Par 12-37 Page 12-13 SECTION 3. GROUND OPERATIONAL CHECKS FOR AVIONICS EQUIPMENT (NON-ELECTRICAL)
12-37. COMPASS SWING. This must be performed whenever any ferrous component of the system (i.e., flux valve compensator, or Standby Compass) is installed, removed, repaired, or a new compass is installed. The magnetic compass can be checked for accuracy by using a compass rose located on an airport. The compass swing is normally affected by placing the aircraft on various magnetic headings and comparing the deviations with those on the deviation cards. Refer to 14 CFR part 23, § 23.2500 and part 91, § 91.205(b), and the equipment or aircraft manufacturer’s manual.
a. A compass swing must be performed on the following occasions:
(1) When the accuracy of the compass is suspected.
(2) After any cockpit modification or major replacement involving ferrous metal.
(3) Whenever a compass has been subjected to a shock; for example, after a hard landing or turbulence.
(4) After aircraft has passed through a severe electrical storm.
(5) After lightning strike.
(6) Whenever a change is made to the electrical system.
(7) Whenever a change of cargo is likely to affect the compass.
(8) When an aircraft operation is changed to a different geographic location with a major change in magnetic deviation (e.g., from Miami, Florida to Fairbanks, Alaska).
(9) After aircraft has been parked on one heading for over a year.
(10) When flux valves are replaced.
b. Precautions.
(1) The magnetic compass must be checked for accuracy in a location free of steel structures, underground pipes or cables, or equipment that produces magnetic fields.
(2) Personnel engaged in the compensation of the compass shall remove all magnetic or ferrous material from their possession.
(3) Use only nonmagnetic tools when adjusting the compass.
(4) Position the aircraft at least 100 yards from any metal object.
(5) All equipment in the aircraft having any magnetic effect on the compass must be secured in the position occupied in normal flight.
Par 12-37 Page 12-14
c. Compass Swing Procedures.
(1) Have the aircraft taxied to the NORTH (0°) radial on the compass rose. Use a hairline sight compass (a reverse reading compass with a gun sight arrangement mounted on top of it) to place the aircraft in the general vicinity. With the aircraft facing North and the person in the cockpit running the engine(s) at 1,000 rpm, a mechanic, standing approximately 30 ft in front of the aircraft and facing South, “shoots” or aligns the master compass with the aircraft center line. Using hand signals, the mechanic signals the person in the cockpit to make additional adjustments to align the aircraft with the master compass. Once aligned on the heading, the person in the cockpit runs the engine(s) to approximately 1,700 rpm to duplicate the aircraft’s magnetic field and then the person reads the compass.
NOTE: (1) For conventional gear aircraft, the mechanic will have to position the magnetic compass in the straight and level position or mount the tail of the aircraft on a moveable dolly to simulate a straight and level cruise configuration. (2) Remember the hairline sight compass is only intended to be used as a general piece of test equipment.
(2) If the aircraft compass is not in alignment with the magnetic North of the master compass, correct the error by making small adjustments to the North-South brass adjustment screw with a nonmetallic screwdriver (made out of brass stock, or stainless steel welding rod).
Adjust the N-S compensator screw until the compass reads North (0°). Turn the aircraft until it is aligned with the East-West, pointing East. Adjust the E-W compensator screw until it reads 90°.
Continue by turning the aircraft South 180° and adjust the N-S screw to remove one-half of the South’s heading error. This will throw the North off, but the total North-South should be divided equally between the two headings. Turn the aircraft until it is heading West 270°, and adjust the E-W screw on the compensator to remove one-half of the West error. This should divide equally the total E-W error. The engine(s) should be running.
(3) With the aircraft heading West, start your calibration card here and record the magnetic heading of 270° and the compass reading with the avionics/electrical systems on then off. Turn the aircraft to align with each of the lines on the compass rose and record the compass reading every 30°. There should be not more than a plus or minus 10° difference between any of the compass’ heading and the magnetic heading of the aircraft.
(4) If the compass cannot be adjusted to meet the requirements, install another one.
NOTE: A common error that affects the compass’ accuracy is the mounting of a compass or instruments on or in the instrument panel using steel machine screws/nuts rather than brass hardware, magnetized control yoke, structural tubing, and improperly routed electrical wiring, which can cause unreasonable compass error.
(5) If the aircraft has an electrical system, two complete compass checks should be performed, one with minimum electrical equipment operating and the other with all electrical accessories on (e.g., radios, navigation, radar, and lights). If the compass readings are not
Par 12-37 Page 12-15 identical, the mechanic should make up two separate compass correction cards, one with all the equipment on and one with the equipment off.
