Flight Management Navigation -
Flight Management Computer (FMC)
Differences between Basic, PIP and Pegasus FMCs, and between Standard and Hybrid CDUs
There are three types of FMC available for the Boeing 757:
Basic FMC The Basic FMC has a navigation database capacity of 200K.
PIP FMC The PIP (Product Improvement Program) FMC is essentially the same as a Basic FMC, except that it has a navigation database capacity of 1-Meg.
Pegasus FMC The Pegasus Advanced FMC has a navigation database capacity of 1-Meg and uses GPS updating in preference to DME-DME updating, thus maintaining a higher degree of accuracy than basic and PIP FMCs.
There are two types of CDU available for the Boeing 757:
Standard CDU The Standard CDU can only be used for passing information to and from the FMCs.
Hybrid CDU The Hybrid CDU has its own 'Alternate Navigation System' that can be used in the event both FMCs fail.
Note: Both the Standard and Hybrid CDUs can be used to control all three types of FMC.
FMC Databases
The FMC contains two or three databases:
- Performance database.
- Navigation database.
- PEGASUS - Airline Modifiable Information (AMI).
The performance database supplies all the necessary performance data to the flight crew. It supplies the FMC with the necessary data to calculate pitch and thrust commands. All necessary data can be shown on the CDU. The database includes:
- Airplane drag and engine characteristics.
- Maximum and optimum altitudes.
- Maximum and minimum speeds.
The crew can enter correction factors for drag and fuel flow to refine the database. The navigation database includes most data usually found on navigation charts. This data can be shown on the CDU or HSI. The database contains:
- The location of VHF navigation aids.
- Airports.
- Runways.
- Other airline selected data, such as sids, stars, approaches, and company routes.
- Transition altitudes.
The FMC contains two sets of navigation data, each valid for 28 days. Each set corresponds to the usual navigation chart revision cycle. The FMC uses the active data for navigation calculations. The contents of the navigation database are periodically updated and are transferred to the FMC before the expiration date of the active data.
On PEGASUS equipped airplanes, the Airline Modifiable Information (AMI) file contains airline specified data. If the FMC senses a conflict in an AMI value after a new AMI data load, the scratchpad shows the message CHECK AIRLINE POLICY.
Thrust Management
The Autothrottle is controlled by the Thrust Management Computer. When VNAV is engaged, the FMC controls the autothrottle by setting the command speeds and thrust reference modes on the thrust management computer.
When VNAV is not engaged, the thrust management is controlled by the flight crew as part of the AFDS system.
Fuel Monitoring
The FMC receives fuel data from the Fuel Quantity System or from manual entries. Fuel quantity values are shown on the PERF INIT page as calculated (CALC), MANUAL, or SENSED. They are shown on PROGRESS page 2 as TOTALIZER and CALCULATED. TOTALIZER and SENSED values are the same data with different names.
The FMC usually uses the calculated value for performance computations. Before engine start, the calculated value is set to agree with the Fuel Quantity indicating system value. When the FMC receives a positive fuel flow signal at engine start, the calculated value is independent of the fuel quantity system and decreases at the fuel flow rate.
The FMC will accept manual entry of the Fuel Quantity. The line title changes to MANUAL and the manual value is then updated by fuel flow rate. When the fuel quantity calculations are based on a manual entry the FUEL QUANTITY DISAGREE message is inhibited. Deleting the manual entry resets the value to equal the totaliser and the title returns to CALCULATED.
If fuel is loaded with the engines running the calculated value will not include the new fuel loaded. Normal operation can be restored by making a manual fuel quantity entry on the Performance Initialization page followed by deletion of the manual entry.
The calculated value is invalid if fuel flow data is invalid. The FMC uses the Fuel Quantity indicating system quantity for performance computations. The line title on the PERF INIT page changes to SENSED and is shown as TOTALIZER on PROGRESS page 2. The fuel used by each engine is calculated with its related fuel flow signal. FUEL USED is also shown on PROGRESS page 2.
Fuel used is reset to zero on the ground after flight when electrical power is removed or when the FMC receives a positive fuel flow at the next engine start.
The scratchpad shows the message FUEL DISAGREE-PROG 2 if the FMC calculates a large difference between the Fuel Quantity indicating system quantity and calculated value. The flight crew should select PROGRESS page 2, and select the fuel value for the FMC to use through landing.
On older FMCs the message reads FUEL QTY ERROR-PROG 2/2.
