IFR GPS terms

[GravityHater]

What is the official, or commonly accepted definitions for the terms:

1. LEG mode
2. OBS mode

when referring to use in an ifr approach-certified GPS?



P.S.
Why is this forum called the "General aviation forum" - was it meant only for "G.A." topics, or for "General" topics that have to do with aviation??

 

[gfvalvo]

What is the official, or commonly accepted definitions for the terms:

1. LEG mode
2. OBS mode

when referring to use in an ifr approach-certified GPS?

Not a formal definition, but:

In LEG mode you're navigating from waypoint to waypoint in the flight plan. The current LEG is a direct route from the previous waypoint to the current waypoint.

Think of OBS mode as putting a virtual VOR at the current waypoint. You can navigate to/from this "VOR" on any desired radial by setting it into the OBS (in the box itself or on the remote NAV head depending on your installation).

 

[paulsalem]

gfvalvo is correct, let me add a couple things.

Leg mode is also used for "Direct To". You are always navigating from point "A" to point "B" in Leg mode, with "Direct To" point "A" is just your present position when you activated the "Direct To" function. In Leg mode the fixes in the flight plan automatically sequence to the next waypoint upon crossing the waypoint you were just navigating to.

OBS mode does not automatically sequence you to the next waypoint. This is why it is used for multiple trips (or a single trip depending on the GPS unit) in a holding pattern.

When you use OBS depends on your GPS unit. For example a Garmin 430 doesn’t require OBS mode to do a procedure turn, however a Kln 89B does.

Hope this helps

 

[GravityHater]

Thanks, that does help you guys.
Next question... where do you learn about gps in general -can you recommend a text? I have the FAA stuff and the manuals but neither are really down to earth, practical.
Won't likely get time to go with an instructor on it.

 

[paulsalem]

Aim chapter 1 has GPS general stuff in it. It's pretty dry and vague. Some units have a simulator you can download for free off of the manufactures web site. What GPS are you using?