Wx Watch: Fear of Freezing

From our earliest days as pilots, we've had instructors, textbooks, training videotapes, and all manner of aviation publications warning us of the dangers of flight in icing conditions. The scare tactics are justified, and for the most part they work. Accident statistics show that comparatively few of us crash as a result of flying into what most would consider a stereotypical ice scenario (i.e., flying along in the clouds, then suddenly picking up a load of killer ice). As the AOPA Air Safety Foundation's recent safety review of general aviation weather accidents between 1982 and 1993 clearly shows, carburetor icing is more of a threat than airframe icing. Carb ice, or misuse of carburetor heat, is one of the top five causes among the 637 "icing" accidents in that time period. Nevertheless, continuing or initiating flight into areas having forecast and/or reported icing conditions leads the list of fatal icing crashes — at a more or less steady rate of 12 accidents per year. Many of the pilots involved in these types of accidents had more than 1,000 hours of total flying time, as well as instrument and multiengine ratings.

News like this elicits instant speculation. How many of these airplanes were equipped with deice boots or other ice protection equipment? The data doesn't provide this kind of analysis, but when it comes to the dangers associated with icing conditions it doesn't really matter. Ice accumulations are bad news — whether your airplane is equipped with ice protection equipment or not. And pilots flying airplanes certified for flight in known icing conditions should not, under any circumstances, believe that they have carte blanche to safely mill around in icing conditions indefinitely. Ice protection devices merely buy you some extra time to come up with an escape plan- -they don't inoculate you from falling out of the sky.

That said, it's important to emphasize that airframe and ice protection equipment manufacturers go to a lot of trouble to make sure that those airplanes certified to fly in known icing conditions are up to the job. That's why a "known icing" package can tack an extra $35,000 or more to an airplane's base price.

Earning known-ice approval starts with an analysis of where supercooled droplets strike an airplane's wing and stabilizer leading edges. This task usually takes place in an icing wind tunnel, where scaled-down components or full-sized flying surfaces are placed in a stream of air chilled to icing temperatures and filled with supercooled droplets. In the tunnel, various angles of attack can be simulated by inclining the wing and stabilizer chords at different angles to the onrushing air. In this way, wing leading edges can be examined to see where their impingement points lie. An impingement point is the location on a leading edge where ice first accumulates.

Using this information, designers of deice boots and anti-ice panels decide how to size their products; the main question is, how far back on the wing chord should the boot or anti-ice panel extend in order to afford protection at a wide range of angles of attack?

Naturally, other components in a known-icing package are also tested for their ability either to prevent ice from forming (an anti-ice function), or shed it once it's accumulated (a deicing method). Some of these components can also be tested in an icing wind tunnel. These include such items as electrically heated pitot tubes, angle-of-attack probes, windshields or windshield plates, and static ports.

Of course, you can't certify an airplane for flight into known icing without having a complete system of components, and it only makes sense that the operation, reliability, and design of each component be verified to the extent possible before a test airplane takes up the whole works for certification trial flights.

The goal of flight test programs, obviously, is to learn how the airplane and its proposed ice protection system will behave in real-world icing conditions. This means flying in natural icing conditions for specified distances and times, within a prescribed range of outside air temperatures and altitudes, and in clouds having water droplet sizes and liquid water contents falling within certain defined limits.

The FAA requires compliance with two different natural icing test scenarios in two different meteorological envelopes. These envelopes are published in FAR Part 25, Appendix C, and if manufacturers want known icing certification, they have to fly enough times in each envelope to satisfy the FAA that the ice protection system can take on each set of conditions.

One envelope, called intermittent maximum, is intended to replicate the kinds of icing conditions encountered in cumuliform clouds. This envelope calls for test flights in temperatures between 32 and minus 40 degrees Fahrenheit, at altitudes between 4,000 and 22,000 feet, and in conditions where water droplets have mean diameters between 15 and 50 microns. (Just for comparison's sake, the average diameter of a human hair is 25 microns, or about a thousandth of an inch. Droplets of freezing rain are roughly 1,000 microns in diameter, and the average raindrop is about 5,000 or so microns in diameter).

Another parameter, liquid water content (LWC, for short) is also specified. LWC is reported in terms of grams of water per cubic meter. If you could squeeze all the water out of a one-cubic- meter parcel of air, that amount of water would be the LWC value. The intermittent maximum envelope calls for LWCs between 0.25 and 2.8 grams of water per cubic meter.

The intermittent maximum rules call for test flights on runs having average distances of 2.6 nautical miles. That doesn't seem like an adequate distance for gathering a good, healthy dose of ice, but perhaps the thinking is that cumulus clouds may be smaller and therefore less time would be spent in them. In any case, those are the rules.

