david b.
Topic Author
Posts: 2894
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Autopilot Functions

Tue Feb 19, 2002 3:10 am

Can someone explain the autopilot functions?
Two of them I like to know about are "NAV" and "HEADING HOLD". Heading hold I would guess is used to hold the aircraft in the direction is heading in. How about GPS holding? Is that what the button is for?

NAV hold I assume is used for ILS landings, to lock on to the localizer.
Teenage-know-it-alls should be shot on sight
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RE: Autopilot Functions

Tue Feb 19, 2002 3:22 am

NAV, I believe will track the current selected course in a given VOR.

HEADING HOLD will hold your current heading, although you can also select HDG mode in which you select the heading you want with a knob.

And GPS holding I guess will turn the aircraft to a heading that intercepts the GPS route.

Experts correct me if I'm wrong!

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RE: Autopilot Functions

Tue Feb 19, 2002 3:46 am

Very much depends on the aircraft type and complexity. I can give you a pretty good rundown of 757/767 Automatics - but that may be way more than you need or want to know.
I used to love the smell of Jet-A in the morning...
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RE: Autopilot Functions

Tue Feb 19, 2002 4:02 am

The names are pretty descriptive. From the standpoint of a light aircraft autopilot, heading hold will hold the current heading or a heading that has been preselected. If the current heading is different from the one selected, the autopilot will turn the aircraft to the selected heading when the autopilot is engaged. Nav will track a selected VOR radial or ILS course. To complicate the issue a bit, I often use heading hold to track a VOR radial by setting a heading that will track the radial and adjusting as necessary to compensate for winds. This smooths out some of the kinks that are present in VOR signals.
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Tue Feb 19, 2002 4:11 am


Greetings! I am very interested in how you got to be a pilot on such big airliners so quickly! I am a student pilot right now, here in the USA and will have my license in a couple weeks hopefully. Here in the US, it is very hard to get into the airlines, and work your way up to big aircraft. Most pilots don't get to fly planes such as the 757/767 until atleast their mid-30s and that is in the right seat! How did you move to your current career position so quickly, I really admire that!


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RE: Autopilot Functions

Tue Feb 19, 2002 4:41 am

Sounds like a Flight Simmer!!!
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RE: Autopilot Functions

Tue Feb 19, 2002 4:53 am

Never heard of GPS hold.....

Heading Hold is the same as the Heading (HDG) mode. The aircraft holds the heading selected by the HSI heading bug.

Like mentioned before the NAV function guides the aircraft on the course (not heading) selected on the HSI. The deviation from the course is displayed by the deviation needle on the HSI. The deviation error is slowly washed out by the autopilot.

When the NAV mode is initially engaged, the autopilot enters the HDG mode and NAV ARM is anunciated until the course is intercepted (captured), at which time the autopilot begins tracking the selected course and NAV CAP is annunciated.

More later....
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RE: Autopilot Functions

Tue Feb 19, 2002 1:20 pm

continued.....(sorry I ran out of time on the last post)

The GPS or FMS is coupled to the autopilot in many different ways depending on the type of the equipment, and age of the aircraft.

The simplest GPS/FMS interface is identical to the NAV mode and may even use the same mode switch along with a second selector to chose between GPS and VOR/ILS. Newer AFCS (automatic flight control systems) have an "LNAV" mode or "FMS" mode that is separate from the NAV mode for this.

The most effective interface uses "roll steering."A roll steering interface provides steering commands directly to the AFCS system instead of just course deviation. It allows for much more complex AFCS coupled procedures like hold patterns, DME arcs or procedure turns.

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RE: Autopilot Functions

Wed Feb 20, 2002 6:12 am

>Two of them I like to know about are "NAV"

Autopilot navigates using flight plan data. Most commonly limited to path-over-the-ground [no vertical guidance control]. What actually computes this ground track varies with installed equipment.


Autopilot will hold the current heading without compensation for wind or ground track. Can be either manually set heading or whatever heading the autopilot senses when it rolls wings level --again, it depends upon installed equipment.

>How about GPS holding? Is that what the button is for?

Personally, I've never heard/seen this function. Suspect it is just another form of Flight Management System functionality that permits creation of a holding fix based upon GPS data only.

>NAV hold I assume is used for ILS landings,
>to lock on to the localizer.

I've never seen/heard of "NAV hold" either. ILS approaches normally have a function switch(es) labeled "ILS"; "LOC" and "G/S"; or "APP" (for approach).

*NO CARRIER* -- A Naval Aviator's worst nightmare!
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RE: Autopilot Functions

Wed Feb 20, 2002 7:26 am

Labeling and function varies slightly from aircraft to aircraft-and some dont even have some of the modes.. here are the general basic functions- note this is quite simplified and not specific.. just general information.

Heading (HDG): you select the heading.. it flies it.

Nav (NAV): this is usually couple to the GPS, INS, or VOR.... it is also horizontal navigation like the heading hold function, but the navigational device of your choice supplies the lateral data for it to hold onto.

Altitude(ALT): you select the altitude.. it holds it.

Vertical speed (VS): Select the vertical speed. the plane pitches for it.

IAS (indicated airspeed): you selected the airspeed.. it pitches for it.... this is generally used in lieu of vertical speed except for acceleration situations.

Approach (APP): Used to fly an ILS...couples with the glideslope and localizer. In some airplanes this can also be used on the VOR and GPS/RNAV approaches.

Backcourse (REV): similar to NAV....but used when tracking a localiser backcourse with reverse sensing on the CDI indications.... this way the airplane doesnt get confused and start flying circles around itself.

Hope this helps. See ya.
Chicks dig winglets.
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RE: Autopilot Functions

Wed Feb 20, 2002 11:39 am

"NAV" signifies to me autopilot coupled to FMS to fly inserted flightplan, same as "L NAV" (lateral navigation).

as opposed to

"PROFILE" or "V NAV" which couples autopilot to FMS for programmed climb and descent.
You're only as good as your last departure.

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