Deviation, unlike variation, depends on the aircraft heading. Also unlike variation, the aircraft’s geographic location does not affect deviation. While no one can reduce or change variation error, an aviation maintenance technician (AMT) can provide the means to minimize deviation error by performing the maintenance task known as “swinging the compass.”
Figure 8-33. Isogonic lines are lines of equal variation.
To swing the compass, an AMT positions the aircraft on a series of known headings, usually at a compass rose. [Figure 8-34] A compass rose consists of a series of lines marked every 30° on an airport ramp, oriented to magnetic north. There is minimal magnetic interference at the compass rose. The pilot or the AMT, if authorized, can taxi the aircraft to the compass rose and maneuver the aircraft to the headings prescribed by the AMT.
As the aircraft is “swung” or aligned to each compass rose heading, the AMT adjusts the compensator assembly located on the top or bottom of the compass. 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.
The adjustments position the compensating magnets to minimize the difference between the compass indication and the actual aircraft magnetic heading. The AMT records any remaining error on a compass correction card like the one in Figure 8-35 and places it in a holder near the compass. Only AMTs can adjust the compass or complete the compass correction card. Pilots determine and fly compass headings using the deviation errors noted on the card. Pilots must also note the use of any equipment causing operational magnetic interference such as radios, deicing equipment, pitot heat, radar, or magnetic cargo.
The corrections for variation and deviation must be applied in the correct sequence as shown below, starting from the true course desired.
Figure 8-34. Utilization of a compass rose aids compensation for deviation errors.
Figure 8-35. A compass correction card shows the deviation correction for any heading.
Step 1: Determine the Magnetic Course
True Course (180°) ± Variation (+10°) = Magnetic Course (190°)
The magnetic course (190°) is steered if there is no deviation error to be applied. The compass card must now be considered for the compass course of 190°.
Step 2: Determine the Compass Course
Magnetic Course (190°, from step 1) ± Deviation (–2°, from correction card) = Compass Course (188°)
NOTE: Intermediate magnetic courses between those listed on the compass card need to be interpreted. Therefore, to steer a true course of 180°, the pilot would follow a compass course of 188°.
To find the true course that is being flown when the compass course is known:
Compass Course ± Deviation = Magnetic Course ± Variation= True Course
Dip Errors
The Earth’s magnetic field runs parallel to its surface only at the Magnetic Equator, which is the point halfway between the Magnetic North and South Poles. As you move away from the Magnetic Equator towards the magnetic poles, the angle created by the vertical pull of the Earth’s magnetic field in relation to the Earth’s surface increases gradually. This angle is known as the dip angle. The dip angle increases in a downward direction as you move towards the Magnetic North Pole and increases in an upward direction as you move towards the Magnetic South Pole.
If the compass needle were mounted so that it could pivot freely in three dimensions, it would align itself with the magnetic field, pointing up or down at the dip angle in the direction of local Magnetic North. Because the dip angle is of no navigational interest, the compass is made so that it can rotate only in the horizontal plane. This is done by lowering the center of gravity below the pivot point and making the assembly heavy enough that the vertical component of the magnetic force is too weak to tilt it significantly out of the horizontal plane. The compass can then work effectively at all latitudes without specific compensation for dip. However, close to the magnetic poles, the horizontal component of the Earth’s field is too small to align the compass which makes the compass unuseable for navigation. Because of this constraint, the compass only indicates correctly if the card is horizontal. Once tilted out of the horizontal plane, it will be affected by the vertical component of the Earth’s field which leads to the following discussions on northerly and southerly turning errors.
Northerly Turning Errors
The center of gravity of the float assembly is located lower than the pivotal point. As the aircraft turns, the force that results from the magnetic dip causes the float assembly to swing in the same direction that the float turns. The result is a false northerly turn indication. Because of this lead of the compass card, or float assembly, a northerly turn should be stopped prior to arrival at the desired heading. This compass error is amplified with the proximity to either magnetic pole. One rule of thumb to correct for this leading error is to stop the turn 15 degrees plus half of the latitude (i.e., if the aircraft is being operated in a position near 40 degrees latitude, the turn should be stopped 15+20=35 degrees prior to the desired heading). [Figure 8-36A]
Figure 8-36. Northerly and southerly turning errors.
