Wednesday, February 22, 2006

Ice encounters of the worst kind: in a C172

John, the Freight Dog, has been writing lately about his daily ins and outs as a professional pilot driving a Cessna Caravan moving freight. Work has to be done icing or no icing, even though the FAA is apparently concerned about airworthiness in icing conditions. So it is no surprise that he feels only too glad to be on the ground after icy night flights. And remember, he has all the dicing bells and whistles... I did not, 7 years ago when I encountered unexpected icing in a Cessna 172 at 13,000 ft.
What was I doing at 13,000 IFR on a 172? Well, there was an airshow in Acapulco beaches, 2 hour flight from Mexico City where I am based and the weather briefing said VFR all along the route. The weather is usually good all year round in Mexico so I had no doubts in my mind about this flight. But weather surprises are not alien to flying. 30 minutes into the flight I found cloud formations that blocked the mountain pass VFR flights have to use to clear the 12,000 ft mountains south of Mexico. The temperature was +10C at 11,000 ft just below the clouds and the clouds themselves did not appear bad ones so I naturally asked the controller for an IFR clearance which he gave me right away. So far so good, I was probably going to be in the soup for 10 minutes or so to cross the mountains, and then enjoy the usual stable weather typical of the southern part of Mexico facing the Pacific.

Entering the clouds, pitot heat on, stay on the gauges, fly the plane, no turbulence, all good. Being inexperienced, I did not checked the OAT right away. I could not have imagined a temperature dropping 12C just entering the clouds! But that is exactly what happened, though I did not noticed at first. My only passenger and girlfriend at that time, was the first to notice the instant frost in the windshield. She said calmly... "Is that supposed to happen?" I turned from the instruments right away and I could not believe what I saw. In front of my eyes, ice was accumulating at an alarming rate! I checked the temp gauge, and the wing. I was in deep trouble now because when 3 seconds later my eyes were back to the instruments, they found that the airspeed was decreasing and the plane was going down at 500 ft/m. Yes, you read correctly, the speed was decreasing. The reason was clear but confused me for several seconds. I was pulling the yoke to get Vy (best climb speed) but the RPM´s were slowly but steadily diminishing. All this within the boundaries of what we now call A airspace of Mexico City International airport. How could so many things go wrong so fast? I felt my brain froze for like a thousand years, but I guess it was only a couple of seconds. After that I heard myself declaring an urgency (PAN, PAN, PAN... ). I requested an immediate vector from the controller away from the mountains, which were certainly below me by now, and out of the icy clouds. My confusion was so high because of the sudden high workload, that it took me an eternity to comply with the vector issued by the controller. Two things were invaluable here: I suddenly remembered the 1-axis autopilot in this 1997 172R, so the heading bug did the job of getting us out of the clouds while I troubleshooted the dying engine. The other was the immediate decision to get out of the ice. The whole encounter could not have lasted more than a minute and a half, but I easily had half an inch of ice on the wings by the time we cleared the clouds in a heading that was perpendicular to my original trajectory and towards Mexico City International Airport, where the skies were clear, just 18 NM to the north. I lost 1,500 ft before the engine started to get back enough power to stop descending and I was also fortunate that just outside of the clouds the air was at 10C like before, which shed the ice very fast. 3 minutes after or so, both me and the controller were calm again, and the damn C172 was flying like if nothing had ever happened! With my shaky, inexpert knees trembling, the only option was to get back to Atizapan (MMJC), my home airport, to live another day. I did not even thought of trying a different way to Acapulco, I have had enough flying adventure for that day. Or so I thought... Because after taking off with calm wind, the conditions for landing half an hour later were so turbulent that it took me 3 tries and two scares to firmly put the plane on the ground at 90 Kts. The schools flying at the time were lucky to bring back the planes they had in the pattern that day, because the instructors had my same problems for landing.

So you see, I understand perfectly the feeling of Freight Dog loving to fly but thinking how good it is being on the ground when the ice is up.

What had happened? Well, 1999 is remembered in Mexico City because that winter night we had something very rare in these latitudes: a hell of a snow storm. It was not forecasted and took even the airliners by surprise, because of the severe turbulence it caused later that night, just before the storm. It did not actually snowed in downtown Mexico, but it did on all the mountains around. All over central Mexico, hills around were absolutely and immaculately white the next morning.

I will not bother you with the lessons learned, which should be obvious. Instead let me tell you about the satisfactions: the next day, the girl still went with me to the airshow and the cristal clear air left by the snowstorm allowed us to see all of the 5 major volcanoes, all snowed, at the same time while enroute. This is a thing I have never seen before or since. It was worth the price.
But of course, respect ice. It can accumulate at unbelievable rates...


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