Wednesday, April 13, 2011

People are Pendantic

Recently I participated in a debate around pitot heaters. Reading the FAR's (CFR 14, what used to be called FARs when I started flying (not that long ago)), there is nothing about having a heated pitot in little airplanes. Even though most manufactured airplanes have heated pitot tubes. Some people believe everyone must have one, no certifications (experimental or otherwise) can be granted without one.

(this gets a little technical, flying talk, there are hints at the end)

Now I am not advocating not having them, but I will say that if you stay out of moisture when it is cold, you should never have pitot icing trouble. There isn't any air flowing into a pitot tube, it wouldn't work if there was air flowing through it. It measures pressure, so the air gets packed into the tube, and the airspeed indicator reflects the pressure, and that is how it works.

The air can't make things colder than the air temprature, that is physics. Flying at high speed, the wind chill only counts as a feel, not as reality. If you  have a windchill factor of 40degrees below zero C, and the air is 5degrees C, the air can only cool metal to 5degrees C. Now if you are flying and the air temperature is above 0C, then there shouldn't be much freezing going on. Of course, the temperature may vary along the route, and a 3 degree lapse rate may allow you to go below zero, if there are any altitude deviations.

Having a heated pitot tube will allow you to know how fast you are going, even if the airplane picks up ice. Is knowing how fast you are going with a load of ice good news or bad? It may not matter, if you get too much ice. The FAA just issued a SAIB for icing, that seemed to show some common sense in a document. They caught some flak for it, since it was common sense, something  everyone should know. Probably they get tired of getting called out to muddy farm fields, to look at broken airplanes, and are wishing that problem would just go away.

So what does any of this have to do with building an engine monitor?

The engine monitor may want to know the temperature of the air going into the engine. Or at least around the engine to know the change in temperature (deltaT) that is happening. Are the CHTs higher today than yesterday, is it because it is warmer out today? It will be a baseline to know how well the instrument is performing.

I expect people will be different or smarter than me. I hope I can stir some thinking.

Watch those temps

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