Descent on the ILS

[sudden twang]

It’s not without some trepidation that I ask this so please be gentle and offer verifiable answers rather than opinion.

You are flying into an airfield surrounded by terrain. You’re cleared via a STAR which ends at the IAF you are cleared to descend to the altitude at the IAF which on the chart is the same figure with a line underneath it a distance of 18 nm to touchdown and is approximately a 3 degree slope. You are cleared ILS.
But it’s hot ISA + 10 you hit the IAF at the defined altitude intercepting the LOC with a half scale fly down on the GS.
There are 3 other step down altitudes on the chart each with a black line below them.
The last of the 3 is the IF at 10 nm
Do you
a) arm the GS capture the glide from above?whilst that will certainly mean the indicated ( as opposed to a baro corrected altimeter) will be below the charted altitude
or
b) use FMC /FMGC to descend ensuring you are at/above the charted altitudes until after the FAF/P then correct to the GS?

I appreciate this isn’t a terrain issue but what do ATC expect outside the FAF could they clear low level VFR traffic below you at say 10/15 nm?

[rudestuff]

A verifiable answer requires a verifiable question. Where exactly is this approach? It doesn't seem all that realistic that you would have an at-or-above constraint that's above the glide slope.

[SW1]

What he mentions sound very much like Alicante runway 10 ILS Z

I would hit the 18 DME at the required altitude with at flaps 1 then arm the approach as you’re cleared. You should be hitting the step down altitudes at approximately the altitudes you need to be at.

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[FullWings]

What SW1 says.

If you are cleared for the ILS / cleared descent with the glide slope, then you would expect ATC to have checked for any conflicting traffic. Established on an ILS in ISA+ conditions, you will always indicate below charted altitudes at checkpoints irrespective of how far from the airfield you are, but you know that in advance and why that is.

[sudden twang]

It doesn’t matter where it is the principle is the same ( except perhaps local variations).
in this case the IAF altitude in ISA is above the glide presumably for terrain.

The Q is outside of ie. before the FAF can you follow the GS if your indicated altitude is below that on the chart when it has a black not below line on the chart because it’s hot?

[sudden twang]

What SW1 says.

If you are cleared for the ILS / cleared descent with the glide slope, then you would expect ATC to have checked for any conflicting traffic. Established on an ILS in ISA+ conditions, you will always indicate below charted altitudes at checkpoints irrespective of how far from the airfield you are, but you know that in advance and why that is.

we crossed FW
i think cleared descent with the GS is different do you have a reference?

[sudden twang]

What he mentions sound very much like Alicante runway 10 ILS Z

I would hit the 18 DME at the required altitude with at flaps 1 then arm the approach as you’re cleared. You should be hitting the step down altitudes at approximately the altitudes you need to be at.

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/1500x2000/img_0129_2c2ac4c7b8fbe592492f93b95e59e09f71c8b58c.png
it’s not ALC but it’s similar I agree but the fixes outside the FAF are a little less challenging. If y the 4200 at D14.6 was say 5000 would you accept descent on the GS if at 14.6 the baro read 4600’.

[8314]

Yes, because it’s indicated, physically you‘ll still be at 5k.

[sudden twang]

Yes, because it’s indicated, physically you‘ll still be at 5k.
yes of course 8314 logically speaking but is there a reference ? I cant find it it Pans Ops

[Bradley Hardacre]

It's not spelt out in PANS Ops per se but you need to know which parts apply, eg where a MHA differs from a MORA and the difference between a FAF and a FAP. I wouldn't be arming the approach at 18d in ALC as the GS DOC is only 10nm.
If you enter the ISA deviation into the FMC it will correct the vertical profile, but that won't help with the hard altitudes as they correspond to baro derived indicated altitudes and the ISA + would have you at an true altitude which is higher than the indicated altitude and the FMC vertical profile and the raw GS would show you are high on profile because you are.
It's the same problem as a cold temperature correction to indicated altitude, but has an energy issue rather than a terrain threat, maybe use a higher drag profile in order to avoid the risk of high energy as you will need a higher descent rate until inside the GS DOC.
It is also possible that the hard altitudes apply only to the Localiser procedure and not the full ILS as the FAP is not a hard altitude unlike the FAF even if they are co located, the FAF is for the non precision approach and the FAP is for the precision approach where it is assumed that the electronic GS inside the DOC is the more accurate as it is not subject to temperature and pressure errors.
Cleared descent with the GS when established on the localiser is a UK difference from ICAO due airspace, elsewhere you will be cleared for the approach from before the IAF.

