Safety Investigation Report 2018:1 Factual Information/1.6/1.6.8 Aircraft Systems/10
SAFETY INVESTIGATION REPORT MH370 (9M-MRO)
The Navigation systems of interest include Global Positioning System (GPS), Air Data Inertial Reference System (ADIRS) and the Flight Management System (FMS).
- a) Global Positioning System
The Left and right GPS receivers are independent and use navigation satellites to supply very accurate position data to the FMC. One is powered by the 115V AC Standby bus and the other by the 115V AC Transfer bus. They pass data to aircraft systems including the ADIRS via the AIMS. GPS tuning is automatic. If the Air Data Inertial Reference Unit (ADIRU) becomes inoperative during flight, the EICAS displays the message NAV ADIRU INERTIAL and the FMC uses only GPS data to navigate.
- b) Inertial System
The ADIRS calculates aircraft altitude, airspeed, attitude, heading, and position data for the displays, flight management system, flight controls, engine controls, and other systems. The major components of ADIRS are the ADIRU, Secondary Attitude and Air Data Reference Unit (SAARU), and air data modules. The ADIRU supplies primary flight data, inertial reference, and air data. The ADIRU is fault-tolerant and fully redundant. The SAARU is a secondary source of critical flight data for displays, flight control systems, and other systems. If the ADIRU fails, the SAARU automatically supplies attitude, heading, and air data. SAARU heading must be manually set to the standby compass magnetic heading periodically. The ADIRU and SAARU receive air data from the same three sources. The ADIRU and SAARU validate the air data before it may be used for navigation. The three air data sources are the left, centre, and right pitot and static systems.
- c) Flight Management System
The FMS aids the flight crew with navigation, in-flight performance optimisation, automatic fuel monitoring, and cockpit displays. Automatic flight functions manage the aircraft lateral flight path (LNAV) and vertical flight path (VNAV). The displays include a map for aircraft orientation and command markers on the airspeed, altitude, and thrust indicators to help in flying efficient profiles. The flight crew enters the applicable route and flight data into the CDUs. The FMS then uses the navigation database, aircraft position, and supporting system data to calculate commands for manual and automatic flight path control. The FMS tunes the navigation radios and sets courses. The FMS navigation database supplies the necessary data to fly routes, SIDs, STARs, holding patterns, and procedure turns. Cruise altitudes and crossing altitude restrictions are used to calculate VNAV commands. Lateral offsets from the programmed route can be calculated and commanded.
The basis of the flight management system is the flight management computer function. Under normal conditions, one Flight Management Computer (FMC) accomplishes the flight management tasks while the other FMC monitors. The second FMC is ready to replace the first FMC if system faults occur. The FMC uses flight crew-entered flight plan data, aircraft systems data, and data from the FMC navigation database to calculate aircraft present position and pitch, roll, and thrust commands necessary to fly an optimum flight profile. The FMC sends these commands to the autothrottle, autopilot, and flight director. Map and route data are sent to the NDs. The EFIS control panels select the necessary data for the ND. The mode control panel selects the autothrottle, autopilot, and flight director operating modes.
Crew Procedure on the operations and programming of the Flight Management System safeguards and protects against incorrect execution of erroneous Information for the Navigation and Performance Data Input. Different levels of verification and cross checking between the Captain and Co-Pilot ensure that any error would be captured and corrected during the crew preparation.
In addition, system logics will also prevent the crew against selection of the wrong co-ordinates from the stored Navigation Database if a particular waypoint code happens to be used by many different places worldwide.