Wx Watch: The Worst Ice

Trust the weather always to bring surprises. That goes for its changeability, to be sure, as well as for our understanding of how weather works. For example, just when we think we understand all about a particular phenomenon, something comes along to remind us how little we really know about the environment in which we fly.

Take airframe icing. For decades pilots have been told that icing comes in four varieties: clear, rime, mixed, and frost. To refresh the memory briefly, clear ice is a tenacious, transparent coating most often caused by the kind of large, supercooled water droplets found in cumulus clouds. Clear ice typically forms in temperatures between 0 and minus 10 degrees Celsius, or 32 and 14 degrees Fahrenheit. The absolute worst form of clear ice accretions is caused by encounters with freezing rain. Freezing rain is very often associated with conditions ahead of a winter warm front, in which case rain aloft falls from the frontal surface into colder air below. Fly in these conditions and a thick sheet of clear ice can form over your airplane's leading edges in a matter of seconds. Clear ice is always bad news, but freezing rain can cripple even the largest, most powerful airplanes — including those equipped with full ice protection systems and certified for flight in known icing. Simply put, no airplane can beat freezing rain.

Rime ice shows up as a milky, rough-surfaced buildup. It's usually found when flying in stratus clouds with outside air temperatures between minus 10 and minus 20 degrees Celsius, or 14 and minus 4 degrees Fahrenheit. As with any form of icing, the onset of rime icing is a warning. The proper response is to exit icing conditions immediately. Climb (if airplane performance or cloud tops permit) or descend out of clouds or precipitation, fly to warmer temperatures, reverse course — whatever it takes to make sure that the ice goes away.

Mixed icing is just that — a mixture of clear and rime. It's caused by a mish-mash of cloud droplet sizes and temperature zones. These formations frequent frontal zones overlying mountainous areas and terrain leeward of the Great Lakes, where lake-effect conditions load the atmosphere with lots of moisture.

Frost happens when nighttime temperatures drop to freezing and dewpoints are below the freezing mark. This provides enough moisture to crystallize over almost the entire airframe. Radiational cooling associated with high pressure systems and clear nights is most conducive to frost formation. As with any other type of icing, frost must also be dealt with immediately. Here, however, there is the luxury of being able to remove it on the ground, as a part of other preflight chores.

So much for conventional icing wisdom.

Recent icing research now informs us that clouds composed of very large supercooled "drizzle drops" can cause huge accumulations of mixed icing in very short periods of time. These drizzle drops can be as large as 200 to 400 microns in diameter — or up to 10 times the size of the normal, 10- to 20-micron-sized supercooled water droplets that cause the garden variety of clear and rime icing. When droplets this big hit airfoils, ice builds up with angular projections and pronounced ridges.

This new information comes to us via an investigation of the October 31, 1994, crash of an American Eagle ATR-72 near Roselawn, Indiana. It bears mentioning that the National Transportation Safety Board has yet to reach final determinations of the causes and factors behind the crash.

Preliminary information, however, indicates that the airplane had been at 10,000 feet and in a holding pattern for 32 minutes at the Lucit intersection, some 50 nautical miles southeast of Chicago. The hold was issued to use up time until an arrival slot became available at the Chicago-O'Hare International Airport. The ship's flight data recorder (FDR) showed that the ATR's airspeed ranged from 160 to 175 knots in the hold. Flaps were set at 15 degrees — the first notch for an ATR-72 and its smaller stablemate, the ATR- 42.

The airplane's cockpit voice recorder (CVR) apparently indicated that the ATR pilots knew they were taking on ice and had been activating the airplane's leading-edge boots and other ice protection equipment. Just before they were cleared to descend to 8,000 feet in the holding pattern, one of the pilots said, "We still got ice."

During the descent, airspeed rose to between 180 and 185 knots. The audible flap-configuration overspeed warning sounded, and at 9,400 feet the crew retracted the flaps.

