TAKEOFF OUTBOUND OF A LOC FRONT COURSE

By Captain Michel Brandt

 

 

1. INTRODUCTION

 

There is in the world a fair number of SID’s which require the pilot to monitor the flight path after takeoff, using the LOC signal of the opposite ILS. In most cases the FMS in NAV mode will steer correctly the aircraft on the desired path, but in general the pilot is expected to monitor the raw data of the prescribed LOC or ILS.

A famous example was runway 13 out of the old Hong Kong Airport where you had to find your way between two island-hills just after takeoff.

The procedures associated to the old Hong Kong Airport are now part of the legend, but SID’s requiring monitoring the opposite ILS will continue to be published and used.

With the introduction on the A330/A340 FMGS of a new feature for LOC Back Course Approach, some confusions have been made in the documentation on the procedure to monitor the flight path after takeoff using the LOC of the opposite runway. This procedure was sometimes incorrectly called "LOC Back Course Takeoff" when in fact the purpose is to takeoff outbound of a LOC FRONT Course.

The objective of this presentation is to bring some clarifications (hopefully) in this matter, mainly for the A330/A340. But the discussion has been generalized to include other Airbus types.

 

2. LOC, TERMININOLOGY AND PRINCIPLES

Any ILS includes a GS and a LOC front beam, some LOC, depending on the type of installation and the type of antenna, have a LOC back beam (Fig 1).

Figure 1 – LOC Terminology and principles

 

 

The LOC front beam is used normally to perform a LOC approach or an ILS approach

if it is associated with a GS.

The LOC Back beam, if it exists and is reliable, is used to perform LOC Back Course approaches. Do not use the LOC back beam if it is not published associated with an approach procedure. In figure 1, the LOC installation shown emits signals from the antenna located at the end of runway 27 including a FRONT BEAM for LOC approach runway 27 and a BACK BEAM for LOC BACK COURSE approach runway 09.

The LOC antenna emits signals with two different modulations, 150 Hz on one side of runway axis, 90 Hz on the other side. The difference of modulation is proportional to the angular deviation indicated by the LOC Deviation Bar.

The difference of modulation is zero on the LOC axis and the algebric sign of the difference of modulation defines two sectors on both the front beam and the back beam.

When the 150 Hz modulation is greater than 90 Hz modulation (in the 150 Hz sector) , the PFD LOC diamond is displayed to the left. The LOC deviation bar on ND also moves to the left relative to the direction of the course indication (ILS course pointer). During an ILS approach if the aircraft is on the right of the axis in the 150 Hz sector, LOC indications are as shown in Figure 2.

Figure 2 – Standard ILS approach

 

 

Note in the example of figure 1 that the 150 Hz sector is to the right of the front beam for a LOC RWY 27 approach and to the left of the back beam of the LOC BACK COURSE RWY 09 approach.

To restore a correct indication on the PFD during a LOC Back Course Approach, it is necessary to inverse the polarity of the LOC signal. This feature is available on A330/A340. When selecting a LOC Back Course approach in the NAV database or in inserting on the RAD NAV page the approach course with an initial B, the polarity of the LOC signal for display is reversed on PFD and ND. This is indicated by B/C flags on ND and PFD (see figure 3).

Figure 3 – LOC Back Course approach A330 / A340

 

The other Airbus models do not have this feature. To fly a LOC Back Course procedure the PFD indication is inversed and should be remove from view.

A correct deviation bar indication can be restored on the ND in selecting the inverse course (the LOC front course) as shown in figure 4.

Figure 4 – LOC Back course approach

This discussion on the LOC B/C approach was necessary for the A330/A340 as the selection of an approach in the FM flight plan before takeoff has an influence on the LOC display during takeoff.

In summary, up to this point, we suggest that the words BACK COURSE remain associated to the BACK BEAM of a LOC installation which is used to fly a BACK COURSE approach.

For the A330/A340 the possibility to insert the LOC BACK COURSE with the initial B has been designed for this usage only.

When we use the LOC of the opposite runway to monitor the flight path after takeoff, we actually use the FRONT BEAM of a LOC. The terms like "LOC B/C TAKEOFF" or similar are totally inappropriate and should be proscribed.

 

3. TAKEOFF, MONITORING LOC OF OPPOSITE RUNWAY

 

In figure 5 the aircraft is taking off on runway 09 and the pilot intends to monitor its flight path with the FRONT beam of the LOC runway 27. The aircraft is shown on the right of its flight path. It is then in the 90 Hz sector of the LOC beam which moves the PFD and the ND LOC deviation indications to the right (which is the 90 Hz sector for the indicators).

Figure 5 – TO monitoring LOC opposite runway

These indications are incorrect (inversed). Note that in this example the selected LOC course was 090, which is the direction of the takeoff but not the direction of the LOC FRONT course.

