If your King Air is a senior citizen built before the late 1970s, I am betting that you are quite familiar with the use of the “Man Cool” and “Man Heat” modes of operation of your environmental mode selector. Why? Because the kinks associated with the Auto mode of environmental control had not yet been worked out and hence the Auto mode was so erratic in its operation that few pilots used that mode. But, by the late ’70s, the Auto mode had been quite well-perfected and therefore it was rarely necessary to use the manual modes. But there will come the inevitable day when the Auto mode fails and you must revert to the manual modes. This article’s intent is to review and remind you of how to best do that.
For the Auto mode to work properly – giving you the desired cabin temperature – it must know three things: (1) the desired cabin temperature; (2) the current cabin temperature; and (3) the temperature of the air flowing into the cabin. Let’s say you are filling the bathtub before the kids get in to bathe. Those same three parameters (but now dealing with the bath water temperature instead of the cabin air temperature) are being checked by you. (1) you know from past experience what the desired water temperature is; (2) you swish your hand in the tub to see what it currently is; and (3) you place your wrist under the faucet to gauge the temperature of the new water flowing in. Right?
In the King Air, the desired cabin temperature (1) is set by the position of the Cabin Temp rotary knob. It is quite common to find that placing the pointer on the knob between the 6 o’clock and 8 o’clock positions will give a comfortable, 72-degree-like, cabin temperature.
The current cabin temperature (2) is detected by a sensor almost always located in the cabin’s headliner. This sensor has a small fan attached to it that draws cabin air up and over the sensor to allow it to get a more accurate reading. Since the sensor is only a couple of inches away from the aluminum skin at the top of the fuselage, at cruise altitude it would tend to feel a temperature significantly colder than actual cabin temperature were it not for the fan.
Finally, the temperature of the incoming air (3) is detected by a sensor located in the duct that comes from the mixing plenum under the copilot’s rudder pedals. The mixing plenum, or chamber, is so-named since it is where new incoming bleed air mixes with recirculated cabin air. The temperature sensor is installed in the branch going from the plenum to the cabin floor outlets.
If any of these three sensors goes bad, it makes the Auto mode operate poorly. In fact, one of the most common malfunctions is caused by failure of the fan in the ceiling. This leads to that sensor feeling too cold and thus the Auto system makes the cabin too hot as it tries to correct the too cold cabin temperature it now senses. Given enough hours flying the King Air, I can guarantee that your Auto mode will malfunction and lead you to use Man Heat or Man Cool until the problem is found and corrected. Let’s talk about those modes.
There are a few meaningful differences in the operation of the manual modes depending on the King Air model being discussed. Specifically, the model 200- and 300-series differs from the 90- and 100-series. I will begin by discussing the 90- and 100-series. From the first C90, LJ-502, to the latest C90GTx – including all E90s, all of the F90-series, and all of the 100-series – their environmental systems are virtually identical.
Three components can and do affect the cabin temperature in this group of airplanes. First, bleed air. The air that has been compressed – and thereby heated – by the engine’s compressor is known as “P3 Air” since it originates from the engine station #3, the “Combustion Chamber Inlet” which is also the compressor outlet. A relatively small portion of P3 air is tapped off of the engine and sent to the Flow Control Unit (flow pack) mounted inside the cowling on the forward side of the firewall. The flow pack often mixes some ambient air with the bleed air before sending it to a junction in the leading edge of the wing’s center section. Following one of the two paths at the junction finds the air going directly to a device called the bypass valve. Following the second of the two paths finds the air flowing across the metallic fins of an air-to-air heat exchanger. Flowing across the other side of the fins is ambient air that has entered the ductwork containing the heat exchanger from a large hole in the wing’s center section leading edge. Similar to an automobile’s radiator – where the engine’s hot water transfers energy to the colder ambient air flowing across its fins – the air-to-air heat exchanger allows the bleed air’s thermal energy to be transferred into the ambient air which heats up and then blows harmlessly overboard through slots in the bottom of the wing’s center section.
The bypass valve is a Y-shaped device, shaped like a slingshot. One of the two tops of the Y is fed by the air from the flow pack that followed the path that did not contain the heat exchanger. Of course, this air is relatively hot since it never transferred any of its energy into ambient air. The second of the Y’s two tops is fed by the air that flowed across the heat exchanger’s fins, lost energy to the ambient air flowing across the other side of the fins and is therefore relatively cold.
