Now that Old Man Winter is holding much of the country in his icy grasp, I thought it would be an appropriate time to review how to warm up the cabin most effectively after engine start. Operators of the King Air model 90, A90 and B90 have only two choices: Either select auto or manual heat on the Cabin Temp Mode rotary selector switch. If your auto mode works well, you will probably use that position. If it is rather unreliable – and that is quite common in these older birds – then “man heat” is the position to select. In either case, the combustion heater should activate and turn a minor amount of kerosene into a major amount of cabin heat, pronto!
The vast majority of King Airs now use bleed air as their source of incoming cabin air – eliminating the supercharger that is found on 90s, A90s and B90s – and supplement this source of heated air with an electric heater of some type. Let’s discuss the options.
There are still pilots who are somewhat lazy in their approach to bleed air … leaving the left and right switches on at all times, summer and winter, including during startup and shutdown. I am happy that this lazy technique is disappearing and being replaced with a much more correct and scientific approach. It has been found that bleed air flow control packages (flow packs) tend to experience some oil mist contamination when they are on at start and shutdown. This is because the labyrinth-style bearing seals in PT6 engines do not seal well until the engine is running and producing internal air pressure; so now most King Air pilots are turning off the flow packs before shutdown and not turning them on until the next start is completed.
Realize this: If you want heat, turn on the flow packs right after starting. For those of us living in climates that are generally warm, it becomes standard practice that the bleed air switches remain off until runway lineup. But when faced with cold weather operation, that is no longer optimal. Instead, as soon as the engines are running, the bleed air switches are moved to the up, open position. Granted, at idle compressor speed, the air is not as compressed and the bleed air is therefore not as hot as when lots of power and compressor speed are experienced. However, the bleed air entering the cabin, even at idle, will be warmer than the trapped, frigid, cabin air, so allow it to enter and start providing warmth right away.
It is exactly because the PT6 does not produce an overabundance of hot bleed air at idle that King Airs contain a supplemental electric heater. The electric heat system varies quite a bit depending upon the King Air model being discussed. I will start my discussion with the electric heat system that is contained in C90s, E90s, F90s, 100s, A100s and B100s. This includes all later C90 variants: the C90A, C90B, C90SE, C90GT, C90GTi and C90GTx.
For these models, the first thing to do is to ensure the engine speed is high enough to support the very large electrical demand that the heater system requires. This means we must have a minimum of 57 percent N1. Since Low Idle for the standard three-blade propeller models in this group is about 50 percent, you will need to adjust both condition levers forward to attain about 60 percent N1. On the other hand, the four-blade propeller models already idle near 60 percent – to avoid the “reactionless vibration” mode that can be harmful to these props – so further tweaking of the condition levers may not be required.
Make sure the N1 speed is near 60 percent and now select auto or man heat with the mode selector. Next, reach over and position the electric heat switch to the up, “Grd Max” position … and standby for heat! Running all eight electric heat grids – four for the normal heater and an identical four for the ground maximum system – really puts out the BTUs!
Three important comments need to be made here. First, with that much electrical load – about 300 amps total heater demand – it is common to see that the Fuel Control Unit (FCU) has permitted some sag in N1 speed. This may be great enough that the four-blade prop models have even allowed their idle Np to drop below the limit. If this has happened, simply move the condition levers forward enough to get the necessary propeller speed.
Second, make your final condition lever adjustments to match propeller speed, Np, not compressor speed, N1. By doing so, the airplane sounds better – less out-of-sync drumming – and tends to track straighter while taxiing. Requiring a 1 or 2 percent split in N1 speeds to match Np speeds is common. If much more than that is required, it indicates that the maintenance shop needs to do a better job of setting the propellers’ low pitch stops to the correct and same, left and right blade angle.
The third comment concerning the use of the heater on the ground is to remember that certain, more important, in-flight users of electricity take priority over the comfort provided by heater operation. These “heater lock-out” items are windshield heat, prop deice and engine lip boot heat, if installed. The pitot cowls fitted to the C90A and after models use exhaust gases to heat the inlet lip, but previous C90s, B90s and A90s – as well as 100s, A100s and F90s – use an electric heating boot on the cowling inlet lip. When any of these lock-out items is operating, all heater operation is prevented. (Lip boot heat will not operate on the ground since the boots can get too hot without sufficient, in-flight airflow over them. Thus, the lip boot lock-out function should only occur when airborne.)
If you are departing immediately into icing conditions, you will want all anti-ice and deice systems on prior to takeoff. But wait until runway lineup before doing so unless you wish to forgo electric heater operation as you taxi out and conduct your equipment checks.
It is common that full heater output is so effective that the cabin begins to get too warm while still taxiing. If this occurs, move the electric heat switch from the top “Grd Max” position down to the center “Normal” position. Doing so kills the extra four heat grids and allows the normal four grids to continue to be available … operating if manual heat is selected and standing by to operate as needed if in the auto mode.
It is rare that electric heat is needed when flying since bleed air heat should be sufficient. However, there are a couple of situations in which in-flight use of electric heat is desirable. The first of these is caused by a very weak or totally dead flow pack. One pack alone should be able to provide full pressurization, but with cold OATs at altitude a single pack won’t provide enough heat to keep the cabin at a comfortable temperature. So, when the cabin is chilly, even with the cabin temperature rheostat cranked up fully clockwise, make sure the electric heat switch is in the center “Norm” position and turn off the lock-out items. Now the four normal heat grids can come on to add to the total heating capacity.
