11.4 Instrument Installation, Magnetic Compass Swing & Regulatory Inspections

11.4 Instrument Installation, Magnetic Compass Swing & Regulatory Inspections

FAA Airframe Subject Matter Focus: Proper installation, calibration, and regulatory compliance guarantee instrument reliability. Aviation maintenance technicians must master instrument panel shock mounting, non-magnetic hardware requirements, dial vs cover glass range markings, white slippage marks, magnetic compass errors (variation, deviation, ANDS/UNOS dip errors), compass swinging calibration on surveyed compass roses, and mandatory 14 CFR §91.411 / Part 43 Appendix E inspection tolerances and static system leak tests.

1. Instrument Panel Shock Mounting & Electrical Bonding

Aircraft instrument panels isolate delicate flight instruments from destructive engine vibration frequencies and airframe structural deflections.

                 INSTRUMENT PANEL SHOCK MOUNT & BONDING JUMPER

             Airframe Structural Bulkhead
           ═══════════════════════════════════════
                          │
                   ┌──────┴──────┐
                   │ Elastomeric │ (Rubber / Silicone
                   │ Shock Mount │  Vibration Absorber)
                   └──────┬──────┘
                          │
           ═══════════════╪═══════════════════════
             Floating Instrument Panel
                          │
           ┌──────────────┴──────────────┐
           │ Flexible Braided Copper     │ (MANDATORY BONDING JUMPER!)
           │ Bonding Jumper Strap        │ Resistance < 0.003 Ohms
           └──────────────┬──────────────┘
                          │
           ═══════════════╧═══════════════════════
             Airframe Structural Ground

Shock Mount Engineering & Maintenance Practices

1. Vibration Isolation: Flight instrument panels are supported on elastomeric rubber or silicone shock mounts engineered to absorb high-frequency, low-amplitude engine and propeller vibrations (frequencies above 500 to 1,000 RPM) that would otherwise cause brinelling of delicate jewel pivot bearings and gear teeth.
2. Mandatory Bonding Jumpers: Because elastomeric rubber shock mounts are electrical insulators, the floating instrument panel becomes electrically isolated from the main airframe ground. In accordance with FAA AC 43.13-1B and 14 CFR §23.1309, flexible braided copper bonding jumpers must be installed across each shock mount:
   • Static Discharge: Prevents dangerous precipitation static (P-static) charge accumulation that causes radio interference and avionics microprocessor crashes.
   • Current Return & Shielding: Provides a low-resistance electrical return path (maximum resistance $\le 0.003\text{ ohms}$ / 3 milliohms) to protect against lightning strike attachment and electromagnetic interference (EMI).
3. Non-Magnetic Fasteners: Standard steel machine screws possess residual magnetism that induces severe localized magnetic deviation. When installing instruments on or near the instrument panel (especially within 12 to 18 inches of the magnetic compass or flux valve), technicians must strictly use non-magnetic brass, titanium, or 18-8 non-magnetic stainless steel screws, nuts, and lock washers.

2. Range Markings & The Glass Slippage Mark Mandate

In accordance with Federal Aviation Regulations (14 CFR §23.1545 and §25.1545), instrument dials and cover glasses must be clearly marked with standardized color-coded radial lines and arcs to indicate safe and hazardous operating limits.

                    INSTRUMENT COVER GLASS SLIPPAGE MARK

                   ┌──────────────────────────────┐
                   │      Instrument Dial Face    │
                   │                              │
                   │    Green Arc   Yellow  Red   │
                   │  (Normal Oper) (Caut) (Limit)│
                   │  ══════════════        │     │
                   │                        │     │
                   │  ──────────────────────┼───  │ ◄── Outer Bezel Rim
                   │                        │     │
                   └────────────────────────┼─────┘
                                            │
                                   [ WHITE SLIPPAGE ]
                                   [ INDEX LINE     ]
                                   (Painted from Outer Glass
                                    onto Metal Bezel Rim)

