASA (gltrs.grc.nasa.gov/reports/1999/TM-1999-208901.pdf) and others, the procedure is to retract flaps, pull back fully on the yoke, and reduce thrust as needed.

Unless you can quickly escape to warm air, you’ll have to plan a zero-flap approach and landing. Try for an ILS with a continuous descent (bonus: a runway with an ILS should be long enough). You’ll have to come in fast, and this is no time to calculate the safe speed; do it at home. Ice can increase your stall speed by 40 percent, so take your clean, max gross stall speed and add 40 percent. Fly your approach at 1.3 times that speed, slow to 10 knots over that speed at the threshold, and don’t pull to idle until you are low enough that a sudden stall would be acceptable. Skip the flare, use the brakes, and count your blessings—after you pat your plane on the windshield.