Page 20 - Volume 14 Number 9

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 (ΔP), 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.
Cleveland Wheels & Brakes
28 17 24
18 • KING AIR MAGAZINE
SEPTEMBER 2020