Mounted inside each of the two tops of the Y is a “butterfly valve.” The dictionary defines butterfly valve as: A valve consisting of a disk rotating on an axis across the diameter of a pipe to regulate the flow, as in the throttles of many engines. The two butterfly valves in the bypass valve assembly are operated through 90 degrees of travel by a single electric motor and a gearbox assembly mounted on the top of the bypass valve, the Y-shaped assembly. As one of the two butterfly valves is rotating to open, the other is rotating closed. The motor can stop both valves’ travel at any place. Therefore, the flow that leaves the bypass valve assembly – flowing out the handle of the slingshot, you might say, and then on into the cabin – can consist of 100% hotter air that bypassed the heat exchanger and 0% of the cooler air that flowed across the heat exchanger’s fins. Or it could be 83% hot and 17% cold; or 50/50; or 25/75; or full cold: 0% hot and 100% cold.
I often teach that a good analogy of a bypass valve is a modern single faucet feeding a sink. By moving the handle, we can control how much water from the hot water line under the sink and how much water from the cold water line mix to feed the faucet. That’s what the two – left wing and right wing – bypass valves accomplish in the airplane: They control the temperature of the air feeding into the cabin. At cruise altitude we never need an electric heater nor a refrigerant air conditioning system since the heat we want is readily available from the proper amount of bleed air heat. Do you see why the bypass valves are our “Mighty Little Giants?” They alone control cabin temperature during the majority of our flight time.
Say we are in cruise using our normal procedure of being in Auto mode, the cabin is colder than we desire, and turning up the Cabin Temp rheostat control has had no effect. Time to try Man Heat mode.
We rotate the mode selector to Man Heat and then hold the Man Temp switch up to the “Incr” position for 15 seconds. Why 15? The exact number is not critical but it’s a good place to start. To drive both left and right bypass valves from one extreme to the other – full hot to full cold bleed air or vice versa – requires one full minute, 60 seconds. Since going to either extreme is likely unnecessary, that’s why I suggest starting with one-fourth of the full travel: 15 seconds. If we use less than that, the change may be so little that we cannot tell if our attempt achieved anything. If we use much more we may overshoot and find that we have a very
Well, hallelujah! It is indeed getting a little warmer! So, we wait 15 minutes or so to see what final cabin temperature we have. Still a little chilly? Then let’s hold the Man Temp switch up to “Incr” for, oh, 10 seconds. A half-an-hour later, the cabin is a little too toasty? Then hold down to “Decr” for 5 seconds. Ah … perfect!
Each bypass valve requires 30 seconds of continuous motor running time to drive the butterfly valves from one extreme to the other. The two bypass valves, left and right, run sequentially not simultaneously. For heating – getting more bypassed air and less air from the heat exchanger – the left bypass valve heats first and the right bypass valve heats second. For cooling, it’s the opposite: Right side goes full cold before the left side leaves the full hot position. As a pilot there is little need to know which valve heats or cools first. Yet, it explains why the Emergency Procedure for Environmental Smoke in the cabin directs us to close the left Bleed Air switch first. You see, that is always the hotter of the two sides unless they are both in the full hot position … and that occurs very rarely.
What’s the difference between Man Heat and Man Cool? In both modes we have identical manual control of bleed air temperature as we have been discussing. The difference involves the other components that can contribute to cabin temperature control: The freon air conditioning system and the electric heater system.
In the example I just presented – of having a chilly cabin in cruise and reverting to Man Heat mode to correct – I never mentioned that the electric heater would also be activated and the cabin would likely get much too warm. Well, wouldn’t it?! Probably not. Why? Because the electric heater didn’t come on. Quiz time: Why not?
If you, like most King Air pilots, turn the windshield heat on before the OAT drops below freezing – many of us do it passing 10,000 feet – then that action locks out the electric heater. Remember that the electric heater pulls so much electric power that the system designers never wanted this “comfort” item to rob electricity from more important “safety of flight” items. Hence, there are two or three “lock out” items that render the heater inoperative: Windshield Heat, Prop Heat and – for the earlier “Chin Style” cowling airplanes – Lip Boot Heat. On the other hand, if none of these lock out items are on, then yes, you should move the Electric Heat switch down to Off before selecting Man Heat so that the heater cannot come on and subsequently make the cabin too warm.
