Control Display Unit
The CDUs are the primary means for interaction between the flight crew and the FMC.
Each CDU page displays up to 6 data lines, along with respective line titles. Line select keys are used to modify the data on any given line; selecting it copies the data into the scratchpad so that they can be modified.
It is also possible to enter data into the scratchpad first and then press a line key to copy the data into that line.
Note that thrust reductions for climb are capped at 25% N1. CLB-1 indicates a 3% thrust reduction (or 10% N1), CLB-2 indicates a 6% reduction (or 20% N2).
Active CDU data being used by the autoflight system is displayed in magenta.
CDU Conventions
Data entry is accomplished using different methods:
- Via scratchpad with alphanumeric keys
- Transferred to scratchpad from a data line
Data from the scratchpad is transferred to a data line by selecting the adjacent LSK.
Note that the scratchpad must be empty in order to use the LSKs to copy data into it. Additionally, typing characters will overwrite FMC-generated messages on the scratchpad.
Pages containing boxes indicates that data entry into that field is mandatory, such as the PERF INIT page during preflight since performance calculations are not possible until entries are made into these fields.
Dash prompts indicate data that is either:
- Optional
- Used for additional calculation or information
Large fonts displayed on the CDU indicate manually entered information or mandatory altitude restrictions. Small fonts indicate predicted altitude values. Note that only large font altitude data values can be deleted.
When large font values are deleted, they are replaced by a predicted value based on constant angle descents.
Entering altitudes can be done in the FL200 or simply 200 format. For altitudes below 1,000 ft a leading zero is required.
An A suffix indicates altitudes to be flow at-or-above. B suffixed indicate at-or-below. For restrictions between to altitudes enter lower limit first, then upper limit, i.e. 100A120B.
Multiple element entries can be placed as a singly entry by separating fields with a slash. For instance, a cruise wind of 100 degrees at 75 kts would be entered via the scratchpad as 100/75.
To change single elements in multi-element values requires either a leading or trailing slash, depending on whether the data is on the left or the right of the CDU. To put it simply, include the slash as shown in the data field relative to the entry: XXX/ for items on the inner right side of the scree, while items on the inner left side prepend the slash - /8.0. Data closest to the LSK can be entered without slashes.
Elements close to LSK do not requires slashes, while elements further from the LSK require a slash in the appropriate position.
Specific keys
The functions keys give direct access to various CDU pages. An important one is the EXEC key, which executes any modification that has been entered into the CDU. A pending modification is indicated by a light bar above the EXEC key.
| Key | Page | Includes |
|---|---|---|
INIT REF | System initialisation and performance data menu | Position and performance |
RTE | Route pages - page 1 includes origin and destination, as well as company routes | Other pages include route segments or direct routings |
DEP ARR | Used to modify departure and arrival procedures | |
FIX | Displays fix page. Used to display and create fixes on current route | |
LEGS | Used to modify lateral and vertical nav data. Page is called MOD RTE when pending modification | Includes waypoints, speed and altitude at waypoint, RNP/ANP values |
HOLD | Hold page. Holding procedures for missed approach are displayed here | |
PROG | Dynamic flight information data, waypoint and destination, ETA’s, fuel remaining | Wind data, RTA (required time of arrival), RNP data |
Alert Messages
There are 4 alert messages on the sides of each CDU:
- MSG → Message displayed on scratchpad
- OFST → LNAV function is giving lateral offset routing
- CALL → Subsystem other than FMC requests control of CDU
How do you select VREF speeds on the FMC?::On the APPROACH REF page, which is accessed with the INIT REF key during flight.
How do you enable pending modifications on the FMC?::If the light bar above EXEC is on, there is a pending modification. Press EXEC to execute said modification.
As the aircraft reaches ToC, the FMC/CDU auto sequences from CLB page to CRZ page.::True
FPA is the actual flight path angle based on present ground speed and ___ . :: Vertical speed.
During a reduced thrust takeoff, the maximum reduction that the FMC will allow is ___ N1.::25%
During a CLB-2 reduced thrust climb out, N1 is reduced by ___ .::6%
Any page containing boxes indicates that data entry into that field is mandatory.::True.