Surviving an inadvertent icing encounter

It's an unusually warm winter day with a surface temperature hovering around 38 degrees Fahrenheit. The winds are relatively calm. You haven't been able to fly for a week or two and you really want to go somewhere. A great day, you decide, to fly from Chautauqua County/Jamestown Airport in northwestern New York to Willoughby Lost Nation Municipal Airport in northeastern Ohio, a distance of 99 nm. The weather looks good along your route, except for Ashtabula County Airport in Ohio, 20 miles west of the Pennsylvania state line -- the Jefferson VOR, one of your checkpoints, is just three miles from there -- but it still should be well within VFR limits.

Survival guide When you recognize that you have ice, there are a few simple things to keep in mind: Assume you have tailplane ice; keep your airspeed 5 to 10 kt above normal landing speed; and do not use full flaps. With ice on the airframe, the stall warning system may not work properly, because it is calibrated to a clean wing. An ice-contaminated wing changes all that. You must use your senses to differentiate between a wing stall and tail stall. In approaching a tail stall you will feel buffeting or lightening of the controls when flaps are lowered, have difficulty trimming, or experience pilot-induced oscillations. In a full tail stall the nose and yoke pitch forward simultaneously and may require a large amount of pull force to recover. Raise flaps to the previous setting. That is, undo your last flap input and consider landing with no or reduced flaps. Differentiate airframe buffet (wing stall) from yoke buffet (tail stall). Make pitch changes slowly, especially nose down. Become acutely aware of the symptoms. Do not use the autopilot if one is installed on the airplane.

There's an overcast layer along the route at 4,000 feet and no tops reported; however, you will be in the clear at 2,500 feet. At your departure time, all the airports along your route of flight reported visibilities better than five miles except for Erie, Pennsylvania, which is next to Lake Erie and showed lower weather coming in off the lake. It indicated that Ashtabula County may have a few clouds at 3,000 feet and occasionally three to five miles' visibility in mist.

About 55 nm after leaving Chautauqua County, visibility begins to decrease because of mist. As you get closer to Ashtabula the mist thickens, and now it's questionable whether you're maintaining VFR at 2,500 feet. You tune the Ashtabula ASOS to get the current weather information, and it reports a few clouds at 2,100 feet.

Concerned that these few clouds could evolve into something more, you decide to descend. A quick check of the temperature gauge shows minus 2 degrees Celsius. Now you notice little streaks of water running back on the windshield, then a thin rim of ice forms at the base of the windshield. A minute or two later, the visibility hasn't improved, and you decide to divert to Ashtabula County.

Ashtabula County is a nontowered airport, so you announce your presence on the unicom and set up for a straight-in landing. You've noticed some ice on the wing and know, from what you have heard and read about ice, to land about 5 to 10 kt faster than normal. You're a little high and fast, so you add 10 degrees of flaps. At 500 feet agl and a half-mile out, you're still slightly high and faster than you want at 75 kt. You move the flap lever to 20 degrees, and the airplane behaves a little erratically. Now you add full flaps and have a hard time maintaining a steady approach angle. You bump down and take a deep breath after your harder-than-usual landing. What happened?

NASA knows ice

Your first inadvertent icing encounter was "an emergency situation. The pilot did the right thing by landing at the nearest airport," says Kurt Blankenship, a research pilot at the NASA Glenn Research Center in Cleveland. "You have to look at all your outs. As a pilot you'll have to assess and make the decision as to which course of action will get you out of icing the quickest. In this case, the pilot chose to land at the airport which he could see, versus doing a 180-degree turn and staying in icing longer," he adds.

William Rieke, a NASA research pilot and chief of aircraft operations at NASA Glenn, says to consider the 180-degree turn only if you have no other options -- like an airport in front of you. "If you are getting ice and you have to do a 180 to get out of it and in the process go back through [the ice] and accumulate more -- or worst case, if the air mass is moving in the direction you just turned to, you may be in it even longer than your initial encounter." If terrain clearance or clouds were not an issue, Rieke would prefer to descend 3,000 feet or, if the airplane was powerful enough to climb with the weight and drag of accumulated ice, climb 3,000 feet in an effort to get out of the icing conditions. (Most training aircraft would not be able to climb if they were carrying an appreciable quantity of ice.)

