5.1 — Electronic Instrument Systems

Electronic instrument systems have replaced traditional analogue "steam gauge" instruments in modern aircraft. These systems use computers, digital data buses, and electronic displays to present flight, engine, and systems information to the crew. Understanding the architecture, components, and layout of these systems is essential for any aircraft maintenance engineer working on current-generation aircraft.

Evolution from Analogue to Digital

Traditional cockpits used individual electromechanical instruments — each driven by a dedicated sensor and displaying a single parameter on a dial or pointer. A typical analogue cockpit might contain over 100 individual instruments.

Modern glass cockpits replace these with a small number of large electronic displays driven by digital computers. This provides:

• Reduced weight and space — fewer instruments, less wiring.
• Improved reliability — fewer moving parts.
• Flexibility — displays can be reconfigured, and information presentation can change with flight phase.
• Reduced pilot workload — integrated displays combine related information.
• Built-in test — digital systems can continuously monitor themselves and report faults.

Typical Glass Cockpit Layout

A modern transport aircraft cockpit typically features the following main displays:

Display	Abbreviation	Function	Typical Location
Primary Flight Display	PFD	Attitude, airspeed, altitude, vertical speed, heading, flight director, ILS deviation	Directly in front of each pilot
Navigation Display	ND	Map, route, waypoints, weather radar, TCAS traffic, VOR/DME information	Beside each PFD (inboard)
Engine/Systems Display	EICAS or ECAM	Engine parameters (N1, N2, EGT, fuel flow), crew alerting, systems status	Centre panel, upper
Multi-Function Display	MFD	Secondary systems pages, checklists, synoptic diagrams	Centre panel, lower

EFIS — Electronic Flight Instrument System

EFIS is the system that generates and displays the primary flight information. A typical EFIS installation includes:

• Display units — LCD screens showing PFD and ND information.
• Symbol generators (SGs) — computers that receive sensor data, process it, and generate the display imagery. Each PFD and ND has its own SG for redundancy.
• EFIS control panel — allows the pilot to select ND range, mode (MAP, VOR, ILS, PLAN), and display options (weather, traffic, terrain).
• Sensor inputs — air data computers (ADCs), inertial reference systems (IRS), radio navigation receivers, FMS.

EICAS and ECAM

System	Manufacturer	Primary Function	Key Feature
EICAS	Boeing	Engine parameters + crew alerting	Messages colour-coded by severity (red = warning, amber = caution, white = advisory)
ECAM	Airbus	Engine parameters + crew alerting + system synoptics	Automatically displays the relevant system page when a fault occurs; provides interactive checklists

Redundancy and Reliability

Electronic instrument systems use multiple levels of redundancy:

• Dual (or triple) symbol generators — if one fails, another takes over.
• Display switching — any display unit can show information from any SG.
• Independent power supplies — displays fed from different electrical buses.
• Standby instruments — independent analogue or self-contained electronic instruments for attitude, airspeed, and altitude.
• BITE (Built-In Test Equipment) — continuous self-monitoring with fault logging for maintenance.