mb339
Posts: 235
Joined: Fri Jun 01, 2001 8:37 pm

HDG Or TRK Mode?

Fri Oct 18, 2002 1:04 am

I know that some aircraft, such as A320 and B777 can change the HDG mode into TRK mode.

My question is: In the event of a radar vectoring for an approach, does the pilot use HDG mode or TRK?
Is the pilot or the controller to adjust the degrees of a vector in case of strong crosswind?

Thanks

mb339
 
ThirtyEcho
Posts: 1409
Joined: Tue Jan 01, 2002 1:21 am

RE: HDG Or TRK Mode?

Fri Oct 18, 2002 1:24 am

All autopilots, even GA autopilots, can shift from heading to track modes. In order to track, you have to track SOMETHING such as a VOR radial or an ILS localizer. In that case, steering commands from the autopilot turn the aircraft as necessary to keep the aircraft flying on the radial or localizer.

In the case of vectors for an approach, the pilot flies what ATC has assigned, which is a heading.

ATC will compensate for a crosswind as the idea is to have the airplane arrive at a specific point in space or to intercept a desired radial or localizer outside, on or inside a specific fix.

Personally, I prefer to use HDG mode when tracking something enroute. The intercept is usually smoother and with less bracketing and the HDG knob tends to keep the pilot more in the loop than TRK does. This makes for a steadier course as it also avoids scalloping on the VOR radial.
 
Rick767
Posts: 2613
Joined: Sun Jan 30, 2000 8:11 pm

RE: HDG Or TRK Mode?

Fri Oct 18, 2002 2:16 am

Whenever we are in receipt of an ATC radar vector we use HDG SEL mode (Heading Select). That is exactly what we are being asked to fly, a magnetic heading.

The only time the aircraft is flying track is, as ThirtyEcho correctly says, when flying a VOR radial / LOC course or programmed route (magenta line). When ATC issue a vector they are normally trying to get us to intercept a localiser course or something, so there is nothing to "track" as such. They will compensate for the wind and will be able to deduce our approximate track across the ground from their screens.
I used to love the smell of Jet-A in the morning...
 
Jeff G
Posts: 438
Joined: Fri Jan 18, 2002 9:56 am

RE: HDG Or TRK Mode?

Fri Oct 18, 2002 8:46 am

You can use the TRK mode in other ways. The A320 TRK mode actually flies a ground track using the FMGC to make good the track, correcting for winds. You don't have to be tracking a radial or other course first, so scalloping isn't a factor. In fact, the A320 can't track a radial using raw data; track information is all from the FMGC. The TRK mode is very useful when flying a non-coupled non-precision approach. A coupled approach is when the flight guidance system flies the approach directly from the FMGC database. But you can also fly the approach (with or without the autopilot engaged) using raw data for reference, and steering the aircraft in TRK mode. This way, once the needle is centered on the approach, just match the TRK bug to the approach heading and the airplane corrects for wind drift all the way in, in theory keeping the needle centered. Similarly, in the TRK mode, you can descend at a constant angle (FPA mode), which also corrects for wind drift. Therefore, at the FAF, begin a descent at the appropriate angle and you'll arrive at the MAP at the MDA, in a position to go around or land with minimal further maneuvering. It's pretty slick. You also have the option to dive and drive, but if the weather isn't right at minimums, FPA is the way to go.

But, no, you don't use TRK for vectors. ATC will do all the wind correction for you, and if you try to "help", you'll just screw them up.
 
CX Flyboy
Posts: 6030
Joined: Sun Dec 26, 1999 6:10 pm

RE: HDG Or TRK Mode?

Fri Oct 18, 2002 10:02 am

We can fly both TRK or HDG on the 777, whenever we want, however, as mentionned, we only use HDG SEL if asked to fly a heading.

However, when we are avoiding weather in the area outside of the terminal area, and we ask for say, 20 miles left or right of track, we will use TRK SEL to go around the weather.

When a SID says fly runway heading, we will also use TRK SEL to do it, unless we are in the US or Canada, where Runway Heading means just that...a heading, flown without adjustment for wind.
 
Rick767
Posts: 2613
Joined: Sun Jan 30, 2000 8:11 pm

RE: HDG Or TRK Mode?

Fri Oct 18, 2002 3:17 pm

TRK SEL, cool. Quite a useful tool in the situations you suggested I'm sure. On the SIDs we have to put the heading in, and use HDG SEL then look at what the wind is doing once we're up there (unless in LNAV when it clearly does all that for us).
I used to love the smell of Jet-A in the morning...
 
ThirtyEcho
Posts: 1409
Joined: Tue Jan 01, 2002 1:21 am

RE: HDG Or TRK Mode?

Sat Oct 19, 2002 12:53 am

In general, ATC vectors are progressive in nature when flying towards an intercept such as a localizer. When you are far away from the intercept, ATC will get you towards the desired course in the quickest way possible; that is to say, you will be vectored directly towards it even if the intercept angle is large. As you near the intercept, ATC will steadily reduce the intercept angle and then turn the rest of the navigation back to the pilot for the final part of the intercept. In the case of a DME arc to a localizer, once you are established on the arc, getting onto the localizer is done without further vectors from ATC. In the case of a basic autopilot operation, all of this is done in HDG mode until you are in position for the autopilot to capture and track. More sophisticated autopilots give you far more options to automate this process.

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