November 1, 2003
By Thomas B Haines
Editor in Chief Thomas B. Haines has been flying general aviation airplanes for more than 25 years.
There it was, no surprise, a little frosting on the leading edge of the wing. It started as just the narrowest band, no broader than your little finger. Within a minute or so, it had widened, covering probably 50 percent of the chord of the leading edge. Having just read Editor at Large Tom Horne's manuscript on tailplane icing (see " Wx Watch: Iced-Up Tails," page 155), I looked back at the tail. Horne, in the right seat, was also checking out the tail. As near as we could tell, there was little to no accumulation back there. The bottom-center portion of the windshield turned opaque; actually more opaque since we were by now fully in the frosty cloud.
The Aeronautical Information Manual says trace icing accumulates barely faster than sublimation. Light icing might cause a problem if flight in it continues for a prolonged period — more than an hour. Moderate and heavy icing are obviously much more serious. The AIM directs pilots flying non-deicing-equipped airplanes to immediately exit the areas of precipitation or find areas where the temperatures are above 2 degrees Celsius. The AIM says icing is most likely to occur between 2 degrees and minus 10 degrees. Except in cases of freezing rain, I have never seen ice accumulate at anything higher than zero degrees Celsius.
On this early morning in late September the temperature was right at zero degrees. The pitot heat and defroster were on. I've found that in most light airplanes, one generates more heat through the friction of pulling the defroster knob than actually comes out the defroster vents. But I felt better having it on.
Horne and I weren't on some suicide mission. We were just two general aviation pilots trying to cross half the continent that morning and doing what we could to stay out of weather trouble. We ventured into the cloud more out of curiosity than necessity. The air was relatively dry, there were no reports of ice, and we had lots of outs — a must-have when dealing with icing. We had departed Frederick, Maryland, 45 minutes earlier into gray, overcast skies. Minutes later, we darted out the top of the clouds into a brilliant blue sky with unlimited visibility and were tooling along at 8,000 feet into a strong headwind with the tops of a broken layer several thousand feet below us.
As we marched westward over the West Virginia mountains, a few cloud tops gradually rose up toward us, but we could see that for 50 miles or more ahead it was only the rare cloud that would reach us — deviations or a climb to 10,000 feet would easily steer us clear of the clouds. So when one isolated cloud top loomed on the horizon I took little notice, except to turn the pitot heat back on and pull the defroster placebo. Flight service had not forecast icing conditions, but the outside air temperature was right at zero degrees so ice was certainly a possibility. Shortly after entering the cloud we noticed frost on the leading edges, then the thin band of mixed ice.
Enough for me. We exercised our option and asked for a climb to 10,000 feet. ATC agreed, accepted our pilot report about the icing, and 500 feet later we were back on top. As we leveled off at 10,000 feet, the temperature remained at about zero. Forty-five minutes later the trace of ice had sublimated away.
If the FAA had its way, we would all be flying around 24/7/365 with the pitot heat on — even on those dog days of summer in West Texas. Our trip west was to Garmin headquarters in Olathe, Kansas, near Kansas City. There we flew the Diamond DA40, the first airplane equipped with the Garmin G1000 integrated flight deck. As Garmin Chief Pilot Doug Carlson pointed out, not having the pitot heat light on causes a continuous caution annunciator on the G1000 panel. Recent amendments to FAR Part 23 aircraft certification standards, which the DA40 complies with, require annunciation when the pitot heat is off. The change stems from a series of Piper Malibu accidents from 1989 through 1991. Six of the high-performance, pressurized singles came apart in flight after encounters with weather at high altitude. The NTSB never determined exact causes, but it is believed that the pilots' failure to turn on the pitot heat may have caused the tubes to ice up, causing faulty airspeed indications. It is believed that the autopilot then attempted to compensate for the faulty airspeed, causing overspeeds. The pilots, recognizing the problem, then probably overstressed the airplane in attempting to recover, causing the in-flight structural failures.
As a result of the accidents and the special certification review that followed, the FAA now wants an advisory light lit on the panel anytime the pitot heat is not on. Most pilots slumming around the pattern on a hot summer day will probably not flip the pitot heat on, but certainly anytime you're flying in or near moisture — at any air temperature — it's safest to use pitot heat.
Having endured 20 to 25 knots of headwind on the westbound leg to Kansas City, you'd think that the next day we'd deserve a hefty tailwind going home. You'd think wrong.
A low overcast, occasional drizzle, and a bitterly cold wind faced us as we prepared to leave Kansas City. The freezing level was forecast for 4,000 to 5,000 feet, so Horne filed for 3,000, which you can quite safely fly in the Midwest. We flew for probably 90 minutes in the clouds with occasional glimpses of the ground. The temperature hovered at 1 to 2 degrees, but no ice. The "tailwind" was about a five-knot headwind, this despite flight service assurance of push. As we passed northeast of St. Louis, the clouds began to break up and we could see clear, blue skies above. But then we entered an area of light blowing snow. Light snow does not cause the worry that ice does, since snow typically won't stick to the airframe. The greater concern in heavier snow is that it packs into air inlets, starving the engine of oxygen — thus the need for an alternate air induction system.
We soon flew out of the weather and enjoyed a higher, smooth ride in the clear for an uneventful landing back home that evening. The tailwind finally materialized in the last hour or so, but only at 10 to 12 knots. So much for paybacks.
Early the next morning, I departed for a flight to northwestern Pennsylvania. The weather that had dogged us in the Midwest had found its way to the mid-Atlantic. The freezing level was forecast to be at 5,000 to 6,000 feet msl. There were no reports of ice. Cloud bases were at between 2,500 and 4,000 feet agl with excellent visibility underneath. My trip across the Appalachians required at least 5,000 feet msl for a few miles in order to maintain minimum safe altitudes.
At 4,000 feet shortly after takeoff, I was soon ensconced in juicy clouds. ATC cleared me for the climb to 6,000 feet. I watched the temperature probe carefully throughout the climb, wanting to know where my safe haven might be if things got dicey. Level at 6,000, the temperature varied between 1 degree and zero. I was in solid clouds with occasional light rain and a plan to immediately turn around if ice began to form.
A few tense minutes later, just southeast of Johnstown, Pennsylvania, I popped out of the clouds into clear conditions with a few clouds at low altitude and a few cirrus clouds above. I filed a pilot report of no icing to help the next pioneer.
Late that afternoon as I preflighted to head home, an isolated rain shower doused the departure airport, Greenville Municipal. This occurred while the sun was shining brightly, forming just east of the airport the most perfectly defined rainbow that I have ever seen. As I took off I felt like I was going to fly right underneath the big bow, but of course I never caught it.
En route home I enjoyed a smooth ride below just a few scattered clouds. The setting sun generated that liquid light that seems so prominent in the fall. Under the soft light, fence rows and tall trees in the middle of the fields generated shadows that seemed to stretch for miles.
Welcome to fall flying — weather challenges and some of the most beautiful vistas you'll encounter all year.
E-mail the author at firstname.lastname@example.org.
AOPA Editor in Chief Tom Haines joined AOPA in 1988. He owns and flies a Beechcraft A36 Bonanza. Since soloing at 16 and earning a private pilot certificate at 17, he has flown more than 100 models of general aviation airplanes.
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