DME Height Check
 

How are the DME Height's mentioned below the plan view calculated?

Should imagine the maths is as follows....
Distance x Sin(glideslope angle)


eg... For 1 nm... 6076.12 x sin 3deg = 317.999


Then maybe rounded up to 318 ft.

DME RIBBON
 

Thanks for the reply, but it's not accurate. I want to know , How does jeppesen deriventhe DME Ribbon altitudes.

Its not accurate
 

Phiggsbroadband answered your question with remarkable accuracy.

Do you think you may be able to fly the aircraft to that degree of accuracy?

When I saw your question I initially ignored it because I would expect anybody with a primary school understanding of arithmetic to be able to work out the answer.

Anyway best of luck to you.

These forums are going down the drain.

Fairly certain that the DME information is provided by the government source, Jepp does not calc.

The altitude rings typically assume a nominal slope angle of 3 degrees. How the accuracy of the ground station (185m) is calc'd is is a bit of an art.

Yes it is.

Jeppesen don't normally derive anything - they use the data supplied by the State authority. Different States use different methodologies.

The advisory altitudes are calculated from the TCH, so the DME distance at the runway threshold has to be calculated. The DME distance at the approach OCA is then determined using tan 3 deg function. This value is then rounded and 320ft/NM (3 deg slope) used for the subsequent DME/altitude values. It ain't rocket science.

Concur,

There are different methods to deal with the issues of accuracy of the ground station, slant range,the 3% accuracy of the system at distance, and rounding...

EDIT: About the same amount of voodoo as calc the MSA...

Thanks a lot everyone. Well ExEng.. you were unable to answer the question with your primary school education, or whatever level you managed to reach. Also, it had nothing to do with flying of the aircraft.

Either way, I was already aware of the formula, phiggsbroadband did mention. I didn't mean to be rude to him/her. But the answers are close, but not exact.

I am looking for the method the authorities/Jeppesen use to determine these values. The only thing, that makes sense, is they add some kind of a margin for obstacle clearance, depending on the runway, for which the approach is designed or as reynoldsno1 mentioned, different methodologies in place.

The accuracy of the ground station is 185m...sortof a pretty big margin in the grand scheme of things...

SIN or TAN??

SOHCAHTOA










The 6072 is the horizontal distance = adjacent
The the hight = opposite.
so:
opposite / adjacent = tangent.








6076 x tan3 degrees = 318.42.

Sin or tan.

DME distance is line of sight, so you can use the SIN function.

If you want distance to threshold (RNAV distance) then TAN.

Or just get a life :E

Really..c'mon now...it was an honest question.

What is the altitude at the DME rings based on.
You will NEVER get the same answer as on the charts by simply using tan 3 degrees.

As you all are aware, the closer one gets to the DME, the less accurate it is.
Its not until you are about 2nm that the errors balance out..fly 3 degrees parallel, and well...you get the picture (I hope) I wont even mention geologic conditions.. One variable that is not in the equation, obstacles, the DME ring is not an obstacle clearance surface.

Safe to say, as I and others have noted, it is a voodoo mix of many varibles that the State source uses that result in the altitudes you see on the charts, much like the MSA circle...

exeng - time for a Victor Meldrew quote?

Thanks for the correction re. Tan vs Sin. Tan is obviously slant range and Sin is actual surface distance... For small angles these work out to be practically equal... 317.99 and 318.42.


As for terrain separation, this could result in a non-standard glideslope angle.. 3.5 degrees for my local airfield, with some going to 5 degrees in more difficult regions.


From a practical point of view, and maybe a trick question... How long do you spend at each DME mark when traveling at over 120 knots?

T'other way around, ain't it? I'm feeling kefuddled again :hmm:

Yes. Phiggsbroiadband had it right the first time then was bluffed into changing his mind.

DME is a slant range distance (hypotenuse) to get the height of the opposite side you multiply the slant range by the sine of the angle.

Of course if you screw up and use the Tangent of the angle for angles around 3 degrees, the error will be about a tenth of a percent, which isn't significant in this application

my goodness...is there a common consensus on how to calc a GPA?
The DME is based on a nominal 3 degree GPA, then with a myriad of factors, the alt at the rings is provided..

aside from that, the issue remains the same.

With all of your 'calcs' what is the altitude at the DME ring distances?:
1. What is your point of origin?
a. the elevation of the DME (if known)
b. the elevation of the DME plus the accuracy factor.
c. the DME broadcast with slant angle deviation.
d. None of the above (ie read what is on the chart and accept)

Yes DME is the slant range distance...and it is exactly that, distance along a 3 degree GPA..surface distance, irrelevant. That is another issue.

It'll be in here somewhere:8260.3B - United States Standard for Terminal Instrument Procedures (TERPS) ? Document Information

I happen to have a question regarding this fact.

If those of you who have access to Jeppview could take a look at the VOR 30R approach in Montpellier (LFMT), and then compare it to the official one published by the French authority: https://www.sia.aviation-civile.gouv..._AD-2.LFMT.pdf (last page).

Jeppesen has voluntarily displaced the final descent point from 6.6NM of FJR to 6.3 NM (the FAF stays at the same place though), thus changing the final approach path angle from 3.00° to 3.13°. I have absolutely no clue to why they would do such a thing, and this is the only case I am aware of.

I agree it doesn't change much, but I don't understand why Jeppesen would go to the trouble of calculating a new approach path, altitudes, etc...

Many thanks to those who can help me shed some light on this.