Standard Range Marking Color Codes (14 CFR §23.1545)

Cover Glass vs. Dial Face Markings & The White Slippage Mark

• Dial Face Application: Applying range markings directly to the internal dial face plate is preferred because the markings cannot rotate or shift relative to the pointer movement.
• Cover Glass Application & The Slippage Mark Rule: If range markings are applied to the outer cover glass of an instrument, a solid white index line (slippage mark) must be painted extending continuously from the outer glass surface across the joint onto the fixed metal instrument bezel rim:
  • The Safety Hazard: Instrument cover glasses are held in place by friction rings or retaining bezels. Thermal cycling or vibration can loosen the glass, allowing it to rotate.
  • Inspection Criteria: During preflight and 100-hour/annual inspections, any misalignment or break in the continuity of the white slippage line indicates that the glass has rotated. This immediately alerts the pilot and mechanic that the painted color arcs and red limit lines are misaligned with the internal dial mechanism, presenting a severe flight-safety hazard.

3. Direct-Reading Magnetic Compass Mechanics & Errors

The direct-reading magnetic compass is the sole self-contained directional instrument on the aircraft that requires zero electrical, vacuum, or pitot-static inputs.

                      MAGNETIC COMPASS INTERNAL STRUCTURE

                             [ Compass Housing Bowl ]
                           (Acid-Free White Kerosene)
                                       │
                               ┌───────┴───────┐
                               │ Float Assembly│
                               │ (Aluminum)    │
                               └───────┬───────┘
                                       │
                     ┌─────────────────┴─────────────────┐
                     │  Two Cobalt-Steel Bar Magnets     │ (Parallel North-South)
                     └─────────────────┬─────────────────┘
                                       │
                                       ▼
                             [ Hardened Steel Pivot ]
                                       │
                                       ▼ Riding in
                             [ Sapphire Jewel Cup ]
                                       │
                                       ▼
                        [ Card Reading Lubber Line ]
                                       │
                        [ Thermal Expansion Diaphragm ]

Internal Construction & Fluid Mechanics

• Fluid Damping: The compass bowl is filled with an acid-free clear hydrocarbon liquid (white mineral spirit or compass kerosene). The fluid provides buoyancy that reduces the resting weight of the float assembly on the jewel pivot to a fraction of an gram, minimizing pivot friction and damping rotational oscillations from turbulence.
• Thermal Expansion Bellows: A corrugated sylphon bronze expansion diaphragm or bellows at the rear of the compass bowl accommodates volumetric expansion and contraction of the fluid across extreme temperature swings ($-50^\circ\text{C}$ to $+70^\circ\text{C}$) without cracking the glass bowl or forming air bubbles.

The Three Major Magnetic Compass Errors

                     MAGNETIC COMPASS ERROR CLASSIFICATIONS

      1. VARIATION                     2. DEVIATION                 3. MAGNETIC DIP ERRORS
 (True vs Magnetic North)       (Airframe Magnetic Fields)          (ANDS / UNOS Dynamics)
           ▲                                ▲                                ▲
           │                                │                                │
    Isogonic Lines                  Compensating Magnets             Vertical Flux Pull
    "East is Least (-)"             N-S / E-W Brass Screws           on Pendulous Float
    "West is Best (+)"              Compass Correction Card          at Mid/High Latitudes