Icing prediction, research and forecast Early in the morning of a cold, gray February day, the icing research team at the NASA Glenn Research Center in Cleveland is conferring with the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, working out a flight plan that will take their highly instrumented de Havilland Twin Otter into ice. Not just any ice-they want to burden the airplane's wings, tail, and struts with some of the thickest, nastiest ice that winter can throw at them. (The Great Lakes region and the Canadian Maritimes have some of the world's most prevalent icing conditions.) As they finalize the day's flight plan that will take them southeast of Cleveland into the worst of the icing conditions, the talk turns to ceilings and cloud tops. They want to determine the areas where clouds will be thick and contain enough liquid to provide consistent icing conditions, because the ultimate end of this mission is to collect ice shapes for simulation validation. They are also seeking information on escape routes to exit the icing conditions if necessary. The flight plan will take them south to Mansfield, east to Akron, back toward Mansfield, then home. Once in the air, they'll stay in the heaviest and most consistent icing conditions by communicating with NCAR via satellite link. NCAR meteorologists will look at their remote sensing sources to update the Twin Otter crew on where to find ice. The Twin Otter is a specialized aircraft, designed to withstand the stresses of ice contamination and instrumented to collect a variety of cloud data, including drop size, liquid water content, and temperature. These data tell the scientists the intensity of icing conditions. The researchers look for conditions to replicate in NASA Glenn's simulation facilities, then collect that icing cloud data and allow ice accretion on the wings, wing cuffs, and test airfoils as well as all the unprotected surfaces on the aircraft. "We document specific ice shapes then try to replicate them in our simulation facilities using the Icing Research Tunnel and LEWICE, GRC's icing prediction software," says Thomas Ratvasky, an aerospace engineer at the Glenn Research Center. The icing missions and research have tangible results, among them advances in understanding tailplane icing. The scientists and pilots, after a thorough exam of cause and effect, helped to develop guidelines for certification testing, as well as education and training specific to tailplane ice contamination. GRC's aircraft icing project has also developed a significant flight program for super-cooled large droplet icing that has enabled NCAR to develop widely available weather forecast tools. That research is the basis for new aircraft certification standards. At press time, Ratvasky and representatives of industry, pilot associations, and governmental agencies were meeting at the request of the FAA's Aviation Rulemaking Advisory Committee to develop new rules that incorporate research on supercooled large droplets conducted by NASA Glenn and the Meteorological Service of Canada. These new data indicate that the current icing certification envelope does not fully describe the icing environment found in natural flight test, says Ratvasky. Future works In the works is ground-based equipment using a combination of multifrequency radars and radiometers. "We are working to develop technology to remotely detect icing conditions using ground-based equipment with the eventual goal of in-flight capability," says Ratvasky. These are the candidate technologies to develop an instrument package composed of off-the-shelf items to locate areas of severe icing conditions so pilots can circumnavigate them. "Icing is fairly unpredictable; you may have encountered it and handled it, but the next encounters you experience could be significantly different. Icing is a very dynamic event -- don't let one instance make you complacent," warns Ratvasky, a private pilot.

To prevent the possibility of aircraft icing, it's important to stay out of visible moisture any time temperatures are below 0 degrees Celsius. And remember, on a standard day you lose about 2 degrees Celsius per thousand feet. In this case, the precipitation was almost invisible.

The ice collected while flying through the freezing mist/drizzle accreted in a variety of places, including the horizontal tail or stabilizer. Here, Blankenship says, airflow started to separate at the tail, causing the controls to be heavy in one direction and light in another. This leads to overcontrolling the pitch of the aircraft because of the control imbalance. The pilot's difficulty flying the approach and landing was not caused by nerves. Rather, Blankenship explains, the pilot most likely had trouble controlling pitch on the approach because of pilot-induced oscillations (PIO).

