Ask the Expert: The Crossover Duct … and Why it was Eliminated

“Out of sight, out of mind.” That can be one description of the topic for this month’s article: The crossover duct. Take a look at the image below, showing the forward portion of a model 90’s environmental system.

Do you see that dark blue and light blue tube that crosses from left to right, on the most forward portion of the diagram? It sits forward in the nose section, just aft of the bulkhead that has the radar antenna bolted onto it. It is totally inaccessible without removing lots and lots of equipment and panels. I have heard more than one King Air mechanic opine that Beech creates the air conditioning (AC) system first, then builds the airplane around it! Well, not really, but gaining access to the environmental components – air conditioner and heater – in the nose area is indeed a labor-intensive and time-consuming task. Keep your fingers crossed that there will be no repairs needed in this section during your next Phase inspection!

The fan that circulates air through the environmental components goes by the name of Vent Blower. Personally, I don’t like the “vent” portion of this label since I believe “venting” tends to involve air exiting some location or container. “Let’s open the kitchen window to vent out the smell of those rancid eggs.” Or, “I’ll open the valve on the bottle to vent some oxygen out to keep the pressure from being excessive.” Yet this particular fan merely picks up cabin air to recirculate it across the cooling and heating components before it flows through the outlets back into the cabin to repeat the process again. So I’d vote for just labeling it “blower,” but the Beechcraft engineers decided to label it “vent blower” so that’s the name we’ll use.

The vent blower is located under the cockpit floor, just between and in front of the pilot’s rudder pedals. (Surprisingly, it is not shown nor labeled in the picture you have examined from the POH.) It picks up the air that is there (better wear clean, non-smelly socks, eh?) and sends it forward through a hole in the forward pressure bulkhead. It now enters the Evaporator Plenum that is located in the cavity beneath the avionics or baggage compartment floor on the left side of the nose wheel well. It passes through a filter before flowing across the coils of the evaporator. As the name indicates, this is where the air conditioning’s Freon boils, or evaporates, from liquid to vapor. Just as it takes plenty of heat energy to boil a pot of water on the kitchen stove, so also does it take energy to boil the Freon. That energy comes from the cabin air, as it sends energy, heat, from the air into the Freon, causing the air to cool. Unlike the water on the stove, however, here the pressure of the Freon is very low and hence its boiling temperature is also low enough that it evaporates without high temperature involved.

As the cabin air loses heat energy and cools, less water can be held in suspension so some condensation of water almost always takes place. That is why the evaporator plenum (plenum is just another word for chamber or space) contains a drain hole on the bottom. That puddle you see on the ramp under the left side of the nose at the end of summer flights is merely the condensed water that exits the drain. To avoid an undesirable pressurization air leak, the drain contains a rubber seal that closes when a positive pressure differential is present, but relaxes to the open position when unpressurized. The evaporator plenum also contains a temperature-sensing switch that will shut down the AC before the plenum gets so cold as to cause the condensation to freeze, thus blocking the flow of air through the plenum. This is the “freeze switch,” set to operate when it senses the temperature approaching 32°F.

So now the air from the vent blower proceeds past the evaporator plenum to continue its circulation pattern. Keep in mind that most of the time in cruise the AC is not operating because it’s not needed. So even though the air always flows across the coils of the evaporator, no cooling may take place. During these times, the plenum merely serves as an inert component that directs the air forward so that it can continue its circulation. The diagram you examined is obviously a top-view, looking down at the environmental components. Hence, there is no way to see or visualize any vertical differences in the tubing’s location. It’s obviously not a 3-D drawing!

So, to the un-informed newcomer, there is no way to know that the dark blue and light blue crossover duct is not just a horizontal tube from left to right. That cannot be, however, because the nose wheel well gets in the way. So, this duct moves vertically upward to get above the wheel well, then goes horizontally from left to right, then drops vertically down to reach the electric heater (or kerosene-fired heater on the older models prior to the C90).

As the circulating air continues its flow, now moving aft on the right side of the nose wheel well, below the right side of the avionics bay, it branches into two parts. The larger part goes through the electric heater that contains eight large grids – coils of exposed wire – that are available to heat the air, if needed. Four of these grids are referred to as the “Normal” heat grids and the other four identical grids comprise the “Ground Max” heater. As you know, except for winter ground operation, rarely are any heat grids needed to supplement the bleed air heat. So, just as with the evaporator plenum, the air’s temperature may or may not be changed here, depending upon the need for heating.

