Page 15 - Volume 15 Number 8

Of course, icing avoidance is always a good idea! A slight detour around a cloud buildup or even a major course modification to avoid the worst of a line of thunderstorms shows a conservative, proper and safety- conscience attitude. Sometimes, however, the decision will be made to fly into visible moisture when the OAT is +5° or below. Do you recognize the latter portion of that sentence? It is in all of Beech’s POMs and POHs as the definition of icing conditions: Visible moisture when the OAT is +5° or below.
“ ... when properly configured and flown, the King Air is one of the most desirable flying machines in which to ride when icing becomes a concern.”
Beech has never specified if the OAT to which they refer is the actual Outside Air Temperature – OAT or SAT (Static Air Temperature) – or is it the Indicated Outside Air Temperature – IOAT or RAT (Ram Air Temperature). Some pilots worry that since there can be as much as an 8°C “ram rise” difference between OAT and IOAT, it could be ill-advised to use IOAT since it actually would be below freezing outside while the IOAT remained in the above freezing range.
Fret not. We instructors at the Beechcraft Training Center always taught this as IOAT: Simply what you read off the OAT gauge. You see, if the OAT probe itself is experiencing a temperature increase due to the compressing of the air as it encounters the probe, would it not be logical to assume that the rest of the airframe also is experiencing some compressibility heating? Whether that theory is 100% correct or not I believe no one truly knows. However, over 57 years of King Air operation – 1964 to 2021 – have verified that problems are not caused by using the reading on the gauge, not corrected for compressibility.
Not until the last couple of decades or so have any King Airs been equipped with OAT/SAT readings on their EFIS displays. Yet, these airplanes also have the IOAT readout on the sidewall by the pilot’s left elbow. Both for the temperature reference in looking up cruise power torque settings, as well as for deciding if icing conditions exist, the elbow gauge is the one to use. Now, if there is a difference more than the ram rise value of around 7°, you may want to have that looked at by the shop when convenient. For many, many years this mechanical gauge looked almost exactly like our moms’ turkey thermometers. In fact, it was not uncommon to call it by that name. Later King Airs have the mechanical dial
AUGUST 2021
face replaced with a digital electronic readout, yet it is still just displaying the temperature of the metal probe. In most cases, the probe has been relocated from the sidewall to the lower, left portion of the nose skin next to the left nosewheel gear door.
Here’s another piece of historical trivia you may find of interest: Before the OAT gauge was located by the pilot’s elbow, it was in the cockpit overhead. For many years, it was just above the pilot’s windshield. All the pilot had to do was tip his head back a bit and he/she would be looking right at it. Complaints started being received, however, about a whistling noise that was created by this probe when it collected some ice. The Beech design team corrected that by moving the probe back from near the windshield’s top edge to a place behind the lighting control panel. Now the pilot had to twist and crane his neck to get the reading – not nearly as convenient as it had been in its original location.
Also, to see the gauge at night, it has a post light next to it with a momentary push-on switch nearby. To avoid the neck-craning contortion, Beech eventually moved the gauge to the left sidewall. But guess what? It took a couple of years for the switch to get moved there also. There are quite a few King Airs in which, at night, you first have to locate the light switch in the overhead panel, rest your finger on it, then turn your attention to the gauge by your elbow. Now push the switch and take the reading. Hey, even Beechcrafts aren’t always 100% perfect!
How many switches or push-pull controls must be activated in icing conditions according to your POM/ POH? A heckuva lot! The lowest number is 10 and the highest is 14! We will be counting them out in the upcoming paragraphs.
KING AIR MAGAZINE • 13