Scale ID Annunciation (RW/APP CRS Error)
/Scale ID Annunciation display in upper left hand corner of the Primary Flight Display
The Scale ID annunciation (often called the approach reference), displayed in the upper left of the Primary Flight Display (PFD), is one of a suite of displays that comprise the PFD Navigation Performance Scales (NPS) Indications.
In the image a runway approach course error (RW/APP CRS Error) is being displayed. The airport is Hobart, Tasmania and the ILS approach is to runway 12. The error has been generated because the CRS window in the MCP has the incorrect approach course (140 degrees). If the approach course was correct, the display would be coloured white - not amber with a strike-through line.
The Scale ID Annunciation display provides, the for the selected approach type, the following approach reference information:
Airport identifier;
Runway approach course;
Distance to the runway threshold; and,
Approach type.
The display also indicates whether a runway approach course error (RW/APP CRS) has occurred.
Possible approach type displays include:
LNAV/VNAV (LNAV and VNAV deviations).
LOC/VNAV (Localiser with VNAV deviation).
FAC/VNAV (IAN final approach course with VNAV deviation).
LNAV/G/S (LNAV deviation and glideslope).
LNAV G/P (LNAV deviation with IAN glidepath).
ILS (ILS approach).
GLS (GLS approach).
Airport Identifier and Display Colour
The airport identifier comprises the identifier and airport name (abbreviated). The identifier will change depending upon the approach type. For an ILS (and IAN approach) the identifier will be the letter I followed by the airport abbreviation. For example, Hobart airport is YMHB. In this case for an ILS approach the airport identifier will be IHB.
The identifier is displayed in two colours: white and amber; amber being cautionary. The later also incorporates a strike-through line (this line dissects the airport identifier and approach course).
White indicates that all the parameters required for the approach have been completed correctly. An amber colour/strike-through indicates that one or more of the required parameters have not been met.
Colour Combinations
The following colour combinations can be observed (further information is discussed later in the article).
Frequency and approach course displayed in white:
When the navigation radio is tuned to the ILS frequency, the identifier will initially display the ILS frequency (109.90) for the approach. The frequency will then change to display the airport identifier (IHB). Whether the colour displayed remains white or changes to amber will depend on whether both navigation radios and CRS course windows are set to the correct ILS approach.
If either display is coloured amber it indicates a RW/APP CRS error has occurred.
Airport Identifier displayed in amber:
One navigation radio is tuned to the ILS frequency. Tuning the second radio to the same frequency will cause the display to change from amber to white.
Approach course displayed in amber:
One or both courses in the CRS course windows (MCP) is not set to the correct ILS approach course.
DME and approach type:
The DME and approach type (ILS) are always displayed in white. The DME will display the distance to the runway when the glideslope is captured by the aircraft.
Pre-Approach Tasks
Prior to commencing an approach, the following should be carried out:
The correct frequency entered into to the navigation radios (NAV 1 & NAV 2);
The correct approach course (for the runway selected) entered into the Captain and First Officer side CRS course windows in the MCP;
An appropriate approach selected from the FMS database (depends on the approach type being used); and,
The approach course for the runway entered into the heading window in the MCP.
Delay
The logic controlling the scale ID annunciation periodically interrogates that data entered into the navigation radios and MCP. This means that a delay is often observed between the annunciation changing colour from white to amber or back again. I am unsure of the timing.
Discussion
The indication that a RW/AP CRS error has been triggered doesn’t alwasy preclude an approach from being carried out (although it’s not recommended). The annunciation indicates that, for the selected approach, something hasn’t been completed with regard to the configuration of the avionics. It's rarely the case that the frequency hasn't been correctly entered into to the navigation radio; more often than not the cause of the annunciation is a CRS course discrepancy, or failure to configure the second navigation radio to the same frequency as the controlling navigation radio.
Using the ILS approach as an example. To correctly configure the instruments for an ILS approach and not receive a cautionary warning, the following must be completed:
Enter the correct ILS frequency into the BOTH navigation radios; and
Enter the correct approach course into BOTH the CRS course windows in the MCP.
It’s also recommended, but not mandatory to:
Enter the approach course into the heading window in the MCP; and
Enter an appropriate approach into the CDU/FMC.
If you enter the ILS frequency into the controlling navigation radio, and enter a different frequency into the other navigation radio, an amber-coloured RW/APP CRS annunciation will be generated. Likewise, a caution will occur if the Captain-side and First Officer side CRS windows don’t display the identical ILS approach course.
IMAGE A-1: ILS approach into runway 12 for Hobart, Tasmania (IHB). The approach course for this approach is 120 degrees. The controlling navigation radio (Captain-side/not shown) has been set to the correct ILS frequency (109.90). The heading that the aircraft is flying is 120 degrees, and the compass rose is offset to the course direction that is displayed in the Captain-side CRS window (140 degrees)
Example (Hobart, Tasmania IHB)
Image A-1 shows an ILS approach into runway 12 for Hobart, Tasmania (IHB). The approach course for this approach is 120 degrees. The controlling navigation radio (Captain-side/not shown) has been set to the correct ILS frequency (109.90). The heading that the aircraft is flying is 120 degrees, and the compass rose is offset to the course direction that is displayed in the Captain-side CRS window (140 degrees).
In the example, a RW/APP CRS annunciation has been triggered for an ILS approach. The airport identifier and approach course are coloured amber with a strike-through line. The DME is 9.4 miles and is coloured white (correct data).
This approach can be flown despite the discrepancy between the four courses (120, 180, 130 & 140 degrees) and a RW/APP CRS annunciation. This is because the ILS approach course (120 degrees) is coupled to the ILS frequency set in the controlling navigation radio – not the course as indicated in the CRS windows in the MCP.
In the example you can see that the localiser has been captured (this is identified by the magenta-coloured course deviation line being centered/in-line with the course pointer) despite the CRS window displaying a course of 140 degrees. Once the aircraft has captured the localizer it will fly the localiser heading no matter what course is displayed in the CRS window (provided it does not exceed 90 degrees).
While this example holds true for an ILS approach other approach types may behave differently.
Important Points:
The scale ID annunciation is an amber-coloured display that annunciates when the avionics have not been correctly configured for the selected approach. The display is a cautionary.
The approach cannot be flown If the CRS course discrepancy is greater than 90 degrees from the ILS approach course. This is because the aircraft will follow the direction of the course set in the CRS window (if greater than 90 degrees).
ProSim-TS
The ProSim737 avionics suite replicates the RW/APP CRS logic used in the real aircraft.
Database Inconsistencies
In some instances the annunciation is displayed despite entering the correct information. A possible reason for this is a scenery navigation database inconsistency.
In older scenery designs the physical location of the localiser beacons was part of the scenery file and this information is what the simulator referred to. With the advent of up-to-date navigational points (supplied by Navigraph) the simulator now refers to a navigational database rather than a scenery database. An inconsistency will occur if there is a discrepancy between the location of the localiser beacons in the scenery and the information recorded in the navigational database.
Final Call
The RW/APP CRS annunciation, although confusing to the uninitiated, does not necessarily mean that an approach cannot be carried out. However, it’s prudent before flying the approach to understand why the RW/APP CRS error has been displayed.
In more cases than not, the reason for the cautionary annunciation is a failure to configure the navigation radios to the same frequency and/or enter the same ILS approach course into both the CRS course windows in the MCP.
