Sensors and Instruments

Understanding Sensors and Instruments for ATPL Students

Aircraft sensors and instruments translate complex physical phenomena into reliable cockpit indications that underpin safe flight. For ATPL theory and practical procedures, this domain spans attitude and navigation instruments, radio and radar-based sensors, engine and vibration monitoring, smoke detection, and the electrical generation and conversion equipment that powers avionics. Aviation regulations require redundancy, accuracy, and specific alerting logic, so pilots must understand both the indications and the system architecture that produces them.

Gyroscopic instruments illustrate how sensor physics affects indications. An air-driven artificial horizon (vacuum-driven attitude indicator) is subject to acceleration errors: during the take-off run, longitudinal acceleration can cause a false nose-up indication due to pendulosity and gyroscopic precession—hence the need to cross-check with the airspeed indicator and outside references. For turn coordination, the needle-and-ball (turn-and-slip/turn coordinator) separates turn rate from lateral balance. If the needle shows right and the ball is displaced left, the aircraft is in a skidding right turn—too much right rudder for the bank angle. The correct procedure is to “step on the ball” to center it and maintain coordinated flight, consistent with ATPL handling techniques and instrument cross-check.

Radio/radar-based height sensing is equally procedural. A radio altimeter (RA) operates in the SHF band—typically 4.2–4.4 GHz—and provides height above ground level (AGL), feeding systems like GPWS/EGPWS and autoland. To ensure the RA reads zero when the main wheels touch down, maintenance and avionics procedures include compensation for residual height and cable length (zero-foot or landing gear height correction). Safety systems also depend on robust detection logic: cargo compartment smoke detection commonly uses multiple sensors with a two-sensor agreement (e.g., C1 and C2) to trigger an alert, minimizing false alarms and aligning with Part 25/CS-25 requirements and airline SOPs for fire warning, isolation, and extinguishing.

Reliable electrical power and engine sensing round out the picture. Transport-category aircraft predominantly use 115 V, 400 Hz three-phase AC; three separate stator windings are spaced 120° apart, and an APU-driven generator can supply the same bus for ground or in-flight backup. DC-to-AC inverters power AC loads from DC sources, while generator protection isolates faults by opening the generator breaker and the exciter control relay. In DC machines, reversing a shunt motor’s direction is achieved by reversing the polarity of either the field (stator) or the armature—not both. A capacitor across breaker points prevents arcing in contact-type ignition systems. Battery management matters too: two 12 V/40 Ah batteries in parallel provide 12 V/80 Ah, maintaining voltage while increasing capacity. Engine health monitoring uses accelerometers; their signals are amplified and filtered before cockpit indication. Thrust instruments like EPR display the ratio of turbine outlet total pressure to compressor inlet total pressure, and their accuracy rests on principles of electromagnetic induction—the Lorentz force acting on charge carriers as conductors cut magnetic field lines—fundamental to generators and many aircraft sensors.

What this question bank covers

• Attitude and turn instruments: acceleration errors, needle-and-ball interpretation, coordinated flight procedures.
• Radio altimeters: SHF operation, AGL measurement, zero-foot adjustments, integration with GPWS/autoland.
• Smoke detection: multi-sensor logic, alert thresholds, regulatory context and cockpit procedures.
• Electrical power: 400 Hz AC systems, 3-phase generator geometry, APU power, inverters, generator fault protection.
• DC machines and components: reversing shunt motors, breaker-point capacitors, battery series/parallel effects.
• Engine systems: vibration monitoring signal conditioning, EPR sensing and indication.
• Electromagnetic fundamentals: Lorentz force and induction as they apply to aircraft systems and sensors.