The smaller air circulation line on the right side of the nose bypasses the heater and sends the air directly to the overhead “Eyeball” or “Wemac” outlets. Again, since it is impossible to know height relationships from this top-down view, you need to know that this line splits into left and right sides, moves up in ducts behind the sidewall upholstery behind the crew seats to the ceiling, and then feeds the outlets. As you can figure, this air may be cooled but it is never hot, not having passed through the heater nor being mixed with new bleed air.

To complete our circulation pattern description, the air that exits the heater now passes through another hole in the forward pressure bulkhead to enter the Mixing Plenum. This chamber is under the floor beneath the copilot’s feet and it is where incoming bleed air enters the distribution system. Hence the name: We are mixing bleed air and recirculated air in this plenum before the combination flows to all outlets in the cockpit and cabin except the overhead ones.

As you can well imagine, it would be pointless, and probably impossible, to vent our pressurized air into unpressurized components in the nose and then somehow force it to re-enter the pressurized cabin! Thus, all the nose distribution components and ducts that we have been discussing are pressurized and are experiencing the same ∆P between the inside and outside of the components and ducts that the airplane is experiencing. See the potential for leaks?

An increase in cabin leakage is virtually a certainty as a King Air ages and logs hours. In fact, it is my opinion that excessive leak rates – as compared to factory specifications – is the most universal “squawk” in all King Airs … and probably in every other pressurized airplane ever made!

Some leaks are relatively obvious because they are easy to hear. A bad door or emergency exit seal is usually quite apparent. But when you find that your airplane has a very high leak rate, yet the cabin seems relatively quiet and normal, guess where those leaks likely exist? Bingo! Up in the nose, far from your ears.

Through the five decades of King Air production and operation, it has become very apparent that the environmental components in the nose – especially that big crossover duct with its holding clamps on both ends – have a less-than-stellar reputation for trouble-free operation. Expressed more simply, they can leak like a sieve! So sorry, but one of the facts of operation of a 90- or 100-series King Air is that eventually your maintenance shop will be charging you for some extensive labor hours as they make needed repairs in this hard-to-access area.

By the time the model 200 was being designed in the early 1970s, the weakness in the 90- and 100-series nose-area environmental distribution was quite well recognized. The engineers wanted to improve the design and make it less problematic. Keep in mind that the highest ∆P any King Air model had up until this time was 4.6 psid, yet the 200 was going to have 6.0 psid. More ∆P – more aggravation of leaks.

Take a look at the image above from the B200 POH.

Well, looky there! No crossover duct! In fact, there is nothing beneath the avionics on the left side of the nose, period. (I always wondered when someone would come up with an STC to add an access door into this void space. Maybe a good spot to store the prop restraints and the inlet plugs, eh?)

Now, the vent blower is no longer under the cockpit floor, but instead is forward in the nose. Air comes through a flapper valve and filter in front of the copilot’s rudder pedals, on top of the cockpit floor, gets sucked to the blower through a hole in the pressure bulkhead, then gets expelled through the evaporator plenum and back through the pressure bulkhead beneath the cockpit floor into the mixing plenum.

Well and good, but where is the heater?! It’s gone!

Sadly, Beech made a little boo-boo here. They looked at the higher compression ratio on these big PT6A-41 engines that had never been used before and concluded that bleed air heat would always be sufficient for cabin comfort, no supplemental heat would be required.

Wrong! In cruise flight, sure, there is plenty of heat. But the design team must have totally overlooked the fact that the bleed air is not very compressed and hence is not very hot on the ground with the engines at Idle. Before the flight test program concluded, it was recognized that something had to be done to provide cabin heat on the ground … or even to supplement the bleed air heat in flight, if ever needed. By this time, however, the environmental air distribution design had been finalized, with no provision for any type of grid heater in the ductwork.

What do they do? They come up with the rather puny and ineffective “Radiant Heat Panels” in the cabin overhead. Better than nothing? Absolutely! As good as an electric grid heater? Not even close! But at least they got rid of the potential for leaks in our now non-existent crossover duct. It was not until the model 300 appeared in 1984 that the design was modified to include both a forward and aft grid-type heater buried in the distribution ductwork. This same design was incorporated into the 200-series beginning with BB-1444 in 1993. Nice!

There you have it: More than you ever wanted to know about the infamous crossover duct. It is still in the latest C90GTx-series, but never in any 200- or 300-series, including the B300 (known as the 350).

May all your leaks be small!