In actual practice, FAA pilots (yes, FAA certification test pilots do the flying) are looking for exposures to icing conditions of durations long enough to satisfy them that the system can handle a reasonably healthy accumulation of ice — horizontal distance requirements notwithstanding. This usually amounts to encounters lasting 20 to 45 minutes.

The other envelope is for continuous maximum conditions, which are supposed to reflect the conditions in stratiform clouds.

How do test crews know for sure that they're flying in the "proper" icing conditions? Special instruments quantify the size of the cloud droplets, as well as the ambient liquid water content. A gel-coated slide serves as the droplet size indicator. During a flight, a technician lowers the slide through a slot in the airframe and into the relative wind, then quickly pulls it back. The size of the droplets is determined by the size of the splatter patterns on the slide. LWC indications are gathered by a dedicated pitot-like probe that's affixed to the lower surface of a wing.

In addition to these scientific measurements, video and/or still cameras are used to document the nature and severity of each icing encounter.

It may be difficult to believe, but finding suitable icing conditions can be a major hurdle. Sometimes manufacturers have to fly to distant locations or wait for long periods of time to locate the right kinds of ice. These extra measures and delays can tack huge expenses onto certification costs. (Funny, those of us not engaged in icing certification trials seem to have no trouble at all finding ice).

For those times when actual icing conditions are especially elusive or when specific components need to be put through the wringer, tanker airplanes can be used to meet certification requirements. Icing tankers are airplanes specially modified to carry huge tanks of water and spray water droplets from a boom. The subject airplane flies in formation behind the boom and in the spray. If the air temperatures are in the proper range, the result can be some fairly large ice accretions. For this reason — and because the subject airplane can exit the spray plume at a moment's notice, should safety become an issue — tanker tests can be the tests of choice. However, they are not substitutes for flying in the real McCoy, and there's no way of avoiding the requirements to fly in actual icing conditions.

Earning known-ice certification doesn't end with proof that the ice protection systems work in a variety of icing conditions. Why? Because ice forms on unprotected areas of the airframe. The wing leading edges may be clean, all right, but what about the wing tips, elevator balance horns, and other unprotected surfaces? Any ice accumulations observed on unprotected areas have to be tested to make sure that the airplane's handling, stability, and control are not compromised. To do this, engineers attach styrofoam blocks of sizes and shapes corresponding to any ice accretions observed during natural-ice or tanker tests. After the artificial shapes are affixed to the airplane's unprotected parts (duct tape is a tried-and-true method of attachment), it's time for more test flights.

Only after this raft of tests is successfully completed will the FAA grant certification for flight into known icing conditions. Having this certification can give you peace of mind during certain winter flight conditions and can make icing encounters legal — but not necessarily safe. Just remember, this nice-to-have feature does not mean that you have the power to defeat inch- thick ice accretions. It merely means that you have a fighting chance of escaping ice, and a little extra time to do so.

It bears repeating: Any pilot flying any airplane should plan flights so as to avoid ice in the first place and escape from icing conditions the moment any ice should begin to form.

There's a lot we don't know about the worst icing conditions. The October 31, 1994, crash of an American Eagle ATR-72 proves this point. In this accident, the airplane iced up in conditions where droplet sizes were estimated in the 400 micron-plus range. Those droplets represent sizes 10 times greater than the certification limit. The airplane was certified for flight in known icing and was perfectly legal. Even so, those huge droplets (dubbed supercooled drizzle drops or SCDD) caused a ridge of ice to form aft of the deice boots; this, in turn, generated a huge rolling moment that caused the airplane literally to fall out of the sky. Mitigating factors were also at work, to be sure (the crew's situational awareness was very low; the airplane was being flown with an unapproved 15-degree flap extension). But an essential fact clearly can be deduced: Many of us may be flying in conditions never addressed during certification tests; therefore, many of us have been, or will be, icing test pilots. The ATR was flying in clouds at 175 knots and 9,000 feet when it first ran into trouble — that's an environment familiar to all of us, not just pilots of turboprop commuters.

Does this accident mean that icing certification rules are in for a change? Maybe. Couple the ATR evidence (see "Wx Watch: The Worst Ice," December 1995 Pilot) with some of the recent findings on tailplane icing ("Wx Watch: Tailplane Icing," March 1994 Pilot) and it's easy to see how a movement to change the icing envelopes and test procedures could come about. The FAA has already had preliminary conferences about this very subject.

I'll be sure to keep you posted on any changes in icing issues. The ATR crash shook icing certification out of a 50-year-old stasis, and as a result we may all be affected by new initiatives of unpredictable scope.