Dry-Air Pump and Pressure Systems

Dry-Air Pump Systems
As flight altitudes increase, the air is less dense and more air must be forced through the instruments. Air pumps that do not mix oil with the discharge air are used in high-flying aircraft.
 
Steel vanes sliding in a steel housing need to be lubricated, but vanes made of a special formulation of carbon sliding inside carbon housing provide their own lubrication as they wear in a microscopic amount.
 
Pressure Systems
Figure 3-28: "Twin-engine instrument pressure system" is a diagram of the instrument pneumatic system of a twin-engine general aviation airplane. Two dry air pumps are used with filters in their inlet to filter out any contaminants that could damage the fragile carbon vanes in the pump. The discharge air from the pump flows through a regulator, where excess air is bled off to maintain the pressure in the system at the desired level. The regulated air then flows through inline filters to remove any contamination that could have been picked up from the pump, and from there into a manifold check valve. If either engine should become inoperative, or if either pump should fail, the check valve will isolate the inoperative system and the instruments will be driven by air from the operating system. After the air passes through the instruments and drives the gyros, it is exhausted from the case. The gyro pressure gauge measures the pressure drop across the instruments.
 
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.
 
Venturi tube: A specially-shaped tube attached to the outside of an aircraft to produce suction to operate gyro instruments.
 
Rigidity: The characteristic of a gyroscope that prevents its axis of rotation tilting as the Earth rotates.
 
Precession: The characteristic of a gyroscope that causes an applied force to be felt, not at the point of application, but 90° from that point in the direction of rotation.
 
Inverter: A solid-state electronic device that converts electrical current from d.c. into a.c. to operate a.c. gyro instruments.
 
Suction-relief valve: A relief valve in an instrument vacuum system to maintain the correct low pressure inside the instrument case for the proper operation of the gyros.

Compass Systems

The Earth is a huge magnet, spinning in space, surrounded by a magnetic field made up of invisible lines of flux. These lines leave the surface at the magnetic north pole and reenter at the magnetic south pole.
 
Lines of magnetic flux have two important characteristics: any magnet that is free to rotate will align with them, and an electrical current is induced into any conductor that cuts across them. Most direction indicators installed in aircraft make use of one of these two characteristics.
 
Magnetic Compass
One of the oldest and simplest instruments for indicating direction is the magnetic compass. It is also one of the basic instruments required by 14 CFR part 91 for both VFR and IFR flight.
 
A magnet is a piece of material, usually a metal containing iron that attracts and holds lines of magnetic flux. Every magnet regardless of size has two poles: a north pole and a south pole. When one magnet is placed in the field of another, the unlike poles attract each other and like poles repel.
 
An aircraft magnetic compass, such as the one in figure 3-15, has two small magnets attached to a metal float sealed inside a bowl of clear compass fluid similar to kerosene. A graduated scale, called a card, is wrapped around the float and viewed through a glass window with a lubber line across it. The card is marked with letters representing the cardinal directions, north, east, south, and west, and a number for each 30° between these letters. The final "0" is omitted from these directions; for example, 3 = 30°, 6 = 60°, and 33 = 330°. There are long and short graduation marks between the letters and numbers, with each long mark representing 10° and each short mark representing 5°.
 
Tag: Types of Airspeed, Indicated Airspeed, Calibrated Airspeed, Equivalent Airspeed, True Airspeed, Mach number, Maximum Allowable Airspeed, and Airspeed Color Code.
 
The float and card assembly has a hardened steel pivot in its center that rides inside a special, spring-loaded, hard-glass jewel cup. The buoyancy of the float takes most of the weight off the pivot, and the fluid damps the oscillation of the float and card. This jewel-and-pivot type mounting allows the float freedom to rotate and tilt up to approximately 18° angle of bank. At steeper bank angles, the compass indications are erratic and unpredictable.
 
The compass housing is entirely full of compass fluid. To prevent damage or leakage when the fluid expands and contracts with temperature changes, the rear of the compass case is sealed with a flexible diaphragm, or in some compasses, with a metal bellows.
 
The magnets align with the Earth's magnetic field and the pilot reads the direction on the scale opposite the lubber line. In figure 3-15: "A magnetic compass", the pilot sees the compass card from its backside. When you are flying north as the compass shows, east is to your right, but on the card "33" which represents 330°  (west of north) is to the right of north. The reason for this apparent backward graduation is that the card remains stationary, and the compass housing and the pilot turn around it, always viewing the card from its back side.
 