"The airplane is close to a full tail stall. This typically happens near the full flap setting at the high end of the flap speed range," Blankenship says. A full tail stall is more dramatic and much harder to recover from than the wing stalls practiced in training.

When the wing stalls you feel the buffet, then the break, and the airplane settles and loses lift. The pilot must lower the nose to recover. In a tail stall, however, the nose will pitch forward, and the yoke snatches forward in an aircraft where the elevator is operated by cables or pushrods as the airflow over the horizontal tail completely separates -- a scenario that could have occurred if the pilot used full flaps. "The pilot did the right thing by keeping speed up, but the cost was being at the high end of the flap speed range -- which could set him up for a full tail stall," Blankenship says.

So what should you do if the airplane is not responding properly to elevator inputs or you feel buffet in the controls? Immediately raise the flaps, says Blankenship. "If the yoke is snatched forward, pull back to recover, and retract the flaps. Pulling back on the yoke is opposite to the recovery technique for a wing stall. [Because the tail develops lift in a downward direction, the yoke must be pulled and not pushed.] In icing experiments in our de Havilland Twin Otter, the pilots needed 180 pounds of force to recover from a full tail stall, and they lost over 600 feet." These were experienced test pilots who expected a tail stall caused by ice.

Know the difference

In this scenario, with the added airspeed on approach, a tail stall is more likely than a wing stall. If there is any ice on the wing, there may be far more on the horizontal stabilizer -- and the pilot may not know ice is building until the airplane's configuration is changed. On approach, with flaps extended and the airplane close to the ground and close to its performance limits, recovery is very difficult.

Even a small amount of ice on the leading edges can cause flow separation and a sudden change in attitude when different inputs are applied, says Rieke. It's critically important to remember that what works for wing stall may intensify a tail stall. In a wing stall recovery, the pilot relaxes back-pressure or pushes the yoke forward to reduce the angle of attack and increase airspeed and regain control.

Responses associated with tail stall recovery are counterintuitive to what student pilots are taught. The stall symptoms are similar, but the recovery is absolutely different: Pull the yoke back (which might take great force); raise the flaps, if lowered; and reduce or be judicious with the power. The NASA pilots say a tail stall talks to you through the yoke; a wing stall you feel in the seat of your pants.

Tail stalls almost always occur with flap extension or at the high speed limit for flap extension.

And a tail stall can sneak up on you. Icing doesn't always show up on the windscreen. If you feel buffet in the yoke and a lightening in the controls that leads to PIO, you most likely have tailplane ice. "It's possible to have very little ice on the wing and significant accumulation on the horizontal tail," says Rieke. "Any ice will affect how the airplane flies; to what degree is unknown."

Remember, with ice on an aircraft not certified for flight in icing conditions, "you're a test pilot. The aerodynamics of the wing and tail have changed. You could experience wing stall without warning," cautions Blankenship. Ice must be avoided -- even in small amounts, it changes the aerodynamics of the wings, tail, and propeller, usually resulting in increased drag. The pilot may only notice ice contamination as a loss of airspeed. The bottom line is that it doesn't matter if you've got a wing or tail stall. To avoid both, use reduced or no flaps on approach. This takes the tail stall out of the picture, and you can fly the approach with extra speed to avoid a wing stall, depending on the situation, says Blankenship.

By Leslie Sabbagh

Science journalist and Popular Mechanics contributing editor Leslie Sabbagh has reported extensively on NASA. A student pilot and member of The Ninety-Nines, she lives in suburban Cleveland.

Want to know more?

• Meteorologist Jack Williams discusses how to detect weather patterns favorable for icing conditions.
• The AOPA Air Safety Foundation's Aircraft Icing Safety Advisor explores ice flying strategies for instrument-rated pilots.

Links to these and other resources are available at AOPA Flight Training Online.