one null position of the loop. The one null position always indicates the direction to the transmitting facility.
Another type ADF system uses fixed, ferrite core loops in conjunction with a rotatable transformer called a resolver or goniometer. It operates essentially the same as the rotating loop, except that one of the windings of the goniometer rotates instead of the loop.
A general procedure for performing an operational check of the ADF system is as follows:
(1) Turn on/off switch to the "on" position and allow the radio to warm up. On installations that use the RMI (Radio Magnetic Indicator) pointer as an ADF indicator, assure that the switch has been positioned to present ADF information.
(2) Tune to the desired station.
(3) Adjust the volume control to an appropriate level.
(4) Rotate loop antenna and determine that only one null is received.
(5) Check that the ADF needle points towards the station. If the aircraft is situated among buildings or any other large reflecting surfaces, the ADF needle may indicate an error as a result of a reflected signal.
RADAR BEACON TRANSPONDER The radar beacon transponder system is used in conjunction with a ground base surveillance radar to provide positive aircraft identification directly on the controller's radar scope.
The airborne equipment or transponder receives a ground radar interrogation for each sweep of the surveillance radar antenna and automatically dispatches a coded response. Civil transponders operate in two modes labeled "Mode A" and "Mode AC" which are switch controlled. The flight identification code, a four-digit number, is assigned during the flight planning procedure.
Some aircraft transponders are equipped with an altitude encoding feature. The aircraft's altitude is transmitted to the ground station through the transponder. The mode selector switch is placed in the "AC" mode when it is necessary to transmit altitude information.
There are several different aircraft transponders in use. They all perform the same function and are basically the same electrically. The major differences are in construction; either a single unit or a control unit for remotely operating the transponder.
A typical transponder is shown in figure 13-16.
The front panel of the illustrated transponder contains all the switches and dials needed for operation.
Antenna Transponder control Power source FIGURE 13-16. Typical transponder system.
A short stub or covered stub antenna is used for transponder operation and is usually mounted on the lower surface of the aircraft fuselage.
To ground check the radar beacon transponder the appropriate test equipment must be used.
DOPPLER NAVIGATION SYSTEMS Doppler navigational radar automatically and continuously computes and displays ground speed and drift angle of an aircraft in flight without the aid of ground stations, wind estimates, or true airspeed data. The Doppler radar does not sense direction as search radar does. Instead, it is speed conscious and drift-conscious. It uses continuous carrier wave transmission energy and determines the forward and lateral velocity components of the aircraft by utilizing the principle known as Doppler effect.
The Doppler effect, or frequency change of a signal, can be explained in terms of an approaching 530
and departing sound. As shown in figure 13-17, the sound emitter is a siren located on a moving ambulance and the receiver is the ear of a stationary person. Notice the spacing between the emitter when it is approaching and when it is departing from the stationary receiver. When the sound waves are closely spaced the listener hears a sound that is higher in pitch. The reverse is true when the emitter is moving away from the listener. Doppler radar uses the frequency change phenomenon just described, except in the radio frequency range.
Median spacing if vehicle were stationary Sound waves equally Sound waves equally and densely spaced ... but widely spaced Vehicle * Vehicle - approaching • departing FIGURE 13-17. Doppler effect with sound waves.
The Doppler radar emits narrow beams of energy at one frequency, and these waves of energy strike the earth's surface and are reflected. Energy waves returning from the earth are spaced differently than the waves striking the earth. The earth-returned energy is intercepted by the receiver and compared with the outgoing transmitter energy. The difference, due to Doppler effect, is used to develop ground spced and wind drift angle information.
A ground operational check of a Doppler system consists of setting a precise airspeed, and a deviation angle which will give a distance-off-course reading. Always refer to the equipment manufacturer's instruction manual or the aircraft's operation manual for the proper test procedure.
INERTIAL NAVIGATION SYSTEM The intertial navigation system is presently being used on large aircraft as a long-range navigation aid. It is a self-contained system and does not require signal inputs from ground navigational facilities. The system derives attitude, velocity, and heading information from measurement of the aircraft's accelerations. Two accelerometers are required, one referenced to north and the other to east. The accelrometers (figure 13-18) are mounted on a gyrostabilized unit, called the stable platform, to avert the introduction of errors resulting from the acceleration due to gravity.
