Page 14 - August 2015 Volume 9, Number 8

an exact figure to provide for the temperature of the air that leaves the blower and heads for the cabin, but we know it is much less than the 500 degrees Fahrenheit and higher temperatures that P3 bleed air can provide. An air-to-air heat exchanger – radiator – is located in the left wing center section in front of the main spar to provide a method for outside air to cool the compressed air before it enters the cabin.
Of course, when heat is desired, the cooling of the compressed air is not desirable. Hence, a butterfly valve is installed in the ambient air duct to the heat exchanger and the pilot has a push-pull control in the cockpit, labeled “Pull On – Schgr Heat,” just to the right of his control wheel (Figure 2). By pulling the knob out, he can close the valve, decrease the ambient air flow to the heat exchanger, and retain more temperature in the cabin’s air inflow.
I am sure readers of this magazine are familiar with the Bleed Air Flow Control Package that regulates the flow of environmental bleed into the cabin of later King Air models. The “Flow Pack” can mix bleed and ambient air and serves to stabilize total air mass inflow throughout altitude, temperature, and engine speed changes.
Similar to the flow pack, the 90, A90 and B90 contain a Flow Control Valve in the left wing root, upstream of the heat exchanger. It also serves as a regulator on the incoming airflow, attempting to keep the flow constant over a range of altitude and engine speed changes. This Flow Control Valve is electrically controlled and vacuum operated. With either unavailable – no electric power or no vacuum – the Flow Control Valve dumps the supercharger’s air into the wing and does not send it to the cabin. When electricity opens a normally-closed solenoid valve, vacuum now provides the “muscle power” to overcome spring tension and thereby send the proper airflow into the pressure vessel, or cabin.
How is vacuum derived? In the Queen Air 88 it comes from the old familiar engine-driven vacuum pumps. In the early King Airs, just like in the current-production
Figure 2: Just to the right of the pilot’s control wheel is the heat pull labeled “Pull On – Schgr Heat.”
models of 2015, it comes by sending regulated bleed air through an ejector (venturi tube) in the cabin’s belly. Yes, even though the 90, A90, and B90 did not use “Big P3” Environmental bleed air for cabin pressurization and heating, they did indeed use “Little P3” to provide pneumatic pressure for deice boots and the vacuum- creating ejector.
Figure 3: A three-position switch labeled “Schgr Vent – Press Test,” located in the environmental control grouping on the copilot’s left subpanel.
In the environmental control grouping on the copilot’s left subpanel, these early King Airs contain a three- position switch labeled “Schgr Vent – Press Test” (Figure 3). The switch normally remains in the center position. It will spring back to the center after being held down to the Pressurization Test position, but it will remain up when placed in the Supercharger Vent position.
Follow-up on Brake Deice System Article
A few months back, my article for this magazine dealt with the optional Brake Deice system that is offered on King Airs with dual main tires. An alert reader, Chaz Harris, contacted me with a valid observation and concern that I had overlooked. He, like a lot of us, has experienced numerous instances of the brake deice valves sticking in the open position when tested in flight. His observation is that Beech/Textron Aviation does not address the procedure to use when this rather common abnormality occurs.
There is a published procedure directing us what to do if a Bleed Air Fail or Brake Deice Overheat (F90-
series) annunciator illuminates. And, I suppose, if the valve remains stuck in the open position long enough with the gear retracted, we will probably eventually receive this light as the plastic sense line melts. But before that happens, why don’t we go ahead and turn the respective side’s Bleed Air Valve switch all the way down to the “Inst & Envir Off” position (or “Pneu & Envir Off” position for the later serial numbers), and terminate the bleed air flow upstream of the stuck valve? Well, that’s the procedure we should take and – are you listening, Textron Aviation? – it is the procedure that should be added to the revised AFM Supplement you’ll be publishing soon. Right?
12 • KING AIR MAGAZINE
OCTOBER 2015