Page 20 - March24

ASK THE EXPERT
Pressurization Basics
by Tom Clements
Ikeep observing a disturbing lack of knowledge and understanding of an aircraft’s pressurization system. Let me try to set the record straight ... or at least straighten it out a little bit. I will use the numbers associated with a member of the King Air B200-series. However, what I write, with minor modifications, will apply to any pressurized airplane.
Differential Pressure (∆P, “Delta P”)
Differential pressure is simply the difference between inside and outside absolute pressures. In engineering parlance, the Greek letter Delta (∆) is commonly used to indicate the difference between two measurements. So, expressed as a formula: ∆P = PCABIN – PAMBIENT.
If a positive amount of ∆P exists, the airplane is pressurized with more pressure inside than outside, just like a party balloon. The doors and windows are trying to be pushed open and the structures must be strong enough to withstand these forces. This is the reason why pressurized airplanes are heavier than their unpressurized predecessors.
So, like that party balloon, we push more air in than is let out and the airplane becomes pressurized, right? When doing a test in the maintenance run-up area, yes, that is correct. In a great majority of our flights, however, it doesn’t work that way. In most cases, we set the pressurization controller for a cabin altitude that is higher than the field elevation from which we departed, right? And then after takeoff the cabin climbs to that altitude, right? Well anytime the cabin is climbing, it is decreasing its pressure and the fixed-volume cabin is therefore losing air, not gaining air. The pressure inside the cabin is indeed decreasing but why we are getting pressurized is because it is not decreasing as fast as the ambient pressure outside the cabin.
18 • KING AIR MAGAZINE
MARCH 2024