Flight Instruments and Systems Section 5.153

CHAPTER 5: FLIGHT INSTRUMENTSAND SYSTEMS

The airspeed indicator, the altimeter, the variometer, the yaw string, and thecompass are your primary tools for tracking the glider’s position and movementthrough the air. Because of their crucial role, it is important that you understandhow these instruments work and how you should interpret the information theyprovide.

Several secondary instruments may also prove useful in particular situations.When flying over unfamiliar terrain, a Very high frequency OmnidirectionalRange (VOR) receiver or Global Positioning System (GPS) unit may come inhandy to provide navigation assistance. For motor-gliders that are permitted tofly in clouds, gyroscopic instruments such as an attitude indicator, a turncoordinator, or a turn and slip indicator are required.

Additionally, many modern gliders are equipped with other flight systems, suchas radios, emergency locator transmitters, and oxygen systems.

In this chapter, you will learn about the various flight instruments and systemsthat you may encounter in a glider.

5.1 The AtmosphereMost of the primary flight instruments use properties of the atmosphere toprovide information about the glider’s speed, altitude, and vertical movement. Inorder to understand how these instruments work, we need to first understandthe atmosphere.

Properties of the AtmosphereAir has weight. The higher you are in the atmosphere, the less air there is on topof you pressing down. Therefore, the pressure in the atmosphere decreases asyou increase your altitude.

Air is also compressible, meaning that it will change in density and volume asthe pressure changes. If you raise a parcel of air, it will expand because of thedecreasing pressure, and its density will decrease.

The temperature of any compressible gas will increase as the pressure isincreased, and decrease as the pressure is decreased. These processes are calledadiabatic heating and cooling. You have probably noticed this effect if you haveever used a spray can. The can gets colder as you use it. This is because thepressure in the can decreases as you let the contents out, and the decrease inpressure causes the decrease in temperature. The opposite effect can be observed

Section 5.1 Flight Instruments and Systems54

when using a bicycle pump. As you work the pump, you will notice it warmingup because the air inside is being compressed.

The same effects apply to the atmosphere. As the pressure decreases withaltitude, the temperature decreases.

The Standard AtmosphereThe actual atmosphere can be quite chaotic, with the temperature and pressurevarying considerably with location and altitude. To provide a common referencewith which to compare aircraft performance, the Standard Atmosphere wasdefined. At sea level, the standard temperature is 59° F (15° C), and the standardpressure is 29.92 in. Hg. (inches of mercury).

Since temperature and pressure change with altitude, standard lapse rates werealso defined. (A lapse rate is the change in a value with altitude.)

Figure 5.1 – Standard pressure

As Figure 5.1 shows, the standard pressure decreases nearly linearly from sealevel up to about 10,000 feet. The pressure lapse rate in this range is roughly -1.0in. Hg. per 1,000 feet increase in altitude.

Flight Instruments and Systems Section 5.155

Figure 5.2 – Standard temperature

The standard temperature lapse rate is linear from sea level up to 35,000 feet. Thetemperature lapse rate is -3.6° F per 1,000 feet increase in altitude. Above 35,000feet, the standard temperature is constant at –70° F.

Keep in mind that the real atmosphere is never standard. There are alwaysvariations in temperature and pressure due to weather.

Measuring PressurePressure is the force that air exerts over an area. Still air has an associatedambient pressure that results from the weight of the air above it. Moving air,when brought to a stop, has additional pressure that results from its kineticenergy. The ambient pressure is called the static pressure. The combination of theambient pressure and the “kinetic” pressure is called the total or pitot pressure.

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Figure 5.3 – Measuring static and pitot pressure.

Static pressure is determined by measuring the pressure of the air withoutdisturbing the airflow. Typically, a pair of small static ports, which are usuallyless than 1/8th inch in diameter, are located on either side of the fuselage. Usingtwo ports helps to average out any errors that might be caused by the glider’s notflying perfectly straight through the air.

Pitot pressure is normally measured using a pitot tube. A pitot tube points intothe flow, so that the airflow is brought to a stop at the mouth of the tube. Thepitot tube is usually located on the vertical stabilizer or in the nose of the glider.Pitot pressure is never less than static pressure.

5.2 Primary InstrumentsAltimeterThe altimeter measures the static pressure. Since the static pressure changes withaltitude, the altimeter can be calibrated to display the height of the glider above agiven level. Usually, this level is chosen to be mean sea level (MSL).

How it WorksA schematic of an altimeter is shown in Figure 5.4. The altimeter consists of ananeroid barometer, or bellows, which is sealed so that no air can flow into or outof it. This bellows is located inside a sealed case that is connected to the staticpressure ports on the glider. When altitude increases, the static pressuredecreases, and the bellows expands, driving the indicating pointers through aseries of gears.

Flight Instruments and Systems Section 5.257

Figure 5.4 – Altimeter schematic

Of course, the altimeter will not function correctly if the static ports are blockedor clogged.

Reading the AltimeterMost altimeters have three pointers. The large pointer indicates hundreds of feet,the middle pointer indicates thousands of feet, and the small pointer indicatestens of thousands of feet.

Figure 5.5 – Reading the altimeter

Adjusting the AltimeterSince weather systems will cause the pressure to change, the altimeter isequipped with an adjustment knob that can be used to set it for current atmos-pheric conditions. This knob can be seen in Figure 5.6 on the lower left corner ofthe altimeter. Notice also the Kollsman window on the right side of the altimeter

Section 5.2 Flight Instruments and Systems58

where the number 29.9 is indicated. This is the sea level barometric pressure ininches of mercury.

Figure 5.6 – The altimeter

Federal Aviation Regulations require that below 18,000 feet, you set the altimeterto the current reported setting from a station located within 100 nautical miles ofthe aircraft. If no station report is available, you can set the altimeter to theelevation of the departure airport.

If you have been flying for a long time or over a long distance, the atmosphericpressure will probably have changed, so you should obtain a new pressuresetting from a ground station if you need to know your precise altitude.

Above 18,000 feet, you should set the altimeter to 29.92 in. Hg. (Note: you shouldonly fly above this altitude while in a “wave window, or with Air Traffic Controlclearance.) If all pilots in a given area have their altimeter set the same,temperature and pressure variations will affect all the altimeters the same, so thatseparation between aircraft can be maintained.

Altimeter ErrorsClearly, if the atmospheric pressure changes, the altimeter reading will beaffected. If you think of the atmosphere as a layer of constant density but varyingthickness, it is easy to see how different pressures affect the altimeter. Where thelayer is thick, the pressure is high, and where the layer is thin, the pressure islow. (While technically incorrect, this assumption greatly simplifies thediscussion, while not detracting from its applicability.)

Remember, the altimeter measures pressure, which is essentially the weight ofthe air above you. It doesn’t so much measure how high you are above sea levelas how far you are below the “top” of the atmosphere.