Flying out of El Paso earlier this week I picked up a little airframe ice. It would have been a non-event for a more capable airplane, but the anti-ice equipment on 32A (pitot heat and windscreen defrost) just wasn’t up to the task.
Sometimes the forecast is wrong.
Yes, thanks for asking, I did check the ADDS website for ice forecast earlier that morning. Well, it was eight hours earlier but, hey, close enough, right?
Anyway, I climbed up to our flight-planned altitude of 9,000 feet and noticed the OAT was a balmy 5 degrees C. Hmmm, that seems lower than I expected. Oh well, you fly the weather you get, right? The flight progressed and soon we were on top and life was good.
What’s that? The OAT was down to 4 degrees C. Drat, we were skimming through the tops of the clouds. Now, what did that article say, cloud moisture is at its maximum at the top of the cloud?
OAT was then 3 degrees C and the clouds were taller and was that moisture showing up on the wings? Rats! OAT then down to 2 degrees C. I was really glad I had two more degrees before… aggghh! Was that really ice forming? Didn’t that ice know the rules? A close look at the 310’s wing root and there was no mistaking: a layer of rime ice was hitchhiking. Turn up the defroster!
“Fort Worth Center, 32A needs a climb to 10,000 to get out of ice.” Did I mention this airplane is normally aspirated? The leisurely climb to 9,800 did nothing to get on top or get out of the ice. Time for a different plan.
“Fort Worth Center, this isn’t working, I need 7,000 to try to get out of the ice.” Throttle back, heading down, sort of. Did I mention this airplane doesn’t have speed brakes? OAT still at 2 degrees C.
“Fort Worth, still not working, I need 5,000 for ice.” OAT reading stuck at 2 degrees C. I tapped the gauge. No change. I was going to have to get that thing fixed when I get home!
“Unable 5,000. Minimum IFR altitude is 6,000. Cleared down to 6. Let me know when you get out of the ice.” I was looking forward to fulfilling that request for information!
“Roger 6, if this doesn’t work, we’ll come around and land Hobbs.”
The lower terrain of Texas is a welcome sight for a pilot with ice on the wings.
Just about 6,200 we popped out the bottom of the cloud and the air temp finally crept up to 4 degrees C. Hitchhiking ice tired of the game and went off to entertain the next aviator.
Where do acorns and blind hogs fit into this story?
When planning the flight, I had to choose between swinging north over rising New Mexico terrain (where surface elevations rise to 6000, 7000, and even 8,000 MSL) or swinging south over those beautiful, flat West Texas plains, surface elevation down to 3,500 MSL). I chose the south route. I considered the consequences had we gone north instead of south and felt a chill.
Old saying: “Even a blind hog finds an acorn once in a while.”
Just call me Porky.
- Cleared for the visual—really? - June 17, 2020
- I was a “weather coward,” and I made the right decision - October 2, 2017
- Ice, acorns and blind hogs - June 19, 2017
This is a really good case study of a decisive and aggressive response to the initial ice accretion. In the jets, we consider icing conditions to exist at a total air temperature (TAT) of +6C on the MD-80, +10C on the 767. That’s not the same as OAT, of course. But the open question, which can lead to some interesting dinner table debates, is exactly what temperature are you measuring with the OAT gauge? Are there position errors? Ram rise errors? Has the air being sampled by the temperature probe been heated in any way? And yes, indeed…has the gauge been calibrated recently? None of those questions have ready answers, but the comprehensive way to address them is to consider any temperatures close to freezing to be suspect.
The normal aspiration aspect may have kept you out of more trouble as well. If you have all of the altitude-enhancing technology, then you start trying to do things like fly the northern route you considered. It is not uncommon, in the accident/incident data, to see some guy out there at 17,000 feet, getting hammered with ice while the MEA is 11,000 and the freezing level is underground. With a pair of normally-aspirated carbs, he would never have been there in the first place.