Venturi Tube and Wet-Type Vacuum Pump Systems

Venturi Tube Systems
Aircraft that do not have a pneumatic pump to evacuate the instrument cases can use venture tubes mounted on the outside of the aircraft, similar to the system shown in figure 3-26: "A venture tube". Air flowing through these tubes speeds up in the narrowest part, and according to Bernoulli's principle, the pressure drops. This location is connected to the instrument case by a piece of tubing. The two attitude instruments operate on approximately 4" Hg suction; the turn-and-slip indicator needs only 2" Hg, so a pressure-reducing needle valve is used to decrease the suction. Filtered airflow's into the instruments through filters built into the instrument cases. In this system, ice can clog the venturi tube and stop the instruments when they are most needed.
 
Tag: Electrical system, pneumatic system, venture tube system, wet-type vacuum pump system, dry-air pump system, pressure system,
 
Tag: Types of Airspeed, Indicated Airspeed, Calibrated Airspeed, Equivalent Airspeed, True Airspeed, Mach number, Maximum Allowable Airspeed, and Airspeed Color Code.
 
Tag: Flying instrument, instrument flight, aviation, piloting, instrument rating, instrument flying training, instrument flight rating, instrument rating requirement, instrument rating regulation, aircraft, aero plane, airplane, and aeronautical knowledge.
 
Wet-Type Vacuum Pump Systems
Steel-vane air pumps have been used for many years to evacuate the instrument cases. The discharge air is used to inflate rubber deicer boots on the wing and empennage leading edges. The vanes in these pumps are lubricated by a small amount of engine oil metered into the pump and this oil is discharged with the air. To keep the oil from deteriorating the rubber boots, it must be removed with an oil separator like the one in figure 3-27: "Single-engine instrument vacuum system".
 
The vacuum pump moves a greater volume of air than is needed to supply the instruments with the suction needed, so a suction-relief valve is installed in the inlet side of the pump. This spring-loaded valve draws in just enough air to maintain the required low pressure inside the instruments, as is shown on the suction gauge in the instrument panel. Filtered air enters the instrument cases from a central air filter. As long as aircraft fly at relatively low altitudes, enough air is drawn into the instrument cases to spin the gyros at a sufficiently high speed.

Vertical Speed Indicators (VSI)

The vertical speed indicator (VSI) in figure 3-14: "Vertical speed indicator" is also called a vertical velocity indicator (VVI) and was formerly known as a rate-of-climb indicator. It is a rate-of-pressure change instrument that gives an indication of any deviation from a constant pressure level.
 
Inside the instrument case is an aneroid very much like the one in an airspeed indicator. Both the inside of this aneroid and the inside of the instrument case are vented to the static system, but the case is vented through a calibrated orifice that causes the pressure inside the case to change more slowly than the pressure inside the aneroid. As the aircraft ascends, the static pressure becomes lower and the pressure inside the case compresses the aneroid, moving the pointer upward, showing a climb and indicating the number of feet per minute the aircraft is ascending.
 
When the aircraft levels off, the pressure no longer changes, the pressure inside the case becomes the same as that inside the aneroid, and the pointer returns to its horizontal, or zero, position. When the aircraft descends the static pressure increases and the aneroid expands, moving the pointer downward, indicating a descent.
 
The pointer indication in a VSI lags a few seconds behind the actual change in pressure, but it is more sensitive than an altimeter and is useful in alerting the pilot of an upward or downward trend, thereby helping maintain a constant altitude.
 
Some of the more complex VSIs, called instantaneous vertical speed indicators (IVSI), have two accelerometer-actuated air pumps that sense an upward or downward pitch of the aircraft and instantaneously create a pressure differential. By the time the pressure caused by the pitch acceleration dissipates, the altitude pressure change is effective.
 
Tag: Types of Airspeed, Indicated Airspeed, Calibrated Airspeed, Equivalent Airspeed, True Airspeed, Mach number, Maximum Allowable Airspeed, and Airspeed Color Code.
 
Position error: Error in the indication of the altimeter, ASI, and VSI caused by the air at the static system entrance not being absolutely still.
 
Kollsman window: A barometric scale window of a sensitive altimeter.
 
Calibrated orifice: A hole of specific diameter used to delay the pressure change in the case of a vertical speed indicator.
 
Memory Aid:
When flying from hot to cold, or from a high to a low, look out below!
 
Tag: Flying instrument, instrument flight, aviation, piloting, instrument rating, instrument flying training, instrument flight rating, instrument rating requirement, instrument rating regulation, aircraft, aero plane, airplane, and aeronautical knowledge.