An inertial navigation system is a complex system containing four basic components. They are:
(1) A stable platform which is oriented to maintain accelerometers horizontal to the earth's surface and provide azimuth orientation.
(2) Accelerometers arranged on the platform to supply specific components of acceleration.
(3) Integrators which receive the output from the accelerometers and furnish velocity and distance.
(4) A computer which receives signals from the integrators and changes distance traveled to position in selected coordinates.
The diagram in figure 13-18 shows how these components are linked together to solve a navigation problem. Initial conditions are set into the system and the navigation process is begun. In inertial navigation, the term initialization is used to denote the process of bringing the system to a set of 1 st Integrators 2nd Accelerometer Acceleration Velocity Distance Computer Correction signal to level platform
- Stabilized platform FIGURE 13-18. A basic inertial navigation system.
initial conditions from which it can proceed with the navigation process. These conditions include leveling the platform, aligning the azimuth reference, setting initial velocity and position, and making any computations required to start the naviga.
tion.
Although all inertial navigation systems must be initialized, the procedure varies according to the equipment and the type aircraft in which it is installed. The prescribed initialization procedures are detailed in the appropriate manufacturer's manuals.
From the diagram it can be seen that the accelerometers are maintained in a horizontal position to the earth's surface by a gyro-stabilized platform. As the aircraft accelerates, a signal from the accelerometer is sent to the integrators. The output from the integrators, or distance, is then fed into the computer, where two operations are performed. First, a position is determined in relation to the preset flight profile, and second, a signal is sent back to the plattorm to position the accelerometer horizontally to the earth's surface. The output from high-speed gyros and accelerometers, when connected to the flight controls of the aircraft, resists any changes in the flight profile.
AIRBORNE WEATHER RADAR SYSTEM Radar (radio detection and ranging) is a device used to see certain objects in darkness, fog, or storms, as well as in clear weather. In addition to the appearance of these objects on the radar scope, their range and relative position are also indicated.
Radar is an electronic system using a pulse transmission of radio energy to receive a reflected signal from a target. The received signal is known as an echo; the time between the transmitted pulse and received echo is computed electronically and is displayed on the radar scope in terms of nautical miles.
A radar system (figure 13-19) consists of a transceiver and synchronizer, an antenna installed in the nose of the aircraft, a control unit installed in the cockpit, and an indicator or scope. A waveguide connects the receiver/transmitter to the antenna.
In the operation of a typical weather radar sys.
tem, the transmitter feeds short pulses of radio-frequency energy through a waveguide to the dish antenna in the nose of the aircraft. In one typical installation the antenna radiates the energy in a beam 3.8° wide. Part of the transmitted energy is reflected from objects in the path of the beam and Indicator Recelver tranunitter FicURE 13-19. Weather radar system diagram.
is received by, the dish antenna. Electronic switching simultaneously connects the antenna to the transmitter and disconnects the receiver during pulse transmission. Following the completion of pulse transmission, the antenna is switched from the transmitter to the receiver. The switching cycle is performed for each transmitted pulse.
The time required for radar waves to reach the target and reflect to the aircraft antenna is directly proportional to the distance of the target from the aircraft. The receiver measures the time interval between transmission of radar signals and reception of reflected energy and uses this interval to represent the distance, or range, of the target.
Rotation or sweep of the antenna and radar beam gives azimuth indications. The indicator sweep trace rotates in synchronization with the antenna.
The indicator display shows the area and the relative size of targets, whose azimuthal position is shown relative to the line of flight.
The weather radar increases safety in flight by enabling the operator to detect storms in the flight path in order to chart a course around them. The terrain-mapping facilities of the radar show shorelines, islands, and other topographical features along the flight path. These indications are presented on the visual indicator in range and azimuth relative to the heading of the aircraft.
An operational check consists of the following:
(1) Tow or taxi the aircraft clear of all buildings and parked aircraft.
(2) Apply power to the equipment, and allow sufficient warmup time. Altimeter
(3) Tilt the antenna to an upward position. indicator
(4) Check the scan on the radar scope for an indication of targets.
