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The Pitot Static System

These are my study notes on the Pitot-Static system. If they are helpful to you in studying to become a pilot please feel free to make use of them. If any part is confusing make sure you bring it up with your instructor, and feel free to drop me a line at rob@robchipman.net.

The pitot static system runs three of the six-pack instruments – the altimeter, the vertical speed indicator & the air speed indicator. The system makes use of a pitot tube and a static port.

The airspeed indicator uses the difference between ram air and static air to mechanically calculate airpseed. The pitot accepts dynamic ram air. The static port measures static air pressure. The two are connected though the gauge itself, with a sealed aeneroid capsule separating the two pressure zones. The capsule, or bellows, is an aeneroid capsule sealed at sea level pressure. As the higher ram air pressure compresses the capsule a simple, but very precise mechanical linkage moves the airspeed needle to indicate the correct speed.

Obviously, blocking either port makes measuring the pressure difference impossible. On the pitot side of the system the pitot tube has a heater to combat freeze up. On walk around you should check that the pitot heat is functioning by turning the pitot heat on and touching the pitot tube. You also have to check that nothing like dirt or a bug has gotten into the tube opening. A pitot tube cover, made of red vinyl so that you don’t forget to remove it, helps with non-ice blockages.

The static port can get blocked by bugs or dirt, or if ice forms over the top of it. There is usually an alternate static port to be used in the case of emergency. Some planes have two static ports.

The positioning of the pitot is set for maximum performance. It has to work through an attitude range from normal flight to a stall attitude and still provide ram air pressure for the system to function. It’s placement is, therefore, strategic.

The static port is positioned out of the airflow and away from form generated turbulence so that it can read static air pressure as accurately as possible.

The static port allows the air sped indicator, the vertical speed indicator and the altimeter to measure and use static air pressure as the plane flies. A blocked static port will cause a frozen altimeter, an even vertical speed indicator and a low reading air speed indicator.

As mentioned, the static port has an alternate port for emergencies in order to remedy these issues. A second static port also helps avoid errors due to slipping (when you’re in a slip that exposes the static port to ram air pressure the airspeed will read low in error). Errors also occur if the density of air varies from what the capsule was calibrated for, as a result of icing or water, or from a contaminated or blocked pitot. Do not try to suck a blockage out of the static port – they are very sensitive.

The density of air malfunction can be fixed by calibration.

A blocked pitot turns the air speed indicator into an altimeter, meaning going up increases indicated speed and diving means slowdown in indicated speed, even in a high speed dive. This is because the ASI aeneroid capsule will compress as the plane ascends and expand as it descends, since there is no ram pressure from the pitot to counter it. Of course, indicated airspeed with a blocked pitot will be random.

The ASI is the only one of the three pitot static three pack that uses both static and ram air pressure.

The altimeter works on standard pressure of 29.92 @ 15 degrees centigrade, which is what 14.7 pounds of atmosphere works out to be in be in inches of mercury at sea level. It also has a sealed aneroid capsule that expands and contracts with changes in altitude. As you ascend the capsule expands, moving a needle mechanically, registering an increase in altitude.

If pressure was always the same (29.92) then that simple altimeter would always be accurate. Atmospheric pressure varies, however, meaning the starting line gets moved with the weather. In other words, if you’re in a high pressure zone you need to move the baseline inside the altimeter up, and if you’re in a low pressure zone you need to dial the benchmark inside the altimeter down. How do you do this?

The Kollsman window (invented by Paul Kollsman in 1928) is the answer. It allows you to adjust the altimeter for variations in barometric pressure and pressure altitude. The window is where the
little knob goes through the altimeter and allows you to adjust the elevation/pressure. Once you start using it you’ll see that it makes sense. Setting the pressure at the reported airport pressure will move the indicated altitude needle pretty much exactly where the airport altitude is; conversely, if you set the needle where the airport altitude should be you’ll be pretty close to the pressure that the ATIS gives you. (Don’t adjust the altimeter this way – use the ATIS).