The Basic Flight Instruments

Aircraft became a practical means of transportation when accurate flight instruments freed the pilot from the necessity of maintaining visual contact with the ground. Safety was enhanced when all pilots with private or higher ratings were required to demonstrate their ability to maintain level flight and make safe turns without reference to the outside horizon.
 
The basic flight instruments required for operation under visual flight rules (VFR) are an airspeed indicator, an altimeter, and a magnetic direction indicator. In addition to these, operation under instrument flight rules (IFR) requires a gyroscopic rate-of-turn indicator, a slip-skid indicator, a sensitive altimeter adjustable for barometric pressure, a clock displaying hours, minutes, and seconds with a sweep-second pointer or digital presentation, a gyroscopic pitch-and-bank indicator (artificial horizon), and a gyroscopic direction indicator (directional gyro or equivalent).
 
Tag: Flying instrument, instrument flight, aviation, piloting, instrument rating, instrument flying training, instrument flight rating, instrument rating requirement, instrument rating regulation, aircraft, aero plane, airplane, and aeronautical knowledge.
 
Aircraft that are flown in instrument meteorological conditions (IMC) are equipped with instruments that provide attitude and direction reference, as well as radio navigation instruments that allow precision flight from takeoff to landing with limited or no outside visual reference.
 
The instruments discussed in this chapter are those required by Title 14 of the Code of Federal Regulations (14 CFR) part 91, and are organized into three groups: Pitot-static instruments, compass systems, and gyroscopic instruments. The chapter concludes with a discussion of how to preflight these systems for IFR flight.

The Factors Affecting Aircraft Performance: Speed Stability

Normal Command
The characteristics of flight in the region of normal command are illustrated at point A on the curve in figure 2-7. If the aircraft is established in steady, level flight at point A, lift is equal to weight, and the power available is set equal to the power required. If the airspeed is increased with no changes to the power setting, a power deficiency exists. The aircraft will have the natural tendency to return to the initial speed to balance power and drag. If the airspeed is reduced with no changes to the power setting, an excess of power exists. The aircraft will have the natural tendency to speed up to regain the balance between power and drag. Keeping the aircraft in proper trim enhances this natural tendency. The static longitudinal stability of the aircraft tends to return the aircraft to the original trimmed condition.
 
An aircraft flying in steady, level flight at point C is in equilibrium. [Figure 2-7: Regions of speed stability] If the speed were increased or decreased slightly, the aircraft would tend to remain at that speed. This is because the curve is relatively flat and a slight change in speed will not produce any significant excess or deficiency in power. It has the characteristic of neutral stability; the aircraft's tendency is to remain at the new speed.
 
Reversed Command
The characteristics of flight in the region of reversed command are illustrated at point B on the curve in figure 2-7. If the aircraft is established in steady, level flight at point B, lift is equal to weight, and the power available is set equal to the power required. When the airspeed is increased greater than point B, an excess of power exists. This causes the aircraft to accelerate to an even higher speed. When the aircraft is slowed to some airspeed lower than point B, a deficiency of power exists. The natural tendency of the aircraft is to continue to slow to an even lower airspeed.
 
This tendency toward instability happens because the variation of excess power to either side of point B magnifies the original change in speed. Although the static longitudinal stability of the aircraft tries to maintain the original trimmed condition, this instability is more of an influence because of the increased induced drag due to the higher angles of attack in slow-speed flight.
 
Tag: Flying instrument, instrument flight, aviation, piloting, instrument rating, instrument flying training, instrument flight rating, instrument rating requirement, instrument rating regulation, aircraft, aero plane, airplane, and aeronautical knowledge.
 
Static longitudinal stability: The aerodynamic pitching moments required to return the aircraft to the equilibrium angle of attack.
 
Slow Airspeed Safety Hint
Be sure to add power before pitching up while at slow airspeeds to prevent losing airspeed.

Medical Factors on Flight Instruments

Tag: Flying instrument, instrument flight, aviation, piloting, instrument rating, instrument flying training, instrument flight rating, instrument rating requirement, instrument rating regulation, aircraft, aero plane, airplane, and aeronautical knowledge.
 
A "go/no-go" decision is made before each flight. The pilot should not only preflight check the aircraft, but also his/ herself before every flight. As a pilot you should ask yourself, "Could I pass my medical examination right now?" If you cannot answer with an absolute "yes," then you should not fly. This is especially true for pilots embarking on flights in IMC. Instrument flying can be much more demanding than flying in VMC, and peak performance is critical for the safety of flight.
 