RADIO ALTIMETER Radio altimeters are used to measure the distance from the aircraft to the ground. This is accomplished by transmitting radio frequency energy to the ground and receiving the reflected energy at the aircraft. Most modern day altimeters are pulse type and the altitude is determined by measuring the time required for the transmitted pulse to hit the ground and return. The indicating instrument will indicate the true altitude of the aircraft, which is its height above water, mountains, buildings, or other objects on the surface of the earth.
The present day generation of radio altimeters are primarily used during landing and are a Category II requirement. The altimeter provides the pilot with the altitude of the aircraft during approach. Altimeter indications determine the decision point whether to continue to land, or execute a climb-out.
A radio altimeter system (figure 13-20) consista Radio altimeter Power source Receiver Transmitter antenna antenna FIcuRE 13-20. Typical radio altimeter system diagram.
of a transceiver, normally located in an equipment rack, an indicator installed in the instrument panel and two antennas located on the belly of the aircraft.
EMERGENCY LOCATOR TRANSMITTER (ELT) Emergency locator transmitters are self-contained, self-powered radio transmitters designed to transmit a signal on the international distress bands of
121.5 MHz (civilian) and 243 MHz (military).
Antennas Control/charger unit Transmitter FIGURE 13-21. Emergency locator transmitter (ELT).
Operation is automatic on impact. The transmitter may also be activated by a remote switch in the cockpit or a switch integral with the unit. If the "G" force switch in the transmitter is activated from impact it can be turned off only with the switch on the case. (See figure 13-21.)
824072 Transmitter The transmitter may be located anywhere within the aircraft, but the ideal location is, as far aft as possible but just forward of the vertical fin. It must be accessible to permit monitoring the replacement date of the battery and for arming or disarming of the unit. A remote control arm/ disarm switch may be installed in the cockpit.
The external antenna must be installed as far as practicable from other antennas to prevent interaction between avionics systems.
Batteries Batteries are the power supply for emergency locator transmitters. When activated, the battery must be capable of furnishing power for signal transmission for at least 48 hours. The useful life of the battery is the length of time which the battery may be stored without losing its ability to continuously operate the ELT for 48 hours. This useful life is established by the battery manufacturer; batteries must be changed or recharged as required at 50% of the battery's useful life.
This gives reasonable assurance that the ELT will operate if activated. The battery replacement date must be marked on the outside of the transmitter.
This time is computed from the date of manufacture of the battery.
Batteries may be nickel-cadmium, lithium, mag nesium dioxide, dry-cell batteries. Wet cell batteries have an unlimited shelf life until liquid is added. At that time their life in an ELT is regulated the same as dry cell batteries-change at 50% of shelf life. When replacing batteries use only those recommended by the manufacturer of the ELT. Do not use flashlight type batteries as their condition and useful life are unknown.
Testing Tests should False Alarms False alarms have caused many of the problems with ELT's. Battery failures, with resulting corrosion of the unit results in either a complete failure or an unwanted transmission. Another type of unwanted transmission is the result of careless handling by the operators of the aircraft.
Test Equipment Two monitors are available for identifying and/or locating unwanted ELT transmissions.
miniature scanning receiver may be mounted in the cockpit to warn the pilot if his ELT is transmitting. The other is a small portable ELT locator for use at general aviation airports to assist in finding an aircraft whose transmitter has accidentally become activated.
The operation of an ELT can be verified by tuning a communication receiver to the civil emergency frequency (121.5 MHz) and activating the ELT.
Turn the ELT off immediately upon receiving a signal in the communication receiver.
In all maintenance and testing of ELT's the manufacturers instructions must be followed.
INSTALLATION OF COMMUNICATION AND
NAVIGATION EQUIPMENT ust the ip- In sweeps.
for ELT audio sweeps. weight of the equipwithstanding the diately.
Machine screws and self-locking nuts.
Rear case support Rivets or machine screws* and self-locking nuts.
FICURE 13-22. Typical radio installation in a stationary instrument panel.
If the radio is to be mounted in the instrument panel and no provisions have been made for such an installation, determine if the panel is primary structure prior to making any cutouts. To minimize the load on a stationary instrument panel, install a support bracket (figure 13-22) between the rear of the radio case or rack and a nearby structural member of the aircraft.
