DME Height Check

[B737NG_Pilot]

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

[phiggsbroadband]

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.

[B737NG_Pilot]

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

[exeng]

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.

[underfire]

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.

[reynoldsno1]

it's not accurate

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.

[underfire]

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...

[B737NG_Pilot]

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.

[underfire]

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

[Capt Fathom]

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


SIN or TAN??

[DutchOne]

SOHCAHTOA










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








6076 x tan3 degrees = 318.42.

[JayGatsby]

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

[underfire]

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...

[BOAC]

exeng - time for a Victor Meldrew quote?

[phiggsbroadband]

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?

[Kefuddle]

Tan is obviously slant range and Sin is actual surface distance
T'other way around, ain't it? I'm feeling kefuddled again :hmm:

[A Squared]

Tan is obviously slant range and Sin is actual surface distanceT'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

[underfire]

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.

[oggers]

I am looking for the method the authorities/Jeppesen use to determine these values.

It'll be in here somewhere:8260.3B - United States Standard for Terminal Instrument Procedures (TERPS) ? Document Information (http://www.faa.gov/regulations_policies/orders_notices/index.cfm/go/document.information/documentid/11698)

[LLZDME]

Jeppesen don't normally derive anything - they use the data supplied by the State authority

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.fr/aip/enligne/PDF_AIPparSSection/IAC/AD/2/1403_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.

[BOAC]

My initial guess is that Jepp declare it a 'CFDA' approach and I would surmise that 3.13 fits better with the FJ check at 600'?

[reynoldsno1]

The only thing, that makes sense, is they add some kind of a margin for obstacle clearance
The obstacle clearance calculations are separate to the DME/altitude calculations. The advisory altitudes will always remain above the required OCA for any particular approach segment down to the MDA, and may require an increase in the VPA. PANS OPS requires 150m (500ft) obstacle clearance in the intermediate segment and 75m (250ft) in the final segment. This does not preclude an obstacle in the missed approach segment controlling the OCA in the final segment.
VNAV procedures use a different methodology ....:)

[underfire]

LLZME,

I guess I should clarify..Jepp uses State data to build their charts.
They use the data available and put that into their database, and they use their database to create the charts and navdatabase. This is the same for the other providers.

When I said they dont derive anything, I guess I did oversimplify, as I was speaking about the altitude of the DME itself. Sorry about that.

They do interpolate, and of course, Jepp designs procedures.

I have been involved with multiple providers and the State source, and frequently find differences, some very significant.

Reynolds, in the final approach segment, typically, the ROC tapers from 200' feet to 500' at the FAF.

[reynoldsno1]

Reynolds, in the final approach segment, typically, the ROC tapers from 200' feet to 500' at the FAF.
For a non-precision approach, using ICAO criteria, the final approach segment MOC (minimum obstacle clearance) is 75m, or about 250ft. The minimum obstacle clearance height is also 75m. The 'tapering' you are referring to is, I assume, the secondary areas? There the MOC goes from 75m to zero - ISTR this is different to TERPS , which I haven't looked at for some time.

[Kefuddle]

The tapering Underfire is referring to is the reduction in the primary area OCH from FAF to minimums/MAPT. The OCH also reduces from 1000' to 500' from the IF to the FAF, and then reduces further from there.

[BOAC]

All fascinating stuff, but nothing to do with the OP's question which was related to advisory (NB Not MINIMUM) heights on an established glidepath.

[Check Airman]

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


You're both right.

Tan A = Sin A/Cos A

When A is small, Cos A ~ 1

thus Sin A ~ Tan A


QED:ok:

[Natstrackalpha]

BOAC is right.






Maybe we can use:




(Sq rt of Ht 1 + Sq rt of Ht 2) x 1.25

[BOAC]

Thanks NTA - I began to wonder if anyone had noticed.

RegardingI 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, - it begs the question - does he/she understand what it is all about?:confused:

[Capn Bloggs]

I began to wonder if anyone had noticed.
I did!

a 'CFDA' approach
or was that a CDFA?

[BOAC]

I began to wonder if anyone had noticed:p

[aditya104]

I was in the middle of writing this yesterday when my internet connection crashed.

To OP

This is not a bad question since there are many approaches where this calculation doesn't match the published altitudes.

The published altitudes are recommended altitudes to allow a constant rate of descent at the published descent angles. This descent angle will lead you to the threshold at threshold crossing height(sometimes 50ft). The DME is not always situated right at the touchdown point. If it were, then the published altitude and the calculated altitude would be the same.

http://i.imgur.com/1eATSvC.png
http://i.imgur.com/WHrAS9I.png
Fig.1 : In this figure, you will see that 5.9D from IIDR is not 5.9D from threshold. So, if you calculate using 5.9D, that answer will not be accurate. The plan view shows how much is 5.9D from the threshold. It is 0.8+3.0+1.9=5.7. So, I have calculated using 5.7, and it comes accurate enough for me(only 2ft deviation).


