THE PITOT-STATIC SYSTEM

Applicants for practical tests should be prepared to demonstrate knowledge about any system installed in the glider used for the test. Fortunately for most applicants that may be only the pitot/static system, but even so, enough fail in this area to make it one of the top ten failure subjects.
The pitot/static system consists of sensors to measure two different air pressures and the instruments to interpret those pressures in terms of airspeed, altitude and vertical speed. Pitot pressure is measured through a forward facing port, where its value increases as airspeed increases. Static pressure is measured through a port located so that its value is independent of speed, generally perpendicular to the direction of flight.



Pressure, density and temperature of the atmosphere all generally decrease as altitude increases. Pressure decreases at a fairly predictable one inch of mercury per thousand feet altitude. Thus, if we measure this pressure, we can determine our altitude, and that is exactly what an altimeter does. To do so, it utilizes one or more small cookie shaped, thin metal containers, illustrated schematically here as a bellows, that can expand and contract as the pressure around them varies. For an altimeter the containers are sealed and static pressure is introduced to the area outside the containers. As the glider climbs, the static pressure decreases (1" hg per 1,000'), the container expands and moves a pointer connected through suitable linkage. The linkage in a real altimeter is more complex than the rack and pinion shown here, but the concept is the same.



For the altimeter to indicate correctly, it must be adjusted for the atmospheric pressure existing at the time. That usually only requires that the pilot set the altimeter to the correct field elevation just prior to takeoff. If the flight is long or covers a long distance it may be necessary to reset the altimeter during flight. That can be done by obtaining the current sea level pressure for the area, and setting that value in the small barometric scale window on the altimeter.
The pressure measured by the altimeter results from the weight of the air above it. It is helpful to visualize the ocean of air in which we fly as similar to an ocean of water; the deeper the water (air) the greater the pressure. There is more air above a given altitude when the pressure is high than when it is low. If an aircraft maintains the same altimeter reading while flying from an area of high pressure to an area of low pressure, the actual altitude of the aircraft will decrease. A simple rule to help remember this is "From High to Low, Look Out Below".
A similar condition exists when flying from a warm area to a colder area. The colder air occupies less space, so for equal surface pressures the "top" of the air must be lower. As in the case of an aircraft flying from high pressure to low pressure, one flying from high temperature to low temperature while maintaining the same altimeter reading will also descend. The same "High to Low" rule applies.



Pitot pressure is the sum of static pressure and dynamic pressure. The difference between pitot pressure and static pressure would then be dynamic pressure, and dynamic pressure is related to airspeed. The airspeed indicator is fundamentally similar to the altimeter, and we can use the same schematic illustration to show how it works. The essential difference is that for the airspeed indicator we introduce pitot pressure to the inside of the pressure sensing device (bellows in our illustration). As airspeed increases so does dynamic pressure, and the bellows expands, moving the indicator through suitable linkage.
Dynamic pressure varies with air density as well as airspeed. Because density decreases with altitude, dynamic pressure for the same true airspeed also decreases with altitude. Therefore, true airspeed is higher than indicated airspeed when flying above standard sea level conditions. A good approximation of true airspeed is to increase indicated airspeed by two percent for each 1,000 feet of altitude. It is also important to remember that all performance speeds, such as stall speed, occur at the same indicated airspeed regardless of altitude.



If we have a container with a small opening in it, the pressure inside the container will soon equal that outside the container by allowing air to pass through the small hole. If we then raise the container, outside pressure (static pressure) will decrease and air will flow out of the container at a rate that varies with how fast we are raising the container. Similarly, air will flow into the container when it descends.
Most variometers indicate vertical speed by measuring the flow of air into and out of the container, called a reference chamber. It is also possible to measure vertical speed by measuring the pressure difference between the static source and the reference chamber. Most airplane VSIs do this, and their reference chamber is the instrument housing itself.

Gliders often have a separate reference chamber which is essentially a thermos bottle connected to the variometer. Our illustration shows an early type of variometer in which air flowing out of the reference chamber lifts a green pellet, and air flowing into the reference chamber lifts a red pellet. Early glider pilots referred to rising air as "green" air. Modern variometers use other mechanical or electronic methods of measuring air flow but the concept is the same as the old pellet vario.




