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Sari Ils Dme Rw31

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Sari Ils Dme Rw31

Old 4th Jan 2011, 16:13
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Sari Ils Dme Rw31

Hello everyone!

I have been practicing the following plate in the sim and have a few questions, if someone can please explain them.



1. On the briefing strip point number 2 mentions “Attention: MSA values include the Cataratas CTR”. Now I understand that the MSA provides a 1000 feet vertical obstacle clearance within a 25NM radius from the VOR but mentioning that it includes the Cataratas CTR…wouldn’t it always include the CTR of an aerodrome given that the CTR has only got to cover from the centre of the airfield 5NM in the directions of approach and departure?
2. In the vertical cross-section of the procedure it mentions that the glide slope needs to be checked at LMM (310 kHz) at 1170 feet, which corresponds to D0.9 IG/D2.2 IGU. Checking at the bottom of the plate it mentions that the minimums are at 1116 feet. Is it me or do you guys think it is pointless in checking the glide slope a mere 54 feet above minimums?
3. In the table of rates of descent it mentions “FAF to MAP”. Looking once again at the vertical cross-section of the procedure am I correct in saying that it would be from D4.0 IG/D5.4 IGU? If so, when I apply rough calculations they don’t seem to match with the distance to the runway:
120 knots = 2’ 48” = 2.8’ = (2.8 minutes x 2 NM/minute) = 5.6 NM
D4.0 IG/D5.4 IGU to THR = 3.1 + 0.6 = 3.7NM
I think I’m also correct in saying that it can’t be from D6.5 IG/D7.9 IGU because that is the FAP.

Thanks in advance.

Last edited by Mohit_C; 4th Jan 2011 at 16:31.
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Old 4th Jan 2011, 16:44
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1- some CTRs are very big. Although this note maybe means the MSA is only valid in CATARATAS CTR, and not beyond it (note the borders).

2- 3000 ft at 6,5 from the ILSDME is enough for G/S check. We don't check G/S at the MM or equivalent, but if we want to do it, we have the info on the plate.

3- the FAF is depicted a a maltesse cross, which in this case is the fix at 6,5 miles. the 4.0 miles fix is just a stepdown for the loc only approach. It means that you can't go below 2200 ft before that point.

4- I wish I could fly that approach in VMC!
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Old 4th Jan 2011, 17:19
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Mohit C:

The non-precision (LOC) MAP occurs .5 miles prior to the threshold.

The descent gradient for the LOC procedure is measured from the non-precision FAF (6.5/7.9) to the threshold at TCH:

3,000 - (916 + 52) divided by 6.2 (distance from NPA FAF to threshold) = 327.74' feet per mile.

The descent gradient from the NPA FAF to the 2,200 stepdown is slightly different than the gradient from that point to the threshold. But, the rates shown are for the entire NPA segment and are rounded (unlike the precision "GS" descent gradients).

The procedure does not require checking the GS at the MM. The "1170" is just there for reference, should you choose to use it.
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Old 4th Jan 2011, 21:37
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I think several factors are interfering with your calculations.

The IG/IGU DME ranges aren't the same as the distance to the threshold because the DME antenna aren't at the threshold. The two different DME's antenna aren't even in the same place. Think of the way to read it as 'When the DME I'm using reads x I will be 3.1nm from the LOC only MAP/3.7nm from the threshold'.

DME distances are rounded to the nearest .1 nm so a slight error may occur. Compare the IG vs IGU ranges vs distance between the IG 4.0 stepdown and the LMM.

DME is a slant range, not an along track distance. The geometry changes as you get closer so that can also affect DME readings vs actual distance.

Speaking of geometry changes, if the DME antenna is located elsewhere on the field than lateral geometry changes will also affect DME reading vs actual distance.

Also the glideslope is typically to the aiming point on the runway ie 300m/1000' further along the runway from the threshold so glideslope calculations will be slightly out if you use the threshold distance.
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Old 5th Jan 2011, 15:44
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Tinstaafl:

DME is a slant range, not an along track distance. The geometry changes as you get closer so that can also affect DME readings vs actual distance.
Slant range is so slight at IAP altitudes that it is ignored in procedure construction.
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Old 5th Jan 2011, 16:36
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How long is the runway, Mohit? Does the aerodrome chart tell you where the ILS DME is?
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Old 5th Jan 2011, 16:58
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The runway is 10,827' in length. The VOR is located to the left side fairly near the far end. The difference in DME distances and the symbol in the plan view shows the DME for the ILS to be near the approach end of Runway 31.
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Old 5th Jan 2011, 22:29
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DME for the ILS to be near the approach end of Runway 31
I would expect it to be co-located with the GP aerial - TCH is 52ft or 15.85m. With 3 deg GP, GP origin is at 15.85/tan 3 deg = 302.4m (0.16NM) after the THR.
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Old 5th Jan 2011, 23:24
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reynoldsno1:

I would expect it to be co-located with the GP aerial - TCH is 52ft or 15.85m. With 3 deg GP, GP origin is at 15.85/tan 3 deg = 302.4m (0.16NM) after the THR.
I would agree.

The FAA does it differently. When an ILS has a DME it is generally located to the side of runway near the far end.

Then, some countries bias the ILS DME to read zero at the threshold.
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