Almost immediately, the first reports say, the FDR showed the airplane's angle of attack increasing. When the airplane's angle-of- attack indicator reading reached six degrees, the autopilot disengaged. A quarter-second after that, the ailerons deflected to near maximum in the right-wing-down direction.

A right rolling motion began, but it was corrected when the angle of attack was slightly reduced and left-wing-down aileron inputs were introduced. The correction was short-lived, however. The ATR once again rolled right, then entered a steep nose-down, inverted attitude. During the subsequent dive, airspeeds reached some 350 knots. Impact occurred only 20 seconds after the first roll to the right. Everyone aboard died in the crash.

The NTSB noted that the weather conditions at the time of the accident involved temperatures near freezing and visible moisture, and "probably supercooled cloud." We all know that this is a recipe for icing, but the NTSB let us in on something that we didn't know. Another American Eagle ATR, this one a -42 model, also had its autopilot disconnect and also experienced uncommanded rolls under similar weather conditions. This incident took place on December 12, 1988, at Mosinee, Wisconsin. The pilots were able to recover but lost 600 feet in the process. And that's not all. According to the NTSB, there were "other" cases of ATRs with roll excursions in the six-year period from 1988 to 1994.

Both the Mosinee and Roselawn events brought Federal Aviation Administration responses. The Mosinee incident caused the FAA to ban the use of the ATR-42's autopilot when freezing rain was forecast or reported. "Ice accretion due to freezing rain may result in asymmetric wing lift and associated increased aileron forces necessary to maintain coordinated flight," the FAA said. "Whenever the aircraft exhibits buffet onset, uncommanded roll, or unusual control wheel forces, immediately reduce angle of attack and avoid excessive maneuvering."

(Hold on a second, I hear you say. No airplane is authorized to take on freezing rain. True, but the recommendations are intended to give guidance to crews that find themselves stumbling into, or trapped in, this kind of a severe icing situation. According to the official definition, no airplane is able successfully to take on severe icing, and this includes freezing rain.)

The autopilot ban was withdrawn after manufacturers complied with a service bulletin asking that vortex generators be installed on the wing's upper surfaces, ahead of the ailerons. Vortex generators, it was hoped, would preserve boundary layer airflow and aileron effectiveness, should ice contaminate the wing leading edges.

Right after the Roselawn accident, the FAA came out with some recommendations for ATR pilots holding in icing conditions: Keep the speed up, keep the propeller rpm up, keep the flaps retracted, and don't use the autopilot — even if vortex generators are installed. The NTSB agreed but said that pilots flying the ATR manually may experience changes in aileron control forces. Onset of a roll problem, the NTSB added, may come so suddenly that a crew may not be able to recover.

The FAA also issued a notice to air traffic controllers, telling them to provide expedited service to ATR pilots who request route, altitude, airspeed, or any other deviations to avoid icing conditions.

To learn more about the ATR's behavior in severe icing, the NTSB recommended that a special certification review be conducted, including flight and/or wind tunnel tests. Until the results of these tests were known, the NTSB also recommended that ATRs not be flown in known or reported icing conditions.

The airlines flying ATRs — Continental Express, American Eagle, Trans World Express, Atlantic Southeast, and cargo operator Summit Aviation — complained about the restrictions. They asked the FAA to withdraw any proposed limitations, saying that they had long experience flying ATRs in ice and that coping with ice was more of a pilot technique problem than an airframe design problem. But the FAA didn't buy the argument.

The FAA issued an airworthiness directive (AD) prohibiting flight in icing conditions and also decided to go ahead with a special certification review. At Edwards Air Force Base, California, six flights lasting a total of 15 hours were conducted. During these tests, an ATR-72 was flown behind an icing spray tanker. A nozzle trailing from the tanker sprayed droplets of water varying in size from 40 microns (the maximum required by certification rules) to 400 microns.

