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How do pilots pinpoint location using DME on IFR chart?


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Hi,

 

Might be really basic and even stupid question, but really appreciate someone shed some light on this.

 

For example here, after taking off north, turn right to intercept MYV on R-154, and then turn left to heading 120 at LIAMM.

 

How do pilots know they are at LIAMM using DME? on the chart it says 30nm, but where is this 30nm measured from? Is it from MYV? If that is the case, my understanding is that DME is the distance between aircraft and ground beacon, so if you fly at different altitude, the DME reading will be different, isn't it?

 

I am aware I can set NAV2 for SAC to see when the aircraft is at R-006 to know I am at LIAMM, but I really want to know how DME is used in situation like this?

 

Thanks,

 

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Hi.

You can get better information / help if you post more details, complete information on the departure / plate, in this case FROGO-SIX.

If You have 2 VORs you can have one on SAC and the other on MYV. They both are capable of transmitting DME.

To verify intersection LIAMM you can use R154 MYV and R006 SAC and in addition on your DME you can read the distance of 30 DME from MYV, as a back up / additional information.

The DME is given based on your altitude in this case 1500ft., and the slant is taken into account.

Some birds may not have a DME. You can also use the 24 DME and R090 from SAC for your next fix.

If you have only one VOR you have to switch back and forth. TV

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How do pilots know they are at LIAMM using DME?

They know by using the definition provided by the chart. It says the 154º radial off MYV at 30 DME, so that is the actual definition of LIAMM, not a rough approximation. LIAMM is NOT a ground location, rather it is defined in the air by what you see on the chart.

 

Note the arrow attached to the 30 DME indication. It is coming from the Marysville VORTAC, so once that DME reads 30 AND if you are on the 154º radial of MYV then, by definition, you are at LIAMM.

 

If that is the case, my understanding is that DME is the distance between aircraft and ground beacon, so if you fly at different altitude, the DME reading will be different, isn't it?

 

The DME does, in fact, read slant range (that is, to the ground station), but it's not as if a foot or two, or even 100-200 feet will make an appreciable difference in where you are (it only reads to the nearest 10th of a mile anyway), so that radial/DME definition is perfectly workable. After all, the angle at 30 miles for the piddling thousand or two feet of altitude makes it a very, very shallow angle (if you understand trig, this is obvious), so the difference is minute. Don't try to be so precise -- the only time precision to that degree matters in aviation is when you are near the ground, which you certainly should not be at that intersection.

 

Please believe that those who designed and made the approach chart know all those factors, and they are more than accounted for. Relax, and believe what the chart says.

 

R145 MYV and R006 SAC

Typo -- that should be the R154 off MYV...

 

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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