Southerly Turning Errors
When turning in a southerly direction, the forces are such that the compass float assembly lags rather than leads. The result is a false southerly turn indication. The compass card, or float assembly, should be allowed to pass the desired heading prior to stopping the turn. As with the northerly error, this error is amplified with the proximity to either magnetic pole. To correct this lagging error, the aircraft should be allowed to pass the desired heading prior to stopping the turn. The same rule of 15 degrees plus half of the latitude applies here (i.e., if the aircraft is being operated in a position near 30 degrees latitude, the turn should be stopped 15+15+30 degrees after passing the desired heading). [Figure 8-36B]
Acceleration Error
The magnetic dip and the forces of inertia cause magnetic compass errors when accelerating and decelerating on easterly and westerly headings. Because of the pendulous-type mounting, the aft end of the compass card is tilted upward when accelerating and downward when decelerating during changes of airspeed. When accelerating on either an easterly or westerly heading, the error appears as a turn indication toward north. When decelerating on either of these headings, the compass indicates a turn toward south. A mnemonic, or memory jogger, for the effect of acceleration error is the word “ANDS” (Acceleration-North/Deceleration-South) may help you to remember the acceleration error. [Figure 8-37] Acceleration causes an indication toward north; deceleration causes an indication toward south.
Figure 8-37. The effects of acceleration error.
Oscillation Error
Oscillation is a combination of all of the errors previously mentioned and results in fluctuation of the compass card in relation to the actual heading direction of the aircraft. When setting the gyroscopic heading indicator to agree with the magnetic compass, use the average indication between the swings.
The Vertical Card Magnetic Compass
The vertical card magnetic compass eliminates some of the errors and confusion encountered with the magnetic compass. The dial of this compass is graduated with letters representing the cardinal directions, numbers every 30°, and tick marks every 5°. The dial is rotated by a set of gears from the shaft-mounted magnet, and the nose of the symbolic aircraft on the instrument glass represents the lubber line for reading the heading of the aircraft from the dial. [Figure 8-38]
Lags or Leads
When starting a turn from a northerly heading, the compass lags behind the turn. When starting a turn from a southerly heading, the compass leads the turn.
Figure 8-38. Vertical card magnetic compass.
Eddy Current Damping
In the case of a vertical card magnetic compass, flux from the oscillating permanent magnet produces eddy currents in a damping disk or cup. The magnetic flux produced by the eddy currents opposes the flux from the permanent magnet and decreases the oscillations.
Outside Air Temperature (OAT) Gauge
The outside air temperature (OAT) gauge is a simple and effective device mounted so that the sensing element is exposed to the outside air. The sensing element consists of a bimetallic-type thermometer in which two dissimilar materials are welded together in a single strip and twisted into a helix. One end is anchored into protective tube and the other end is affixed to the pointer, which reads against the calibration on a circular face. OAT gauges are calibrated in degrees °C, °F, or both. An accurate air temperature provides the pilot with useful information about temperature lapse rate with altitude change. [Figure 8-39]
Chapter Summary
Flight instruments enable an aircraft to be operated with maximum performance and enhanced safety, especially when flying long distances. Manufacturers provide the necessary flight instruments, but to use them effectively, pilots need to understand how they operate. As a pilot, it is important to become very familiar with the operational aspects of the pitot-static system and associated instruments, the vacuum system and associated instruments, the gyroscopic instruments, and the magnetic compass.
Figure 8-39. Outside air temperature (OAT) gauge.
Chapter 9
Flight Manuals and Other Documents
Introduction
Each aircraft comes with documentation and a set of manuals with which a pilot must be familiar in order to fly that aircraft. This chapter covers airplane flight manuals (AFM), the pilot’s operating handbook (POH), and aircraft documents pertaining to ownership, airworthiness, maintenance, and operations with inoperative equipment. Knowledge of these required documents and manuals is essential for a pilot to conduct a safe flight.
Airplane Flight Manuals (AFM)
Flight manuals and operating handbooks are concise reference books that provide specific information about a particular aircraft or subject. They contain basic facts, information, and/or instructions for the pilot about the operation of an aircraft, flying techniques, etc., and are intended to be kept on hand for ready reference.
The aircraft owner/information manual is a document developed by the aircraft manufacturer and contains general information about the make and model of the aircraft. The manual is not approved by the Federal Aviation Administration (FAA) and is not specific to an individual aircraft. The manual provides general information about the operation of an aircraft, is not kept current, and cannot be substituted for the AFM/POH.
An AFM is a document developed by the aircraft manufacturer and approved by the FAA. This book contains the information and instructions required to operate an aircraft safely. A pilot must comply with this information which is specific to a particular make and model of aircraft, usually by serial number. An AFM contains the operating procedures and limitations of that aircraft. Title 14 of the Code of Federal Regulations (14 CFR) part 91 requires that pilots comply with the operating limitations specified in the approved flight manuals, markings, and placards.
Originally, flight manuals followed whatever format and content the manufacturer felt was appropriate, but this changed with the acceptance of Specification No. 1 prepared by the General Aviation Manufacturers Association (GAMA). Specification No. 1 established a standardized format for all general aviation airplane and helicopter flight manuals.
The POH is a document developed by the aircraft manufacturer and contains FAA-approved AFM information. If “POH” is used in the main title, a statement must be included on the title page indicating that sections of the document are FAA approved as the AFM.
