| How to recognize a blocked static port. |
| If you encounter a static source block while you are flying you can recognize it by referencing your other instruments. If you are descending with power at idle, you should not be increasing airspeed if you decrease your pitch attitude. Look at your other instruments. Ask yourself if they all agree in what information they are presenting to you. If they don't, see which one is in error. |
| Introducing alternate air into the static system! | |
| How to correct a blocked static source. |
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Some aircraft are equipped with an alternate static source. This is installed in order to compensate for a blocked static source. The source air is taken from inside the cockpit. This will induce a small amount of error into the
static instruments. The correction can be found in the aircraft's owner's manual. If the aircraft is not equipped with an alternate static source, the pilot may choose to induce static pressure into the block static system by breaking the glass face of the Vertical Speed Indicator (VSI). The VSI is choosen as the instrument to break because of the fact that when the glass is broken, it may lodge against the indicating needle and render the instrument inoperative. This instrument has been determined to be the least needed instrument and would have the least affect on the remainder of the flight. |
| Strike the center of the glass face of the VSI to introduce an alternate static air source to the blocked static system! |
| Pitot Tube Blockage |
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If the pitot tube becomes blocked, any total pressure contained in the expandable capsule will be trapped. If the aircraft maintains its present altitude and does not change velocity, no errors will occur in the airspeed indicator.
If the aircraft descends, the static pressure will begin to increase. Only the pressure surrounding the expandable capsule will change which will cause an imballance between the static pressure inside the capsule and the static pressure outside.
This imballance will cause the expandable capsule to be compressed. This compressing of the capsule will draw the rod backward. Through this, the airspeed indicator will show a decrease in airspeed. The descent is not nearly as critical as a pitot blockage during a climbing. In this instance, the static pressure would decrease leading to the expandable capsule being able to expand further. This would drive the airspeed indicator to show a higher airspeed. If you do not recognize the blockage, your initial reaction may be to reduce power or increase pitch attitude. Both of these reactions could lead to decreasing actual airspeed, eventually resulting in a stall. |
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Be careful with a pitot blockage: you may stall in a climb or overspeed in a descent. |
| Types of Airspeed |
There are several different types of airspeeds used when flying an airplane. Each of these are listed and explained below.
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| Indicated Airspeed |
| Indicated airspeed is the simplest to understand. It is simply the airspeed that is read directly off the airspeed indicator. This airspeed does not take into account any corrections for altitude, temperature or compressibility. |
| Calibrated Airspeed |
| Calibrated airspeed is indicated airspeed corrected for errors associated with the instrument and position. |
| Equivalent Airspeed |
| Equivalent airspeed is calibrated airspeed corrected for compressibility error. Compressibility is when the aircraft reaches faster speeds where air starts to pile up in front of the pitot tube. It no longer is able to register correctly. This is only relevant for aircraft moving at speeds nearing 200 knots and greater. |
| True Airspeed |
| True airspeed is equivalent airspeed corrected for density altitude. As the aircraft ascends in the atmosphere, the true airspeed increases at a rate of approximately 2 knots per thousand feet. True airspeed is important in performing navigation computations. |
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