Errors with altimeters have to do with pressure differentials. Since you set the altimeter to a specific barometric pressure that is identical to your starting ambient pressure, and since you can fly into a different ambient pressure by flying to a different pressure zone ( high pressure/low pressure/high/low temps), the altimeter will not read accurately without intelligent re-adjustment.

To remedy this you re-set the altimeter with updated altimeter settings that come from FSS or ATIS. If you can’t acquire these you can reason that hot atmosphere means less dense air, and therefore the aircraft is higher than indicated.

At standard atmospheric temperature (15 degrees centigrade) the plane is at true altitude. Colder temperatures mean denser air, meaning that the plane will lower than indicated UNLESS THE ALTIMETER IS ADJUSTED. Being too low, especially in bad visibility, and especially in mountains, is a hazard, and mountains in bad weather are often colder rather than warmer.

Remember, hot air is less dense and allows the static aeneroid capsule to expand more. Cold
air is denser and so the capsule expands less.

Again, the static port can ice over or get blocked otherwise. If this happens the altimeter won’t work. There will be no change in the static pressure so capsule neither expands nor contracts. It stops where the blockage occurs. A blocked static port will cause a frozen altimeter.

Low pressure zones rotate counter clockwise. If you notice persistent rightward drift you are likely flying into a low pressure zone. Rightward drift therefore indicates that you’re flying into lower pressure, which can mean lower temperatures, meaning denser air, which can mean you are flying lower than indicated altitude.This gives rise to the saying “high to low, look out below”. If you are able to keep an eye on outside temperature you can also get hints from that. (Cabin temperature can be very comfortable even when its below freezing outside, but don’t let this lead you to assume its always warm outside at 5,000 feet AGL).

Abnormally high pressures (over 31.00) don’t register on the altimeter. That reading requires a very dry high pressure. You will know when it happens and must subtract 100 feet for every .1 inch of mercury over 31.00.

Mountain effect, in altimeter terms, means that winds deflect off terrain effecting micro pressure environments. Think of the Bernoulli effect and how a Venturi works. Local pressure can ary enough to effect the instruments. Although air waves can extend 100 miles from the mountain the big key is to realize that when you’re flying in mountainous areas you have to keep a sharp eye on your instruments to detect possible errors.

There are four types of altitude: true, absolute, pressure and density.

True altitude is actual height over mean sea level.

Absolute altitude is actual height above ground.

Pressure altitude represents physical distance above sea level and is measured in inches of mercury, millibars or hectopascals.

Density altitude is changed by pressure and temperature. Density altitude represents variation from molecule density.

THe altimeter measures actual altitude, but to really understand what it’s telling you you have to understand the other concepts.

The Vertical Speed Indicator measures the rate of climb or descent. It’s a trend and rate instrument. That means that it will indicate whether you are trending up (ascending) or down (descending). When the needle is above the baseline you’re trending up, and when its below the baseline you’re trending down, or descending. It’s not exact, but its still very useful.

The rate is the speed of the climb/descent (the speed of the trend), measured in feet per minute.

Again, this instrument works with the static port and an aenoroid capsule. The aneroid capsule has a calibrated hole that allows the bellows to expand and contract as it catches up to ambient static pressure. This makes the VSIread 0 at level flight. There are predictable errors and malfunctions.

There is a lag error that can sometimes be 6 to 8 seconds. THis occurs because the capsule has to equalize pressure, and this takes some time. The reading on the dial comes from the capsule equalizing (when its equalized it will read 0).

Reversal error occurs when the needle goes down first after indicating an ascent. Additionally, a sudden pitch change can make a vertical speed indicator change in the opposite direction.

My name is Rob Chipman and I’m a realtor, pilot and all around goof off based in Vancouver, BC. I really enjoy flying, real estate and the Chilcotin.¬† My company is¬†Coronet Realty Ltd., located at 3582 East Hastings Street, Vancouver, BC, V5K 2A7. I have a C-150L that I own with two other pilots, based out of Pitt Meadows. Do not hesitate to contact me by email if I can help you do anything, especially if its likely to be interesting.

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