Ask the Expert: A Supercharger on a King Air?!

Why, yes indeed! All straight 90s, A90s and B90s use a single supercharger, driven by the left engine, as the source of air inflow for cabin pressurization. The PT6A-6 and PT6A-20 engines that these models used had lower compression ratios and delivered less air than later members of the PT6A family, yet in 1971 when the B90 ceased production and the C90 took its place, Beech did indeed use bleed air, tapped equally from both engines – the same old PT6A-20s – as the pressurization air source. So if sufficient air had always been available, why was it not used on the first three King Air models?

After all, not only does bleed air have the advantage of redundant, left and right supplies, but also it is so much warmer than the air from the supercharger that it becomes the primary in-flight heat source for the cabin. This allows the Jet-A-burning combustion heater (Janitrol heater) to be replaced by a much simpler and more reliable electric heater. With these advantages in mind, again we wonder why Beech ever made King Airs with superchargers.

The answer to this enigma is that Beech had always planned a piston-engine-powered “King Air,” and no bleed air was available from that type of powerplant. By going with the supercharger, a common system could be fitted to both models. The decision was made to proceed with the turbine version first. The piston-powered one, the Queen Air 88, did not make its appearance until a couple of years later. The 88 was never a hot-seller and less than 50 were made during its brief four-year production run. (Prospective customers had been spoiled with the much better performance of the turbine version, the venerable King Air. The 88 earned the not-so-affectionate nickname “Lead Sled” among the factory pilots.)

The PT6A-6 on the original 90, as well as the IGSO-540 on the Model 88, both used the same method for driving the supercharger: The engine had a hydraulic pump that circulated fluid to a hydraulic motor that spun the supercharger. This cumbersome arrangement was replaced on the A90 and B90 models by a much simpler mechanical drive gearbox attached to the left engine’s accessory case. I doubt that any 90s remain that have not been upgraded to the mechanical drive.

So what exactly is this supercharger? Wikipedia states, “The Roots type supercharger or Roots blower is a positive displacement lobe pump which operates by pumping a fluid with a pair of meshing lobes not unlike a set of stretched gears. Fluid is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust. It is frequently used as a supercharger in engines, where it is driven directly from the engine’s crankshaft via a belt, chain, or gears.

It is named after the American inventors and brothers Philander and Francis Marion Roots, founders of the Roots Blower Company of Connersville, Indiana, who first patented the basic design in 1860 as an air pump for use in blast furnaces and other industrial applications. In 1900, Gottlieb Daimler included a Roots-style supercharger in a patented engine design, making the Roots-type supercharger the oldest of the various designs now available. Roots blowers are commonly referred to as air blowers or PD (positive displacement) blowers, and can be commonly called “huffers” when used with the gasoline-burning engines in hotrod customized cars.”

Wow! A design from 1860 still doing service as an air supply for some pressurized airplanes! Google search it if you’d like and you can find some fascinating drawings and even videos of the blower in action.

Figure 1 shows a two-lobe rotor model which is used in the King Air. The pump body has external fins to help dissipate the heat caused by compression. I do not have 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 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.

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. (Notice both the Schgr Vent – Press Test switch on the top row, as well as the Press Control circuit breaker on the lower right.)

When the landing gear squat switch – or WOW, weight-on-wheels, switch – is activated, electric power is removed from the Flow Control Valve’s solenoid, causing the vacuum to be removed and hence stopping the inflow of supercharger air. Selecting the “Press Test” position of the switch in the cockpit causes the Flow Control Valve to receive electricity and vacuum, making it send its normal air supply into the airplane. At the same time, the Dump and Preset solenoids are de-energized to allow the pressurization controller to operate normally. (The Preset solenoid did not appear until the B90 model,
LJ-318 and after.) With the pressurization controller set for a lower-than-field-elevation altitude, the cabin should start descending to verify a proper pressurization test.

In a similar manner, moving the cockpit switch up to Schgr Vent does half of what Press Test does. Namely, it causes the Flow Control Valve to send proper airflow into the cabin, but it has no effect on the outflow or safety valves. Thus, the airplane will be receiving air from the supercharger to ventilate, not pressurize, the cabin…explaining the name, Supercharger Vent(ilation).

Once airborne, of course the outflow and safety valves should be doing their respective things and the supercharger should already be supplying proper inflow. That leads to the conclusion that you can select any position at all – up, center, or down – with this strange switch when you are flying and nothing changes whatsoever. It only has functionality while on the ground.

You may be wondering, “When would one use Supercharger Ventilation anyway? What purpose does it serve?” The simple answer is that it serves almost no purpose and most pilots of these airplanes have probably never used it! But its most common use in days past was smoke removal! “Huh? Smoke removal? Why would I still be taxiing out with smoke in the airplane?!” Because the boss and his friends in back are smoking cigars and cigarettes! Yes, back in the ‘60s when these airplanes were being produced, smoking was still amazingly widespread and passengers thought nothing of lighting up after they took their seats. It could get pretty unpleasant, especially for any non-smoking pilots, when all of that “polluted” air was trapped in a pressure vessel with no inflow and no outflow. By selecting Schgr Vent, bringing the outside air in and forcing flow out of the safety valve, the smoke level in the airplane could be decreased significantly. (If you are lined up behind a row of airliners on the taxiway, you have the choice of smelling the cigarette smoke or the kerosene fumes, eh?) As I said, nowadays the switch is rarely ever moved to its Up position.

What if we encounter “real” smoke in the airplane while flying? Maybe our Roots Blower developed a leak that allowed oil to mix with the air. How do we terminate the cabin air inflow when we need to?

On the pilot’s left subpanel is another push-pull knob, labeled “Press Air – Firewall Shut-Off Pull,” just to the left of the control wheel. When the knob is pulled, a mechanical shut-off valve located on the left engine compartment firewall stops the flow of air heading toward the Flow Control Valve and instead dumps the air back into the engine compartment through a hole in the firewall, where it can escape harmlessly overboard through a vent on the left side of the nacelle.

While conducting flight training in these “mature” King Airs, I have found – more often than I’d like – that this firewall shut-off valve cannot be closed by the push-pull control. The lack-of-use combined with some corrosion probably has provided too much resistance. If you operate one of these old birds, please make sure you and/or your maintenance providers exercise this control with some degree of regularity to ensure its freedom.

However, what if we encounter environmental smoke in flight, try the knob, and find it’s stuck? Is there a workaround? Of course. Locate the “Press Cont” CB on the copilot’s left subpanel and pull it. Doing so will de-energize the Flow Control Valve and stop cabin air inflow, dumping the air into the wing where it can leak overboard. This method is not as desirable as use of the push-pull control since it allows the air to pass through the firewall and enter the wing. In most cases, that will be no big deal since typically the smoke comes from a minor supercharger oil leak. On the other hand, were there an actual fire … hmmm, I hope that doesn’t happen and also that your push-pull control is not stuck.

Most of our readers will never have the opportunity to fly one of these old King Airs equipped with a supercharger. But for those who are flying one now or who will be flying one at some time in the future,
I hope you’ve found this history lesson worthwhile.

If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at kblonigen@cox.net.

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?