Never has a truer contradiction been stated. First of all, we all know that “air” is in fact a gaseous mixture in the scientific context. And “primordial,” wasn’t that a movie from the 1970s starring Raquel Welch? In fact the true definition of primordial is “primary or fundamental component”. It just so happens that air is the one fundamental component that makes our business a reality.
So just what is air? According to the American Heritage Dictionary, air is a colorless, odorless, tasteless, gaseous mixture consisting of mainly nitrogen (78 percent) with a bit of oxygen (21 percent) and a sprinkling of argon, carbon dioxide, hydrogen, neon, helium, and a few others.
The troposphere is the portion of the atmosphere that extends outward from the earth’s surface around seven to 10 miles.
This is the region in which most aircraft operate. It is also in this area where varying amounts of particulate matter and moisture are prevalent. Air density is also greatest closest to the ground. This is due to the earth’s gravity and compressibility of the air and can be observed by noting the rapid pressure decrease with altitude increase. Atmospheric pressure will reach about one half its sea level value at about 18,000 feet. The air in the troposphere is in constant motion with both horizontal and vertical wind currents and in this region temperature changes with altitude at an average rate of 3.6 F per 1,000 feet. This means, at the outer reaches of the troposphere, -70 F can be expected.
So why the review of elementary school science? Well, our industry is all about air pressure. If the pressure on the top of a wing increases and begins to equalize with lower wing pressure then the aircraft assumes the glide path of an aerodynamic crowbar.
The challenge here is to measure variations in air, provide correction factors where needed, and then deliver the information in a way that can be interpreted by the flight crew. In fact interpretation is a big part of the issue; many of the terms may have different implications based on context. For example, airspeed is only a partial term, indicated airspeed (IAS) is considered to be uncorrected for instrument error and the effects of air density; calibrated airspeed (CAS) which compensates for instrument error; and true airspeed (TAS) which is CAS plus compensation for air density and temperature variation. They do all relate to the measured forward movement of the aircraft through the air. Temperature is another area that includes words like true air temperature (TAT) and static air temperature (SAT). Do they mean the same thing? Probably not.
So what information is it exactly that needs to be gathered and how do we accumulate it?
The History of the Pitot
In the mid-1700s a French physicist named Henri Pitot concluded that by placing a hollow tube directly in the path of flowing water, a certain pressure would be present. By then factoring the reference pressure of the water with the sensing pressure on either side of his tube, a fairly exact calculation could be made of the water’s velocity as it flowed. As a result of his efforts Pitot has earned himself a place in aviation.
The pitot tube as we know it today varies little from the original application. Of course refinements and adaptations had to be made to accommodate aircraft installation. Location and alignment on the airframe is most important. It is imperative that the tube have an unobstructed sampling of relative air. As these devices are frequently in prominent positions, they often become the recipient of hangar rash. Remediation of mechanical damage to a probe may involve sophisticated measurement and alignment procedures using in some situations laser levels or specially manufactured alignment fixtures. Routine inspection of pitot tubes should include looking for deformities in the probe or supporting mast as well as obstructions and anomalies at the sensing port. Many of these devices are designed with a great deal of sensitivity and in some cases a minor amount of corrosion or even erosion in the wrong place could have an undesirable result on the pressure being measured. Manufacturers’ references regarding inspection criteria should always be consulted during probe evaluation.
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