Avionic interview prep

Embedded System

Computer system that is part of a larger system or machine

What are the key subsystems of a typical avionics suite?

• Communication (COMM): VHF/HF/SATCOM

• Navigation (NAV): VOR, ILS, GPS, Inertial Ref Systems

• Flight Control: Autopilot, Flight Management System (FMS)

• Displays: Primary Flight Display (PFD), Multi-Function Display (MFD)

• Monitoring: Engine Indication & Crew Alerting System (EICAS/ECAM)

• Surveillance: TCAS, Weather Radar, Transponder

ARINC 429

Point-to-point, unidirectional (12.5/100 kbps), deterministic, legacy standard.

AFDX

Switched Ethernet network (10/100/1000 Mbps), bidirectional, full-duplex, uses Virtual Links (VLs) for deterministic bandwidth and latency, modern standard (A380, B787).

DO-254

Assurance Guidance for Airborne Electronic Hardware

DO-178C

Software Considerations in Airborne Systems and Equipment Certification. Defines software levels (A-E, with A most critical) and lifecycle processes

RTCA/EUROCAE

Industry consortia that develop technical standards (e.g., DO-xxx, ED-xxx) for aviation

EASA

European Union Aviation Safety Agency (regulatory authority for Europe, like FAA in the US).

What is an LRU (Line Replaceable Unit)

A modular avionics component designed to be quickly swapped on the flight line for maintenance.

How does a VOR work?

Ground station transmits two signals: one omnidirectional, one rotating. The aircraft receiver compares the phase difference to determine its radial (line of position) from the station.

Localizer

Provides left/right guidance to the runway centerline (horizontal).

Glideslope

Provides up/down guidance for the ideal descent path (typically 3°).

GPS

Absolute position from satellites, accurate but susceptible to jamming/interference.

INS

Dead-reckoning using accelerometers & gyros. Self-contained, drifts over time, immune to external signals. Combined in an INS/GPS.

What is SATCOM used for?

Long-range, beyond line-of-sight communication for voice and data (ACARS, passenger connectivity, operational data).

Describe TCAS/ACAS.

Traffic Alert and Collision Avoidance System. Uses transponder signals to detect nearby aircraft. Issues TA (Traffic Advisory) and RA (Resolution Advisory – commands like "Climb" or "Descend") to avoid mid-air collisions.

PFD

Primary Flight Display – Shows attitude, airspeed, altitude, heading (the "basic T").

MFD

Multi-Function Display – Shows navigation maps, weather radar, systems status pages, etc.

What are the primary inputs to an Air Data Computer (ADC)

Pitot pressure (airspeed), static pressure (altitude), and temperature. Computes Calibrated Airspeed, True Airspeed, Pressure Altitude, Mach Number.

What does the FMS do?

The aircraft's "brain." Manages navigation (lateral & vertical flight plans), performance optimization, and interfaces with autopilot and auto-throttle.

Explain Fly-By-Wire (FBW)

Replaces mechanical flight controls with electronic interfaces. Pilot inputs are interpreted by flight computers which command actuators, providing stability augmentation and flight envelope protection.

Flight Control Law

Control algorithm in an FBW system (e.g., Pitch: C* law, Roll: Direct law). Can be normal, alternate, or direct based on system failures.

What is MIL-STD-1553?

A military-standard time-division multiplexed data bus. Has a Bus Controller (BC), multiple Remote Terminals (RTs), and optional monitors. Used in many military and some civilian aircraft.

deterministic

Guaranteed maximum latency and bandwidth. Critical for safety-critical systems where response time must be predictable (e.g., AFDX Virtual Links).

What is a Data Concentrator?

A unit that aggregates data from various sensors/subsystems and places it onto a high-speed data bus (like AFDX) for distribution.

What is the purpose of an IMA (Integrated Modular Avionics) architecture?

Replaces multiple dedicated LRUs with shared processing modules in a rack. Applications run on common hardware in partitions for resource efficiency, weight savings, and flexibility.

What is "partitioning" in IMA?

Strict isolation (in time and space) between software applications on the same hardware to prevent one application from affecting another (critical for safety and certification).

