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The Flight Control Unit of modern Airbus aircraft has a pushbutton for selection of either

  • heading / vertical speed (HDG-V/S), or
  • track / flight path angle (TRK-FPA)

guidance for the Autopilot and Flight Director. I know in principle what these modes mean (i.e. difference of heading vs. track, or between vertical speed and flight path angle) and how the guidance on the Primary Flight Display changes from Pitch/Roll bars to the Flight Path Vector depending on the selected mode:

HDG-V/S versus TRK-FPA

What I would like to ask is when is each mode used. How do pilots decide which guidance mode suits them most in a given situation? For example, is it related to

  • personal preference of the Pilot Flying?
  • specific flight phases or situation/circumstances?
  • procedures that require a certain selection?
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It's mostly up to points 2 and 3 on your list rather than personal preference:

Note that most ATC instructions (beit on departure,cruise or initial approach) will ask of you to fly a specific heading and/or a specific climb rate (v/s). This makes the hdg/vs mode the 'standard' mode to be used durring flight. This is also the mode where the F/Ds provide an easy to use guidance for the pilots to follow.

The reason for this Hdg preference for ATC is beyond the scope of this question, and is quite a nice narative on how historically we arrived at using heading for separation, but I will say this much: imagine ATC software attempting to interpolate the track of two converging a/c , to determine minimum separation and/or collision conflicts. Now imagine there is (unkown to the atc) changing wind along route, and both a/c will change hdg to maintain proper ground track. The math required is mind-blowingly complicated...(because changing hdg will change time of arrival at intersection,etc )

Anyway, once you look at some final approaches you will see that there is usually a final approach Track and a (3deg usually) glide path angle posted on the chart. The reason is that being closer to the runway, you start to become a bit more interested in your path wrt to ground rather that wrt to other trafic.

TLDR

TRK-FPA, as part of normal operating procedures, is only used when flying the final part of a non-precision approach, and is usually engaged about 1 NM before the FAF.

Mind you, it's use is not prohibited outside of that, just there is kinda little use for it.

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  • $\begingroup$ Using track would probably still be simpler for ATC (for separation, altitude is better anyway, and for vectoring the controller has to estimate the wind). The problem is that until recently it was impossible for pilots, and most aircraft still don't have prominent display of track to use for vectoring. $\endgroup$
    – Jan Hudec
    Mar 22, 2019 at 21:29
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    $\begingroup$ actually, using constant track for two converging traffic is amazingly difficult to compute min separation. This is why ATC might temporarely give you a “continue on present heading” for a few minutes. Moving on a constant heading means that both aircraft are now moving together with the airmass, taking the wind (and wind change) out of the equation. Google it on the net, I was amazed how complex the equations are to compute the minimum separation of two aircraft $\endgroup$
    – Radu094
    Mar 22, 2019 at 22:22
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The standard-operating mode for lateral navigation is the NAV mode, where the aircraft is tracked inbound RNAV-Waypoints, heading mode is of no use because you would always have to correct for the wind. ATC clearances may assign a heading to the aircraft, then the HDG mode is used for lateral navigation, how would you fly that with the TRK & FPA mode with changing winds. Also vertically V/S is used rather than FPA. Flying a vertical speed (feet per minute) is a lot easier to handle to optimize your descent or climb profile. There are also other lateral and vertical modes used like CLIMB / OPEN CLIMB, DESCENT / OPEN DESCENT. Only occasion TRK & FPA is used is during some Non-Precision Approaches to track Radials or Bearings and to fly a vertical Path (trajectory). Selecting the TRK & FPA mode puts the "BIRD" on the PFD, with the Flight directors off and in manual flight this is a good aid (showing you the actual trajectory of the aircraft) for flying raw data approaches or visual approaches without the autoflight system.

Cheers

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    $\begingroup$ Welcome to aviation.se! Please consider using a linebreak here and there to facilitate reading. Also, please be aware that salutations are not needed here, and usually are considered distracting. $\endgroup$
    – Federico
    Jun 24, 2017 at 19:39

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