I’m a South Florida lady, and so is my fine flying machine. It’s relatively fat wing, tapered tips, relatively thin horizontal and vertical tail surfaces and elevator with “horns” are made of .032 aircraft-grade aluminum coated with paint (and not even that much paint). And even though I’m at a balmy 30 degrees North latitude, a momentary jaunt through the middle of a building cumulous cloud at the right altitude and outside temperature can easily coat my aircraft in a shiny glazing of thick, clear ice.
The aerodynamic results of accumulating even a thin coat of ice, whether clear, rime or mixed, can be a bona fide emergency, depending on the airplane’s design characteristics. Among the choices designers make when planning a new aircraft are the wing’s size, shape and airfoil. As a rule, a thicker airfoil will carry more ice than a thin one, all things being equal. “All things” are seldom equal, however, and how an airplane reacts to ice can vary from flight to flight. Here’s why.
The crew of an Embraer EMB-120 Brazilia learned why firsthand on the way back from the Bahamas to Fort Lauderdale, Fla. The EMB-120 has a thin, “go fast” wing, and this commuter aircraft’s short jaunt that day through a building cumulus at 17,000 feet might have been just a rock ’n roll ride for a few minutes, designed to give the pilots their “jollies,” Gary Larson style. I’d like to believe they just couldn’t get around towering cumulus in all quadrants and were finally forced to go through one.
Inside, they hit a phenomenon that scientists are still struggling to understand—supercooled liquid droplets (SLD)—which turned the EMB-120 into an ice cube full of people, and one that didn’t fly very well, either. In fact, the NTSB report shows the cloud spit them out in a spin, which they recovered from at or around 10,000 feet when the flight controls suddenly began to work again. They diverted to West Palm Beach (Fla.) International airport and landed safely.
Experience has shown me that even a thin coating of ice can be damaging. I will never forget flying VFR just below a cloud layer in a Cessna 172 many years ago, and picking up a light dusting of rough, white rime ice in the clear air. It was barely 1/8-inch thick and really did not affect the flight capabilities of the aircraft. “How quaint,” I thought.
That was, until I looked back at the nav-radio “whisker” antennas on the tail, which were bouncing with ever-increasing amplitude—a vibration set up by the airframe and the unbalanced state of the antennae, now weighted with ice. Yikes! I descended in a hurry below the freezing level before an antenna departed the airframe.
Mountain ice can be some of the worst. My husband, a freight pilot early in his career, learned about ice one night on a milk run he’d done 50 times before, often in weather, from Oakland, Calif., to Reno, Nev. His Piper Aztec hit a “patch” of icing in the clouds near Truckee that nearly did him in. The mixed rime and clear ice accumulated so quickly that the Aztec’s deicing boots were useless, and the airplane, already at gross weight with freight and now weighed down well beyond that with ice, began to descend.
Reno Approach wished him luck when he dropped off their radar, and luck, combined with dogged survival instincts and some skill, got him onto the localizer back course, which he followed to the end of the runway, all the while carrying full power and descending. He made it. He also never again made the mistake of taking an airplane with that power-to-weight ratio into weather that even hinted at icing.
A Perennial problem
The examples above show how treacherous and insidious unexpected icing conditions can be—and they are the “happy” endings, where the pilots could be debriefed. The NTSB annals are full of proof that icing can and has, throughout the history of human flight, been a killer problem.
Charles Lindbergh iced up during his 1927 crossing of the Atlantic—his journals show he nearly turned back because of the ice. The U.S. Air Mail Service and Army did some research on icing in flight during the mid-1920s, but it took the National Advisory Committee for Aeronautics (NACA), which in 1928 built a small refrigerated wind tunnel at its Langley Memorial Aeronautical Laboratory in Virginia, to really study how ice formed on an airfoil. That was the beginning of decades of icing research.
We continue flying with NACA airfoils on many GA aircraft today, and we still haven’t solved the problem of how to keep ice off of light aircraft lifting surfaces. So, 80 some years and countless scientific studies later, what do we know? One of the keys to escaping the situation is aircraft performance, and some of that is tied up in an aircraft’s wing shape. Why wing shape? Well, different wing shapes, control surfaces and tailplanes handle the ice better than others.
The three airfoils used in a recent NASA study on airfoil performance with ice were the venerable NACA 23012, NLF 0414 and the NACA 3415. These airfoils exhibit a distinct range of aerodynamic characteristics (see the sidebar above for details on NACA airfoil designations). For example, the NACA 23012 is a traditional forward-loaded section with a low pitching moment. It’s used on the Aero Commander Shrikes, Mudry CAP 10B and Helio Super Couriers, among other GA aircraft. Boundary layer transition on the upper surface of this wing is close to the leading edge at moderate to high lift coefficients.
On the other hand, the NLF 0414 airfoil was designed to maintain a laminar boundary layer over a majority of the upper surface at moderate lift coefficients (Alan and Dale Klapmeier, co-founders of Cirrus Design, chose it for their seminal VK-30 design in the 1980s). As a result, the pressure loading is more uniform over the leading-edge and mid-chord regions. Both of these sections (or similar airfoils from those families) are currently flying on both turboprop and piston-engine aircraft.
Meanwhile, the NACA 3415 airfoil has aerodynamic characteristics that lie between the other two. While not nearly as popular for aircraft use as the NACA 23012, this airfoil has aerodynamic characteristics similar to NACA six-series airfoils, which are used on the Cessna 210 and Mooney airplanes.
