Chapter 13: Digital Aircraft Systems II — ECAM, EICAS, EFIS, GNSS, TCAS, IMA, Cabin and Information Systems

ECAM — electronic centralised aircraft monitoring (Airbus)

ECAM monitors the engines and systems, shows their status on the two centre displays, and when something fails tells the crew what has failed, what to do about it and what the consequences are. Chapter 1 described where its displays sit; here is how it works.

• SDACs (system data acquisition concentrators) acquire system data — analogue, discrete and digital — and send it to the DMCs and FWCs.
• FWCs (flight warning computers) detect failures and abnormal configurations, generate the warning and caution messages with their procedures, the memos, and the aural alerts, and light the master WARNING and master CAUTION lights. They receive the most critical signals (such as engine fire) directly.
• DMCs (display management computers) generate the E/WD and SD images, as well as the PFDs and NDs.
• The ECAM control panel selects system pages and has CLR (clear), RCL (recall), STS (status) and EMER CANC (emergency cancel) keys.

• Display modes: the SD normally shows the page for the current flight phase automatically; a failure brings up the related system page (failure-related mode); a parameter drifting out of its normal range triggers an advisory; and the crew can call any page manually.
• The FWCs divide the flight into phases (ten on the A320) and inhibit less important alerts during critical phases such as take-off and landing, so the crew are not distracted.

EICAS — engine indication and crew alerting system (Boeing)

• The upper display shows the primary engine indications (N1 or EPR, EGT) and the crew alert messages; the lower display (or an MFD) shows secondary engine indications — N2, N3, fuel flow, oil pressure, temperature and quantity, vibration — and system synoptics, status and maintenance pages.
• EICAS computers (two on the 757/767; functions of the AIMS cabinets on the 777; the common core system on the 787) generate the displays and alerts.
• Alert levels: warnings (red, master WARNING light and an aural — bell, siren or voice — immediate action), cautions (amber, master CAUTION light and a beeper — prompt action), advisories (amber, indented, no light or aural — action when time permits), memos (white reminders of normal states such as PARKING BRAKE SET), status messages (which affect dispatch, linked to the MEL) and maintenance messages (ground only).
• If a display fails, a compacted format squeezes the essential engine data onto the remaining screen.

EFIS as a system

• EFIS consists of the display units, the symbol generators or DMCs, the EFIS control panels and the source-selection switching (Chapter 1).
• BITE: display units and generators test themselves continuously. A display test (from the EFIS control panel or the maintenance system) draws a test pattern and lists faults. Comparator monitors check that both pilots' attitude, heading, altitude and speed agree and flag a disagreement (for example an amber ATT, HDG or ALT comparison message).
• Faults are reported to the central maintenance system; most are cleared by replacing a DU or generator, but check cooling and connectors first.

GNSS — global navigation satellite systems

• Constellations: the US GPS (at least 24 satellites in six orbital planes, about 20 200 km up, orbiting twice a day), Russia's GLONASS, Europe's Galileo and China's BeiDou. Each system has a space, a control (ground stations) and a user segment.
• Principle: each satellite broadcasts its orbit data and a time from its atomic clock. The receiver measures each signal's travel time — the pseudo-range. Four satellites are needed for a three-dimensional position plus the receiver's clock error; the result is also an extremely accurate time reference.
• Integrity: RAIM (receiver autonomous integrity monitoring) uses spare satellites to cross-check — five to detect a faulty satellite, six to exclude it. SBAS (satellite-based augmentation — WAAS in the USA, EGNOS in Europe, GAGAN in India) broadcasts corrections and integrity data from geostationary satellites, allowing approaches with vertical guidance (LPV). GBAS does the same from a ground station at an airport for GLS approaches.
• Installation: antennas on top of the fuselage with a clear view of the sky; a stand-alone receiver, a multi-mode receiver (MMR, combining ILS, GLS and GPS), or a receiver inside the IRS for a hybrid GPS/inertial solution. GNSS feeds the FMS, IRS, terrain awareness, ADS-B, clocks and the ELT.
• Limitations: weak signals that can be jammed or spoofed, satellite geometry, ionospheric effects, and antenna masking in steep turns. Antennas must not be painted over except with approved paint, and must be correctly sealed and bonded.