(6) When the compass is satisfactorily swung, fill out the calibration card properly and put it in the holder in full view for the pilot’s reference.
d. Standby (wet) Compass. Adjustment and compensation of the Standby Compass may also be accomplished by using the “compass swing” method.
12-38. PNEUMATIC GYROS.
a. Venturi Systems. The early gyro instruments were all operated by air flowing out of a jet over buckets cut into the periphery of the gyro rotor. A venturi was mounted on the outside of the aircraft to produce a low pressure, or vacuum, which evacuated the instrument case, and air flowed into the instrument through a paper filter and then through a nozzle onto the rotor.
(1) Venturi systems have the advantage of being extremely simple and requiring no power from the engine, nor from any of the other aircraft systems; but they do have the disadvantage of being susceptible to ice, and when they are most needed, they may become unusable.
(2) There are two sizes of venturi tubes: those which produce 4 inches of suction are used to drive the attitude gyros, and smaller tubes, which produce 2 inches of suction, are used for the turn and slip indicator. Some installations use two of the larger venturi tubes connected in parallel to the two attitude gyros, and the turn and slip indicator is connected to one of these instruments with a needle valve between them. A suction gage is temporarily connected to the turn and slip indicator, and the aircraft is flown so the needle valve can be adjusted to the required suction at the instrument when the aircraft is operated at its cruise speed.
(See Figure 12-1.)
FIGURE 12-1. VENTURI SYSTEM FOR PROVIDING AIRFLOW THROUGH GYRO
INSTRUMENTS
Par 12-38 Page 12-16
b. Vacuum Pump Systems. In order to overcome the major drawback of the venturi tube, that is, its susceptibility to ice, aircraft were equipped with engine driven vacuum pumps and the gyro instruments were driven by air pulled through the instrument by the suction produced by these pumps. A suction relief valve maintained the desired pressure (usually about 4 inches of mercury (inHg)) on the attitude gyro instruments, and a needle valve between one of the attitude indicators and the turn and slip indicator restricted the airflow to maintain the desired 2 inches of suction in its case. Most of the early instruments used only paper filters in each of the instrument cases, but in some installations, a central air filter was used to remove contaminants from the cabin air before it entered the instrument case.
(1) The early vacuum pumps were vane-type pumps of what is called the wet type one with a cast iron housing and steel vanes. Engine oil was metered into the pump to provide sealing, lubrication, and cooling, and then this oil, along with the air, was blown through an oil separator where the oil collected on baffles and was returned to the engine crankcase. The air was then exhausted overboard. Aircraft equipped with rubber deicer boots used this discharge air to inflate the boots. But before it could be used, this air was passed through a second stage of oil separation and then to the distributor valve and finally to the boots. (See Figure 12-2.)
FIGURE 12-2. INSTRUMENT VACUUM SYSTEM USING A WET-TYPE VACUUM PUMP
Par 12-38 Page 12-17
(2) The airflow through the instruments is controlled by maintaining the suction in the instrument case at the desired level with a suction relief valve mounted between the pump and the instruments. This valve has a spring-loaded poppet that offsets to allow cabin air to enter the pump and maintain the correct negative pressure inside the instrument case.
(3) The more modern vacuum pumps are of the dry type. These pumps use carbon vanes and do not require any lubrication, as the vanes provide their own lubrication as they wear away at a carefully predetermined rate. Other than the fact that they do not require an oil separator, systems using dry air pumps are quite similar to those using a wet pump. One slight difference, however, is in the need for keeping the inside of the pump perfectly clean. Any solid particles drawn into the system through the suction relief valve can damage one of the carbon vanes, and this can lead to destruction of the pump, as the particles broken off of one vane will damage all of the other vanes. To prevent particles entering the relief valve, its air inlet is covered with a filter, and this must be cleaned or replaced at the interval recommended by the aircraft manufacturer.
c. Positive Pressure Systems. Above about 18,000 ft, there is not enough mass to the air drawn through the instruments to provide sufficient rotor speed, and, to remedy this problem, many aircraft that fly at high-altitude use positive pressure systems to drive the gyros. These systems use the same type of air pump as is used for vacuum systems, but the discharged air from the pump is filtered and directed into the instrument case through the same fitting that receives the filtered air when the vacuum system is used. A filter is installed on the inlet of the pump, and then, before the air is directed into the instrument case, it is again filtered. A pressure regulator is located between the pump and the in line filter to control the air pressure so only the correct amount is directed into the instrument case.
d. System Filters. The life of an air driven gyro instrument is determined to a great extent by the cleanliness of the air that flows over the rotor. In vacuum systems, this air is taken from the cabin where there is usually a good deal of dust and very often tobacco smoke. Unless all of the solid contaminants are removed from the air before it enters the instrument, they will accumulate, usually in the rotor bearings, and slow the rotor. This causes an inaccurate indication of the instrument and will definitely shorten its service life. Dry air pumps are also subject to damage from ingested contaminants, and all of the filters in the system must be replaced on the schedule recommended by the aircraft manufacturer, and more often if the aircraft is operated under particularly dusty conditions, especially if the occupants of the aircraft regularly smoke while flying. (See Figures 12-3 and 12-4.)
