B737 Navigation

Navigation Systems

• FMS
• Radio Navigation

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.

Radio Navigation

The aircraft has two DME systems installed. These are auto tuned by the FMC for necessary position updating. Geographical position can be acquired from dual DME or from a collocated VOR/DME.

The status of FMC calculations based on recievers can be seen on the NAV STATUS page.

The aircraft is also equipped with two ISL recievers.

IRU

Inertial Reference Unit

The inertial reference system is comprised of the IRU and the Mode Select Unit, and computes aircraft position, ground speed, and attitude data for:

• DU
• FMC
• Auto flight
• Other system

It consists of a self-contained system once fully initialised. It allows the aircraft to continue navigating even if all other sources of path information are lost.

There are two IRS installed on the aircraft, labelled Left IRS and Right IRS.

The IRU itself provides all information required for avionics:

• Pitch and roll
• Heading
• Acceleration
• Wind data
• Calculated data, including:
  • Flight path angle
  • Roll/Pitch rates
  • Vertical speed
  • Yaw data

Its primary sources for data are 3 internal laser gyros and 3 inertial accelerometers.

Thus the only external inputs required for the IRS are:

• Initial Position
• Barometric altitude to stabilize vertical navigation
• True Airspeed to calculate wind speed and direction

Alignment process

Normal alignment is initiated by placing the IRS Mode Selector switch to NAV. After a short power test, the ALIGN light illuminates. It must be fully aligned prior to entering navigation mode.

Note that ADIRU alignment must not be attempted at latitudes greater than 78 degrees 15 minutes.

To initalize position, use the CDU via the POS INIT page. IRS alignment can be autofilled from GPS data, or by manually entering coordinates. Alignment times varies from 5 to 17 minutes, depending on position. Between stops, it is possible to achieve a 30-second position update by placeing the mode select switch on ALIGN and manually entering the GPS coordinates on teh IRS Display panel.

Full alignment is completed once the ALIGN lights extinguish and the PFD and Navigation Display provide all appropriate information with no flags.

ATT mode is used to level the system to provide attitude andd possibly heading information in case of a navigation failure.

Power sources

Each IRU has two power sources: a 115V AC and a 28V DC. Although each is sufficient for operation, the 28V DC power is a back-up source.

The left IRU is powered by the AC Standby Bus, while the right IRU is powered by the AC Transfer Bus 2.

Either or both systems can be switched to the Hot Battery Bus. When this occurs, however, the right IRU will only remain active for 5 minutes.

Link to original

GPS

The aircraft is equipped with 2 independent GPS receivers that provide information to the FMC/FMS. Their data is also provided to various other systems.

GPS operation is automatic, but it can be manually selected off on CDU NAV PAGE 2.

With the FMC Source Select in the NORMAL position, both CDUs are controlled by the ___ FMC.::Left. RNP values can be manually entered into the CDU by the flight crew.::True. What is displayed on the CDU scratchpad if ANP exceeds the limit RNP value during an approach?::UNABLE REQD NAV PERF-RNP. It is also displayed on the Navigation Display. Actual Navigation Performance is the FMC’s estimate of the quality of its position determination.::True. The FMC select ___ first as the primary update to the aircraft’s position.::GPS. The IRS ON DC light illuminates when::The IRS is being powered by the switched hot battery bus. At cruise altitude, an amber GPS light illuminating with an IRS annunciator and MASTER CAUTION indications means what?::Both GPS receivers have failed. What cause loss of IRS alignment?::1) AC and DC power failure, 2) Aircraft moved during alignment, 3) IRS Mode Selector switch moved to OFF.