Pilot performance can be seriously degraded by both prescribed and over-the-counter medications, as well as by the medical conditions for which they are taken. Many medications, such as tranquilizers, sedatives, strong pain relievers, and cough-suppressants, have primary effects that may impair judgment, memory, alertness, coordination, vision, and the ability to make calculations. Others, such as antihistamines, blood pressure drugs, muscle relaxants, and agents to control diarrhea and motion sickness, have side effects that may impair the same critical functions. Any medication that depresses the nervous system, such as a sedative, tranquilizer, or antihistamine, can make a pilot much more susceptible to hypoxia.
 
Title 14 of the Code of Federal Regulations (14 CFR) prohibits pilots from performing crewmember duties while using any medication that affects the faculties in any way contrary to safety. The safest rule is not to fly as a crewmember while taking any medication, unless approved to do so by the Federal Aviation Administration (FAA). If there is any doubt regarding the effects of any medication, consult an Aviation Medical Examiner (AME) before flying.
 
Alcohol
14 CFR part 91 prohibits pilots from performing crewmember duties within 8 hours after drinking any alcoholic beverage or while under the influence. Extensive research has provided a number of facts about the hazards of alcohol consumption and flying. As little as one ounce of liquor, one bottle of beer, or four ounces of wine can impair flying skills and render a pilot much more susceptible to disorientation and hypoxia. Even after the body completely metabolizes a moderate amount of alcohol, a pilot can still be impaired for many hours. There is simply no way of increasing the metabolism of alcohol or alleviating a hangover.
 
Tag: Flying instrument, instrument flight, aviation, piloting, instrument rating, instrument flying training, instrument flight rating, instrument rating requirement, instrument rating regulation, aircraft, aero plane, airplane, and aeronautical knowledge.
 
Fatigue
Fatigue is one of the most treacherous hazards to flight safety, as it may not be apparent to a pilot until serious errors are made. Fatigue can be either acute (short-term) or chronic (long-term). A normal occurrence of everyday living, acute fatigue is the tiredness felt after long periods of physical and mental strain, including strenuous muscular effort, immobility, heavy mental workload, strong emotional pressure, monotony, and lack of sleep. Acute fatigue is prevented by adequate rest, regular exercise, and proper nutrition. Chronic fatigue occurs when there is not enough time for a full recovery from repeated episodes of acute fatigue. Recovery from chronic fatigue requires a prolonged period of rest. In either case, unless adequate precautions are taken, personal performance could be impaired and adversely affect pilot judgment and decision making.
 
IMSAFE Checklist
The following checklist, IMSAFE, is intended for a pilot's personal preflight use. A quick check of the items on this list can help the pilot make a good self-evaluation prior to any flight. If the answer to any of the checklist questions is yes, then the pilot should consider not flying.
 
Illness—Do I have any symptoms?
Medication—Have I been taking prescription or over-thecounter drugs?
Stress—Am I under psychological pressure from the job? Do I have money, health, or family problems?
Alcohol—Have I been drinking within 8 hours? Within 24 hours?
Fatigue—Am I tired and not adequately rested?
Eating—Have I eaten enough of the proper foods to keep adequately nourished during the entire flight?
 
Tag: Flying instrument, instrument flight, aviation, piloting, instrument rating, instrument flying training, instrument flight rating, instrument rating requirement, instrument rating regulation, aircraft, aero plane, airplane, and aeronautical knowledge.
 
Hypoxia: A state of oxygen deficiency in the body sufficient to impair functions of the brain and other organs.

Inner Ear Orientation on Flight

The inner ear has two major parts concerned with orientation, the semicircular canals and the otolith organs. [Figure 1-1: Inner ear orientation] The semicircular canals detect angular acceleration of the body while the otolith organs detect linear acceleration and gravity. The semicircular canals consist of three tubes at right angles to each other, each located on one of the three axes: pitch, roll, or yaw. Each canal is filled with a fluid called endolymph fluid. In the center of the canal is the cupola, a gelatinous structure that rests upon sensory hairs located at the end of the vestibular nerves.

[Figure 1-2: Angular Acceleration] illustrates what happens during a flight turn. When the ear canal is moved in its plane, the relative motion of the fluid moves the cupola, which, in turn, stimulates the sensory hairs to provide the sensation of turning. This effect can be demonstrated by taking a glass filled with water and turning it slowly. The wall of the glass is moving, yet the water is not. If these sensory hairs were attached to the glass, they would be moving in relation to the water, which is still standing still.

Tag: Flying instrument, instrument flight, aviation, piloting, instrument rating, instrument flying training, instrument flight rating, instrument rating requirement, instrument rating regulation, aircraft, aero plane, airplane, and aeronautical knowledge.