Par 12-38 Page 12-18 FIGURE 12-3. INSTRUMENT VACUUM SYSTEM USING A DRY-TYPE AIR PUMP F IGURE 12-4. INSTRUMENT PRESSURE SYSTEM USING A DRY-TYPE AIR PUMP 12 -39.–12-50. [RESERVED.]
Par 12-51 Page 12-19 SECTION 4. PITOT/STATIC SYSTEMS
12-51. GENERAL. In order for the pitot-static instruments to work properly, they must be connected into a system that senses the impact air pressure with minimum distortion and picks up undisturbed static air pressure.
a. Pitot pressure is ram air pressure picked up by a small open-ended tube about a ¼ inch in diameter that sticks directly into the air stream that produces a pressure proportional to the speed of the air movement. Static pressure is the pressure of the still air used to measure the altitude and serves as a reference in the measurement of airspeed.
b. Airspeed requires pitot, altimeter, rate of climb, and transponder-required static.
12-52. SYSTEM COMPONENTS. The conventional design of the pitot system consists of pitot-static tubes or pitot tubes with static pressure parts and vents, lines, tubing, water drains and traps, selector valves, and various pressure actuated indicators or control units such as the altimeter, airspeed and rate-of-climb indicators, and the encoding altimeter connected to the system. (See Figure 12-5.)
FIGURE 12-5. PITOT/STATIC SYSTEM FOR A SMALL AIRCRAFT 12 -53. PITOT/STATIC TUBES AND LINES. The pitot tube (see Figure 12-6) is installed at the leading edge of the wing of a single-engine aircraft, outside the propeller slipstream or on the fuselage of a multiengine aircraft with the axis parallel to the longitudinal axis of the aircraft, unless otherwise specified by the manufacturer.
Par 12-53 Page 12-20 FIGURE 12-6. PITOT/STATIC TUBE HEAD 12 -54. STATIC PORTS AND VENTS.
a. Static ports and vents (more modern trend) should be mounted flush with fuselage skin.
One port is located on either side of the fuselage, usually behind the cabin.
b. Inspect for elevation or depression of the port or vent fitting. Such elevation or depression may cause airflow disturbances at high speeds and result in erroneous airspeed and altitude indications.
12-55. HEATER ELEMENTS. A heating element is located within the tube head to prevent the unit from becoming clogged during icing conditions experienced during flight. A switch in the cockpit controls the heater. Some pitot-static tubes have replaceable heater elements while others do not. Check the heater element or the entire tube for proper operation by noting either ammeter current or that the tube or port is hot to the touch. (See Figure 12-6.)
Par 12-56 Page 12-21
12-56. SYSTEM INSPECTION.
a. Inspect air passages in the systems for water, paint, dirt, or other foreign matter. If water or obstructive material has entered the system, all drains should be cleaned. Probe the drains in the pitot tube with a fine wire to remove dirt or other obstructions. The bottom static openings act as drains for the head’s static chamber. Check these holes at regular intervals to preclude system malfunctioning.
b. Check to ensure the water drains freely. If a problem is experienced with the pitotstatic system drainage or freezing at altitude, and the tubing diameter is less than 3/8 inch, replace it with larger tubing.
c. Check the pitot tube for corrosion.
(1) The pitot probe should not have any corrosion within 1/2 inch of the probe tip.
(2) Make sure there is no flaking which forms pits and irregularities in the surface of the tube.
NOTE: It is essential that the static air system be drained after the airplane has been exposed to rain.
12-57. SYSTEM LEAK TEST.
a. Pitot-static leak tests should be made with all instruments connected to assure that no leaks occur at instrument connections. Such tests should be accomplished whenever a connection has been loosened or an instrument replaced.
b. After the conclusion of the leak test, return the system to its normal flying configuration. Remove tape from static ports and pitot drain holes and replace the drain plug.
12-58. STATIC SYSTEM TESTS. These tests must comply with the static system tests required by § 91.411 and be performed by an appropriately rated repair station with the appropriate test equipment. If the manufacturer has not issued instructions for testing static systems, the following may be used:
a. Connect the test equipment directly to the static ports, if practicable. Otherwise, connect to a static system drain or tee connection and seal off the static ports. If the test equipment is connected to the static system at any point other than the static port, it should be made at a point where the connection may be readily inspected for system integrity. Observe maintenance precautions given in paragraph 12-60 of this section.
b. Do not blow air through the line toward the instrument panel. This may seriously damage the instruments. Be sure to disconnect the instrument lines so no pressure can reach the instruments.
c. Apply a vacuum equivalent to 1,000 ft altitude, (differential pressure of approximately
1.07 inHg or 14.5 inches of water) and hold.
