The pitot tube generally hangs from the wing with the opening forward and positioned forward of the area where air becomes displaced by the wing. This device senses pitot (or ram) pressure and is used by the airspeed indicator. The pitot tube has a drain at the back to release any moisture from the air that may otherwise accumulate and affect the readings. Where there's moisture, there's the potential for ice, and so the pitot tube is equipped with a heater that can be activated from the cockpit. How do you know when there's ice (or another blockage), though? That depends...
If the pitot intake is blocked, but the drain is still open, then the air already in the system will have a path to escape, resulting in a gradual reduction in pressure. Lower pressure is usually a sign of less air being forced into the pitot tube, so the airspeed indicator will show a slower speed.
If the pitot intake and drain are both blocked, or if there's a block between the pitot tube and the instruments, then the air in the system is trapped and the pressure will remain constant. With no changes in altitude or atmospheric pressure, the airspeed indicator would remain unchanged, and so the pilot may not suspect a problem.
A problem will become evident, however, as altitude changes. Generally, airspeed decreases during a climb and increases during descent. The airspeed indicator uses not only the ram pressure but also the static (ambient) pressure to determine airspeed; basically, the static pressure fills an area of the instrument, and the ram pressure tries to inflate something within the area -- less inflation means slower speed. (Incidentally, an airplane with higher groundspeed at a higher altitude can show the same indicated airspeed as an airplane with lower groundspeed at lower altitude, thanks to air density.) So imagine that the airplane is indicating 140 knots when the static pressure increases, say upon descent. Increased static pressure with constant ram pressure will cause the device to deflate a bit, which will result in lower indicated airspeed, which is typical of a climb. This observation should raise a red flag.
The static pressure is sampled by a static port, mounted usually along the fuselage and in such a way that it just senses the ambient air pressure. Air pressure is measured in inches of mercury (" Hg), standard of 29.92" Hg at 0 MSL, and broadcast as part of the METAR for local airfields. As part of preflight, you hop in the plane, tune in the local AWOS or ATIS, and dial the current barometric reading into the altimeter. In fact, they announce it as "altimeter 3-0-0-1." If all goes well, adjusting the barometer will result in an altimeter reading that matches the airport's altitude above sea level.
When the static port gets clogged, that affects the airspeed indicator, the altimeter and the vertical speed indicator (VSI). The altimeter will just get stuck since the pressure is trapped inside. The VSI will get stuck at 0 since it registers changes, and no changes will occur. The airspeed indicator, though, will give incorrect measurements. Typically the static pressure and ram pressure are "in sync," so to speak -- high pressure means high air density in the chamber and lots of air molecules funneling through the pitot tube into the inflatable portion of the airspeed indicator; low pressure means low air density in the chamber and fewer air molecules to be rammed. When the static port is blocked, the static pressure is held constant regardless of outside air pressure, so it's no longer coordinated with the ram pressure. Say it gets blocked at 3000' (that is, at a specific air pressure). On a normal climb from here, the ram pressure would be decreasing and so would the static pressure, but now the ram pressure will decrease but the static pressure will not, resulting in slower than usual indicated airspeed readings. On a normal descent from 3000', the ram pressure would increase and so would the static pressure, but in this case only the ram pressure will increase and the airspeed indicator will read faster than expected.
Those are considerations when there's a malfunction of some sort. Pilots also need to consider what happens just when moving from region to region when the air pressure changes. The saying goes "High to low, look out below." Let's dissect that. Your altimeter is set for a barometer of 30.01" Hg as Charlotte Approach just gave you. In the vicinity of Charlotte, you'd expect your altimeter to be fairly accurate. Flying at 2600', you head west toward the mountains, where the pressure is dropping. The standard conversion is 1" of mercury for every 1000' of altitude. If you don't adjust your altimeter and try to maintain your 2600' of indicated altitude as you fly into an area with a pressure of just 29.01" Hg, your true altitude is now just 1600' -- look out below! As the terrain comes up, you really don't want to be going down. Lower pressure means less dense, which under standard conditions means higher altitude; under non-standard conditions it means update your altimeter!
That's it for today. I started off today with Rod Machado's Private Pilot Handbook. It is so dreadfully corny, and way too distracting for me to use as a serious refresher aid. Sigh. I like my old Guided Flight Discovery textbook, but in places it's just so flat. I also did an experiment today and decided to study from home instead of at the library. That just doesn't work! My focus was definitely less honed. The dishwasher needs unloading, the laundry needs folding, there's a bug on the window, a lizard just crawled up the deck railing, the cat wants to be petted, there's that new "green monster" smoothie recipe I was thinking of trying, the Keurig needs more water, .... Back to the library on Wednesday. With one of these tasty new smoothies.