The radio equipment must be securely mounted to the aircraft. All mounting bolts must be secured by locking devices to prevent loosening from vibration.
Adequate clearance between the radio equipment and the adjacent structure must be provided to prevent mechanical damage to electric wiring or radio equipment from vibration, chafing, or shock landing- Do not locate radio equipment and wiring near units containing combustible fuids. When separation is impractical, install baffles or shrouds to prevent contact of the combustible fluids with radio equipment in the event of a plumbing failure.
Cooling and Moisture The performance and service life of most radio equipment is seriously limited by excessive ambient temperatures. The installation should be planned so that the radio equipment can dissipate its heat readily. In some installations it may be necessary to produce an airflow over the radio equipment, either with a blower or through the use of a venturi.
The presence of water in radio equipment promotes rapid deterioration of the exposed components. Some means must be provided to prevent the entry of water into the compartments housing the radio equipment.
Vibration Isolation Vibration is a continued motion caused by an oscillating force. The amplitude and frequency of vibration of the aircraft structure will vary considerably with the type of aircraft.
Radio equipment is sensitive to mechanical shock and vibration and is normally "shock-mounted" to provide some protection against in-flight vibration and landing shock.
When special mounts (figure 13-23) are used to isolate radio equipment from vibrating structure, such mounts should provide adequate isolation over the entire range of expected vibration frequencies.
When installing shock mounts, assure that the equipment weight does not exceed the weight-carrying capabilities of the mounts.
Shock mount FIcURE 13-23. Typical shock-mounted base.
Radios installed in instrument panels do not ordinarily require vibration protection, since the panel itself is usually shock-mounted. However, make certain that the added weight can be safely carried by the existing mounts. In some cases it may be necessary to install larger capacity mounts or to increase the number of mounting points.
Radio equipment installed on shock mounts must have sufficient clearance from surrounding equipment and structure to allow for normal swaying of the equipment.
Periodic inspection of the shock mounts is required, and defective mounts should be replaced with the proper type. The factors to observe during the inspection are: (1) Deterioration of the shock- 535
absorbing material, (2) stiffness and resiliency of the material, and (3) overall rigidity of the mount.
If the mount is too stiff, it may not provide adequate protection against the shock of landing. If the mount is not stiff enough, it may allow prolonged vibration following an initial shock.
Shock-absorbing materials commonly used in shock mounts are usually electrical insulators. For this reason, each electronic unit mounted with shock mounts must be electrically bonded to a structural member of the aircraft, similar to that shown in figure 13-24. This may also be accomplished by using sheets of high-conductivity metal (copper or aluminum) where it is impossible to use a short bond strap.
Shock mount Bonding jumper FIcURE 13-24. Typical shock mount bonding jumper.
REDUCING RADIO INTERFERENCE Suppression of radio interference is a task of first importance. The problem has increased in proportion to the complexity of both the electrical system and the electronic equipment. Almost every component of the aircraft is a possible source of radio interference. Radio interference of any kind deteriorates the performance and reliability of the radio and electronic systems.
Isolation is the easiest and most practical method of radio noise suppression. This involves separating the source of radio noise from the input circuits of the affected equipment. In many cases, the noise in a receiver may be entirely eliminated simply by moving the antenna lead-in wire just a few inches away from the noise source. Some of the sources of radio interference in aircraft are rotating electrical devices, switching devices, ignition systems, propeller control systems, a.c. powerlines, and voltage regulators.
An aircraft can become highly charged with static electricity while in flight. If the aircraft is improperly bonded, all metal parts will not have the same amount of charge. A difference of potential will exist between various metal surfaces. The neutralization of the charges flowing in paths of variable resistance, due to such causes as intermittent contact from vibration or the movement of the control surfaces, will produce electrical disturbances (noise) in the radio receiver.
Bonding provides the necessary electrical connection between metallic parts of an aircraft. Bonding jumpers and bonding clamps are examples of bonding connectors. Bonding also provides the low-resistance return path for single-wire electrical systems.