The ear was designed to detect turns of a rather short duration. After a short period of time (approximately 20 seconds), the fluid accelerates due to friction between the fluid and the canal wall. Eventually, the fluid will move at the same speed as the ear canal. Since both are moving at the same speed, the sensory hairs detect no relative movement and the sensation of turning ceases. This can also be illustrated with the glass of water. Initially, the glass moved and the water did not. Yet, continually turning the glass would result in the water accelerating and matching the speed of the wall of the glass.

The pilot is now in a turn without any sensation of turning. When the pilot stops turning, the ear canal stops moving but the fluid does not. The motion of the fluid moves the cupola and therefore, the sensory hairs in the opposite direction. This creates the sensation of turning in the opposite direction even though the turn has stopped.

The otolith organs detect linear acceleration and gravity in a similar way. Instead of being filled with a fluid, a gelatinous membrane containing chalk-like crystals covers the sensory hairs. When the pilot tilts his/her head, the weight of these crystals causes this membrane to shift due to gravity and the sensory hairs detect this shift. The brain orients this new position to what it perceives as vertical. Acceleration and deceleration also cause the membrane to shift in a similar manner. Forward acceleration gives the illusion of the head tilting backward. [Figure 1-3: Linear acceleration]

Instrument Flying Rating Regulations

Although the regulations specify minimum requirements, the amount of instructional time needed is determined not by the regulation, but by the individual’s ability to achieve a satisfactory level of proficiency. A professional pilot with diversified flying experience may easily attain a satisfactory level of proficiency in the minimum time required by regulation. Your own time requirements will depend upon a variety of factors, including previous flying experience, rate of learning, basic ability, frequency of flight training, type of aircraft flown, quality of ground school training, and quality of flight instruction, to name a few. The total instructional time you will need, and in general the scheduling of such time, is up to the individual most qualified to judge your proficiency—the instructor who supervises your progress and endorses your record of flight training.

Holding the Instrument Rating does not necessarily make you a competent weather pilot. The rating certifies only that you have complied with the minimum experience requirements, that you can plan and execute a flight under IFR regulations, that you can execute basic instrument maneuvers, and that you have shown acceptable skill and judgment in performing these activities. Your Instrument Rating permits you to fly into instrument weather conditions with no previous instrument weather experience. Your Instrument Rating is issued on the assumption that you have the good judgment to avoid situations beyond your capabilities. The instrument training program you undertake should help you not only to develop essential flying skills but also help you develop the judgment necessary to use the skills within your own limits.

Once you hold the Instrument Rating, you may not act as pilot in command under IFR or in weather conditions less than the minimums regulations prescribed for VFR, unless you meet the recent flight experience requirements.

Instrument pilots rely strictly on instrument indications to precisely control the aircraft; therefore, they must have a solid understanding of basic aerodynamic principles and regulations in order to make accurate judgments regarding aircraft control inputs.

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Aviation Instrument Rating

Is an instrument rating necessary? The answer to this question depends entirely upon individual needs. Pilots who fly in familiar uncongested areas, stay continually alert to weather developments, and accept an alternative to their original plan, may not need an Instrument Rating. However, some cross-country destinations may take a pilot to unfamiliar airports and/or through high activity areas in marginal visual or instrument meteorological conditions (IMC). Under these conditions, an Instrument Rating may be an alternative to rerouting, rescheduling, or canceling a flight. Many accidents are the result of pilots who lack the necessary skills or equipment to fly in marginal visual meteorological conditions (VMC) or IMC conditions and attempt flight without outside references.

Pilots originally flew aircraft strictly by sight, sound, and feel while comparing the aircraft’s attitude to the natural horizon. As aircraft performance increased, pilots required more inflight information to enhance the safe operation of their aircraft. This instrument flying information has ranged from a string tied to a wing strut, to development of sophisticated electronic flight information systems (EFIS) and flight management systems (FMS). Flight instrument interpretation and aircraft control have advanced from the “one, two, three” or “needle, ball and airspeed” system to the use of “attitude instrument flying” techniques.

Navigation began by using ground references with dead reckoning and has led to the development of electronic navigation systems. These include the automatic direction finder (ADF), very-high frequency omnidirectional range (VOR), distance measuring equipment (DME), tactical air navigation (TACAN), long range navigation (LORAN), global positioning system (GPS), instrument landing system (ILS), microwave landing system (MLS), and inertial navigation system (INS).

Perhaps you want an Instrument Rating for the same basic reason you learned to fly in the first place—because you like flying. Maintaining and extending your proficiency, once you have the rating, means less reliance on chance and more on skill and knowledge. Earn the rating—not because you might need it sometime, but because it represents achievement and provides training you will use continually and build upon as long as you fly. But most importantly—it means greater safety in flying.