1. Magnetic Variation: The angular difference between True Geographic North and Magnetic North. Isogonic lines connect points of equal magnetic variation on aeronautical charts; the agonic line connects points of zero variation. Navigation conversion rule: "East is Least (subtract east variation), West is Best (add west variation)": $\text{Magnetic Heading} = \text{True Heading} \pm \text{Variation}$
2. Magnetic Deviation: Compass error induced by localized magnetic fields originating from the aircraft itself (ferrous structural steel engine mounts, electrical wiring currents, alternator fields, radios). Corrected by compass swinging.
3. Magnetic Dip Errors (ANDS & UNOS in the Northern Hemisphere):
   • Because the Earth's magnetic flux lines point downward toward the magnetic pole (vertical dip component), the compass float's center of gravity is placed below the pivot point with a counterweight. This pendulous suspension creates dynamic errors during acceleration and turning:
   • Acceleration Errors (ANDS Rule on East or West Headings):
     • Accelerate North: Accelerating on an East or West heading causes inertia to hold the counterweight back, tilting the float and causing the compass to falsely indicate a turn toward the North.
     • Decelerate South: Decelerating on an East or West heading causes the float to swing forward, causing the compass to falsely indicate a turn toward the South.
   • Turning Errors (UNOS Rule on Turns from North or South Headings):
     • Undershoot North: Turning from a North heading, centrifugal force swings the float outward, causing the compass to momentarily lag or indicate a turn in the opposite direction (roll out of turn prior to reaching North).
     • Overshoot South: Turning from a South heading, centrifugal force accelerates the compass card ahead of the turn, causing it to lead the actual turn (pass South before rolling out).

4. Compass Swinging Procedure & Correction Cards

Compass swinging is the precise physical calibration and compensation procedure performed to neutralize aircraft internal magnetic deviation fields and document residual deviation on the cockpit Magnetic Compass Correction Card.

                  COMPASS SWINGING FOUR-STEP COMPENSATION SEQUENCE

             1. ALIGN AIRCRAFT NORTH (000°)          2. ALIGN AIRCRAFT EAST (090°)
             ══════════════════════════════          ═════════════════════════════
             Adjust N-S Compensating Screw           Adjust E-W Compensating Screw
             to eliminate 100% of North Error        to eliminate 100% of East Error
                           │                                       │
                           ▼                                       ▼
             3. ALIGN AIRCRAFT SOUTH (180°)          4. ALIGN AIRCRAFT WEST (270°)
             ══════════════════════════════          ═════════════════════════════
             Adjust N-S Compensating Screw           Adjust E-W Compensating Screw
             to eliminate EXACTLY 50% of Error       to eliminate EXACTLY 50% of Error
                           │                                       │
                           └───────────────────┬───────────────────┘
                                               │
                                               ▼
                               5. SWING ALL 30° INCREMENTS
                            (Record Residual Deviation on Card)
                            (Maximum Allowable Deviation ≤ 10°)

Step-by-Step Compass Swinging Protocol

1. Prerequisites & Environmental Standards:
   • Performed on a certified, non-magnetic surveyed compass rose located well away from steel-reinforced concrete runways, metal hangars, underground high-voltage cables, or parked vehicles.
   • Aircraft must be in normal flight attitude (tailwheel aircraft raised to level flight attitude).
   • Engines running, electrical systems energized, radios, avionics, and navigation lights ON to replicate operational in-flight magnetic fields.
   • All adjustments must be made using a non-magnetic brass, aluminum, or beryllium screwdriver.
2. The Calibration Sequence:
   • Step 1 (Heading North - 000°): Align aircraft magnetic heading precisely North ($000^\circ$). Adjust the N-S brass compensating screw until the compass indicates exactly $000^\circ$ (100% error removed).
   • Step 2 (Heading East - 090°): Turn aircraft precisely East ($090^\circ$). Adjust the E-W brass compensating screw until the compass indicates exactly $090^\circ$ (100% error removed).
   • Step 3 (Heading South - 180°): Turn aircraft precisely South ($180^\circ$). Note the remaining heading error. Adjust the N-S screw to remove EXACTLY HALF ($50%$) of the error (splitting the difference between North and South).
   • Step 4 (Heading West - 270°): Turn aircraft precisely West ($270^\circ$). Note the remaining heading error. Adjust the E-W screw to remove EXACTLY HALF ($50%$) of the error (splitting the difference between East and West).
3. Recording Residual Deviation:
   • Taxi the aircraft around the compass rose in $30^\circ$ increments (000°, 030°, 060°, 090°, 120°, 150°, 180°, 210°, 240°, 270°, 300°, 330°).
   • Record the compass readings on the cockpit Magnetic Compass Correction Card (FOR: 000° STEER: 002°).
   • FAA Regulatory Limit (14 CFR §23.1327): The maximum allowable residual deviation on any heading after compensation must NOT exceed $10^\circ$.

