In this lesson from Free Pilot Training, we'll be discussing the pitot-static instruments and we'll explain how those work. In the last few lessons, we've been discussing the gyroscopic instruments, but today we're going to talk about three of those primary flight instruments that use air pressure to give us information. These are the airspeed indicator, the altimeter, and the vertical speed indicator. These instruments are provided air pressure from the pitot tube and the static port, and they turn it into information that's really important for pilots in flight.

There are three types of air pressure that come into play when we talk about these instruments: static air pressure, dynamic air pressure, and total air pressure. Static pressure, which is also called ambient air pressure, is pressure that's caused by our atmosphere. This is always present, whether the airplane is stopped or moving, and although it might seem unnoticeable to you, these air molecules push on everything in our atmosphere. The closer you are to the center of the Earth, the more tightly these air molecules are packed, and because they're packed so tightly at lower elevations, the lower your altitude, the higher your static air pressure is. Our airplanes use static ports to sense this type of air pressure. These are usually placed on the side so they're not affected by relative wind. Static air pressure is measured in inches of mercury. On a normal standard day at sea level, the average air pressure is 29.92 inches of mercury. Now, the air pressure isn't always 29.92 inches of mercury there, but this is about the average, so this is what we call standard air pressure. Then every thousand feet above sea level, the pressure reduces by one inch of mercury.

In the next lesson, we're going to go into a lot more detail about this and how the altimeter works, so if you haven't already subscribed, please consider doing so now. Anytime an object like an airplane moves through the air, it's impacted by the air molecules that it is traveling through, and this creates a certain amount of pressure on the object that's equal and opposite to the airplane's direction of travel, and this is called dynamic air pressure. In order to understand how the pitot-static system works, you have to understand that this pressure is directly related to the speed at which the aircraft is flying.

Next, we have the total air pressure, and this is a combination of dynamic and static air pressure. While this may seem obvious at first, you'll see why we talk about this here in just a minute. Now, the altimeter uses a static port to sense the static pressure outside the aircraft, and because the static pressure changes one inch of mercury every thousand feet, the altimeter is able to give us an altitude reading because it senses that pressure. As I mentioned before, static ports are placed on the side of aircraft, and this keeps the instruments from being affected by dynamic air pressure, and this keeps the readings as accurate as possible.

The other instrument that uses a static port is the vertical speed indicator. This instrument allows air to escape or come in through a small hole, which is vented through the static port. By allowing air to come in or go out, this instrument senses the pressure differences as we climb or descend, then it gives us a reading in feet per minute. The airspeed indicator is a little bit different animal. It also receives air from the static port but not for reasons you might think. The airspeed indicator measures the ram air that comes from relative wind through the pitot tube, but this doesn't give us an accurate measurement because static air actually makes its way into the pitot tube as well, and this is that total air pressure we were talking about earlier. And by taking static air from the static port, the airspeed indicator is able to offset any static air that might come in through the pitot tube to keep the airspeed indicator as accurate as possible.

The people who designed your airplane usually put the pitot tube in a spot where it won't be affected by the prop wash, but there is something that can affect your airspeed indicator, and that's the angle of the pitot tube in relation to the relative wind. High angles of attack and different airplane configurations can affect the accuracy of your airspeed indicator, and that's because weird angles can make it difficult for the ram air to enter the pitot tube. Every airplane is different, but every airplane has some kind of error caused by this, and the manufacturers account for this by putting a chart in the POH. This is what we call calibrated airspeed. Once you make these corrections, something else you could run into that would cause these instruments to get errors is if the pitot tube or the static port got clogged up, and there's a lot of different things that could clog up these things—anything from bugs, mud, ice, and that's just to name a few.

One thing that shouldn't clog up your pitot tube, though, is water. That's because of this little drain hole here on the back, I call this the weeping hole. This is there so if water enters the front of your pitot tube, it'll just slide right out the back here. But if the water freezes in or on the pitot tube, then it can start clogging it up again, but that's what these little heating elements are for. You just turn the pitot heat on in your cockpit, and it should heat these up enough to melt that ice. So what happens if these things get clogged? Well, if the pitot tube gets clogged, the only instrument that's going to be affected is your airspeed indicator, and if you remember, that's because this is the only instrument that uses dynamic air pressure to give us a reading.

For the airspeed indicator, three possible problems could happen. The first would be if the hole in the front of the pitot tube for the ram air got clogged. As long as your drain hole doesn't get clogged, what will happen is your airspeed will drop to zero, and that's because the pitot tube is no longer taking in ram air but the static port and the drain hole are still both taking in static air which allow them to offset each other. The second possibility is if the front of the pitot tube and the drain hole get clogged. If this happens, any air pressure in the pitot system will be trapped, and your airspeed indicator is frozen at the last indication it was giving you. From here, if the static port is also plugged, you won't see any change, but if the static port is clear, then the airspeed indicator will make minor changes with altitude. I like to tell students that your airspeed indicator just turned into an altimeter at this point.

The third thing that could possibly happen is only the static port gets clogged. If this happens, the pitot tube is still taking in the dynamic and the static pressure from the front of the pitot tube, but the static port is unable to offset that static air that's also coming in the front. This means that your airspeed indicator is going to give you faster airspeeds than what you're actually going. Now, because the altimeter and the vertical speed indicator both use a static port to get their information, if this gets clogged, it's going to affect both of those too. For the altimeter, if the static port gets clogged, it's going to freeze at the last altitude that it was indicating. But the vertical speed indicator will only display 0 because it can no longer sense a climb or a descent. If you're lucky enough to have a clogged static port, I do have some good news for you, though.

On most newer training airplanes, they have something called an alternate static source. Typically, this is an alternate static port that's on the inside of your cockpit, and you can select this alternate static port by flipping a switch labeled "alternate static" on most airplanes. Just keep in mind when you use this alternate static source, it's not quite as accurate as the one on the outside of the airplane, and that's because the pressure difference on the inside of the cab is slightly different than outside. Now, if you don't have an alternate static source, one thing you can do is you can break the glass on any of the static instruments. If you do this, I recommend breaking the glass on the vertical speed indicator, and that's because it's by far the least important of the instruments.

Once again, if you do this, this is nowhere nearly as accurate as a static port on the outside of the airplane, but something is better than nothing, and in an emergency, this could save your life. I hope you enjoyed today's video. If you did, would you click that like button for me? Also, if you're really enjoying this training from the Free Pilot Training channel, be sure to check out our t-shirt store in the description below, then watch this video right here. You know you want to.