### The combined meeting of the ISPACG and IPACG FANS Interoperability Teams (FIT)![](media/image1.jpeg){width="1.3291666666666666in" height="0.7368055555555556in"}

Honolulu, USA, 25^th^ January -- 27^th^ January 2006

###### 

###### Problems with CPDLC Route Clearances

(Presented by the Airservices Australia)

+-----------------------------------------------------------------------+
| # SUMMARY                                                             |
|                                                                       |
| This Working Paper describes some of the idiosyncrasies associated    |
| with CPDLC route clearances. Aircraft manufacturers are requested to  |
| confirm and/or clarify some of the issues raised.                     |
+-----------------------------------------------------------------------+

1\. **Introduction**

1.1 New DARP (i.e. CPDLC re-route) procedures were added to the FOM
during its last update (Version 2). It should be noted that there are
many idiosyncrasies associated with CPDLC route clearances. A route
clearance that is not properly formatted can have undesired results,
ranging from the inability for the pilot to load the clearance at all to
the loading of a clearance differing from what was intended by the
controller. These problems are exacerbated by slightly differing
functionality in different avionics.

1.2 This WP attempts to highlight some of these problems, as well as
seeking clarification regarding some of the functionality.

1.3 The problems considered in this working paper are:

• The inclusion of the optional sub-fields within the *routeclearance*
variable; and

• The contents of the *routeinformation* field within the
*routeclearance* variable.

**2. Discussion**

2.1 **Optional fields within the *routeclearance* variable**

2.1.1 The *routeclearance* variable contains a number of optional
fields; the most commonly used being the *routeinformation* field which
determines the actual route of flight. Other (optional) fields may
contain information concerning the Departure airport, Arrival airport,
Runway information as well as Arrival and Departure procedures.

2.1.2 Different aircraft types have different requirements concerning
these optional fields. In fact, some fields, although optional, are 'not
to be used' under certain circumstances. The inclusion (or failure to
include) certain fields may result in the inability of the pilot to load
the clearance as issued.

2.1.3 To provide guidance to ATSUs when issuing CPDLC route clearances
(e.g. DARPs), avionics/aircraft manufacturers are requested to provide
the following data:

+-------+-------------+--------------+-------------------------------+
| Acft  |             | **UM 79      |                               |
| type: |             | CLEARED TO   |                               |
|       |             | (*position*) |                               |
|       |             | VIA          |                               |
|       |             | (*routec     |                               |
|       |             | learance*)** |                               |
+-------+-------------+--------------+-------------------------------+
| Sub-  |             | Optional (O) | Comment                       |
| field |             |              |                               |
| name  |             | Mandatory    |                               |
|       |             | (M)          |                               |
|       |             |              |                               |
|       |             | Do not use   |                               |
|       |             | (N)          |                               |
+-------+-------------+--------------+-------------------------------+
| ADEP  |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| DRWY  |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| DEP   |             |              |                               |
| Proc  |             |              |                               |
| (+    |             |              |                               |
| t     |             |              |                               |
| ransi |             |              |                               |
| tion) |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| A     |             |              |                               |
| irway |             |              |                               |
| inte  |             |              |                               |
| rcept |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| ARR   |             |              |                               |
| Proc  |             |              |                               |
| (+    |             |              |                               |
| t     |             |              |                               |
| ransi |             |              |                               |
| tion) |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| APP   |             |              |                               |
| Proc  |             |              |                               |
| (+    |             |              |                               |
| t     |             |              |                               |
| ransi |             |              |                               |
| tion) |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| ADES  |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| ARWY  |             |              |                               |
+-------+-------------+--------------+-------------------------------+

> **Table I -- UM 79**

+-------+-------------+--------------+-------------------------------+
| Acft  |             | **UM 80      |                               |
| type: |             | CLEARED      |                               |
|       |             | (*routec     |                               |
|       |             | learance*)** |                               |
+-------+-------------+--------------+-------------------------------+
| Sub-  |             | Optional (O) | Comment                       |
| field |             |              |                               |
| name  |             | Mandatory    |                               |
|       |             | (M)          |                               |
|       |             |              |                               |
|       |             | Do not use   |                               |
|       |             | (N)          |                               |
+-------+-------------+--------------+-------------------------------+
| ADEP  |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| DRWY  |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| DEP   |             |              |                               |
| Proc  |             |              |                               |
| (+    |             |              |                               |
| t     |             |              |                               |
| ransi |             |              |                               |
| tion) |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| A     |             |              |                               |
| irway |             |              |                               |
| inte  |             |              |                               |
| rcept |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| ARR   |             |              |                               |
| Proc  |             |              |                               |
| (+    |             |              |                               |
| t     |             |              |                               |
| ransi |             |              |                               |
| tion) |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| APP   |             |              |                               |
| Proc  |             |              |                               |
| (+    |             |              |                               |
| t     |             |              |                               |
| ransi |             |              |                               |
| tion) |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| ADES  |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| ARWY  |             |              |                               |
+-------+-------------+--------------+-------------------------------+

