6.6 Instrument Systems: Pitot-Static, Gyros, and Displays

Instrument Systems: Indications Need Trustworthy Inputs

Aircraft instruments turn pressure, temperature, position, magnetic direction, rotation, electrical signals, and computer data into information for the flight crew. The Airframe ACS includes annunciators, magnetic compasses, pressure and temperature instruments, position indications, gyros, vacuum systems, pitot-static systems, fuel quantity, range markings, electronic displays, built-in test equipment, and static system leak-check requirements. The exam theme is that an indication is only useful when its source, installation, and calibration are trustworthy.

Pitot-static systems supply pressure information to instruments such as airspeed indicators, altimeters, and vertical speed indicators. A leak, blockage, damaged line, loose fitting, water intrusion, or incorrect connection can create misleading indications. Maintenance must protect openings, avoid forcing water or air into instruments improperly, and perform required leak checks by the approved procedure.

Use this instrument troubleshooting framework:

• Identify which indications are affected and what inputs they share.
• Check power, circuit protection, sensor source, plumbing, bonding, and connectors.
• Inspect pitot tubes, static ports, drains, alternate static source, lines, and fittings.
• Verify instrument range markings, placards, and installation suitability.
• Handle mechanical gyros gently and protect electronic units from electrostatic discharge.
• Use built-in test equipment when available, but confirm faults with maintenance data.
• Treat intermittent warnings as real until the cause is found.

Static system leak checks are important because a small leak can create altitude errors. The rules and procedures depend on aircraft operation and installed equipment, so the mechanic follows the applicable regulation and maintenance instructions. A technician should not improvise pressure or suction because instruments can be damaged by excessive pressure or rapid changes.

Gyroscopic instruments may be vacuum, pressure, or electrically driven. Mechanical gyros can be damaged by shock, contamination, incorrect suction, or mishandling during removal and installation. Vacuum filters and regulators influence gyro performance. Electronic flight displays add different concerns such as software configuration, cooling, connectors, bonding, and electrostatic-sensitive devices.

Annunciator systems communicate warning, caution, and advisory information. An intermittent light can be tempting to dismiss, but the ACS risk elements identify this as a maintenance risk. The fault may be a sensor, switch, wiring problem, lamp, display unit, power issue, or a real intermittent system condition. Troubleshooting should preserve evidence and follow the wiring and system logic.

Magnetic compasses require inspection for fluid, bubbles, leakage, security, markings, lighting if installed, and deviation correction as required. Compass swinging is not a casual adjustment; it follows a procedure and records deviations on a compass card.

For the exam, choose the answer that protects indication reliability. That usually means checking shared inputs, using the approved test set or procedure, preventing instrument damage, and documenting system condition after removal, installation, or adjustment.

Instrument removal and installation also create risk. Pinched lines, swapped connectors, loose mounting screws, missing bonding, blocked cooling, and incorrect range markings can create new faults. After any disturbance, the mechanic should verify security, configuration, indications, and required tests before deciding that the panel is airworthy.