Page 17 - Volume 12 Number 6

means that the outflow valve would be positioned by the controller to allow nine ppm to escape (14 ppm in, 5 + 9 ppm out) ... we’re in balance and the cabin is holding its altitude, maintaining a constant cabin pressure.
Now let’s make the leaks add up to 20 ppm at 6.5 psid. (Don’t ask me how we got to 6.5, because we won’t be staying there, as you’ll see.) Since now, even with the outflow valve totally closed, there is more air exiting (20) than entering (14) a net loss of cabin air is taking place and the cabin must be losing air molecules, losing pressure, and hence climbing. As the cabin climbs while the airplane flies level, however, ∆P is decreasing and hence the mass flow through the leaks is also decreasing. As the cabin goes up and ∆P goes down, eventually a perfect balance will be reached, wherein the leaks total 14 ppm, equal to the inflow. At that point, the cabin stops climbing. But now you see two common but incorrect indications: First, the cabin is higher than the altitude you’ve dialed into the controller, and second, your maximum attainable ∆P is well below the correct 6.5 psid value.
Inflow
The flow packs attempt to provide constant air mass flow regardless of altitude, outside air temperature, or compressor speed (N1 or Ng). If compressor speed is too low, however, the flow cannot keep supplying the pounds of air that it should ... the air pump isn’t turning fast enough. A quick and unscientific check of your inflow and outflow is this: Can you maintain maximum ∆P with both power levers pulled back far enough to just trigger the landing gear warning horn? If the answer is no, then you can be sure that your air inflow is too low (weak or dead flow pack) or your air outflow is too high (excessive leaks) or a combination of both.
As you reduce power aggressively for a descent – either to comply with an ATC request or to keep the speed down due to turbulence – you may
JUNE 2018
observe the cabin starting to climb. In fact, I tend to watch the cabin’s vertical velocity indicator (VVI), more than torque or fuel flow, when I reduce power significantly. You may need to push the power levers back up a bit to keep supplying enough inflow to prevent the cabin from ascending. On the other hand, if you need to come down steeper, it’s time for landing gear extension and maybe, if it’s not overly turbulent, approach flaps too. Remember that the maximum allowable load factor limit is reduced when flaps are extended.
Even the relatively small por- tion of air that is bled from the engine’s compressor for cabin pres- surization and heating in a King Air typically causes the engine to run about a 10 to 20 degrees hotter ITT than if the bleed air were shut off and allowed to remain in the engine. That explains why leaving the bleed air valve switches closed sometimes allows more takeoff power to be achieved.
Outflow
The pressurization control system is made by Honeywell Aerospace. Honeywell is the name that has survived from a long line of company acquisitions and mergers. The control system we use evolved from the very first installations used on B-29s in the latter days of World War II. The company that designed and manufactured that system was Garrett AiResearch. So even today, most of us say it is an AiResearch control system.
The system is mechanical, using springs and vacuum. Electricity plays a minor role. In the King Air, the system uses electric power primarily for Dumping: Opening a normally- closed solenoid valve that permits vacuum to suck open the Safety Valve and thereby create an opening (hole) so large that cabin pressure quickly equalizes with ambient pressure. In fact, the reason that a total loss of electric power in flight always leads to a lack of pressurization is not because the control system fails.
KING AIR MAGAZINE • 15