Ask the Expert: Heater Operation (and ADF Tuning)

Ask the Expert: Heater Operation (and ADF Tuning)



Darn it, the heater is out!

Now that we have been in winter’s icy grip (well, here in Phoenix, it’s more like a warming caress!), your King Air’s heater is probably getting some use. This discussion applies to the electric heater in C90s (and all the C90 variants), E90s, F90s, 100s, A100s, and B100s. In these models, the electric heater is a supplement to bleed air heat. Although most useful on the ground, it may also be used in flight whenever bleed air alone is insufficient to comfortably heat the cabin.

On the ground, bleed air is usually quite cool since the compressors are turning slowly at Idle. In fact, there may be no bleed air at all: With the engines not yet started, it surely is nice to be able to heat the cabin using the electric heater in conjunction with a Ground Power Unit (GPU).

Remember that the heater is composed of eight identical elements or grids, four wired together in parallel to make up the Normal heater and an identical four wired in parallel to create the Ground Maximum heater. When all eight grids operate on the ground, it is satisfying how quickly the cabin warms even on the most frigid of winter mornings.

Yet there are some operators who have never fully utilized this wonderful system due to a lack of understanding and/or a lack of recent practice with it. Let’s review how to make the heater operate properly and effectively and remind ourselves of some reasons why the heater may not be cooperating today.

First, the vent blower. The heat grids – each using about 36 amps of current – get so hot that they would damage themselves and the heater casing if that heat energy were not carried away by sufficient airflow. Yes, the heater has an overtemperature protection switch, but it is located in the heater’s discharge duct, not in the heater core itself. Thus, it will not feel the excessive internal temperature quickly enough to prevent damage. Therefore, the vent blower has an airflow pressure sensor (think pitot tube) that will not allow the Normal or Ground Max heater to operate unless the device senses a good amount of air flow exiting the blower. If ever your vent blower dies, the heater will die with it but then come back to life when the vent blower is replaced.

I suggest that you always position the vent blower switch to the High position when using the electric heater. This not only provides an increased volume of airflow across the grids to move the heat energy into the cabin more quickly, but also ensures that the airflow pressure sensor is more likely to function correctly. In fact, in most serial numbers – LJ-620 and after, LW-120 and after, B-208 and after, as well as in all F90s and B100s – the blower automatically kicks into High speed mode whenever the heater is operating. To prolong the life of the blower, remember to move its switch back to Low or Auto once airborne when the abundance of bleed air heat causes the heater to no longer be operating.

Do a little math with me: 36 amps per grid times four Normal grid elements equals 144 amps. When the Ground Max elements are also running, we double that to 288 amps. Yet, the heater cannot operate without the Vent Blower, so that adds another 20 amps or so. Over 300 amps required to get full heater operation!

With the exception of the momentary demand of the starter motor, this is by far the largest electrical load that the airplane ever experiences in normal operations. A few consequences of this huge demand: First, the GPU must have enough capacity to handle this demand. Using a simple battery cart to run the heater is not a good idea. A powerful GPU is needed.

Second, a single generator has insufficient output to do the job, since its maximum continuous rating is 250 amps. Make sure both generators are on line before running the heater. Also, for you people with three-blade props and hence have your Low Idle speeds set near 50 percent N1, you will be exceeding generator cooling limits if you apply this much generator load while still at Low Idle. Set both condition levers for about 60 percent N1 before turning on the heater! And for you people with four-blade props, you may also need to tweak your condition levers a bit forward if the generator load bogs down the engine enough such that the propeller speed drops below the minimum Np (propeller speed) limit.

Third, the system designers ensured that the high electrical demand of this comfort item – the electric heater – would never rob power from more important safety of flight items. When switching on any of these anti-ice systems, a relay is activated that prevents the heater from operating. The heater “Lock Out” systems are (1) windshield heat, (2) propeller heat, and, if so equipped, (3) lip boot heat.

The heater is rarely needed in flight due to bleed air heat. In fact, if you find that the cabin is chilly in cruise and you need to operate the supplemental electric heat to stay warm, the reason is virtually always two weak bleed air flow packs or a single totally dead pack. If you are in cold clouds and need the heater in flight, tough. You’re going to be chilly. On the other hand, when you break out on top you can get the heater to join in once your turn off all two or three of your Lock Out items. Remember to get them back on before penetrating clouds in the descent.

Here is how it should go on your next icy winter flight: After starting, make sure your idle speeds are near 60 percent N1 and then reach for the environmental controls on the copilot’s left subpanel. The order is not critical, but here’s what we need to do: Bleed Air switches – OPEN, Vent Blower switch – HIGH, Cabin Temp Mode Selector – AUTO; Electric Heat switch – GND MAX.

