Very curious!

[Flying Illini]

Just thought I would toss this Q out there:

The flying club I belong to has a very nicely equipped 172 SP...only problem is the GPS database is over a year old and very much out of date. This aircraft has no other DME equipment on board. Is it legal to use the GPS DME? I have flown this aircraft several times and the DME seems to be right on (meaning it reads 0.0) when you are above a navaid or airport that was entered into the unit. I assume that this does not qualify as DME when an instrument approach requires DME. Little help!

Very curious,
Flying Illini

 

[ksu_aviator]

My understanding is this...

You can use an uncertified GPS for DME substitution provided the database is current.

If the database isn't current, you can use GPS (certified or not) for situational awareness, but you need to cross check your waypoint's coordinates with a current chart.

I've never seen this actually published anywhere, that is just how it was explained to me.

 

[Deftone45075]

You can use it for situational awareness, however not for any appoach that requires DME. DME is normally a sant range distance. If your GPS is reading 0.0 above a navaid then it is not giving slant range.

 

[Deftone45075]

I understood that you could use an IFR-certified GPS in place of DME for instrument approaches

You could be right as far as disregarding the slant range, however in the original post, the GPS database is not current, and it may or may not be ifr approved, thus it can not be used to provide DME on an IAP.

 

[brianjohn]

Hey KSU-
Is that what Mr. Gross taught you?
I treat his words as gospel...

 

[metrodriver]

Since when does GPS give slant range DME?

 

[Khirsah]

AIM 1-1-21 (GPS)

specifically Table 1-1-8

IFR Enroute and Terminal, IFR Oceanic/Remote, and IFR Enroute, Terminal, and Approach approved GPSes may be used in lieu of ADF or DME provided they have a current database. Handheld and VFR panel mount GPSes may not be used in lieu of ADF or DME, regardless of whether or not the database is current.

 

[ksu_aviator]

Nah, BG didn't teach me a lot about GPS. However, if he told me the sky was red, I'd believe him. He knew what he was talking about.

The slant range difference is negligible on an instrument approach (distances under 30 NM, altitude below 5000 feet).

Maybe you've caught this already, but what you said is backward. Slant range is most effected at close range rather than long range. The rule of thumb I was taught was that for every thousand feet of altitude (agl) that is how far out you need to be for DME to be accurate. I know that is hard to understand so I'll say it this way, if you are at 3,000 ft AGL and inside of 3 miles, DME is inaccurate, but if you are outside of 3 miles DME is suppose to be accurate.

 

[brianjohn]

KSU- I don't think I have ever met a more practical, no-nonsense, cut- the- baloney guy than BG.
He sure knows his stuff though. A very down to earth guy too.

 

[Iceman21]

Who in the world is BG?

 

[brianjohn]

BG= chief at KSU.