Par 12-58 Page 12-22
d. After 1 minute, check to see that the leak has not exceeded the equivalent of 100 ft of altitude (decrease in differential pressure of approximately 0.0105 inHg or 1.43 inches of water).
12-59. TEST PITOT SYSTEM. Tests should be in accordance with the aircraft manufacturer’s instructions. If the manufacturer has not issued instructions for testing pitot systems, the following may be used:
a. Seal the drain holes and connect the pitot pressure openings to a tee to which a source of pressure and manometer or reliable indicator is connected.
b. Restrain hoses that can whip due to applied pressure.
c. Apply pressure to cause the airspeed indicator to indicate 150 knots (kts) (differential pressure 1.1 inHg or 14.9 inches of water), hold at this point and clamp off the source of pressure. After 1 minute, the leakage should not exceed 10 kts (decrease in differential pressure of approximately 0.15 inHg or 2.04 inches of water).
CAUTION: To avoid rupturing the diaphragm of the airspeed indicator, apply pressure slowly and do not build up excessive pressure in the line.
Release pressure slowly to avoid damaging the airspeed indicator.
d. If the airspeed indicator reading declines, check the system for leaky hoses and loose connections.
e. Inspect the hoses for signs of deterioration, particularly at bends and at the connection points to the pitot mast and airspeed indicator. Replace hoses that are cracked or hardened with identical specification hoses. Any time a hose is replaced, perform a pressure check.
WARNING: Do not apply suction to pitot lines.
12-60. MAINTENANCE PRECAUTIONS. Observe the following precautions in all pitot-static system leak testing:
a. Before any pitot/static system is tested, determine that the design limits of instruments attached to it will not be exceeded during the test. To determine this, locate and identify all instruments attached to the system.
b. A system diagram will help to determine the location of all instruments as well as locate a leak while observing instrument indications. If a diagram is not available, instruments can be located by tracing physical installation.
c. Be certain that no leaks exist in the test equipment.
d. Run full range tests only if you are thoroughly familiar with the aircraft instrument system and test equipment.
Par 12-60 Page 12-23
e. Make certain the pressure in the pitot system is always equal to, or greater than, that in the static system. A negative differential pressure across an airspeed indicator can damage the instrument.
f. The rate of change or the pressure applied should not exceed the design limits of any pitot or static instruments connected to the systems.
g. When lines are attached to or removed from the bulkhead feed through fitting or at a union, ensure the line attached to the opposite end is not loose, twisted, or damaged by rotation of the fitting. Such fittings normally are provided with a hex flange for holding the fitting.
12-61. REPLACING LINES. If necessary to replace lines, observe the following installation:
a. Attach lines at regular intervals by means of suitable clamps.
b. Do not clamp lines at end fittings.
c. Maintain the slope of lines toward drains to ensure proper drainage.
d. Check the lines for leaks.
12-62. RELOCATION OF PITOT TUBE. If pitot tube relocation is necessary, perform the relocation in accordance with approved data and consider the following:
a. Freedom of aerodynamic disturbances caused by the aircraft.
b. Location protected from accidental damage.
c. Alignment with the longitudinal axis of the aircraft when in cruising flight.
12-63. TROUBLESHOOTING THE PITOT/STATIC PRESSURE SYSTEM.
a. If instruments are inoperative or erratic operation occurs, take the following action:
(1) Check for clogged lines. Drain lines at the valves (especially after aircraft has been exposed to rain). Disconnect lines at the instruments and blow them out with low-pressure air.
(2) Check lines for leaks or looseness at all connections. Repair as required.
b. If the pitot heating element(s) are operative, check the following:
(1) Are circuit breaker(s) tripped?
(2) Reset the circuit breaker to determine if:
(a) The system is OK, or
(b) The circuit breaker trips again, if so:
Par 12-63 Page 12-24
1. Check the wiring continuity to the ground. If the switch(s) is defective, repair as necessary.
2. Check the heating element; replace it if it is defective.
TABLE 12-1. COLOR CODES FOR PITOT-STATIC SYSTEMS CODE ABBR. DEFINITION COLOR
PP PITOT PRESSURE NATURAL
SP STATIC PRESSURE (PILOT) RED
STATIC PRESSURE (CO-PILOT) GREEN
STATIC PRESSURE (CABIN) YELLOW
STATIC PRESSURE (STANDBY) BLUE
12-64.–12-69. [RESERVED.]