If we now select the course 270, the LOC deviation indication of the ND becomes correct as shown in figure 6.

Figure 6 – TO monitoring LOC opposite runway

 

The proposed procedure is then:

Enter the LOC (or ILS) IDENT of the opposite runway on the RAD NAV page. For the A300/A310 the selection is made manually on the ILS control box.

Verify that the course indicated on the RAD NAV page is the LOC front beam of the opposite runway.

Disregard and clear the RWY/ILS MISMATCH" message on MCDU (A320 /A330 /A340),

Keep PFD ILS display out of view (on A300 / A310 disregard PFD LOC indication)

Select rose ILS mode on one ND,

When on the runway, check LOC IDENT and signal consistency.

 

4. FURTHER DISCUSSION ON THE A330/A340 DESIGN

The B/C function of the A330/A340, designed for LOC BACK COURSE approaches has induced some incorrect interpretations in relation with the monitoring of LOC indication during takeoff outbound of a LOC FRONT beam. The error was in fact coming from a wrong terminology when referring to a "BACK COURSE TAKEOFF". The words "Back Course" induce us to attempt using the B/C function of the A330/A340 FMGC during such Takeoff. Here the purpose is to monitor the takeoff with the front course of the opposite LOC and not to use the LOC back beam.

The B/C function should not be used, as due to its implementation for LOC Back Course approach, it can interfere with the display at takeoff under certain conditions.

The intent of the design is to activate the B/C function (inversion of the polarity of LOC signal) when a back course approach is selected. Consequently if the Back Course approach is selected before takeoff on the FM flight plan, it has an influence on the ND and PFD LOC displays.

When a LOC monitoring after takeoff is intended the first rule is then : Do not select in the FM F-PLN any approach at destination.

The second question is to determine which LOC course should be entered for on the RAD NAV page and which initial letter, B or F?

Taking the example of Fig 6 we can enter the LOC courses F270 in the RAD NAV page. In that case we are in the same situation as for the other Airbus types:

the PFD (LOC indication) is incorrect and should be kept out of view, or disregarded (A300 / A310)

the ND LOC deviation bar is moving on the correct side (see Figure 6).

 

This is a satisfactory situation as the navaid being monitored is a LOC front (F) beam oriented with a course of 270 degrees as indicated on the approach plate of the opposite runway.

For the purpose of the discussion we have evaluated the other option which would be to enter the course B090 in the RAD NAV page. The B activates the B/C function which inverts the polarity of the LOC signal and then both the PFD and the ND LOC deviation indications become in the correct direction as shown on figure 7.

Figure 7 – TO monitoring LOC opposite runway

 

But in addition, even if it is not the correct procedure, we have to look at the effect of the selection in the FM flight plan of an approach procedure. The table of figure 8 gives the results of this investigation.

 

Figure 8 – A330/A340 effect of FM entries

 

In our example to select the LOC course B090 may appear to be a better choice, but if any (except a LOC B/C approach) approach is selected at destination then the LOC indications at takeoff will be incorrect both on ND and PFD

If F270 is entered, the ND will remain correct if any approach procedure (except a LOC B/C approach) is selected at destination. This has been found to be a better choice more "error resistant".

The other reasons not to recommend the selection of B090 in our example are in addition :

The B/C flag would be displayed on ND and PFD.

This flag should be reserved for LOC Back Course approach when the LOC Back Beam is being used.

It is more logic when monitoring a LOC FRONT beam even flying outbound, to select on the RAD NAV page its frequency and its course which is a front course (F).

When entering the IDENT of the LOC (or ILS) of the opposite runway, the course which will automatically come up is the F course.

Commonality of procedure with the A320 family is kept.

To conclude, on the A330/A340, taking off outbound of a LOC FRONT beam we recommend to use the same procedure as in the paragraph 3 above for the other Airbus types (Figure 6).

The B/C function of the A330/A340 is reserved for LOC B/C approach.

 

5. CONCLUSION

When, after takeoff, the monitoring of the LOC of the opposite runway is intended:

Do not select any approach at destination in the FM F-PLN (A330/A340).

Enter the LOC (on ILS) IDENT of the opposite runway on the RAD NAV page. For A300/A310 this selection is made manually on the ILS control box.

Disregard and clear the RWY/ILS MISMATCH message on MCDU. (A320 / A330 / A340)

Verify that the course indicated on the RAD NAV page is the course of the LOC front beam of the opposite runway. For A330/A340 this course is shown following the letter F (A330/A340). On A300/A310 this selection is made manually on the ILS control box.

Keep PFD ILS display out of view (on A300 / A310 disregard PFD LOC indication).

Select ROSE ILS mode on one ND.

When on the runway check LOC IDENT and signal consistency.