Now, take a look at the image below. These are the distances from IID DME which is located beyond upwind end of RWY 25. At 5.0D IID, you have to be 2840'. From IIDR this altitude was 3420'. Both these DME distances are not from threshold. Your calculated error will be greater the farther away the DME is from the threshold.
http://i.imgur.com/6jVFaHt.png
http://i.imgur.com/NU7YOEY.png
Fig.2

____________________________________________________________ ________________________________________________

@ LLZDME (http://www.pprune.org/members/394075-llzdme)

Can you could post the chart in question?

[LLZDME]

Yes, here it is:

http://i.imgur.com/Dw2Tt36.jpg?1

[Capn Bloggs]

Out of interest, LLZDME, what Procedure Altitude is shown in the Briefing Strip for that approach?

[underfire]

the procedure ref in post #20...VOR 30R approach in Montpellier (LFMT)

http://i58.tinypic.com/27y7kon.jpg

LLZME,

I see your point, on the Jepp chart, it seems they moved the FAF to what the State shows as the IAF. TOD on the Jepp chart is not identified...and with a different GPA than the State 3 degrees.

for ref, I checked the coordinates...
FJ is approx 0.15nm from threshold
FG is approx 0.45nm from threshold

Perhaps Jepp can explain...

EDIT: loc of RWY per State source (on Google Earth)
http://i57.tinypic.com/kb2g9.jpg

[reynoldsno1]

The tapering Underfire is referring to is the reduction in the primary area OCH from FAF to minimums/MAPT. The OCH also reduces from 1000' to 500' from the IF to the FAF, and then reduces further from there.

Understood - but I certainly wouldn't refer to it as "tapering", as the primary surfaces involved do not slope...
Also, beware of comparing ILS check heights with non-precison heights. ILS heights should take into account earth curvature, which can have more of an effect than you might think.

[underfire]

From Jepp on LFMT:

"I have compared these two items in question to the state VOR 30R chart. The FAF placement at 6.6 DME is correctly charted as the state chart shows. The level segment is added after the FAF and the descent angle is increased due to the stepdown fix. With an angle of 3.00°, the stepdown fix altitude requirement of 600’ will not be met. As a result, the angle is increased on the chart to show the angle that is needed to clear this altitude. Since the angle is raised, a level segment is needed to be added to show the appropriate place to start the descent."

Interesting...

[BOAC]

See? I told you it was a 'CFDA':p

[Capn Bloggs]

The FAF placement at 6.6 DME is correctly charted as the state chart shows.
The state chart has the FAF at 6.5DME FJR and 6.2DME FG.

The level segment is added after the FAF and the descent angle is increased due to the stepdown fix. With an angle of 3.00°, the stepdown fix altitude requirement of 600’ will not be met.
The minimum segment altitude is 580ft from the FAF all the way in to FG. The 600 (594) at FG looks to me like the actual 3° profile altitude (note a similar depiction at the OM). The italics are confusing...the limiting altitudes should not be italicised.

Jepp needs to re-think it's chart, methinks! :ok:

[underfire]

Bloggs, all,

Concur, I asked them to clarify the addition of the level segment beyond the FAF. When they added the level segment beyond the FAF, it forced the 3.13° to make it work.

The State procedure works fine with the 3° profile. (assuming a 50' TCH, which, well, even they appear to assume)
The ALT AD of 17 clears up the descrepancies. I dont deal with these type of procedures, so the ALT AD, REF HT ALT AD feature is not something I am familiar with. All of the altitudes have been rounded up by 17.

Is this due to the VOR offset/elevation?

I have asked Jepp to clarify ALT AD as well.

Here is the entire State Chart...Can someone post the entire Jepp Chart?

http://i61.tinypic.com/5bpi5u.jpg

[Capn Bloggs]

Underfire, the chart you posted isn't the same as the earlier one you posted. It has the FAF at 6.6 DME FJR (as Jepp says) and no profile check at the OM. :confused:

[Capt Fathom]

Can't quite work out what most of this has got to do with the original post?

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

[underfire]

Bloggs,

You are correct. The second one I posted is the correct chart...sorry about that

Jepp has the FAF in the same location, the issue is the level segment past the FAF and different GPA.

[Capn Bloggs]

That's OK, UF. Really odd Jepp using 3.13°. The chart clearly shows 3° misses the FG step by 20ft.

[underfire]

no worries...

What is ALT AD of 17? I note all of the altitudes have 17 added to them...
600 (583), 2000 (1983), etc..

[Capn Bloggs]

What is ALT AD of 17? I note all of the altitudes have 17 added to them...
600 (583), 2000 (1983), etc..
Must be a French thing. Means Aerodrome Altitude 17ft. Threshold altitude is 6ft. The bracketed values are the "AAL" heights.

[underfire]

interesting, and yet the MDA (H) uses 20 feet

Its funny, they have all of this useless data on the chart, yet leave out TCH. :}

[rennaps]

For non-precision approaches the Tan 3° formula is OK for nominal altitudes.
However for Precision approaches the Earth curvature should be taken into account which will raise the nominal glide path altitudes depending on the distance away from the touch down point.