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Mission Statement 

The purpose of FLAPS-2-APPROACH is two-fold:  To document the construction of a Boeing 737 flight simulator, and to act as a platform to share aviation-related articles pertaining to the Boeing 737; thereby, providing a source of inspiration and reference to like-minded individuals.

I am not a professional journalist.  Writing for a cross section of readers from differing cultures and languages with varying degrees of technical ability, can at times be challenging. I hope there are not too many spelling and grammatical mistakes.


Note:   I have NO affiliation with ANY manufacturer or reseller.  All reviews and content are 'frank and fearless' - I tell it as I see it.  Do not complain if you do not like what you read.

I use the words 'modules & panels' and 'CDU & FMC' interchangeably.  The definition of the acronym 'OEM' is Original Equipment Manufacturer (aka real aicraft part).


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If you see any errors or omissions, please contact me to correct the information. 

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Reference Nav Data - CDU Functionality Explained

In past posts, I’ve documented some of the functionality of the Flight Management Computer (FMC) as displayed by the Central Control Unit (CDU).  Following on with this theme, let’s look at four navigation data functions the FMC is capable of: Reference Nav Data, Nav Options, Nav Status and Nav Frequency Changes.

Before continuing, the FMC/CDU is controlled by the avionics suite you are using; whether it is ProSim737, Sim Avionics or whatever.  Each avionics suite provides differing functionality; therefore, if something does not operate as indicated, it maybe a limiting factor of the avionics suite in use.

Note:  This post follows standard terminology.  lsk3R means line select key 3 right.



Occasionally, you will need to cross check information and the frequency of a specific navaid.  

The Reference Nav Data display is part of the Nav Data page and can be assessed by the INDEX page:


The screen will show three available options: Enter WPT Ident, Navaid Ident and Airport Ident.

Example:  Type HB into the navaid Ident.  Two pages will be displayed showing all the HB Idents from the navigation database.  Selection of the appropriate navaid (HB) will present a further page displaying the following information:  Navaid WPT, Airport and Ident code, Latitude, Longitude, Frequency, Elevation and magnetic variance.

NOTE:  If you cannot identify the ident by name use the Longitude and Latitude coordinates.



Following on from the Reference Nav Data page are:  Nav Options and Nav Status.

Nav Options and Nav Status can be assessed two ways:

2:  PROG (progress) / NAV STATUS (lsk6R)  (use when in flight)

Two consecutive pages are available: Nav Options and Nav Status.  By default, Nav Status (page 2/2) is displayed.  Use the PREV and NEXT PAGE keys to cycle between the two pages.

Nav Status - page 1/2

This page provides you with a list of the closest navaids including frequencies.  It also indicates the currently set identifier and frequency for NAV 1 and NAV 2 (as set on the NAV 1/2 radio).

Nav Options - page 2/2

This page can be used to inhibit a particular waypoint or station.  By inhibiting a navaid, it will not be able to be used by the CDU to calculate a navigation solution.  By default all navaid types are activated.  At crew discretion, two VOR and two DME stations can be inhibited.  When you inhibit a navaid it will be removed from page 1/2 and not be visible in the Nav Status page list.  The inhibited navaid will be reset when you reset the CDU.  



In usual practice, crew will alter the navigation, communication, ADF and transponder frequency on the actual panel located in the central pedestal.  However, often you may need to cross check frequencies, dial in a third frequency for positional awareness, or use a frequency from an avionics module not present in the pedestal or that is malfunctioning.

The alter Nav Data screen can be assessed by:

MENU / MAINT (lsk6R) / COM/NAV (lsk3L)

This will display a page showing all idents and frequencies currently being used.

To alter a frequency, type into the scratch pad the frequency of the navaid and upload to the appropriate line.  To upload, select and press the key to the left or right of the nominated radio.  Changing a frequency in the CDU will also cause a corresponding change in the frequency of the selected radio (in the center pedestal).

Flow Route

When you work through the above four functions of the CDU, you will note that the INDEX function is always available.  This allows you to easily develop a flow route as you move between the various pages.  Once you know how the flow route operates, you will discover that the CDU is very much like a book with several hundred pages of information that is easily assessable via a few “select” menu keys.

As with all my posts, if you discover a discrepancy please contact me so it can be rectified.

BELOW:  Montage of images from the CDU showing various pages displayed within the Reference Nav Data.  CDU is manufactured by Flight Deck Solutions (FDS).  Click image to see larger.

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