Note: The FUEL DISAGREE message is inhibited if the fuel quantity on the PERF INIT page is entered manually. Deleting a manual entry sets the fuel quantity back to the totaliser value, changes the line title back to CALC and enables the FUEL DISAGREE message.
The FMC continually estimates the fuel at the destination airport if the active route is flown. The CDU message INSUFFICIENT FUEL is shown if the estimate is less than the fuel reserve value entered on the PERF INIT page.
Note: FMC calculated fuel predictions assume a clean configuration. Flight with gear or flaps extended cause fuel prediction errors. Fuel predictions are accurate after the gear and flaps are retracted.
Loss of FMC Electrical Power
The FMC must have continuous electrical power to operate. When the electrical power is interrupted and returns, the FMC automatically restarts.
After the restart, the performance data shown on the PERF INIT page must be re-entered. The route previously in use is available but must be re-activated.
The flight crew must modify the active waypoint to engage LNAV. Select the applicable active waypoint and proceed direct or intercept a course to the waypoint.
FMC Failure
FMC Synchronisation
If both FMCs are operating, they operate independently but occasionally compare data. If the comparison fails to meet established tolerances, or an FMC detects a potential fault, a resynchronisation is initiated. (Actual faults may result in multiple resynchronisations followed by one FMC shutting down.)
With the NAV selectors in the vertical positions, one FMC is designated the master and the other is the slave. The Left FMC is the master except when either:
- the right autopilot is in command or
- all autopilots are disengaged, the left flight director is OFF and the right flight director is ON. (When operating in LNAV and/or VNAV mode the master FMC provides commands to the AFDS and autothrottle.)
In most cases the master FMC resynchronises the slave FMC. However, if the master detects a potential fault the slave resynchronises the master. During a resynchronisation one FMC stops supplying data while the other FMC loads it with new data. Resynchronisations normally take about 15 seconds, but can take longer if a CDU key is pressed.
Any of the following flight deck indications may appear during a resynchronisation:
- The FMC Light illuminates and EICAS displays FMC MESSAGE and perhaps L or R FMC FAIL.
- One CDU displays the RESYNCING OTHER FMC message while the other CDU display is frozen, or displays the MENU page and TIMEOUT - RESELECT message.
- The SINGLE FMC OPERATION message may appear.
- The HSI on the side being resynchronised displays the VTK flag.
If the slave resynchronises the master while in LNAV and VNAV mode, the autothrottle disengages and the AFDS indicates a mode failure (amber line through mode annunciation, F/D bar removed and an autopilot caution).
Single FMC Failure
The scratchpad shows the message SINGLE FMC OPERATION after loss of a single FMC. The EICAS advisory message L/R FMC FAIL is displayed and the HSI MAP flag is displayed on the side with the failed FMC.
The crew member on the side with the failed FMC selects the opposite FMC with the NAV selector to regain CDU access to the operating FMC and HSI map displays. LNAV and VNAV, if engaged, stay engaged and all flight plan and performance data is kept.
ETOPs
The crew member on the side with the failed FMC selects the opposite FMC with the FMC switch on the instrument selector panel to regain CDU access to the operating FMC and HSI map displays. If the left FMC fails, use the right autopilot to enable LNAV and VNAV operation. If the right FMC fails, use the left or centre autopilot. All flight plan and performance data is kept.
Note: If the MENU page and the scratchpad message TIMEOUT RESELECT is shown, the FMC is no longer connected to the CDU. Use the <FMC prompt on the MENU page to connect the CDU to the FMC.
Dual FMC Failure
In the unlikely event that both FMCs fail, LNAV and VNAV are not available. The EICAS advisory messages L FMC and R FMC are displayed. The HSI MAP display is also not available.
PEGASUS - Selecting CDU-L or CDU-R with the NAV selector on the instrument select panel provides route data to the HSIs. Alternate Navigation using the CDUs is discussed in the next section.
Note: The CDUs display the MENU page but the < FMC prompt is not shown in line 1. Push the LEGS function key to show the IRS LEGS page and the PROG key to show the IRS PROGRESS page.
FMS Resets
PEGASUS - A software reset may occur in dual or single FMC operation. The active route becomes inactive, the performance data is erased, and LNAV and VNAV (if engaged) modes fail. To regain FMC operation, activate and execute the flight plan, enter the necessary performance data, and engage LNAV and VNAV.
