FMS

Flight Management System

FMC Index

• General
• Control and management
• Databases
• VNAV
• RNP
• CDU

General

The FMS is comprised of the following systems: ?

• Flight Management Computers
• Autopilot Flight Director System
• Autothrottle
• Inertial Reference Units
• GPS

The term FMS refers to the concept of joining these independent systems to provide automatic navigation, guidance, and performance management.

The 737 is equipped with dual FMCs, although it is certified for flight with a single FMC. Thus, a secondary FMC is on board that can provide complete navigational functions if the other should fail.

In flight, the FMC calculates position from GPS, navigation radios, and the IRUs. Priority is based on availability of valid data. Position updates from navigation sensor positions are used in the following priority order:

1. GPS
2. DME
3. VOR/DME
4. Loc/DME
5. Localizer

If GPS and all ground-based navigation systems fail, FMC navigation is based solely on IRU position.

Control and management

The primary controls for the FMS are:

• Mode Control Panel
• Control Display Units
• EFIS Control Panels
• FMC Source Selection Switch

The FMS uses a present position to base all navigation guidance. In combination with a database and preflight entries, the FMC can produce accurate calculations. The FMC uses the following variables for computation:

• Gross Weight
• Zero Fuel Weight
• Planned Cruise Altitude
• Temperature and winds aloft
• Centre of Gravity
• Performance management

The FMC will automatically transition between phases of flight in the following order:

1. Preflight
2. Takeoff
3. Climb
4. Cruise
5. Descent
6. Approach
7. Flight Complete

Using the LEGS page the crew can view all of the individual waypoint segments to be used during flight.

The FMC calculates airplane present position, pitch, roll, and thrust commands to fly an optimum flight profile. The commands are sent to the Auto-throttle, flight director, and autopilot.

Internal databases

Performance database

The FMC contains a performance database that eliminates the need for pilots to refer to a performance data manual during flight. It includes:

• Airplane Drag and Engine characteristics
• Maximum and optimum altitudes
• Max and minimum airspeeds
• Fuel computations
• Engine limits
• Anti-ice bleed corrections

Navigation database

In addition to the performance database, the FMC contains a navigation database that includes:

• Location of VHF Nav Aids
• Waypoints
• Airports
• Runways
• SIDs an STARs
• Approaches
• Company Routes

VNAV

VNAV provides vertical guidance during climb, cruise, and descent phases of flight. The FMC and AFDS work in conjunction to calculate and control altitude, airspeed, and thrust lever position. Note that while flying in VNAV, the aircraft will adhere to altitude restrictions set in the LEGS page, regardless of MCP selected altitude unless commanded to do so with DES NOW or ALT INTV (altitude intervention).

Although VNAV is capable of managing most aircraft configuration targeting an idle descent from TOC to touchdown, the CDU may display a DRAG REQUIRED if an unexpected tailwind results in a significant increase in airspeed to maintain path. On the other hand, a DES PATH UNACHIEVABLE message may be displayed if the planned profile cannot be accomplished.

VNAV reverts to VNAV SPD if a limit speed will be exceeded.

RNP

RNP is the accuracy standard an aircraft mus adhere to while navigating through specific airspace. Specific environments require a specific set RNP value.

The FMC supplies default Required Navigation Performance values as follows:

Environment	RNP
Oceanic	12
En Route	2
Terminal	1
Approach	.3

The FMC will also report ANP, or Actual Navigation Performance. This is the FMC’s estimate of the quality of its position determination. It is represented by the word ACTUAL on the FMC, and represents the estimated maximum position error with 95% probability.

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