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Icing at 5 degrees C?

Measuring air temperature in turbine flying is not simple

The operating procedures for the Beechcraft King Air 350i that I fly command that engine ice protection be turned on whenever the temperature is 5 degrees Celsius or below when flying in visible moisture.

Photography by Mike Fizer.
Zoomed image
Photography by Mike Fizer.

I may have dozed through some parts of physics class, but I do remember that the Celsius temperature scale is based on water freezing at 0 degrees and boiling at 100 degrees. Why would the King Air engines need ice protection at 5 degrees above freezing? The answer is that measuring outside air temperature when flying at turbine airplane airspeeds is not a simple task.

For the piston airplanes we all learned to fly in the outside air temperature (OAT) was just that: the temperature of the air outside the airplane. When the OAT dropped to zero we knew icing was possible if there was much moisture in the air. But that basic bit of essential OAT information is distorted by increased airspeed. True airspeed, to be specific.

As true airspeed increases, the air impacting a temperature sensing probe compresses. Of course, compressing any fluid or gas increases its temperature above the ambient. We call this increased air temperature “ram rise” because it is the effect of air being rammed more forcefully against the temp probe that causes the compression, and thus the increased temp report from the probe. That reading is called ram air temperature (RAT).

For many years the only air temperature report available to pilots of any airplane was RAT. At piston airplane speeds RAT may only be one or two degrees above actual air temp. But at turbine speeds RAT can be 10, 20, or even more degrees than the actual temperature. In the decades past many of us carried a small circular computer that could, among other tasks, convert RAT into the actual air temperature when true airspeed or Mach were dialed in.

But when airplanes were certified the FAA didn’t allow manufacturers to force pilots—thank heavens—to convert RAT to an actual air temperature to know when icing was possible. So that’s why turbine airplane operating procedures demand ice protection be activated at 5, or 10, or maybe even more, degrees above 0 degrees C.

If air compression is warming the temperature sensor, isn’t it also warming the entire airplane so water won’t freeze? No. The compression effect of turbine airplane cruise speed is considerable on a small temperature probe, but not nearly enough to significantly warm a large surface such as a wing leading edge. For that to happen you need to fly at transonic, or supersonic speed where friction, not compression, is the dominate warming force.

For example, the ultimate Concorde airspeed limit was its skin temperature. Because it was built primarily of aluminum, its structure could only tolerate the friction heating of the supersonic slipstream up to around Mach 2 before the metal would start to lose its integrity.

RAT distortions are also found in most turbine airplane performance charts. Air temperature, of course, has a huge impact on how fast an airplane climbs, or cruises, or its maximum altitude for a given weight. RAT is the starting point for looking up that information in most turbine airplane flight manuals.

RAT is like those golf course guides that tell you how far it is from, say, a sprinkler head to the pin, but it’s up to you to figure out how far your ball is from the sprinkler head.But when it comes to performance predictions, what we care about most is the air temperature versus ISA (international standard atmosphere). ISA is not really the standard air temperature on any given day, but is an accepted yardstick against which airplane performance is predicted. Flight test data is collected using ISA as the zero reference point. If the air temperature is higher than ISA, performance degrades. Colder than ISA and most performance parameters are better than baseline.

Which brings us all back to RAT. It’s a meaningless temperature until it’s corrected to determine the freezing point, and also corrected to measure the real air temperature versus ISA. RAT is like those golf course guides that tell you how far it is from, say, a sprinkler head to the pin, but it’s up to you to figure out how far your ball is from the sprinkler head. And, as in golf with laser range finders, technology has come to our rescue and RAT can now be more an artifact of past limitations than a fundamental bit of information in turbine airplane flying.

The first air temperature measurement improvement began to show up on business jets somewhere around 1980. It is a small probe mounted near the nose of the airplane, not far from the pitot tubes. The probe has a short mast with a squarish scoop mounted on it. The scoop is shot full of tiny holes. It’s the Rosemount probe, named for the company that developed it.

The very clever and specific shape and hole pattern of the Rosemount probe decelerates the air stream to near static so a temperature sensor in the scoop can measure the real air temperature. This is called static air temperature (SAT)—or sometimes TAT for total air temperature—and is similar to the air temperature a thermometer on your porch would report. It’s the real temperature of air around the airplane and thus a true measure of the air density the airplane is operating in. And SAT is also an accurate measure of when moisture will freeze.

The Rosemount probe works well, but it’s expensive so its installation is limited to more expensive jets. But, once again, modern electronics changed everything in the form of the digital electronic air data computer that has become so inexpensive that it’s now commonly installed in piston singles.

The digital air data computer uses solid-state electronic sensors to measure air pressure in the pitot-static system and thus computes indicated airspeed, altitude, and vertical speed. Because it knows those values, it’s a simple task to apply corrections to a simple RAT measurement to show us SAT. And even more useful, the digital air data computer has plenty capability to calculate the SAT deviation plus or minus ISA, the air temperature information that is critical to turbine airplane flying.

What has become common in newer avionics systems is to display three air temperatures. SAT is the real air temperature corrected for the ram effect of higher airspeeds. We also see the air temperature deviation plus or minus ISA. And RAT is still there because we’re still supposed to configure the airplane for certain conditions based on that temp.

In reality most of us don’t pay much attention to RAT anymore. What really matters most to a trip is temperature deviation above or below ISA. Flight planning programs show forecast of this value because it is so critical to meeting performance predictions. Few of us talk about actual air temperature, SAT, in degrees but instead that it was plus or minus this many degrees.

But, there is still the crisis of conscience that happens often when RAT shows a temperature closing in on freezing and the checklist demands action. In my case what to do when the RAT shows plus-5 degrees and I’m in a cloud. The SAT shows the air temperature is actually above freezing, and there is no ice sticking to the airplane anywhere. Do I turn on the engine ice protection and lose a bunch of power the closed ice vanes cause? The rules say yes. It is written. But my eighth grade physics lesson says no. Water doesn’t freeze above 0 degrees.

RAT at turbine speeds is always wrong. Life was simpler when we didn’t really know SAT. We bowed to the RAT and the checklist. But will moisture really freeze at plus-5 degrees? In the days when all we knew was RAT, the answer is yes.

J. Mac McClellan is a corporate pilot with more than 12,000 hours, and a retired aviation magazine editor living in Grand Haven, Michigan.

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