Bonding radio equipment to the airframe will provide a low-impedance ground return and minimize radio interference from static electricity charges. Bonding jumpers should be as short as possible and installed in such a manner that the resistance does not exceed 0.003 ohm. When a jumper is used only to reduce radio noise and is not for current-carrying purposes, a resistance of
0.01 ohm is satisfactory.
The aircraft structure is also the ground for the radio. For the radio to function correctly, a proper balance must be maintained between the aircraft structure and the antenna. This means the surface area of the ground must be constant. Control surfaces, for example, may at times become partially insulated from the remaining structure. This would affect radio operation if the condition was not alleviated by bonding.
Shielding is one of the most effective methods of suppressing radio noise. The primary object of shielding is to electrically contain the radio frequency noise energy. In practical applications, the noise energy is kept flowing along the inner surface of the shield to ground instead of radiating into space. The use of shielding is particularly effective in situations where filters cannot be used. A good example of this is where noise energy radiates from a source and is picked up by the various circuits that eventually connect to the receiver input circuits. It would be impractical to filter all of the leads or units that are affected by the radiated noise energy; thus the application of effective shielding at the noise source itself is preferred, for it eliminates the radiated portion of the noise energy by confining it within the shield at its source.
Ignition wiring and spark plugs are usually shielded to minimize radio interference. If an intolerable radio noise level is present despite shielding, it may be necessary to provide a filter between the magneto and magneto switch to reduce the noise.
This may consist of a single bypass capacitor or a combination of capacitors and choke coils. When this is done, the shielding between the filter and magneto switch can usually be eliminated.
The size of a filter may vary widely, depending on the voltage and current requirements as well as the degree of attenuation desired. Filters are usually incorporated in equipment known to generate radio interference, but since these filters are often inadequate it is frequently necessary to add external filters.
Static Discharger Wicks Static dischargers are installed on aircraft to reduce radio receiver interference. This interference is caused by corona discharge emitted from the aircraft as a result of precipitation static. Corona occurs in short pulses which produce noise at the radio frequency spectrum. Static dischargers, normally mounted on the trailing edges of the control surfaces, wing tips, and vertical stabilizer, discharge the precipitation static at points a critical length away from the wing and tail extremities where there is little or no coupling of the static into the radio antenna.
Three major types of static dischargers are in use:
(1) Flexible vinyl-covered, silver- or carbonimpregnated braid.
(2) Semiflexible metallic braid.
(3) Null-field.
Flexible and semiflexible dischargers are attached to the aircraft by metal screws and should be periodically checked for tightness. At least 1 in. of the inner braid of vinyl covered dischargers should extend beyond the vinyl covering. Null-field dischargers (figure 13-25) are riveted and epoxy bonded to the aircraft structure. A resistance measurement from the mount to the airfrace should not exceed
0.1 ohm.
INSTALLATION OF AIRCRAFT ANTENNA SYS- TEMS An introductory knowledge of radio equipment is a valuable asset to the aviation mechanic, especially a knowledge of antenna installation and maintenance, since these tasks are often performed by the mechanic.
Antennas take many forms and sizes dependent upon the job they are to perform. Airborne antennas should be mechanically secure, mounted in interference-free locations, have the same polarization as the ground station, and be electrically matched to the receiver or transmitter which they serve.
The following procedures describe the installation of a typical rigid antenna:
(1) Place a template similar to that shown in figure 13-26 on the fore-and-aft center line at the desired location. Drill the mounting holes and correct diameter hole for the transmission line cable in the fuselage skin.
Sufficient size to accommodate transmission line cable WaL-
• No. 18 drill
• Non-conductive yellow nylon cap - Nylon/resistive coating FIGURE 13-26. Typical antenna mounting template.
Set screw Tungsten needle Aluminum retaíner (bonded) Typical wing trailing edge installation FIGURE 13-25. Null-field static discharger.
(2) Install a reinforcing doubier of sufficient thickness to reinforce the aircraft skin.
The length and width of the reinforcing plate should approximate the example shown in figure 13-27.
