Invisible Ice

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A 20,000-hour Boeing 747 captain nearly prangs his Bonanza on takeoff due to a tiny bit of wing ice that he couldn't even see. If it can happen to him, there's a lesson here for all of us.

I write this with considerable personal embarrassment, in the hopes that someone may learn from it.

Sunriver, Oregon, is a lovely summer and winter resort community about 20 miles from Mt Bachelor, a popular ski spot. We rent a condo there for a week every year, during the kids' Spring break. I usually fly down with one or both boys and the dog...and my wife drives the van down with the rest of the stuff. It's almost impossible to get it all in the van, and utterly impossible in the Bonanza, of course.

After four days of great skiing, Thursday turned into a blizzard...all lifts closed. The family was skied out, so we were thinking of going home early, and when we heard a "monster storm" was brewing for the weekend, we moved the departure date to Friday.

The weather Friday morning was zero-zero with snow and fog, a foot of fresh snow on the ground (on top of solid ice), and 15ºF. Hmmm, this is not good.

I drove to the airport Friday morning with my son Alex...carefully, using four-wheel drive! I was very concerned about the snow-covered airplane, but was relieved to find that the snow brushed off easily, with ice only on the wing walk. I was also worried about cold-starting my precious IO-550, but the engine oil pan heater and insulated engine cover I'd used overnight had brought all EGT/CHT/OIL up to 50ºF.

The runway at Sunriver (18-36) is 5,500' long, hard surfaced, at an elevation of 4,159' MSL. Runway 18 is "the usual" runway in use, and appears to have a very slight upslope, with slightly rising terrain off the end, and high terrain left, right and ahead beyond 5 NM. Runway 36 appears slightly downhill, with dropping terrain off the end, and it's a straight shot to DSD VOR, 23nm away. The instrument approach is off that VOR, with minimums of MDA 5,480' (HAA 1,322) and 1-1/4.

The runway was unplowed, but I figured I could charge up and down it a few times and make a path for a takeoff if they didn't plow it first. The wind was 220, varying between near-calm for long minutes at a time, to 20 in the snow squalls blowing through. The weather was varying all over the place: it would pop up to 1,000-and-3 or better for 15 minutes or so when the snow quit, then drop to zero-zero again in snow showers. The temperature had risen to 28ºF, so it was getting a little wet.

Go or no-go?

I reviewed what I needed in order to go:

  • A clean airplane,

  • A 36 departure (implying not much wind from 220, a tailwind),

  • Local weather good enough to circle and come back immediately,

  • Good weather at Redmond OR, 23nm away (ILS to 200' available),

  • A decent pilot report of "no icing" at the lower levels.

The odds against all that happening at once were not good, but what the heck? Alex and I were well-dressed, with snow boots, were having a good time cleaning the airplane off (several times), several locals were sitting around gabbing, and the office was warm and dry for breaks. So we kept the airplane pretty clean, fooled around, and waited. Redmond opened up early and stayed good, according to Flight Service: two thousand overcast and five, with ten miles viz.

A Malibu landed during one of the breaks, I had him in sight 3 miles out. He reported the only bad weather was within 5 NM of Sunriver, that Redmond was wide open, and there was "no icing" at all on the approach to Sunriver. He'd come from Seattle at FL210 with no problems. The Malibu pilot was very quiet and professional, told me exactly what I wanted to know, nothing more, nothing less. Hot dog, solid gold!

A Bonanza came in from the south, with a "hero" pilot talking about sneaking down the river, lots of icing, and advised me how to follow the access road through Sunriver (prohibited, for noise), intercept the main highway, and follow that to Redmond. I didn't ask, but got the feeling he wasn't instrument rated or didn't like flying in actual. When I mentioned just climbing straight out on the VOR approach course, he looked like he'd never heard of that technique. I discounted him entirely.

No snow is falling, so we did the final cleanup, got the airplane absolutely clean...and then it started snowing again, heavily, temperature 28ºF, a little wet, weather back down to zero-zero. Rats!

"OK, tell ya what", I sez to the lineman, "I'd like to fire up, taxi out away from the office, get it warmed up, checked out and all set to go, then come back, shut down and wait for a break in the weather. Then you shoot it with hot (180ºF) glycol, and I'll fire up again and go". Agreed.

So I fired up, and darned if the snow in the prop blast didn't blow right off! Hmmm, I'd thought it was too wet to do that. So I told 'em on Unicom I'd go try a takeoff, and if it didn't blow off by 40 knots I'd stop and come in for glycol. Sounds like a plan.