The gist of the research showed that any airfoil’s pressure load distribution plays a key role in its sensitivity to ice accretion, particularly in the first 20 percent of chord, where ice is most likely to accrete. The results generally indicated that more front-loaded airfoils (e.g., NACA 23012) tended to be more sensitive to these types of ice accretion. The NLF 0414 airfoil, which was the most aft-loaded of the three airfoils tested, was the least sensitive to SLD ice simulations. The “Mama Bear” NACA 3415 sat right in-between on the scale of sensitivity.
But sensitivity to ice isn’t the only factor that stops an airfoil from performing its lifting duties during flight. The same study showed that the aerodynamic effects of ice on an airfoil are a function of the location of the ice with respect to the airfoil’s pressure distribution, the ratio of the ice shape height over the chord length of the wing and the geometry of the ice shape itself. As you might imagine, larger ice shapes can have substantial effects on lift, drag and pitching moment, and SLD, which can stream back and freeze onto areas of the wing where no de-ice can reach it in flight, creates the most problems.
It turns out that the backward streaming of SLD that freeze roughly 20 percent behind the leading edge cause more lift deterioration than even deforming leading edge ice on wings and tail surfaces. And the elevator horns on aircraft such as Cirrus SR20/22 aircraft are absolute magnets for deforming icing. The research showed that close to 30 percent of the total drag associated with an ice encounter remained after all the protected surfaces (leading edges, props, engine air intakes, windshields) of the typical “known ice equipped” aircraft were cleared.
With most airfoils, it takes very little surface roughness to degrade aerodynamics. “Sandpaper” ice can cause a very early and abrupt peak on the lift curve, followed by a precipitous drop in lift. And it’s insidious—until the peak is reached and flow separation occurs, the lift curve may be quite normal—don’t expect to feel any buffet as you get close to aerodynamic stall—no stall warning horn, either.
Larger ice shapes work against lift the same way. And de-icing boots sometimes become the problem. Research by the FAA has shown that large ice shapes can be created by the boots themselves in a mixed rime/glaze condition when the inflation cycle time was three minutes. It turns out that better ice shedding happens when the boots’ cycle time is performed at just one-minute intervals, even in severe icing conditions.
Where else does ice form fast and disruptively? Fly a twin with alcohol deicing props, and you will find yourself jumping out of your skin with every thud of ice shedding off the props and being flung into the fuselage.
Remember, propellers are airfoils, too. Deform them and you’ll lose thrust. Unbalance props far enough with weighty ice and you risk a fracture, or worse. Flat discs and horny ridges of ice are known to form on propeller spinners, too.
Finally, all you have to do is fly a Cirrus or Corvalis without gear fairings and wheel pants to understand how nearly every surface of these aircraft presented to the wind is tuned to help “fly” the aircraft. Now, pollute those carefully honed shapes with heavy, shape deforming ice. Can you spell D-R-A-G?
Pilots stuck descending out of the clouds, their aircraft frozen blocks of ice to the man describe landing with full power, barely maintaining a speed above stall to the runway. These pilots generally survived because they were smart enough not to apply flaps, which can often induce a tail stall in iced-up conditions. Tail stall recoveries are the exact opposite of wing stall recoveries, and bad things happen fast if the pilot can’t rectify the problem by removing flaps and pulling up on the stick immediately.
Speaking of Rules
The FAA’s FAR 91.527 does not prohibit light aircraft from flying in known icing. It does clearly state that any aircraft flying in known icing conditions must be equipped with numerous anti-ice or de-icing strategies.
The regulations don’t go into specifics on how to stay ice-free, but the technologies for doing so aboard piston-powered airplanes include heated windscreens, glycol sprays for prop, windshield, wings and tail surfaces, electric heating or pneumatic-inflated rubber boots on leading edges. Antennas on these aircraft are reinforced or redesigned to handle ice. Turboprops also use boots and the like, while jets usually bleed air from the engines to heat portions of the airframe. There are very few light piston aircraft equipped for known ice, and no airplane is equipped to cruise in known icing conditions: The equipment is there to enable an escape.
Once you find yourself in icing, it might be helpful to categorize it (at least for other nearby pilots and aircraft, since you’ll be getting your little hiney out of there, right?). The Aeronautical Information Manual (AIM) defines how in-flight icing should be reported when filing a Pirep:
Trace: Ice becomes perceptible. Rate of accumulation is slightly greater than the rate of sublimation.
Light: Accumulation may create a problem if flight is prolonged in this environment (over one hour). Occasional use of deicing/anti-icing equipment removes/prevents accumulation. It does not present a problem if the deicing/anti-icing equipment is used.
Moderate: Short encounters become potentially hazardous and use of deicing/anti-icing equipment or flight diversion is necessary.
Severe: Deicing/anti-icing equipment fails to reduce or control the hazard. An immediate flight diversion is necessary.
If All Else Fails, Avoid
I started this cautionary tale with a couple of real-life stories about icing encounters. Both pilots escaped icing encounters without damage to life or property. Though the encounters were different in type of aircraft, scope and severity, we both agree: neither of us wants to tangle with ice, not a little, not a lot, ever again.
The best policy for dealing with ice in the sky? Avoid it. Inadvertently find it? Get out of it.