(Landing at Fairbanks in January using Man Heat? I suggest turning off the lock out items once you are in VMC to allow the electric heater to join in the fun!)
Before I continue to discuss Man Cool, time for another quick quiz. What malfunction can lead to lack of sufficient heat in cruise that cannot be remedied by switching from Auto to Man Heat?
There are two common answers. The first answer is lack of bleed air. If one flow pack is very weak or fails completely so that it is supplying no air into the cabin, very often this failure will be noted first by lack of heat not by lack of pressurization. Either flow pack alone should be able to maintain maximum Differential Pressure (∆P), even though a lot of King Airs have excessive leaks that prevent this from happening. So, realize that a lack of heat that cannot be remedied by use of Man Heat often points to a dead flow pack.
The second common reason for why the use of Man Heat will not solve the problem of a too cool cabin is that a bypass valve motor has failed. When and if that happens then regardless of whether the command for more heat comes from the automatic controller – often referred to as the “Barber-Coleman” box – or from the Man Temp switch, the butterfly valves cannot move. They are locked in their last position. Since the valves move sequentially, the failure of one valve will prevent the other valve from ever receiving a signal to move.
Now for Man Cool. Identical to Man Heat, this mode gives the pilot control of Bleed Air temperature and Bypass Valve position via the “Man Temp Incr – Decr” switch. But now, instead of asking for the electric heater to join in, the air conditioning system is told to operate continuously.
Returning to our discussion of using Man Heat in cruise. As we now begin our descent into Houston in July, we have been holding the Man Temp switch down to Decr periodically until we are sure the accumulated time of use has surpassed 60 seconds … the bleed air is as cool as it can get. Yet the cabin is getting noticeably warm. Time to select Man Cool and let the air conditioning come on.
When you move the mode selector to Man Cool it’s a good idea to watch the load meters. You hope to observe a very major electrical load increase since the compressor motor pulls nearly 150 amps. Ah … here comes the cool air out of the vents! It is unlikely, but possible, that the cabin will actually get a bit too cold. If so, hold the Man Temp switch up to the Incr position for a few seconds to warm up the incoming bleed air. As you continue the descent to your landing it will be typical to again hold the Man Temp switch to Decr until you know the bleed air is as cool as possible.
I often teach that there are actually three, not two, modes of Manual temperature control. The first one is using Man Cool to run the AC continually but with manual regulation of bleed air temperature. The second one is using Man Heat to run the normal electric heat grids continually but with manual regulation of bleed air temperature. The third one – and the one that will likely be used the most – is using Man Heat to allow manual regulation of bleed air temperature but with the Electric Heat switch in the Off position so that no heater grids may operate. Of course, having any of the lockout items on accomplishes the same thing as moving the Electric Heat switch to the Off position.
I will close by discussing the
differences in the 200- and 300-series. (Realize that the B300s [350s] after serial number FL-500 have a totally new and different environmental system: The “Keith” system. Our discussion does not apply to them.) The manual control of bleed air temperature via the Man Temp switch stays the same: 30 seconds per side to go from one temperature extreme to the other; left side heats first, right side follows; right side cools first, left side follows. There is no electric heater in the first 1,400-plus of the BB-series except for the puny radiant heat panels in the cabin ceiling. These play no role in either the Auto or Man Heat modes of operation. They have their own On-Off switch that functions without any tie-in to any other environmental control. When grid heaters in the forward and aft underfloor environmental ducting replaced the radiant heat panels in 1993 – and which all 300s and the earlier 350s have – they were designed to be a ground use item only. They do not operate in flight since bleed air should always be sufficient to supply the desired heat. (But not when a flow pack is dead!)
There is a significant difference when it comes to Man Cool: The AC system only gets a command to operate when the left Bypass Valve is at or very near the full cold position. This seems to make sense – why engage the compressor clutch, start pumping Freon, and add workload to the right engine until the bleed air is cold? Yet the 90- and 100-series, as we have discussed, allow for AC being on even with full hot bleed air coming in! To recap, in most situations in the 200- and 300-series there is no difference between Man Cool and Man Heat … they both provide manual control of bleed air temperature via the Man Temp switch. You only add in AC when in Man Cool and the Man Temp switch has been held to DECR long enough to send the bypass valves fully cold.
I hope this review has given you a good reminder of how the Manual modes operate and some tips that will prove to be worthwhile when and if you must revert to their use.