The plan unravels

After a 20 minute delay for ATC, the snow quit and the viz and ceiling came up nicely. ATC came through on schedule, and we were released. By now, the runway had been plowed and was hard pack snow on top of ice. Lousy braking but no drag.

I briefed Alex, and we started the takeoff roll. We had 1/2" or more of snow on the wings, so I fully expected to abort at 40 knots and go back in to get deiced. But to my surprise, everything blew off beautifully by 40 knots in the first 500' of roll. Hot dog, it all came together...someone up there likes me! I dismissed the wing contamination problem completely, checked for proper airspeed indication, power settings, EGTs, etc.

Just one problem. The leading edges back to the spar of my wing are painted a dark maroon (to help melt ice), and the wing aft of the spar is painted beige. At about 90 knots, just as I was lifting the nosewheel, Alex quietly said "Dad, the leading edges are AFU". I looked, and looked again, and we had about 1/64" of solid clear ice, EXACTLY covering ONLY the maroon part, almost impossible to to see, with tiny bumps every half inch or so, maybe 1/32" high. At the MOST.

It was too late to stop, so I babied it off, stayed in ground effect, got the gear up, and went over the far end of the runway at 110 knots and 5 feet of altitude. Kept it fairly level as the ground fell away...thank heavens we departed runway 36! I managed to gain maybe 100' or 200', did a very gentle buttonhook turnaround, and landed on 18. The 300 hp IO-550 engine was at full throttle the whole time, except backed off on very short final to cross the fence at 110 knots. The airplane was NOT happy, and flew horribly.

Locals came out, glanced at the leading edges, and asked "What'd you come back for?"

"It doesn't fly so good with that ice."

"THAT little bit affected you?" They still don't believe it.

We shot the whole airplane with 180ºF glycol, took another 20-minute ATC delay, and the snow began falling again. Sigh. I didn't figure we'd make it, but tried another takeoff roll, and this time, the mixed water, snow and glycol peeled off nicely at 40 knots. This time, there was no question that the leading edges were totally dry and clean. Believe me, we were looking!

We crossed the far end of the runway at maybe 200 feet and 120 knots, climbed into the clag at better than 1,000 feet-per-minute, and popped out in good (but gloomy) weather over Redmond. We re-entered the soup at 9000' and climbed to 16,000' in clouds. At cruising altitude, OAT was -27ºF, not a sign of icing. We had a beautiful flight to Seattle, in and out of clouds, sometimes between layers, arriving just as full darkness fell. That's the way it's supposed to work!

What happened here?

I'm not positive, but I can take a guess.

Ice and water are funny things. Water can turn to ice in a heartbeat, and 32ºF is not always as significant as most folks think. Other factors come into play.

I suspect the dark leading edges of my airplane subtly changed the dynamics just a tiny bit. What little radiant energy coming through the clouds from the sun (we could barely see traces of the sun once in awhile) might have warmed the maroon areas just a tiny bit, and there might have been a layer of water trapped under the 1/4" of snow. When the airflow began, and the pressure dropped, that thin layer of water instantly froze into clear ice, while the bulk of snow blew away. The main surface of the wing, perhaps a tiny bit cooler, didn't melt the snow enough to make water, and when the snow there blew away, it left the beige portion of the wing clean.

There are two main lessons here. First, take a HARD look at ALL areas of the wing before committing to a takeoff. Two, unless it's happened to you, it's hard to believe how little upper-wing contamination it takes to spoil performance...big time! I thought I had learned that lesson long ago. Be careful out there.

Very, very invisible

And before you jump to any conclusions, let me tell you that there was no "get-home-itis" involved here. I would have been quite content to return to the condo, spend the night, and either try again the next day, or just drive home with the family, and come back another day to get the airplane. I felt no pressure to go at all. The next day, faced with the same choice, perhaps there would have been some pressure, but not enough to make me do something stupid.

I was absolutely, positively CERTAIN that wing was clean, or I would not have continued the takeoff.

Trouble is, I was wrong, and THAT is the point. It is very difficult to see clear ice on a dark-colored leading edge. Almost impossible. The only way I could see it at all (after Alex pointed it out to me) was by catching the reflections just right...and on that gloomy day, there were not many reflections to be had!

This was perfectly CLEAR ice, and the line of demarcation couldn't have been sharper if it had been masked off and sprayed. Ice on the dark part of the wing, none at all on the light part. Very, very invisible.

I think I may take a lesson from the DC-9 crews and tape a small length of yarn on the leading edge, maybe just inboard of my gas cap. If the string is flopping, there's no ice...if it's motionless, there is ice. I thought of that as a joke when I first heard about it...but suddenly, it ain't a joke.