TCAS — traffic alert and collision avoidance system

• TCAS II (the ICAO standard ACAS II, current version 7.1) is independent of ground ATC. It interrogates the transponders of nearby aircraft on 1030 MHz and receives their replies on 1090 MHz, measuring range from the reply time, bearing with a directional antenna, and relative altitude from the altitude each transponder reports.
• From range and closure rate it computes tau, the time to the closest point of approach. A traffic advisory (TA) — "TRAFFIC, TRAFFIC" — is given roughly 20 to 48 seconds before; a resolution advisory (RA) — "CLIMB, CLIMB", "DESCEND, DESCEND", "LEVEL OFF", "MONITOR VERTICAL SPEED" — roughly 15 to 35 seconds before. RAs are vertical manoeuvres only, shown on the PFD or VSI, and are coordinated with the other aircraft's TCAS through the Mode S data link so that the two manoeuvres complement each other.
• Aircraft with Mode C or Mode S transponders give full information; a Mode A-only transponder gives no altitude, so only a TA is possible; an aircraft with no transponder is invisible to TCAS.
• Components: TCAS computer, a directional antenna on top (and below — directional or omni), the Mode S transponders, a control panel (STBY, XPDR, TA ONLY, TA/RA), traffic display on the ND, RA guidance on the PFD or VSI, and aural alerts through the audio system.
• Close to the ground RAs are progressively inhibited — below about 1 000 ft above ground TCAS gives TAs only. A self-test from the control panel shows a test pattern and announces "TCAS test pass" or fail.

Integrated modular avionics (IMA)

• In a federated architecture every function has its own LRU with its own processor, power supply, inputs and outputs. In IMA, standard processing modules in cabinets or racks host many applications, while remote data concentrators or I/O modules near the sensors and actuators convert signals onto the network.
• Applications are partitioned in time and memory (ARINC 653), so that a fault in one cannot affect another hosted on the same module.
• Examples: the Boeing 777 AIMS (Airplane Information Management System) cabinets on ARINC 629; the Airbus A380 and A350 CPIOMs (core processing input/output modules) on AFDX; the Boeing 787 Common Core System with general processing modules and remote data concentrators on its ARINC 664 network.
• Benefits: less weight, volume, wiring and power; fewer spare part numbers; new functions added by software. Maintenance: loading hosted applications and configuration tables (strict software control), module replacement, and fault isolation through the central maintenance function.

Cabin systems

• Cabin intercommunication data system (CIDS) — on Airbus aircraft: two directors (one active, one standby) drive decoder/encoder units on cabin data buses, providing passenger address, cabin and service interphone, passenger calls, the no-smoking and seat-belt signs, cabin and reading lights, lavatory smoke indications, evacuation signalling, and water and waste indications. The flight attendant panel controls it. Its cabin layout is held in a programmable module — airline-modifiable data under configuration control. Boeing aircraft have equivalent cabin services systems.
• In-flight entertainment and connectivity: head-end servers, seat electronic boxes and displays, audio/video on demand, and cabin Wi-Fi linked to the ground by satellite.
• Cabin systems are kept in network domains separated from the aircraft control systems (Chapter 4), and their BITE reports to the cabin or central maintenance system.

Information systems

• Onboard information or network server systems (ATA 46) host electronic documentation, the electronic technical logbook, maintenance applications and data loading, and connect terminals in the flight deck, cabin and maintenance areas.
• The electronic flight bag (EFB) — portable or installed — replaces paper charts, manuals and performance calculations.
• Wireless ground links (gate Wi-Fi or cellular) upload databases and download flight data, ACMS reports and quick-access recorder data at the gate.
• Because these systems connect to the outside world, they rely on security: domain separation, firewalls, controlled software and approved devices only.