Understanding the Pitot-Static Air System and Flight Instruments
Introducing the Pitot-Static System and Instruments – Video Transcript
Today, we are going to talk about the pitot-static system. Specifically, I want to dive into a few different elements – so, the first thing we’re going to start with is, what are these two air sources and what makes them different? The next is, which flight instruments use the pitot-static system and finally, we will look at some different designs that we find in our airplanes and figure out which design you have in your airplane. So, stick around and hopefully by the end of this we will learn something about the pitot-static system.
So, I have got some information on the board here and let us start with our first question. We want to talk a little bit about the difference between the static air and this pitot air. We will start with the static air, and this is really representing the ambient air around us. Why is that important? Well, that is important because the air around us changes as we change altitude. This is very similar to another fluid, like water. If you have ever dove into the deep end of a pool or gone diving in the ocean or a lake, the deeper you go, the higher the pressure that you generally feel in your ears. That works the same way with air. As we go to higher and higher altitudes, the air pressure becomes less and less, and the closer we get to the surface, the greater that pressure is.
Now, that pressure changes at a relatively consistent rate, at least at lower altitudes, so this helps us in ultimately being able to calculate our altitude. So, the static air typically follows what is known as a lapse rate. This lapse rate is measured at approximately one inch of mercury per thousand feet. To put that into perspective, if we had a barometer that would measure pressure and we left it at sea level, the standard sea level pressure is 29.92 inches of mercury. If we then climbed a ladder – or climbed an altitude – by a thousand feet, we would see that pressure would be relatively close to 28.92 inches of mercury and so on. This is valuable information, as we can have a device that would be able to then calculate our change in altitude or to know what our altitude would be.
Our pitot air is also sometimes referred to as our ram air, and this is more or less comprised of two components. Imagine if you were driving down the highway and you stick your hand out the window, you would feel a bunch of air pressure pushing on your hands. Most of that pressure that you’re feeling is what we call the relative wind, or your speed, and the other bit would still be the static air that you are in. So really, the pitot air, or this Ram air, can be described as a combination of the static air pressure that you are in coupled with the velocity, or the speed, pressure that is created by this relative wind. This would be very helpful to us in determining what our air speed would be as we fly through an air mass.
Now, we will move on to question number two. Which instruments use the pitot-static system? So, there is three flight instruments that ultimately use this pitot-static system – the airspeed indicator, the altimeter, and the vertical speed indicator. Starting from the airspeed indicator, we see here that I’ve kind of illustrated that the airspeed indicator uses both the pitot source and the static source. Why would that be the case? Well, the answer, if we go back to our first question, was that the pitot air by itself is comprised of both our speed and the pressure from the ambient static air. So, what that means then, is we want to be able to sort of remove the static air component from that equation, so they ultimately just compare the pitot source to the static source. That way, it is only indicating our velocity. The altimeter only utilizes the static source and this is because once again, based on this lapse rate, we have identified increments of where pressures should be at and therefore, the altimeter has just set up so that it would interpret those static air pressures and then depict the appropriate altitude. Finally, we have the vertical speed indicator and the vertical speed indicator is simply going to use the static source to measure our rate of change in static pressure. So, if the pressure is going down quickly, then we know that we are climbing rapidly. Opposite of that would be if we were descending quickly, we would see that the vertical speed indicator would sense that the pressure was continuing to increase over time and therefore it can indicate whether we are climbing or descending.
The last question is about how the pitot-static system may be designed in your particular airplane. In my experience, there is really two different designs you are going to find across pretty much all airplanes. The more standard design, that we typically see, is on the outside of the airplane. There will be what is called a pitot tube, either hanging below a wing, or sometimes mounted on the side of the airplane or on the nose. Then there is a static port which will usually be sort of flush mounted along the edge of the fuselage, or the empennage, of the airplane. Those air sources then go into the cockpit and connect to the various flight instruments as you see here. I have it arranged as the airspeed indicator, the altimeter, the vertical speed indicator, and in this case, in most examples, will see that the mode c encoder for the transponder also has a static source so that it can determine the altitude that we’re at and send that information along to any transponder requests. In this example, I kind of drew this arrow to identify the direction of flight. So, what that means is, if we are flying this direction, then the relative wind would be forcing this air pressure into this pitot tube.
The other design that we see – and specifically here, at AeroGuard, where we fly all piper aircraft – is in a design slightly modified. Basically, the difference is, instead of having a separate pitot from this static port, it’s all combined into one unit that hangs below the wing called a pitot mast. So on the leading edge of this pitot mast, once again direction of flight, we see that there would be an opening and that would represent our pitot source, or this ram air source, and then on the trailing edge on the backside of this pitot mast there is the static port which is to supply static pressure to the airspeed indicator and the altimeter, the vertical speed indicator, and once again the mode C encoder for the transponder.
So, I hope that this helped clarify any questions you may have had about the pitot-static system. Continue to watch. We’re going to have more videos in reference to how each of the flight instruments work as well and hopefully that will just move you forward in your knowledge.