Explain redundancy in avionic systems

Using multiple, independent channels (2, 3, or 4) to perform the same function. A voting system (e.g., 2-out-of-3) isolates a failed channel and maintains operation.

Fail-Safe

System failure does not cause catastrophe; the aircraft can continue safe flight (may degrade capabilities).

Fail-Operational

The system remains fully functional after a single failure (requires at least dual redundancy).

Define BIT (Built-In Test)

Automated diagnostic tests performed by avionics units: PBIT (Power-Up BIT), IBIT (Initiated BIT), CBIT (Continuous BIT).

What is a FHA

Functional Hazard Assessment. A top-down analysis to identify failure conditions and classify their severity (Catastrophic, Hazardous, Major, Minor, No Effect).

Explain the concept of "derated" requirements

When a component (e.g., a power supply) is used below its maximum rated capacity to increase reliability, reduce heat, and extend lifespan

What are the DALs (Design Assurance Levels)?

DO-178C/DO-254 Levels:
- A (Catastrophic)
- B (Hazardous)
- C (Major)
- D (Minor)
- E (No Effect)

Dictates rigor of development/certification activities.

What is MC/DC and why is it important?

Modified Condition/Decision Coverage. A stringent structural code coverage criteria required for Level A software. Ensures all conditions independently affect the decision outcome.

What is a Requirements-Based Test?

A test case derived directly from a system/software requirement to verify that the requirement is correctly implemented. Traceability is mandatory for certification.

Explain the V-Model in avionics development

A development lifecycle:

• Requirements

• Design

• Implementation

• Integration

• Verification & Validation

Left side is decomposition, right side is integration and testing.

What is configuration management in avionics?

Controlling and tracking all changes to requirements, design, code, and documentation to ensure integrity and traceability throughout the product lifecycle

HIRF

High-Intensity Radiated Fields. External electromagnetic energy that can disrupt avionics. Designs must include shielding, filtering, and hardening to ensure immunity.

Lightning Protection

Use of conductive paths, bonding, and surge protection to safely route lightning current around the aircraft and prevent damage.

Why is 400 Hz AC power used in aircraft?

Allows for smaller, lighter transformers and motors compared to 50/60 Hz for the same power output. Critical for weight savings

What are the typical aircraft power buses?

• AC: 115V 400Hz (primary), 26V AC (for some instruments).

• DC: 28V DC (primary from Transformer Rectifier Units - TRUs), ±5V, ±15V for analog sensors, 3.3V/5V for digital logic.

What is a TRU?

Transformer Rectifier Unit. Converts 115V 400Hz AC power to 28V DC power.

An aircraft's weather radar display is blank. What could be the cause?

Faulty radar transceiver, antenna stabilization failure, incorrect mode selection, display system fault, or loss of cooling (radar generates heat).

How would you troubleshoot intermittent communication dropouts on an ARINC 429 bus?

Check connector integrity and grounding, inspect wiring for chafing, monitor bus with analyzer for signal quality/voltage levels, check LRU termination.

An ADC provides erroneous altitude readings. What are your first checks?

Check the static port for blockage (ice, tape, insects). Verify pitot-static system integrity for leaks.

An FBW aircraft enters alternate control law. What does this imply?

A failure has occurred in the primary systems. Flight envelope protections are lost or reduced, and control may revert to a simpler, direct mode.

During testing, a software module passes unit tests but fails integration. Why?

Interface mismatch (timing, data format), resource contention (memory, CPU), uninitialized global variables, or unhandled error conditions from other modules.

How do you ensure software with no single point of failure?

Implement hardware redundancy (multiple channels) and software diversity (different teams/algorithms), rigorous partitioning, and monitor/compare outputs with a voting mechanism.

What is the importance of traceability in avionics?

Provides an audit trail from requirements to design to code to tests. Essential for certification (proving you've met all safety requirements), impact analysis, and change management.

Verification

Did we build the system right?

Validation

Did we build the right system

Explain the concept of "worst-case execution time" (WCET) analysis.

Determining the maximum possible time a software task can take to execute. Critical for ensuring real-time performance and schedulability in safety-critical systems