(3) Install the antenna on the fuselage, making sure that the mounting bolts are tightened firmly against the reinforcing doubler and 537
Antenna A transmission line connects the final power amplifier of a transmitter to the transmitting antenna. The transmission line for a receiver connects the antenna Fuselage skin to the first tuned circuit of the receiver. Transmission lines may vary from only a few feet to several feet in length.
Transponders, DME, and other pulse type trans- Existing stringers ceivers require transmission lines that are precise in length. The critical length of the transmission lines View A-A provides minimum attenuation of the transmitted or received signal. Refer to the equipment manufacture Reinforcing doubler installation manual for type and allowable length of Alclad 2024-T3 transmission lines.
Approximately one Coaxial cable is utilized in most airborne installainch spacing ot tions for transmission lines. Coaxial cable is an ⅛" min. dia. rivet unbalanced line that functions with a balanced antenna. To provide the proper impedance matching and the most efficient power transfer a balun is used. The balun is an integral part of the antenna and is not visible without disassembling the antenna.
When installing coaxial cable (transmission lines) secure the cables firmly along their entire length at intervals of approximately 2 ft. To assure optimum operation, coaxial cables should not be routed or tied to other wire bundles. When bending coaxial cable, be sure that the bend is at least 10 times the size of the cable diameter.
1½" edge distance min.
FIGURE 13-27. Typical antenna installation on a skin panel, the mast is drawn tight against the gasket.
If a gasket is not used, seal between the mast and the fuselage with a suitable sealer, such as zinc chromate paste, or equal.
The mounting bases of antennas vary in shape and sizes; however, the aforementioned installation procedure is typical and may be used for mast-type antenna installations.
Transmission Lines A transmitting or receiving antenna is connected directly to its associated transmitter or receiver by wire(s) which are shielded. The interconnecting shielded wire(s) are called a coaxial cable which connects the antenna to the receiver or transmitter.
The job of the transmission line (coaxial cable) is to get the energy to the place where it is to be used and to accomplish this with minimum energy loss.
Maintenance Procedures Detailed instructions, procedures, and specifications for the servicing of radio equipment are contained in the manufacturer's operation and service manuals. In addition, instructions for removal and installation of the units are contained in the maintenance manual for the aircraft in which the equipment is installed.
Although installation appears to be a simple procedure, many radio troubles can be attributed to carelessness or oversight when replacing radio equipment. Specific instances are loose cable connections, switched cable terminations, improper bonding, lack of or improper safety wiring, or failure to perform an operational check after installation.
Two additional points concerning installation of equipment needs emphasis. Prior to re-installing any unit, inspect its mounting for proper condition of shock mounts and bonding straps. After installation, safety wire as appropriate.
CHAPTER 14
CABIN ATMOSPHERE CONTROL SYSTEM
NEED FOR OXYGEN Oxygen is essential for most living processes.
Without oxygen, men and other animals die very rapidly. But before this extreme state is reached, a reduction in normal oxygen supplies to the tissues of the body can produce important changes in body functions, thought processes, and degree of consciousness. The sluggish condition of mind and body caused by a deficiency or lack of oxygen is called hypoxia. There are several causes of hypoxia, but the one which concerns aircraft operations is the decrease in partial pressure of the oxygen in the lungs.
The rate at which the lungs absorb oxygen depends upon the oxygen pressure. The pressure that oxygen exerts is about one-fifth of the total air pressure at any one given level. At sea level, this pressure value (3 p.s.i.) is sufficient to saturate the blood. However, if the oxygen pressure is reduced, either from the reduced atmospheric pressure at altitude or because the percentage of oxygen in the air breathed decreases, then the quantity of oxygen in the blood leaving the lungs drops and hypoxia follows.
From sea level to 7,000 ft. above sea level, the oxygen content and pressure in the atmosphere remain sufficiently high to maintain almost full saturation of the blood with oxygen and thus ensure normal body and mental functions.
At high altitude there is decreased barometric pressure, resulting in decreased oxygen content of the inhaled air. Consequently, the oxygen content of the blood is reduced.
At 10,000 ft. above sea level oxygen saturation of the blood is about 90%. Long exposure at this altitude will result in headache and fatigue. Oxygen saturation drops to 81% at 15,000 ft. above sea level. This decrease results in sleepiness, headache, blue lips and fingernails, impaired vision and judgment, increased pulse and respiration, and certain personality changes.