Immediately the Ground Max heater elements should activate. You will observe a large increase in generator load and, in a few moments, you may notice a rather disconcerting smell of burning material. Relax, it is just the accumulated carpet lint and dirt getting burned off the heater elements that haven’t been used in a long time. You will also likely observe in a short time a click sound from another relay and the loadmeters increasing even more. What happened? The Normal heat grids just got added to the Ground Max ones. You see, the Normal heater, in Auto mode, never gets a command to operate until the bleed air has already gotten fully hot. So, it will not come on until the bleed air bypass valves have traveled to the full-hot position. This could take as long as 60 seconds, but it is likely that the bypass valves were probably in a fairly hot position at the end of the last flight (unless a massive cold front passed during the night!) so usually just a few seconds elapse before the Normal grids join with the Ground Max.

I sometimes teach that the Ground Max heat grids are like the portable little electric heater you plug into the outlet to help the furnace heat that chilly winter bathroom. It has no tie-in whatsoever to the house thermostat or heating system but rather operates in a totally independent fashion. When you position the electric heat switch to GND MAX, it’s like you plugged in the portable heater … it operates, period. But the Normal Grids are indeed tied into the environmental system and don’t waste energy until the heat of the bleed air – which must come into the cabin for pressurization anyway – is fully exploited.

The electric heat switch is held in the GND MAX position by an electromagnet that is only energized when the appropriate squat switch is activated … weight on wheels. When we lift off at takeoff, this “independent” member of our heating system says good-bye. Even with mediocre bleed air flow, GND MAX will not be needed in flight.

And if you were to forcably hold the electric heat switch to the GND MAX position once airborne, then what? The answer, for you few trivia buffs, is that doing so will indeed cause the Ground Max grids to operate … but the Normal grids, if operating, will shut off! A landing gear uplock switch – not the Squat switch – prevents all eight grids from operating when the gear is retracted.

Remember when I mentioned the burning smell? That can be scary for the passengers so a trick I use is to burn off the lint and dirt periodically by running each heater element for a minute or so on a deadhead cruise leg. To do so, (1) make sure the Lock Out items are off, (2) position the mode selector to MAN HEAT, (3) place the electric heat switch to the center, NORM position (or verify that it is already there), and (4) verify that the loadmeters jump up and the smell begins. After a minute or so, reach over and hold the heater switch up to GND MAX and keep it there for a minute or so to burn off the other grids. Now let go of the heater switch and return the mode selector to AUTO.

By the way, where should you leave the heater switch on warm days? NORM or OFF? It really makes no difference. Realize that in NORM the Normal heater grids never operate until the Bypass Valves get to the full-hot position. So, in this position the normal heater is just available to operate, not actually operating. On hotter days, it will never be requested by the automatic heating system.

ADF Tuning

What the heck does ADF Tuning have to do with heater operation?! Absolutely nothing!

I decided to use the remaining article space to throw in a little trick that you may not have been taught. This trick applies only to the Collins Pro Line II tuning heads, the ones that were the standard before Pro Line 21 came along around 2005. The ADF tuning control – like the COMM and NAV control heads – allow a frequency to be dialed into the bottom, Standby window and then it is flip-flopped with the active frequency by momentarily tapping the little transfer switch up and releasing it.

But do you realize that these heads offer an active tuning option? By depressing the little white button just below the tuning knobs for two seconds or longer, the frequency in the Standby window is replaced with dashes and now the upper, active frequency is tuned directly.

Last week I was flying from Houston back to Phoenix on a Sunday in LJ-1190, a sweet 1988 C90A with these types of avionics, and the NFL Conference playoff games were in progress. How do I find the appropriate AM radio station that was broadcasting the game?

The trick is this: Put the head into the active tuning mode and use the small tuning knob to dial the last two digits to 10. Now use the larger tuning knob to dial in “5.” The window shows 510. Listen to the ADF – best to use ANT (Antenna) mode for clearer reception – to find if you hear the game. No? Then move the big knob up one click to get a 6 … 610. Likewise, 710, 810, 910, 1010, 1110, 1210, 1310 to the end of the AM spectrum, 1710. If you are still searching, use the small knob to dial the last two digits to 20 and now repeat the search going down: 1720, 1620, 1520, etc. Try 30 and scan up, 40 and go down, etc. That sure is faster than having to flip-flop from Standby to Active each time, eh?

Before I end this discussion, I will state that my using Direct tuning on a COMM or NAV head is very, very rare but it does come in handy occasionally. For example, suppose I have already entered the Tower frequency in the Standby window while still talking to Approach control in the Active window. The wind shifts, the pattern gets rearranged, and now Approach assigns me another Approach frequency. In this situation I can retain the Tower frequency by going to Active tuning – hold the white button for two seconds – dial in the new frequency, then hold the button again for two seconds to find that the Tower frequency is still waiting in Standby. As I said, rare but handy.

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