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 compensator assembly mounted on the top or bottom of the compass allows an aviation maintenance technician (AMT) to create a magnetic field inside the compass housing that cancels the influence of local outside magnetic fields.
 
This is done to correct for deviation error. The compensator assembly has two shafts whose ends have screwdriver slots accessible from the front of the compass. Each shaft rotates one or two small compensating magnets. The end of one shaft is marked E-W, and its magnets affect the compass when the aircraft is pointed east or west. The other shaft is marked N-S and its magnets affect the compass when the aircraft is pointed north or south.

Pitot-Static Systems

Three basic pressure-operated instruments are found in most aircraft instrument panels. These are the sensitive altimeter, airspeed indicator (ASI), and vertical speed indicator (VSI). All three receive the pressures they measure from the aircraft Pitot-static system.
 
Flight instruments depend upon accurate sampling of the ambient atmospheric pressure to determine the height and speed of movement of the aircraft through the air, both horizontally and vertically. This pressure is sampled at two or more locations outside the aircraft by the Pitot-static system.
 
The pressure of the static, or still air, is measured at a flush port where the air is not disturbed. On some aircraft, this air is sampled by static ports on the side of the electrically heated Pitot-static head, such as the one in figure 3-1: "A typical electrically heated pitot static head". Other aircraft pick up the static pressure through flush ports on the side of the fuselage or the vertical fin. These ports are in locations proven by flight tests to be in undisturbed air, and they are normally paired, one on either side of the aircraft. This dual location prevents lateral movement of the aircraft from giving erroneous static pressure indications. The areas around the static ports may be heated with electric heater elements to prevent ice forming over the port and blocking the entry of the static air.
 
Pitot pressure, or impact air pressure, is taken in through an open-end tube pointed directly into the relative wind flowing around the aircraft. The pitot tube connects to the airspeed indicator, and the static ports deliver their pressure to the airspeed indicator, altimeter, and VSI. If the static ports should ice over, or in any other way become obstructed, the pilot is able to open a static-system alternate source valve to provide a static air pressure source from a location inside the aircraft. [Figure 3-2: A typical pitot static-system] This may cause an inaccurate indication on the pitot-static instrument. Consult the Pilot's Operating Handbook/Airplane Flight Manual (POH/AFM) to determine the amount of error.
 
Position Error
The static ports are located in a position where the air at their surface is as undisturbed as possible. But under some flight conditions, particularly at a high angle of attack with the landing gear and flaps down, the air around the static port may be disturbed to the extent that it can cause an error in the indication of the altimeter and airspeed indicator. Because of the importance of accuracy in these instruments, part of the certification tests for an aircraft is a check of position error in the static system.
 
The POH/AFM contains any corrections that must be applied to the airspeed for the various configurations of flaps and landing gear.
 
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.
 
Pitot-static head: A combination pickup used to sample Pitot pressure and static air pressure.
 
Static pressure: Pressure of the air that is still, or not moving, measured perpendicular to the surface of the aircraft.

The Factors Affecting Aircraft Performance: Trim

A trim tab is a small, adjustable hinged surface, located on the trailing edge of the aileron, rudder, or elevator control surface. It is used to maintain balance in straight-and-level flight and during other prolonged flight conditions so the pilot does not have to hold pressure on the controls. This is accomplished by deflecting the tab in the direction opposite to that in which the primary control surface must be held.
 
The force of the airflow striking the tab causes the main control surface to be deflected to a position that will correct the unbalanced condition of the aircraft.
 
Because the trim tabs use airflow to function, trim is a function of speed. Any change in speed will result in the need to re-trim the aircraft. A properly trimmed aircraft seeks to return to the original speed before the change. Therefore, it is very important for instrument pilots to keep the aircraft in constant trim. This will reduce the workload significantly and allow pilots to tend to other duties without compromising aircraft control.
 
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.
 
Reversed Logic
In the region of reversed command, as you slow down you require more power.

Aeronautical Decision Making

Aeronautical decision making (ADM) is a systematic approach to the mental process used by pilots to consistently determine the best course of action in response to a given set of circumstances. ADM builds upon the foundation of conventional decision making, but enhances the process to decrease the probability of pilot error. ADM provides a structure to analyze changes that occur during a flight and determine how these changes might affect a flight's safe outcome.
 
