Page 20 - Volume 11 Number 8

An image from the B200 POH shows the Nose Circulation system has changed and the crossover duct was eliminated.
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
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.
18 • KING AIR MAGAZINE
AUGUST 2017