At 22,000 ft. above sea level the blood saturation is 68% and convulsions are likely to occur. Remaining without an oxygen supply at 25,000 ft. for 5 minutes where the blood saturation is down to 55 to 50% will cause unconsciousness.
COMPOSITION OF THE ATMOSPHERE The mixture of gases commonly called air but more technically termed atmosphere is composed principally of nitrogen and oxygen, but there are smaller quantities of other important gases, notable carbon dioxide, water vapor, and ozone. Figure 14-1 indicates the respective percentage of the quantity of each gas in its relation to the total mixture.
Other gases 1% Oxygen 21% Nitrogen 78% FICURE 14-1. The gases of the atmosphere.
As the altitude increases, the total quantity of all the atmospheric gases reduces rapidly, and, except for water vapor and ozone, the relative proportions of the gaseous mixture remain unchanged up to about 50 miles altitude, or slightly above. Above 50 miles altitude, changes do take place, and different 539
gases and new forms of the gases present at lower.
altitudes appear.
Nitrogen is the most common gas and comprises 78% of the total mixture of atmospheric gases.
However, insofar as man is concerned nitrogen is an inert gas which cannot be used directly for his own life processes. But biologically it is of immense importance because many compounds containing nitrogen are essential to all living matter.
Oxygen and its importance cannot be overestimated. Without oxygen, life as we know it cannot exist. Oxygen occupies 21% of the total mixture of atmospheric gases.
Carbon dioxide is of biological interest. The small quantity in the atmosphere is utilized by the plant world to manufacture the complex substance which animals use as food. Carbon dioxide also helps in the control of breathing in man and other animals.
Water vapor in the atmosphere is variable, but, even under the moist conditions at sea level, it rarely exceeds 5%; yet this gas absorbs far more energy from the sun than do the other gases. Vapor is not the only form in which water occurs in the atmosphere; water and ice particles are nearly always present. These ice particles also absorb energy and, with water vapor, play an important part in the formation of atmospheric and weather conditions.
Ozone is a variety of oxygen which contains three atoms of oxygen per molecule rather than the usual two. The major portion of the ozone in the atmosphere is formed by the interaction of oxygen and the sun's rays near the top of the ozone layer.
Ozone is also produced by electrical discharges, and the peculiar odor of ozone, which is somewhat like that of weak chlorine, can be detected after lightning storms. Auroras and cosmic rays may also produce ozone. Ozone is of great consequence to both living creatures on earth and to the circulation of the upper atmosphere. Ozone is important to living organisms because it filters out most of the sun's ultraviolet radiation.
Pressure of the Atmosphere The gases of the atmosphere (air), although invisible, have weight just like that of solid matter.
The weight of a column of air stretching from the surface of the earth out into space is called the atmospheric pressure. If this column is 1 sq. in., the weight of air at sea level is approximately 14.7 lbs.;
and the atmospheric pressure, therefore, can be stated as 14.7 p.s.i. at sea level. Another common way of stating the atmospheric pressure is to give the height of a column of mercury which weighs the same as a column of the atmosphere of the same cross sectional area. When measured this way, the atmospheric pressure at sea level is normally 1013.2 millibars, or 29.92 in. Hg.
The atmospheric pressure decreases with increasing altitude. The reason for this is quite simple: the column of air that is weighed is shorter. How the pressure changes for a given altitude is shown in figure 14-2. The decrease in pressure is a rapid one, and at 50,000 feet the atmospheric pressure has dropped to almost one-tenth of the sea level value, At a few hundred miles above the earth, the air has become so rarefied (thin) that the atmosphere can be considered nonexistent, but the line of demarcation with space is very vague.
100,000 80,000 60,000 40,000 20,000 Sea levei 10 12 Pressure in pounds per square inch FIcURE 14-2. How the atmospheric pressure decreases with altitude. For example, at sea level the pressure is 14.7
p.s.i; while at 40,000 ft., as the dotted lines show, the pressure is only 2.72 p.a.i.
Temperature and Altitude The variations in atmospheric temperature near the earth are well known and need no discussion.