> **Table II -- UM 80**

+-------+-------------+--------------+-------------------------------+
| Acft  |             | **UM 83 AT   |                               |
| type: |             | (*position*) |                               |
|       |             | CLEARED      |                               |
|       |             | (*routec     |                               |
|       |             | learance*)** |                               |
+-------+-------------+--------------+-------------------------------+
| Sub-  |             | Optional (O) | Comment                       |
| field |             |              |                               |
| name  |             | Mandatory    |                               |
|       |             | (M)          |                               |
|       |             |              |                               |
|       |             | Do not use   |                               |
|       |             | (N)          |                               |
+-------+-------------+--------------+-------------------------------+
| ADEP  |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| DRWY  |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| DEP   |             |              |                               |
| Proc  |             |              |                               |
| (+    |             |              |                               |
| t     |             |              |                               |
| ransi |             |              |                               |
| tion) |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| A     |             |              |                               |
| irway |             |              |                               |
| inte  |             |              |                               |
| rcept |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| ARR   |             |              |                               |
| Proc  |             |              |                               |
| (+    |             |              |                               |
| t     |             |              |                               |
| ransi |             |              |                               |
| tion) |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| APP   |             |              |                               |
| Proc  |             |              |                               |
| (+    |             |              |                               |
| t     |             |              |                               |
| ransi |             |              |                               |
| tion) |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| ADES  |             |              |                               |
+-------+-------------+--------------+-------------------------------+
| ARWY  |             |              |                               |
+-------+-------------+--------------+-------------------------------+

> **Table III -- UM 83**

**2.2 Contents of the *routeinformation* variable**

2.2.1 Whilst considering DARPs procedures, another issue that needs to
be considered is the specific route information contained in the
*routeinformation* variable.

2.2.2 To request an amended route, the pilot has the choice of a single
downlink: DM24 REQUEST \[route clearance\]. However, ATC has a number of
possible CPDLC clearances involving the *routeclearance* variable which
could be used to respond; namely:

-   UM79 CLEARED TO \[position\] VIA \[route clearance\]

-   UM80 CLEARED \[route clearance\]

-   UM83 AT \[position\] CLEARED \[route clearance\]

2.2.3 When a downlink request is received, most ground systems have the
capability to autoload the contents of variables in the downlink request
into the corresponding variables in the uplink response.

2.2.4 If a downlink route clearance request is received, depending on
which uplink clearance message element is used to respond, it may be
necessary to modify the contents of the *routeclearance* variable from
what was requested. This is best described by example -- refer to the
diagram below.

![](media/image2.png){width="0.3700787401574803in"
height="0.3425196850393701in"}

2.2.5 The aircraft's current route clearance is BOOLA CHOWW HELLO HOWDY
BNJUR GIDAY, and is requesting from CHOWW direct to IORNA then BNJUR
then as flight planned.

2.2.6 The following table shows a downlink request, with three possible
ATC responses that all have the same nett result.

+----------------------------------+-----------------------------------+
| Aircraft downlink request        | Possible ATC uplink responses     |
+----------------------------------+-----------------------------------+
| REQUEST (BOOLA CHOWW IORNA BNJUR | AT (CHOWW) CLEARED (IORNA BNJUR   |
| GIDAY)                           | GIDAY); or                        |
|                                  |                                   |
|                                  | CLEARED (BOOLA CHOWW IORNA BNJUR  |
|                                  | GIDAY); or                        |
|                                  |                                   |
|                                  | CLEARED TO (BNJUR) VIA (BOOLA     |
|                                  | CHOWW IORNA)                      |
+----------------------------------+-----------------------------------+

2.2.7 It can be seen that the second example is the only one in which
the *routeclearance* variable in the uplink is a **direct** mapping of
the *routeclearance* variable in the downlink. In the other two
examples, the uplinked *routeclearance* variable is a variation (i.e. a
subset) of the downlinked *routeclearance* variable.

2.2.8 Controllers must be aware of this problem when constructing a
response to a downlink route request. For the route clearance response
to be correctly constructed, the following issues need to be clarified:

-   To what extent does the *routeclearance* variable have to be
    modified?

> For example, would the uplink AT (**CHOWW**) CLEARED (**CHOWW** IORNA
> BNJUR GIDAY) (i.e. where the position "CHOWW" is repeated) be valid,
> or would it cause a 'partial load' error in the avionics? Similarly,
> could example 3 be uplinked as CLEARED TO (**BNJUR**) VIA (BOOLA CHOWW
> IORNA **BNJUR**) (where the position "BNJUR" is repeated)?

-   Does the destination need to be repeated as the last point in the
    route field? Does it matter?

-   Do the answers to the above questions apply to all aircraft types?

**3. RECOMMENDATION**

3.1 As well as providing information to make controllers aware of some
of the problems concerning CPDLC route clearances, this working paper
makes the following recommendations:

-   Avionics manufacturers provide input to the tables above, as well as
    providing responses to the issues raised in paragraph 2.2.8. It is
    recommended that this information be retained in the FOM; and

-   DARPS procedures described in the FOM include guidance material
    concerning the issues described within this working paper.