Simple ;) Jeppesen question.

[D-OCHO]

Following problem occurs.
Our company is flying regularly into an airport in the north of Spain (LEBL).
After intercepting the ILS (LOC and G/S) and checking the mandatory check altitudes (cq. OM) we find out that we are always min. 100 ft. above these altitudes and this with the good QNH set in the Altimeter. This can be of course an dangerous situation. What is now the procedure for your calculation of your D/A. For example do you have to raise your D/A by min. 100 ft. to stay save.
:confused:
We checked with Jeppesen but could not find anything about this subject.

Thank you for your interest and happy landings :}

[bsevenfour]

A very interesting problem. There are a number of possibilities (Forgive me if I'm stating the obvious in them!)

1- You have an overreading altimeter. This can of course be a result of cold weather operations but I think we can rule this out in the north of Spain. Also you obviously do not get the same problem in other airports you fly into so I think we can rule this out.

2- Perhaps the check heights that Jeppessen have provided were incorrectly printed. You can in this case do a rough check. At the bottom of the profile view you should be able to determine the distance the OM is from the runway threshold. Multiply this figure by 316ft for a 3 degree path. The add the threshold crossing height and the TDZE. This should give you a close approximation of the crossing height required.

3- The only other thing I can think of (Once again forgive me if I'm stating the obvious to you but I believe this can be a bit of a trap with the Jepp plates) is that the figures you are looking at are not the glideslope crossing heights of the OM but the minimum crossing heights for a LOC only approach that may well be also shown on the profile view. The glideslope crossing height should have a little 'GS' preceeding them.

Some questions I would like to ask:
On landing did the altimeter reading agree sensibly with the airfield elevation ?

What are you using to confirm the position of the outer marker i.e. are you using sound alone or are you using sound, or a DME reading or needle if it is a LOM ?

[411A]

It is not all that unusual to be plus/minus 100 feet at the final fix/GS height, depends on temperature.
When you are at DA (200agl, for example) your DA will be correct/accurate.

Not necessary to add corrections.

[bookworm]

Can you tell us which approach (i.e. which runway) and the altitudes/heights in question please?

[777AV8R]

The fix crossing altitude on the altimeter may be incorrect because of the outside air temperature.

That said however; when temperatures are below standard, that is when you DO consider making the corrections...not so much a worry until near 0C, but below....then the aircraft will be below DH altitude and thus needs the correction.

I think we've beat this one to death before some time ago.

[bookworm]

Did we also beat to death how we came to have an ISA-20 day in Barcelona in August? :)

[OzExpat]

With due regard for the post by 411A, with whom I can almost always agree, I can't agree on this occasion. All other things being equal, if your altimeter is reading higher than it should at the FAP, in comparison with the chart specification, you need to add that discrepancy to the DA. This is because your altimeter will continue to read high as you reach the real DA.

If you rely on the altimeter, as is usual for Cat 1 ILS, a 100 feet over-read means you will actually be at 100 FT AGL when the altimeter tells you 200 FT (yeah, yeah, I know it don't read height AGL, but you get the picture, okay?). So, being at 100 FT AGL, when you should be at 200 FT, is a bad idea, especially if you still can't see... :eek:

[D-OCHO]

For an plate of the actual approach checkBilbao VOR / DME- ILS / DME RWY 30 (PDF file) (http://ais.aena.es/img/ad2/lebb/lebb-iac4.pdf). This is the Spanish AIP version and as far as I can see this is the same as the Jeppesen version.
Further the aircraft we are flying are brand new (< 2 years old) Bombardier Dash 8's with dual FMS. Altimeters getting there reading trough the ADC. The discrepancy occurs on hot and cold days so temperature is no factor.
8.00 DME BLV/ 6.50 DME ILS crossing altitude 2570 ft. on Jeppesen this is the mandatory G/S altitude on that DME. We are always around 2.650 ft. Altitude on ground is perfect as per plate.
Only other problem is the G/S angle 5,94% / 3,4 degrees so a little bit steeper than usual.

[TopBunk]

D-OCHO

Have you considered that the Spanish Air Pilot is wrong? Maybe the altitude has been wrongly surveyed?

[D-OCHO]

D-OCHO Have you considered that the Spanish Air Pilot is wrong? Maybe the altitude has been wrongly surveyed?
Let say it this way: we hope not.:sad: :rolleyes:
But yes we have been thinking about it.

[411A]

OZEXPAT,

Ah, not always.
There can be very distinct differences between the temperature at 1500agl (for example) and the surface.
Considering inversions and all...very common in the Middle East/South Asia (India for example).
Not likely in Spain however, from my experience.

[LEM]

The temperature deviation doesn't need to be corrected at DH.
There would be a rule if it were the case.
That's because the same temperature daviation is going to "compress", let's say, by a certain percentage the amount of air below you: a 100 feet deviation at the outer marker equals almost nothing at DH.
And these deviations are already considered when they establish the minima. Enroute, for example, a QNH of 960Hpa is considered, if I recall correctly.

[bookworm]

The numbers on this one look rather odd. Isn't the DME usually zeroed to the threshold, even if displaced? FAP-THR 11.60 DME in the timings implies that's the case.

In that case, we should be looking at 1130 and 2400 height at 3.0 and 6.5 DME respectively, rather than 1230 and 2442 as on the chart.

And what's the 540 (412) annotation? It can't be alt(height) at 0 DME as it's too high. It doesn't seem to be related to the LOC only MDA(MDH) of 700(580). Is it on the Jepp plate too?

So what gives?

[reynoldsno1]

Another factor can be earth curvature/beam refraction corrections, which can be up to 100ft at about 12nm from the array...

[TopBunk]

bookworm

I see you post from the UK - do you fly much outside the UK?

If so you would be aware that whilst the UK tend to put in a put the DME at the runway midpoint and then put a time delay factor in to give 0 DME for end touchdown point, this is much less common elsewhere (unfortunately).

[bookworm]

TopBunk

A good point, but in this case:

a) There's a separate DME on the VOR
b) Rwy 30 is the only one with an ILS -- other approaches are VOR/DME on the BLV

It seems a bit daft to dedicate a DME to the ILS and then not zero it to the threshold. I suppose it's possible that the threshold was displaced after the DME was set up? But the aerodrome chart doesn't suggest that. The threshold, BTW, is displaced about 500 m, which would mean a height at runway end of 100 ft above the threshold crossing height.

The earth curvature issues are interesting but doesn't seem to reflect the magintude of the discrepancy here.

[TopBunk]

bookworm

It seems a bit daft to dedicate a DME to the ILS and then not zero it to the threshold
Yes it does, but that is what happens in 80% of the world, ie most of it outside the UK,as I said before. Next time I get on board I'll look at the Aerad, oops, soory Thales or Racal, charts and investigate if there are any explanatory notes that may help further.

[SimJock]

out of interest I tried this on a level D sim and got different results too.

DME BLV ALT RA
10.13 3200
8.00 2460
4.50 1200 1080
2.50 500 380

Our data is based on that supplied by Aerad whose "Europe and Middle East Supplment" shows the ILS to have an angle of 3.35 degrees which is I presume why our heights are lower than the published chart. Jeppesen plates show 3.4 degrees as per the spanish plate. Its enough of a difference in angle to affect the altitudes but difficult to understand the 200ft difference at 2.5d. I'll crank it up to 3.4 degrees and see what difference it makes.

For the record, ILS DME zero at displaced threshold, BLV DME 1.5.