This article is written in hopes that it will serve as a useful reminder about how to configure your King Air for in-flight icing encounters. If you have any significant amount of flight time in a King Air, I suppose that you have become quite comfortable with flying and handling the airplane in icing conditions. At first, many pilots fear that those big wing leading edges will collect ice with a vengeance. Yet, with experience, the operators come to realize that the wing does exceedingly well during ice encounters. In fact, the smaller leading edges of the tail surfaces collect ice significantly faster than the wing. Overall, however, 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.
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.
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 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.
There are two or four switches to the left of the pilot’s control wheel: Two Engine Auto-Ignition switches and, for the later models, two Engine Anti-Ice switches. On the subpanel to the right of the control wheel we find a bunch more: Two Pitot Heat switches, two Fuel Vent Heat switches and a single Stall Warning Heat switch make up five of the switches on the right subpanel. These five switches have become known as the “Hot Five” and many King Air pilots operate them all the time when airborne, turning them on at “Runway Lineup” and off in the “After Landing” procedure. I have no complaint whatsoever with this process. However, I personally don’t do it. It’s not a POH procedure but rather comes from FlightSafety and U.S. Army training tradition. Wrong? No way! Maybe it’s because I have resided in Arizona for the last 34 years, but I just don’t see the need for running the Hot Five when it’s warm and clear, so typical of the United States’ southwestern states. My technique isn’t more “right” than running the Hot Five always, it’s just my habit/preference.
Another piece of historical trivia before I finish counting out the remainder of the ice protection switches: It’s impossible to turn on the Hot Five in a King Air 100 (the original “Straight” 100 made from 1969 to 1971, not the A100 nor the B100). The 100 model has no fuel vent heat! Although most of us think that the heated “standpipe” vent tube is a backup for the recessed vent (and that it is!) that’s not why it’s there. As you know, fuel vents serve two purposes. First, they allow air to enter the tank as fuel is consumed to prevent a vacuum from being created which could collapse a bladder tank. Second, they provide an overboard path for fuel when it expands, usually due to heat buildup, causing thermal expansion.
Apparently, both Beech and the Federal Aviation Administration (FAA) agreed that the recessed fuel vent on each side of a King Air, by its recessed nature, was not prone to becoming blocked by ice. Hence, no backup was necessary. In the model 100 only, enough expansion space exists in its tanks that thermal expansion causing the fuel to overflow the tank is not possible. The 100’s fuel system, although similar to the A90, B90 and C90 system has some significant changes. One of these is that its maximum capacity is 370 gallons, not the 384 of the others. No other King Air fuel system is exactly like it.
All the other King Air models, however, are capable of thermal expansion causing the need to vent fuel overboard. If this fuel were to come out of the recessed fuel vent in flight, it would flow back along the bottom wing skin. A concern arises about the potential for fire caused by this fuel on the wing. Hence, the real reason for the standpipe vent is to expel the expanding fuel far enough away from the wing skin such that it blows away free and clear, posing no increased fire hazard. But, since this standpipe vent is indeed a ready collector of ice, it has to be heated. That’s why all King Air models with the exception of the Straight 100, have the left and right Fuel Vent Heat switches.
As the “Hot Five” become the “Hot Three” on 100s, here’s another interesting tidbit: They become the “Hot Four” on some A90 and B90 models because some of these airplanes have no Stall Warning Heat switch.
The actual stall warning vane (the movable piece) has always been heated. Whenever the battery switch comes on, the heat is there. For a while, that was the only heat associated with the stall transducer. But then some concern arose about the stall warning becoming inaccurate when ice accumulated on the metal plate behind the vane. Safeflight, the manufacturer of the system, began offering heating for this plate and Beech added that on the assembly line and added the Stall Warning Heat switch in the cockpit. The previous airplanes now had a mandatory kit available that added the plate’s heating element. However, rather than force the operator to run an additional wire from the wing to the cockpit and add a new switch on the subpanel, they tied this component into the Right Pitot Heat switch. (Realize that all earlier King Airs had the right pitot tube midway out on the wing, not on the nose.)
Due to the extensiveness of items to cover in this article, it will have two parts. Next month we will continue to discuss the ice protection systems King Airs have and the difference between the various models.