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"Airspeed is life." It's a saying you've probably heard at some point, and it's true. There are a lot of things you can fly without, but airspeed isn't one of them.
And your primary way of determining airspeed? Your airspeed indicator. Obviously.
Your airspeed indicator is a pretty important instrument, and it's a good idea to understand how it works. It's even more important to understand what happens when it fails, so you're prepared if it does. So what happens behind that round dial? Let's take a look.
Your airspeed indicator is actually a pretty simple instrument. And whether you're flying a steam gauge or glass panel aircraft, they use the same principles.
Your airspeed indicator measures dynamic pressure. It's the same pressure caused by your airplane's movement through the air. However, in order for your airspeed indicator to measure dynamic pressure correctly, it needs to measure static air as well. That's because the higher you go, the lower atmospheric pressure is.
So how does the measuring work? It starts with your pitot tube, which measures combination of static and dynamic pressure, otherwise known as "ram air". If you're sitting on the ground, your ram pressure only includes the static component. But once your start moving forward, static and dynamic pressure are measured.
Next up is your static port (or ports). Your static ports connect to your airspeed indicator as well, and they constantly measure the static pressure of the air.
Your airspeed indicator, put simply, is a scale that measures the difference between the static pressure from your static ports, and the ram pressure (dynamic + static) from your pitot tube The static pressures cancel each other out, and you're left with dynamic pressure.
It does this by filling up the case of the instrument with static air, and filling something called an "pressure diaphragm" (the orange thing in the diagram above) with ram air. There are also some gears involved to make your airspeed needle move, but to keep things simple, as the diaphragm fills up with more ram pressure, it expands, and your airspeed goes up.
That's a good question. They use the same principle, comparing and measuring ram air and static air. But beyond that, it's not that important how it happens, unless you're the engineer at Garmin.
Failures never happen. Right?
Ok, maybe they do, and when your airspeed fails, its usually caused by either 1) your static ports getting clogged, or 2) your pitot tube getting clogged.
So if either one gets clogged, how can you tell? We have the answer, and it starts with this scenario.
Your Pitot Tube Stays Open
The most likely scenario here is that your static ports ice over. And when they do, they trap whatever static pressure was in your airspeed indicator at that exact time.
That works out fine as long as your barometric pressure doesn't change, and you stay at the same altitude. But, if that's not the case, things are going start going wrong. Let's look at what happens if you start climbing.
If you climb at a constant airspeed, your the static component of your ram pressure goes down. But, the static pressure in your instrument's casing remains the same. Because you don't have enough ram pressure, your airspeed will decrease, and you'll start flying faster than what's indicated.
What happens if you descend? The exact opposite. You'll fly slower that what's indicated, because you have too much ram air for the static pressure trapped in your airspeed indicator.
Your Static Ports Stay Open
So what happens if your pitot tube ices over, but your static ports stay open? There are a couple different cases here, but let's stay the whole thing iced over, including the drains.
If this happens, your ram pressure gets trapped. And just like the first scenario, if nothing changes, you're fine.
But what happens if you start climbing? Your static pressure decreases, and the trapped static pressure component of your ram air is too great, which means you're indicating a faster speed than you're actually flying. And if you descent, the exact opposite happens.
If this happens, all of your ram air will leak out the drain, and you're left with nothing but static pressure. And if that happens, your airspeed goes to 0.
If your entire pitot tube and static ports ice over, chances are you have bigger problems than just airspeed. But just so we've covered it, your airspeed indicator will freeze in place, because there will be no changes to static or ram pressure.
Keeping your ports clear is obviously important, and the best way to do that is with pitot heat when you're in icing conditions. But beyond that, it's critical that you make sure your ports aren't clogged with anything before you leave the ground.
If you do that, you'll have all the airspeed indications you need for your flight.
Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at email@example.com.