Friday, September 02, 2005

Blockage of the Static Port

Understanding the System

To know what will happen, it is critical to understand the system, and how it is set up. The instruments involved are the airspeed indicator, altimeter and vertical speed indicator. The pitot tube feeds directly to the airspeed indicator, and the static port goes to all three.

Supposing that the static port becomes blocked, this will affect all three instruments.


We can start with the altimeter: It senses altitude based on pressure changes coming from the static port. if the static port is blocked, then the pressure in the line isn't changing. It will continue to show whatever altitude you had at the time of blockage, since that correlates to the internal pressure of the air currently trapped in the line.

Vertical Speed Indicator

The vertical speed indicator works by means of a calibrated leak. That allows it to compare the ambient pressure reading now with what it read a moment ago. Since that pressure reading from the static port is now a constant, the VSI will sense no change, and indicate a constant zero feet per minute.

Airspeed Indicator

Those two are very straight-forward. The reading on the airspeed indicator takes a little more thinking though, since both the static and pitot connect to it. The static port is reading calm-air pressure, while the pitot tube senses ram air. The air entering the pitot tube is forced in as a result of facing into the wind, so higher speed through the air translates to higher pressure in the tube.

A quick note on that: you can get a sense of the difference by sticking your hand out the window of a moving car (taking care not to hit a mailbox along the way, please). As you speed up, the air in front of your hand exerts more force. At any forward speed, the air in front of your hand exerts more force than the air inside the car.

The airspeed indicator gets its reading from a comparison of the two pressures- the pitot and the static. If those two pressures are the same, then you have zero speed indicated. In the case of a blocked static port, that pressure will remain constant, while the pitot pressure will fluctuate with true airspeed. If you remain at a constant altitude after the blockage, the airspeed indicator will read correctly. The only effect occurs after climbing or descending.

Airspeed Indicator After a Climb

Since calm-air pressure decreases as you climb, the airspeed indicator would normally recognize that, but now it doesn't. That means that the actual static pressure is higher than it should be. So the pitot and static pressures are closer together, giving an airspeed indication lower than it should. How much lower depends on how much you have climbed.

Airspeed Indicator After a Descent

The error after descending has the same root cause. Since the air trapped in the static line is at a constant pressure, that pressure will not increase as it should during the descent. So it is lower than it should be, making the comparative pitot and static readings farther apart than they ought to be. That will translate into an indication higher than normal.

So in a sense, the airspeed indicator can become an altimeter of sorts, during a static blockage.