December 1, 2009
Let’s get something out of the way right up front: Avoid icing conditions if at all possible, and escape them immediately should you encounter icing. It’s as simple as that.
In actual practice, however, there can be complications. A front speeds up, the weather changes, and your strategizing may not move as fast as those thickening clouds you’ve been trying to dodge for the past 20 minutes. Soon, you’re in the soup. Of course, you learned about the front during your preflight weather briefing. And let’s say you noticed pireps (pilot reports) of “trace” to “light” icing from airplanes flying in the frontal zone. Does this mean that your risks are lower than if the pireps mentioned “moderate” icing? Not by any means.
The measurement and effects of icing intensity levels have always been a controversial subject. Today, we use the terms trace, light, moderate, and severe to indicate the rate and extent of ice accretions. (The terms rime, clear, mixed, and supercooled large-droplet [SLD] refer to the type of ice accumulation, and do not imply rate, effect, or extent of an icing situation.)
The distinctions between the intensity levels can mean a lot. They inform other pilots of the potential hazard, and give meteorologists information to help confirm or refute assumptions they might have made in previous forecasts. A pirep given to air traffic control (ATC)—and reports of hazardous or unforecast weather as required per FAR 91.183—can pave the way to a helpful vector, climb, or descent away from the ice.
The Aeronautical Information Manual (AIM), in Section 7-1-21, describes the current intensity levels to use when reporting icing conditions.
Trace icing sounds pretty benign. The AIM says that this intensity level can be reported when ice becomes perceptible. It goes on to say that the rate of accumulation is slightly greater than the rate of sublimation. Even though this implies a steady buildup over time, the AIM states that ice protection equipment “is not utilized unless [trace levels of icing are] encountered for an extended period of time (over one hour).”
The potential for trouble kicks up a notch with light ice accumulations. Light ice, in the AIM, means that the rate of accumulation may create a problem if flight is continued past one hour. The definition goes on to say that occasional use of ice-protection equipment removes or prevents light ice, and that this level of icing “does not present a problem” if deicing/anti-icing equipment is used. Moderate icing
In moderate icing, the AIM says, even short encounters become potentially hazardous, and that “the use of deicing/anti-icing equipment or flight diversion is necessary.”
Severe icing—now we have a serious problem. The AIM says that ice protection equipment cannot “reduce or control the hazard. Immediate flight diversion is necessary.”
For years, these icing intensity levels have been criticized. One argument cites that icing intensity is aircraft-dependent. In other words, the intensity categories don’t apply equally to all airplanes. A big, fat Piper Aztec wing accumulates ice at a lower rate than a Mooney’s slender wing cross-section. That’s because ice accretes much more quickly on small-radius projections—which also explains why ice first appears on antennas, rivet heads, elevators, and other small-radius protuberances. So just because a Boeing 737 reports light icing doesn’t mean that your Cessna 182 will experience the same. What’s light icing for one airplane can be moderate to severe for others. In short, we simply don’t know how the intensity reported by one airplane relates to intensities on others.
Notice also that the intensity definitions always refer to ice-protection equipment. In fact, most light general aviation airplanes do not have ice protection. (The exception might be a solitary heated pitot tube.) So, some say that the intensity levels really don’t apply to unprotected aircraft—if you don’t have ice protection, how can you know what its effect might be in your current situation? Other than as suggestions regarding the nature of the icing environment, of what operational value are today’s icing intensity levels?
Furthermore, the AIM makes no distinction between ice protection equipment that’s been installed piecemeal under a supplemental type certificate (STC) and the equipment complement installed in an airplane that’s been certified for flight into known icing (FIKI) conditions. There’s a big, big difference between an airplane that’s fitted solely with aftermarket deice boots and a heated windshield plate, and one that’s met the sort of in-flight testing and simulations required for FIKI approval.
Finally, the warnings about severe icing don’t seem to apply to certain turboprops meeting the requirements of Special Federal Aviation Regulation (SFAR) number 23, section 34, which calls for FIKI certification. These rules affect turboprops with more than 10 seats, and operated under FAR Part 135—commuter and on-demand air charter and air taxi operations. The letter of the law, in FAR Part 91.527, says that these SFAR 23 airplanes can operate in known or forecast severe icing conditions. So what happened to the severe intensity’s warning that ice protection equipment can’t fight off ice?
How did we inherit today’s icing intensity scale? Good question, with some revealing answers. For a complete rundown, see Richard K. Jeck’s technical paper on the FAA’s Web site.
The first intensity scale was developed in the 1940s, but not for airplanes. It was designed for reporting the ice accretions at the weather observatory at the summit of Mount Washington, New Hampshire! It was assumed that airplanes flying through similar conditions would build ice at the same rate, based on an airplane flying at 200 mph/174 knots.
In 1956, the U.S. Air Force came up with an intensity scale that factored liquid water content (LWC, or the number of grams of water per cubic meter) and distance traveled into the equation. It’s worth mentioning that the 1956 scale used a small, one-half-inch-diameter probe for making the ice-over-distance measurements—a bad idea, considering that the probe’s ice-collection efficiency served to overstate the accumulation rate. In addition, the Air Force scale was aimed at typical fighter aircraft.
The intensity scale was altered again by an interagency committee in 1964. It replaced the “severe” with the “heavy” category of ice accretion. In 1968, the current intensity scale came out.
But the problems mentioned previously persisted—chiefly, that ice accretion rates vary by aircraft type and model. An FAA Inflight Icing Plan was drawn up in 1998, and a working group came up with a suggested renaming of icing intensities. The group sought to emphasize and quantify each icing intensity’s level of effect. Specific airspeed losses, climb-rate deteriorations, power requirements, and control input and vibration responses were listed. The table above shows the proposed scheme.
However, a big problem remains with this approach. In order to be meaningful, each airplane would have to undergo extensive in-flight testing to quantify the performance losses and other effects. That would be a huge task. Perhaps that’s why the plan has yet to be adopted.
All of this bureaucratic evolution is interesting, but it shouldn’t distract us from following the avoid-or-immediate-escape admonition that I stated at the beginning of this article. Besides, there’s a big trap in buying into the intensity levels lock, stock, and barrel. “Light” icing, for example, can be a transient thing. Stay in light icing long enough, and you’ve got severe icing.
E-mail the author at firstname.lastname@example.org.
1. Loss of airspeed because of aircraft icing, based on indicated airspeed maintained prior to encountering ice.
2. Additional power required to maintain aircraft speed/performance.
3. Estimated reduction in rate of climb because of aircraft icing.
4. Effect of icing to aircraft control inputs.
5. Vibration or buffet that may be sensed through the aircraft controls (not intended to refer to unusual propeller vibration for airplanes so equipped in icing conditions).
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