Page 20 - August 2015 Volume 9, Number 8

Ask the Expert Pressurization System
Abnormalities
by Tom Clements
One of our readers suggested that I discuss some of the common failures that affect the King Air’s pressurization system. I am grateful for his idea and this article will address those abnormalities.
Let me begin, however, by stating that the great majority of pressurization problems in King Airs are not the ones I will be presenting now! Lack of sufficient inflow – a weak flow pack or two, combined with excessive outflow, too many cabin leaks ... these are the causes of the great majority of pressurization problems. I have said it before and I’ll say it again: I would suggest that the most common, almost universal, weakness in King Airs is that they have leak rates well in excess of what Beech specifies. And you know what? I don’t care! To find a leak rate – peak cabin climb rate when the inflow is stopped while at maximum differential pressure, ∆P – below 2,500 fpm is quite rare, although that’s the specification Beech tells us is correct. But if I see 4,000 fpm or even more, I can happily live with that if (1) I can maintain maximum ∆P on one flow pack alone at cruise power, and (2) that I can reduce power back to the gear horn warning, with both flow packs operating, without the cabin starting to climb (due, of course, to less inflow than outflow).
No, this time we are discussing problems that may exist even with a wonderfully tight pressure vessel and with strong flow packs.
First, the basics: Remember that both the outflow and the safety valves are spring-loaded to the closed position and require vacuum to suck them open. Suppose that the line going to the throat of the bleed air ejector – the vacuum source – disconnects so that we have no vacuum. What will happen?
Well, when the Bleed Air switches are turned on, air flows into an essentially closed container – the pressure vessel, cockpit and cabin. With inflow taking place and no outflow happening (except for the leaks), the cabin is gaining air mass, pressure is increasing, and altitude is decreasing. With high power on the engines – such as at takeoff – it will be normal to see the cabin’s rate-of-
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climb gauge hit the peg at -6,000 fpm! Whew, that gets your, and your ears’, attention!
So before the cabin dives down too far, we’ll grab the Cabin Pressure Control switch and move it forward to Dump, right? Well, sure, feel free to do that. However, in this scenario, it won’t work. Why? Because we have no vacuum, no force to suck the Safety Valve – the dump valve – open.
If we cannot control the outflow, we need to stop the inflow. The solution to this runaway cabin dive is to turn off both Bleed Air Valve switches. With the inflow now stopped, the cabin will continue to leak air out until we become unpressurized as the cabin rises to the airplane’s altitude. This could easily take in excess of 20 minutes.
If there are operators of straight 90, A90, or B90 models reading this, loss of vacuum affects your airplanes differently than what we have discussed which applies to the airplanes with flow packs. Your Flow Control Valve in the left wing center section – that regulates the flow from the supercharger by sending the proper mass flow into the cabin and dumping the rest overboard in the wing – needs vacuum to keep from dumping all of the supercharger’s air overboard. Hence, a total loss of vacuum – the hose disconnecting from the bleed air ejector’s throat – does not lead to a runaway cabin dive, but instead leads to a cabin climb. This is due to the fact that the Flow Control Valve stops sending any air into the pressure vessel, combined with the unavoidable pressure vessel leaks that exist.
The next malfunction I will discuss is failure of the Preset Solenoid Valve. This is the valve (not installed on B90 and earlier models) that goes from its Normally-Open (N.O.) state to a closed condition when we are on the ground and/or when we move the Pressurization Control switch to Dump. The purpose of this valve – as its name indicates – is to allow the pilot to preset the pressurization controller for the desired cabin altitude prior to takeoff. Most of us do this during our Before Takeoff procedure. Yet, with
AUGUST 2015