GPS sensitivity during a hold

[Privateer]

Hi all, I'm fairly certain I read in the AIM about using the terminal (+/- 1Nm) sensitivity during a hold if you are using the GPS to perform the hold. I'm looking for that specific guidance again and can't seem to find it. So was I just dreaming it up? Does anyone know where I can find some direction on this. Thanks

 

[poadeleted20]

I know of no such specific guidance in the AIM or any other FAA document, other than for a hold which is part of a terminal procedure, since all terminal procedures must be flown in the terminal mode. Other than that, I know of nothing that says enroute sensitivity isn't good enough.

 

[jimbtv]

If you were relying on needle deflection on your CDI as an indication of distance from course, you could find yourself closer or farther than you think, depending on how the GPS sensitivity is set.

My method is to fly a specific offset distance from the course line (on the reversal) and rely on the CDI for left or right-of-course indication. Track offset will show how far you are away from the course line.

I don't recall seeing anything in print about manual sensitivity adjustments for holds.

 

[John Collins]

I am not aware of any requirement that the GPS CDI Full Scale Deflection (FSD) sensitivity be set to 1 NM for a hold. Just a comparison between a GPS and the VOR FSD would not indicate that it would be necessary. A non WAAS GPS has a FSD of +/- 1 NM when in the terminal area (within 30 NM of the departure or destination airport) and +/- 5 NM when enroute (outside of the terminal area). A WAAS GPS enroute FSD is +/- 2 NM. A VOR FSD is +/- 10 degrees.

If there were a VOR located at the airport, at 30 NM from the airport a VOR CDI deflection of 2 degrees (1/5th of full scale) would be equivalent to a GPS deflection of +/- 1 NM. In terminal mode, both the Non WAAS and the WAAS GPS would be full scale and therefore be 5 times as precise at this distance. If instead the GPS were in enroute mode, a non WAAS GPS would also be (1/5th scale) and a WAAS GPS would be 1/2 scale, so for the Non WAAS GPS, the CDI sensitivity would be the same as the VOR and for a WAAS GPS the CDI sensitivity would be 2.5 times greater than a VOR.

The pilot would be expected to be in terminal mode when within 30 NM of the departure/destination airport and in enroute mode when outside this distance. If a flightplan has been entered into the GPS, the mode switch will occur automatically.

 

[Privateer]

Thanks for the reply's. Terminal mode is what was sticking in my head, but couldn't find a direct reference for it, other than the one about operating in that environment. Granted for general aviation most our holds are probably done in this area. Thanks again.