However, at high altitudes the atmospheric temperature is not so variable but tends to have a more set pattern.
The meteorologist finds it convenient to define, somewhat arbitrarily, the atmosphere as being made up of several layers. The lowest of these is called the troposphere. The air temperature decreases with 540
increasing altitude in the troposphere and reaches a definite minimum at the top of the layer. The top of the troposphere is called the tropopause. The tropopause reaches its greatest height over the equator (about 60,000 ft.) and its lowest height over the poles (about 30,000 ft.). The tropopause marks the point at which air temperature stops decreasing with increasing altitude, and remains essentially constant.
The atmospheric layer above the tropopause is called the stratosphere. The lower stratosphere is an isothermal (constant temperature) region in which the temperature does not vary with altitude. The isothermal region continues up to about 82,000 to 115,000 ft. altitude. Above this level, the temperature increases sharply at the rate of about 1.5° C per 1,000 ft. The temperature reaches a peak at about 164,000 to 197,000 ft. altitude. Above the 197,000 ft. altitude (approximately), the temperature decreases again, reaching a minimum of -10° F. to -100° F. at about 230,000 to 262,000 ft. altitude. Above this level, the temperature again increases and apparently continues to increase until the edge of space.
The foregoing paragraphs have presented a general knowledge of the atmosphere. It is obvious that a means of preventing hypoxia and its ill effects must be provided. When the atmospheric pressure falls below 3 p.s.i. (approximately 40,000 ft.), even breathing pure oxygen is not sufficient.
The low partial pressure of oxygen, low ambient air pressure, and temperature at high altitude make it necessary to create the proper environment for passenger and crew comfort. The most difficult problem is maintaining the correct partial pressure patiene of oxygen in the inhaled air. This can be achieved by using oxygen, pressurized cabins, or pressure suits. The first and second methods are used extensively in civil aviation.
Pressurization of the aircraft cabin is now the accepted method of protecting persons against the effects of hypoxia. Within a pressurized cabin, people can be transported comfortably and safely for long periods of time, particularly if the cabin altitude is maintained at 8,000 ft., or below, where the use of oxygen equipment is not required. However, the flight crew in this type of aircraft must be aware of the danger of accidental loss of cabin pressure and must be prepared to meet such an emergency whenever it occurs.
PRESSURIZATION When an aircraft is flown at a high altitude, it burns less fuel for a given airspeed than it does for the same speed at a lower altitude. In other words, the airplane is more efficient at a high altitude. In addition, bad weather and turbulence can be avoided by flying in the relatively smooth air above the storms. Aircraft which do not have pressurization and air conditioning systems are usually limited to the lower altitudes.
A cabin pressurization system must accomplish several functions if it is to assure adequate passenger comfort and safety. It must be capable of maintaining a cabin pressure altitude of approximately 8,000 ft. at the maximum designed cruising altitude of the aircraft. The system must also be designed to prevent rapid changes of cabin altitude which may be uncomfortable or injurious to passengers and crew. In addition, the pressurization system should permit a reasonably fast exchange of air from inside to outside the cabin. This is necessary to eliminate odors and to remove stale air.
In the typical pressurization system, the cabin, flight compartment, and baggage compartments are incorporated into a sealed unit which is capable of containing air under a pressure higher than outside atmospheric pressure. Pressurized air is pumped into this sealed fuselage by cabin superchargers which deliver a relatively constant volume of air at all altitudes up to a designed maximum. Air is released from the fuselage by a device called an outflow valve. Since the superchargers provide a constant inflow of air to the pressurized area, the outflow valve, by regulating the air exit, is the major controlling element in the pressurization system.
The flow of air through an outflow valve is determined by the degree of valve opening. This valve is ordinarily controlled by an automatic system which can be set by the flight crewmembers. A few simple minor adjustments are required on the average flight, but most of the time automatic controls need only to be monitored. In the event of a malfunction of the automatic controls, manual controls are also provided. A schematic of a basic pressurization system is shown in figure 14-3.
The degree of pressurization and, therefore, the operating altitude of the aircraft are limited by several critical design factors. Primarily the fuselage is designed to withstand a particular maximum cabin differential pressure. Cabin differential pres- 541