The POH for most light aircraft built after 1975 is also designated as the FAA-approved flight manual. The typical AFM/POH contains the following nine sections: General; Limitations; Emergency Procedures; Normal Procedures; Performance; Weight and Balance/Equipment List; Systems Description; Handling, Service, and Maintenance; and Supplements. Manufacturers also have the option of including additional sections, such as one on Safety and Operational Tips or an alphabetical index at the end of the POH.
Preliminary Pages
While the AFM/POH may appear similar for the same make and model of aircraft, each manual is unique and contains specific information about a particular aircraft, such as the equipment installed and weight and balance information. Manufacturers are required to include the serial number and registration on the title page to identify the aircraft to which the manual belongs. If a manual does not indicate a specific aircraft registration and serial number, it is limited to general study purposes only.
Most manufacturers include a table of contents that identifies the order of the entire manual by section number and title. Usually, each section also contains a table of contents for that section. Page numbers reflect the section and page within that section (1-1, 1-2, 2-1, 3-1, etc.). If the manual is published in loose-leaf form, each section is usually marked with a divider tab indicating the section number, title, or both. The Emergency Procedures section may have a red tab for quick identification and reference.
General (Section 1)
The General section provides the basic descriptive information on the airframe and powerplant(s). Some manuals include a three-dimensional drawing of the aircraft that provides dimensions of various components. Included are such items as wingspan, maximum height, overall length, wheelbase length, main landing gear track width, diameter of the rotor system, maximum propeller diameter, propeller ground clearance, minimum turning radius, and wing area. This section serves as a quick reference and helps a pilot become familiar with the aircraft.
The last segment of the General section contains definitions, abbreviations, explanations of symbology, and some of the terminology used in the POH. At the discretion of the manufacturer, metric and other conversion tables may also be included.
Limitations (Section 2)
The Limitations section contains only those limitations required by regulation or that are necessary for the safe operation of the aircraft, powerplant, systems, and equipment. It includes operating limitations, instrument markings, color-coding, and basic placards. Some of the limitation areas are airspeed, powerplant, weight and loading distribution, and flight.
Airspeed
Airspeed limitations are shown on the airspeed indicator (ASI) by color coding and on placards or graphs in the aircraft. [Figure 9-1] A red line on the ASI shows the airspeed limit beyond which structural damage could occur. This is called the never-exceed speed (VNE). A yellow arc indicates the speed range between maximum structural cruising speed (VN0) and VNE. Operation of an aircraft in the yellow airspeed arc is for smooth air only and then only with caution. A green arc depicts the normal operating speed range, with the upper end at VN0 and the lower end at stalling speed at maximum weight with the landing gear and flaps retracted (VS1). For airplanes, the flap operating range is depicted by the white arc, with the upper end at the maximum flap extended speed (VFE), and the lower end at the stalling speed with the landing gear and flaps in the landing configuration (VS0).
Figure 9-1. Single-engine airspeed indicator.
In addition to the markings listed above, small multi-engine airplanes have a red radial line to indicate single-engine minimum controllable airspeed (VMC). A blue radial line is used to indicate single-engine best rate of climb speed at maximum weight at sea level (VYSE). [Figure 9-2]
Powerplant
The Powerplant Limitations portion describes operating limitations on an aircraft’s reciprocating or turbine engine(s). These include limitations for takeoff power, maximum continuous power, and maximum normal operating power, which is the maximum power the engine can produce without any restrictions and is depicted by a green arc. Other items that can be included in this area are the minimum and maximum oil and fuel pressures, oil and fuel grades, and propell
er operating limits. [Figure 9-3]
All reciprocating-engine powered aircraft must have a revolutions per minute (rpm) indicator for each engine. Aircraft equipped with a constant-speed propeller or rotor system use a manifold pressure gauge to monitor power output and a tachometer to monitor propeller or rotor speed. Both instruments depict the maximum operating limit with a red radial line and the normal operating range with a green arc. [Figure 9-4] Some instruments may have a yellow arc to indicate a caution area.
Figure 9-2. Multi-engine airspeed indicator.
Figure 9-3. Minimum, maximum, and normal operating range markings on oil gauge.
Weight and Loading Distribution
Weight and Loading Distribution contains the maximum certificated weights, as well as the center of gravity (CG) range. The location of the reference datum used in balance computations is included in this section. Weight and balance computations are not provided in this area, but rather in the weight and balance section of the AFM/POH.
Figure 9-4. Manifold pressure gauge (top) and tachometer (bottom).
Flight Limits
Flight Limits list authorized maneuvers with appropriate entry speeds, flight load factor limits, and types of operation limits. It also indicates those maneuvers that are prohibited, such as spins or acrobatic flight, as well as operational limitations such as flight into known icing conditions.
Pilot's Handbook of Aeronautical Knowledge (Federal Aviation Administration) Page 41