The ADM process addresses all aspects of decision making in the cockpit and identifies the steps involved in good decision making. These steps are:
 
1.      Identifying personal attitudes hazardous to safe flight.
2.      Learning behavior modification techniques.
3.      Learning how to recognize and cope with stress.
4.      Developing risk assessment skills.
5.      Using all resources.
6.      Evaluating the effectiveness of one's ADM skills.
 
In conventional decision making, the need for a decision is triggered by recognition that something has changed or an expected change did not occur. Recognition of the change, or non-change, is a vital step in any decision making process. Not noticing the change in the situation can lead directly to a mishap. [Figure 1-7A: Decisions making] The change indicates that an appropriate response or action is necessary in order to modify the situation (or, at least, one of the elements that comprise it) and bring about a desired new situation. Therefore, situational awareness is the key to successful and safe decision making. At this point in the process, the pilot is faced with a need to evaluate the entire range of possible responses to the detected change and to determine the best course of action.
 
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.
 
Figure 1-7B: Decisions making illustrates the ADM process, how this process expands conventional decision making, shows the interactions of the ADM steps, and how these steps can produce a safe outcome. Starting with the recognition of change, and following with an assessment of alternatives, a decision to act or not act is made, and the results are monitored. Pilots can use ADM to enhance their conventional decision making process because it: (1) increases their awareness of the importance of attitude in decision making; (2) teaches the ability to search for and establish relevance of information; (3) increases their motivation to choose and execute actions that ensure safety in the situational timeframe.
 
Aeronautical decision making (ADM): A systematic approach to the mental process used by pilots to consistently determine the best course of action in response to a given set of circumstances.
 
Situational awareness: Knowing where you are in regard to location, air traffic control, weather, regulations, aircraft status, and other factors that may affect flight.
 
 
The DECIDE Model
A tool to use in making good aeronautical decisions is the DECIDE Model. [Figure 1-7C: Decisions making] The DECIDE Model is a six step process intended to provide the pilot with a logical way of approaching decision making. The six elements of the DECIDE Model represent a continuous loop process to assist a pilot in the decision making when faced with a change in a situation that requires judgment. The model is primarily focused on the intellectual component, but can have an impact on the motivational component of judgment as well. If a pilot continually uses the DECIDE Model in all decision making, it becomes very natural and could result in better decisions being made under all types of situations.

Nerves Signals Tell the Pilot His/Her Current Position

Nerves in the body’s skin, muscles, and joints constantly send signals to the brain, which signals the body’s relation to gravity. These signals tell the pilot his/her current position. Acceleration will be felt as the pilot is pushed back into the seat. Forces created in turns can lead to false sensations of the true direction of gravity, and may give the pilot a false sense of which way is up.

Uncoordinated turns, especially climbing turns, can cause misleading signals to be sent to the brain. Skids and slips give the sensation of banking or tilting. Turbulence can create motions that confuse the brain as well. Pilots need to be aware that fatigue or illness can exacerbate these sensations and ultimately lead to subtle incapacitation.

Orientation: Awareness of the position of the aircraft and of oneself in relation to a specific reference point.

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.

Spatial disorientation: The state of confusion due to misleading information being sent to the brain from various sensory organs, resulting in a lack of awareness of the aircraft position in relation to a specific reference point.

Vestibular: The central cavity of the bony labyrinth of the ear, or the parts of the membranous labyrinth that it contains.

Attitude Instrument Flying Techniques

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 in flight information to enhance the safe operation of their aircraft. This 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). Interpretation of the instruments and aircraft control have advanced from the “one, two, three” or “needle, ball and airspeed” system to the use of “attitude instrument flying” techniques.

Basic maneuvers, flown by sole reference to the instruments rather than outside visual cues, for the purpose of practicing basic attitude flying. The patterns simulate maneuvers encountered oninstrument flights such as holding patterns, procedure turns, and approaches.

Instrument flying techniques differ according to aircraft type, class, performance capability, and instrumentation. Therefore, the procedures and techniques that follow will need to be modified for application to different types of aircraft. Recommended procedures, performance data, operating limitations, and flight characteristics of a particular aircraft are available in your Pilot’s Operating Handbook/Airplane Flight Manual (POH/AFM) for study before practicing the flight maneuvers.

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The Need of 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 Aviation 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.