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Understanding Pitot-Static Failures

Boldmethod

The first step to being prepared in an abnormal or emergency scenario is to know your aircraft's systems. Knowing your systems allows you to think through erroneous indications and diagnose the root cause, getting you on the ground safely.

This article will focus on the pitot-static system. If you want to learn about your attitude indicator, click here.

Blockages

Whether you have an all-glass panel or traditional round-dial instrument panel, many of the indications that a port, tube, or drain may be clogged are the same.

Pitot Tube Blockages

The only instrument connected to the pitot system is the airspeed indicator. For this explanation think of the airspeed indicator in simple terms. There is a diaphragm that has ram air pressure (from the pitot tube) on one side, and static pressure (from the static port) on the other side.

If your pitot tube becomes blocked, there is no more ram pressure entering the tube. Any excess pressure leaks out of the drain hole, and you'll be comparing the drain hole's pressure and the static pressure. This means your airspeed indicator will be at zero, just like when you're sitting on the ramp.

Pitot And Drain Hole Blockage

Now, what happens if your drain hole is blocked in addition to the pitot tube? Think of this as trapping the air inside your pitot system. If you don't climb, descend, speed up, or slow down, your airspeed indicator will freeze on the last airspeed before the tube/drain became blocked.

Now imagine your tube/drain blocked and 3,000 feet, and you start a climb to 5,000 feet. The side of the diaphragm that is connected to the pitot system will remain pressurized to 3,000 feet, while the static source side drops pressure as you climb.

This will cause your airspeed indicator to show a faster-than-normal airspeed as you climb. It will also cause it to indicate a slower-than-normal airspeed as you descend. This can be a very disorienting sensation, especially in instrument conditions.

Static Blockage

Since the static port is connected to your airspeed indicator, altimeter, and VSI, they will all be affected by a static blockage.

A static blockage will cause your airspeed indicator to show inaccurate indications. If you climb at a constant airspeed, your ram pressure's static component decreases. Since your static ports are clogged, they have too much static pressure. Essentially, they're stuck at a lower altitude. The difference between ram and static pressure is smaller, and your indicated airspeed decreases. You're now flying faster than your indicated airspeed. The opposite is true if you descend.

Now let's look at your altimeter and VSI. Since your altimeter uses aneroid wafers to compare static pressure to standard pressure (29.92 inHg) if the static port is blocked, the static pressure at the time of blockage gets trapped in the altimeter. With no more changes in static pressure, your altimeter freezes at the altitude the blockage happened.

Your VSI uses a calibrated leak and diaphragm to compare changes in static pressure to determine your climb or descent rate. Without a changing static pressure, your VSI's calibrated leak will allow pressure to slowly equalize in the unit. Your VSI will move to 0 FPM and no longer change, regardless if you're climbing or descending.

What To Do

So you've recognized that something has gone wrong with your pitot-static system. If you're in visual flight conditions, use your outside references and make a plan to land. If you don't have airspeed indications, or you suspect they're inaccurate, rely on your standard pitch and power settings to descend at a safe speed. Use the sound of the air rushing over your cabin as another tool to determine that you're flying at a safe speed. If you have a static blockage and your altimeter is frozen, use your iPad EFB to display your GPS altitude.

If you're in the clouds the situation is a bit more complex. After you identify the issue and transition to a partial panel scan with the aircraft under control, you'll want to notify ATC that you've had an instrument failure (FAR 91.183 & AIM 5-3-3).

Make sure your pitot heat is on, and check your circuit breakers to make sure none of them have popped. You'll also want to make sure your alternate static source is selected.

Have you ever had a pitot or static failure? Tell us about it in the comments.

Nicolas Shelton

Nicolas is a private pilot from Southern California. He is currently studying at Purdue University, where he is working on advanced pilot ratings. You can reach him at nicolas@boldmethod.com.

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