“But what if I am flying in clouds and need to keep all of the ice protection activated, including the lock-out items?” In that case, hand out the blankets; the cabin will remain chilly.
The second situation in which supplemental electric heat is desirable in flight occurs when engine power remains low for a lengthy period. The lower the power, the less the compressor speed, the cooler the bleed air. During a prolonged low-power descent or while drilling holes in the cold sky in a holding pattern, let the normal heat grids help in keeping the cabin comfortable. As discussed above, you must be out of icing conditions so that the lock-out items may be turned off.
Now I’d like to address the supplemental electric heat system on the 300-series (300s and 350s) and the later 200-series, the ones produced in 1993 or after. (This discussion does not apply to the Keith Environmental Control System found on 200s and 350s since about 2006.) Instead of normal and ground maximum heating grids, there are two separate heaters installed in the floor air distribution ductwork – one forward and one aft. On the ground, a solenoid latches the electric heat switch in the up, on position when that position is selected while the rotary mode selector is either in the “Auto” or “Man Heat” mode. At this time, only the forward heat grid begins to operate. But in 100 percent of the cases, you will probably also want the aft heater to operate. By merely moving the aft blower switch to the up, on position, you have achieved that. The heater would get too hot rapidly were there not sufficient airflow over it, so only when the aft blower is running will the aft heater start to operate. Ah, I feel the cabin getting warmer quickly!
The engines installed on the 200- and 300-series have a higher compression ratio than those engines on the smaller King Airs and, because of that, they put out hotter bleed air in flight at typical power settings. Recognizing this, the designers concluded that there would never be a need for supplemental electric heat in flight. Therefore, the electric heat we are discussing now is definitely a ground-only system. The latching solenoid will not work in flight and even if the crew physically held the electric heat switch up, neither forward nor aft heater will operate.
During the runway line-up procedure, we are directed in these models to position the electric heat switch to the “Off” position. This reduces generator load, allowing the engine to be more likely of meeting takeoff power requirements. If we ever forget this step (of course we never would!), it’s not a big deal because (1) the engines will have plenty of power capability due to the cold OAT, and (2) when the strut extends, the switch will turn itself off anyway since the holding magnetic solenoid latch releases.
Temperature sensors installed in the heater ductwork protect the system from getting too hot. The temperature at which the heater shuts off due to this over-temperature protection, about 115°F, is much higher than the temperature at which the system will permit the heater to reset and start to operate again, near 60°F. The practical result of this? Suppose the cabin reaches a comfortable temperature while taxiing out and the crew decides to switch the heater switch off. Unfortunately, there is a lengthy ATC delay, so the King Air must hold short of the runway. Dang! The cabin is getting chilly again, so it’s time to turn the electric heater back on. There is a good chance that it won’t come on, the switch will not latch. Why? Because of that “allow to operate” temperature limit in the ducts. Conclusion? Make darn sure you have a toasty cabin before switching the heater off since there is a high probability you won’t be able to get it back on.
I will conclude this article by discussing the last of the supplemental electric heat systems, the one that was an option on the 200-series from the model’s start in 1974 through 1992. This system is comprised of radiant heat panels in the cabin’s headliner. (Just in the actual cabin; not in the cockpit nor in the toilet and baggage areas.) In my opinion, the systems that we have previously covered – the ones in the 90-, 100-, later 200- and all 300-series – are real winners that do indeed put out the heat! But the radiant heat panels? What a joke!
The single time that the panels can indeed prove useful and effective is when an external power unit can be plugged in an hour or more before the intended flight and then the heat panels can be switched on for an extended period of time. Slowly, the frigid cabin will be warmed nicely.
In the relatively short time between engine start and takeoff on a typical flight, there is not enough time for the radiant heaters to do much. Expect to remain chilly until takeoff power is applied and finally the bleed air gets nice and hot.
If you decide to switch on the panels while taxiing, it’s a good idea to leave the mode selector in the “Off” position. There are two reasons for this. First, leaving the mode selector off means that the vent blower can also be left off. If it is running, all it does is blow cold air out the various vents. Second, the cabin temperature sensor that feeds information to the control board when in the “Auto” mode is, for almost all serial numbers, located in the cabin ceiling. It can get an erroneous indication that the cabin is warmer than it is due to its proximity to the heated panels.
The only way to really get a lot of heat energy into an early 200 is via the bleed air input, but of course the temperature of the bleed air depends upon the speed of the engines’ compressors. I certainly do not recommend taxiing around in the winter at high idle or even more N1 speed – since the aircraft wants to roll so fast – but, while stopped in a run-up area, selecting high idle will indeed contribute to better heating.
A commonly reported annoyance with the radiant heat panels is that, in use, the heat weakens the glue that is holding the panel to its Velcro strip, so one or more panels begin to sag down from the ceiling. That does not contribute to warm and fuzzy feelings in the passengers! Thank goodness it has been rare, but there are also some reported cases of a radiant heat panel catching on fire! As I wrote earlier, this system is kind of a joke!
One last bit of advice before I bring this article to a close: It is never desirable to turn off a powerful duct heater at the same time that airflow through the heater ceases. Not being blown downstream, the residual energy in the heater elements can cause excessive, localized temperatures to result. So never switch off a heater while the vent and/or aft blower is stopped. Instead, switch off the heater while ensuring the blowers remain on for at least 15 seconds more. This important step is actually stated in the 300 and 350 POHs, but it is the proper technique for all models with heating elements buried in the ductwork.
I hope this information helps you King Air aficionados stay nice and warm during your winter flying!
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