The most common reason for failing a practical test because of lack of knowledge about the pitot-static system is inadequate understanding of total energy compensation for a variometer. A glider in flight possesses two kinds of energy. Kinetic energy is due to its speed and potential energy is due to its altitude. The pilot can trade one for the other within limits. Diving will increase speed at the expense of altitude. Pulling up will gain altitude at the expense of speed. The only way to increase total energy is to find a lift source.

The significance of all this is evident when a pilot flying at relatively high inter-thermal speed pulls up to work a thermal at thermalling speed. An uncompensated variometer will show the climb due to the pull up as well as the climb due to the thermal. If the pull up pegs the vario, it will be impossible to assess the thermal strength without wasting a lot of time. If the vario showed only the climb due to the thermal, our problem would be solved, and total energy compensation does just that.

The most popular way to implement total energy compensation is to use a special static port for the variometer. The pressure in a venturi tube decreases as the speed of the air through it increases. If we use a venturi tube for the vario static port it will sense speed as well as static pressure. Such a static source would not be suitable for the altimeter because it would always present a lower static pressure than ambient, but variometers don't care about absolute pressure, only changes in pressure.
With total energy compensation, when we pitch up in still air to trade speed for altitude, the decreasing speed of air through the venturi tube increases air pressure just enough to offset the decrease in ambient pressure. The variometer sees no change in static pressure and indicates no climb. If we are executing the pull up in rising air the venturi pressure offsets only that part of the climb that is due to the speed change. The remaining ambient pressure change is due to rising air. In our illustration we assume still air so the variometer always indicates the glider still air sink rate for the corresponding air speed.
Total energy compensation works the same way in reverse (i.e. when we are diving) but we are usually more interested in using it to remove "stick thermals".

In addition to removing the effect of speed changes from the variometer indication, it is also possible to remove the glider still air sink rate for the current airspeed. This type of total energy compensation is called a "netto" system and shows only the air mass vertical movement. That is probably overkill for most of us, but anyone leaving the traffic pattern can benefit from basic total energy compensation.



OXYGEN SYSTEMS

Although the glider used for the practical test may not carry an oxygen system, it is capable of reaching altitudes where oxygen is required, so the examiner probably expects the applicant to know something about oxygen systems and their application. If the pilot has had the opportunity to experience hypoxia in an altitude chamber the need for this knowledge will be obvious. Like the drunk who thinks he can whip anybody in the bar, the pilot with hypoxia thinks he/she can fly better than anyone else in the sky. If you wait until you think you need oxygen, you won't care, so know the rules and apply them as if your life depended on it - it does.

When gases are mixed, the total pressure they exert is the sum of their partial pressures. Air is about one fifth oxygen at all altitudes so the partial pressure of oxygen is about one fifth of the ambient pressure at any altitude. The ability of the lungs to transfer oxygen to the blood stream depends on the partial pressure of oxygen. We can maintain an acceptable value at higher altitudes by increasing the total pressure or by increasing the percent of oxygen in the air. Either way increases the partial pressure of oxygen. Since we usually don't have pressurized cabins in gliders, we must use supplemental oxygen and increase the percent of oxygen in the air we breathe.
Regulations require pilots to use oxygen after thirty minutes above 12,500 feet and all the time above 14,000 feet. They must provide it for passengers above 15,000 feet. Depending on their physical condition and their aversion to risk, they may choose to use supplemental oxygen at lower altitudes. Many recommend 10,000 feet in the day time and even lower at night. Because the onset of hypoxia is difficult to recognize, pilots should never risk exceeding the altitudes specified in the regulations.

It is equally important to be sure the oxygen system you are using is appropriate for the altitude at which you plan to fly, and that it is working properly. Continuous flow systems, as the name implies, supply oxygen continuously and thus waste some. They are the least expensive system, however, and can be used up to 18,000 feet with a cannula and 25,000 with a mask.

Diluter demand systems sense altitude and supply the appropriate amount of oxygen. They also provide oxygen only when the pilot inhales, so they minimize waste. They are useful up to about 35,000 feet, where they are supplying 100% oxygen to the pilot.

Since more than 100% oxygen is impossible, the only way to increase its partial pressure at higher altitudes is to increase its total pressure. Pressure demand systems supply oxygen to the mask at higher than ambient pressures, and are useful up to about 45,000 feet. If you are determined to go higher, you would be wise to invest in a pressurized cabin aircraft.




2000 Jim D. Burch

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