The ATR handled the 40-micron icing situations well. But when droplets reached the 200-micron size, a ridge of ice formed just behind the aft border of the leading-edge deice boots. At slower airspeeds, aileron effectiveness was diminished with this kind of icing formation. This seems to be consistent with the behavior of the ATRs in the Mosinee and Roselawn events, so attention focused on a new type of icing condition — one with huge droplets that could spread across a wing much farther than originally expected.

To make sure that ATRs could handle this new type of severe icing, Avions de Transport Regional (the French-Italian consortium that builds the airplane) came up with a new deice boot design that extends farther back on the wing and prevents any ridges of ice from building aft of the leading edges. The new boots' area of coverage is twice that of the originals'.

On January 12, the FAA lifted the ban on flying ATRs in icing conditions, with the caveat that crews follow new training and flight procedures. The prohibition against flight in freezing rain and drizzle remained — a rather moot point since airlines do not dispatch their airplanes into areas known to be experiencing these severe conditions anyway.

The new procedures closely resembled the recommendations issued earlier. If caught in freezing rain, pilots were told, do not retract the flaps (if they were extended in the first place), don't use the autopilot, and don't let the airspeed go below 175 knots. To identify freezing rain and the dangerous new "supercooled drizzle drop" icing condition, crews were told to look to the cockpit side windows. If a vertical line of ice forms at the midpoint of the window pane, the advice is to fly out of the area immediately. Of course, the ship's ice detection equipment should also be monitored for any signs of ice buildup.

The new, larger boots passed yet another series of icing tests and were approved; an AD was issued requiring their installation on all ATRs. The first installations began in June, and by September, all 175 U.S.-registered ATRs had the new boots. Even with this extra protection, however, the freezing rain and training provisions of the AD remain in effect.

So ends the ATR icing saga, at least for the time being. The fallout from this chain of events has yet to completely shake out. The discovery of the new, larger droplets has prompted a move to reexamine the icing certification requirements of all transport- category airplanes — turboprops in particular. The existing rules were concocted in the late 1940s. We have learned much since then, but the regulations do not reflect all the advances we've gained in both meteorology and operational experience. The icing envelope is not what we thought it was, and extra-large icing droplets of the 200- to 400-micron size seem to be more common than anyone ever dreamed.

In a move to educate pilots about the dangers of this "new" type of icing, the FAA's Aircraft Certification Service has recently put out a leaflet titled Roll Upset in Severe Icing! It gives some excellent new insights on airplane behavior in severe icing. Copies can be requested by writing to: FAA Office of Public Affairs, Department of Public Inquiries, APA-200, 800 Independence Avenue, S.W., Washington, D.C. 20591.

This all goes to reinforce the warning that icing of any type can drastically affect an airplane's stability and handling — and the larger the droplet, the worse the effects.

It's something to keep in mind during the icing season. Avoiding ice is the first priority. If it's encountered, leave the area immediately. And before taking off, always have a strategy for coping with ice if you should run into it.

Here's another tip. Check the Limitations section of the pilot's operating handbook or information manual for the airplane you'll be flying, plus any appended safety supplements. Procedural recommendations for flying in icing conditions may be published in these documents. Separate safety supplements that issue operational advice for coping with icing conditions may also be published. For some of its airplanes, for example, Beech publishes minimum airspeeds for flight in icing conditions. For Barons and Travel Airs, that airspeed is 130 knots.

This kind of information certainly doesn't constitute an endorsement to fly in icing conditions, but it's nice to know in case you wind up in that predicament. The intent of airspeed recommendations is to help pilots establish an angle of attack that will minimize ice accretions on the wing's underside. Accretions there destroy lift and raise stall speed very quickly; and they are especially dangerous because they are invisible to pilots of low- wing airplanes.

These warnings may not come as news to many. But the Roselawn accident's legacy is to remind us all that nothing is ever cut and dried when it comes to weather and that what we don't know certainly can hurt us. Makes you wonder what else is lurking out there.