5. Regulatory Inspections: 14 CFR §91.411 & Part 43 Appendix E

Federal Aviation Regulations establish strict inspection intervals and pneumatic leak tolerances for pitot-static and altimeter systems operating under Instrument Flight Rules (IFR).

                 REGULATORY INSPECTION INTERVALS & LEAK TOLERANCES

   14 CFR §91.411 ALTIMETER & STATIC SYSTEM          14 CFR §91.413 ATC TRANSPONDER
   Mandatory Every 24 CALENDAR MONTHS                Mandatory Every 24 CALENDAR MONTHS
   for Aircraft Operated in Controlled Airspace      for Any Transponder-Equipped Aircraft
   under Instrument Flight Rules (IFR)               Operating in Controlled Airspace
             │                                                 │
             ▼                                                 ▼
   ┌───────────────────────────────────┐             ┌───────────────────────────────────┐
   │ Part 43 Appendix E Static Test:   │             │ Part 43 Appendix F RF Transponder │
   │ • Unpressurized: 1,000 ft Suction │             │ • Mode A/C/S Reply Frequencies    │
   │   Max Leak ≤ 100 FPM over 1 Min   │             │ • Peak Pulse Power (>125 Watts)   │
   │ • Pressurized: Max Cabin Diff Alt │             │ • Altitude Encoder Digitizer Match│
   │   Max Leak ≤ 2% or 100 FPM        │             │   (Within ±125 ft of Altimeter)   │
   └───────────────────────────────────┘             └───────────────────────────────────┘

14 CFR §91.411 Inspection Scope & Testing Procedures

Under 14 CFR §91.411, no person may operate an airplane in controlled airspace under IFR unless, within the preceding 24 calendar months, each static pressure system, each altimeter instrument, and each automatic pressure altitude reporting system has been tested and inspected in accordance with 14 CFR Part 43 Appendix E.

Static Pressure System Leak Test Standards (Part 43 Appendix E)

1. Unpressurized Aircraft Static Leak Test:
   • Ensure all static ports, drains, and alternate static valves are sealed.
   • Connect a certified pitot-static test set to the static system port.
   • Evacuate the static line to apply a suction equivalent to 1,000 feet of altitude (approximately $1.07\text{ inHg}$ suction).
   • Seal the test set and monitor the altimeter for 1 minute.
   • PASS/FAIL CRITERIA: The static system leakage rate must NOT exceed 100 feet of altitude per minute.
2. Pressurized Aircraft Static Leak Test:
   • Evacuate the static system to a suction equivalent to the maximum certified cabin differential pressure for the aircraft.
   • Seal the system and monitor for 1 minute.
   • PASS/FAIL CRITERIA: The leakage rate must NOT exceed 2% of the equivalent altitude test pressure OR 100 feet per minute, whichever is greater.
3. Pitot System Leak Test:
   • Apply pitot pressure to produce an indicated airspeed of 150 knots (or normal cruising speed per maintenance manual).
   • Clamp the pressure source and monitor the ASI for 1 minute.
   • Leakage must not exceed 1 to 2 knots per minute (or zero leakage depending on airframe maintenance specifications).
4. Altimeter Bench Test Requirements (Part 43 Appendix E):
   • Scale Error: Altimeter scale accuracy tested at specified altitude intervals (e.g., within $\pm 20\text{ ft}$ at sea level, $\pm 50\text{ ft}$ at 10,000 ft, $\pm 100\text{ ft}$ at 20,000 ft).
   • Hysteresis & Friction: Tests wafer response lag during climb vs descent and mechanical gear friction under gentle tapping.
   • Case Leak Test: Instrument case evacuated to 18,000 ft; case leak must not exceed $100\text{ fpm}$.