[8314]

PANS OPS 8168 gives you 25 miles of usable LOC, so this is obstacle checked, so you are good to go. If shorter it should say on your IAC.

[sudden twang]

PANS OPS 8168 gives you 25 miles of usable LOC, so this is obstacle checked, so you are good to go. If shorter it should say on your IAC.

What about airspace checked ? If there is no terrain there could be VFR traffic in uncontrolled airspace

[Vessbot]

In FAA land (the only one I can speak for) all stepdown altitudes prior to the FAF must be complied with. And yes, on hot days this may keep you above the glideslope. Probably comes up very seldom, but we do have notes at some airports to watch out for it (ORD and LAX, not by coincidence both often intercepting the localizer very far out).

[Bradley Hardacre]

PANS OPS 8168 gives you 25 miles of usable LOC, so this is obstacle checked, so you are good to go. If shorter it should say on your IAC.
Yes it does and you are correct, but we're not talking about the LOC, ICAO Annex 10 gives you a range of 10nm for a standard GS unless a different range is published in the individual airfield AIP and since the subject of the discussion is the vertical bit, it is worth noting that it is different to the LOC which covers the horizontal bit.

I would add that the minimum altitudes for the approach are published precisely because the GS cannot be relied upon outside it's DOC.

[vilas]

Yes it does and you are correct, but we're not talking about the LOC, ICAO Annex 10 gives you a range of 10nm for a standard GS unless a different range is published in the individual airfield AIP and since the subject of the discussion is the vertical bit, it is worth noting that it is different to the LOC which covers the horizontal bit.

I would add that the minimum altitudes for the approach are published precisely because the GS cannot be relied upon outside it's DOC.
You are correct. Beyond the distance given glide slope doesn't ensure terrain clearance.

[Bradley Hardacre]

Thanks Vilas, the Glideslope performance criteria is given in ICAO annex 10 Ch 3 ref 3.1.5.3.1 "coverage" and the GS should not be used outside 10nm unless promulgated in the individual Airfield AIP or by NOTAM.

[Voidhawk]

The GS only provides terrain clearnce in the final segment, which starts at the FAP.

Before the FAP, the minimum altitudes of the intermediate and initial segments provide terrain clearance. As the descent gradients in the initial and intermediate segments can be greater than in the final segment, these minimum altitudes could be above the GS - whether it’s a hot day or not.

[galaxy flyer]

The GS only provides terrain clearnce in the final segment, which starts at the FAP.

Before the FAP, the minimum altitudes of the intermediate and initial segments provide terrain clearance. As the descent gradients in the initial and intermediate segments can be greater than in the final segment, these minimum altitudes could be above the GS - whether it’s a hot day or not.

Those charted altitudes assume standard atmosphere, which is why we temp compensate at cold temperatures. No reason not to do so for hotter temps than standard, but it’s not done.

[LapSap]

Let me guess.
VHHH 25R approach?
What an absolute joke.
Waiting for somebody to clip the top of Tai Mo Shan and they can kiss goodbye to their US$20 Billion (yes you read correctly) 3rd runway.

[Domobaby]

It doesn’t matter where it is the principle is the same ( except perhaps local variations).
in this case the IAF altitude in ISA is above the glide presumably for terrain.

The Q is outside of ie. before the FAF can you follow the GS if your indicated altitude is below that on the chart when it has a black not below line on the chart because it’s hot? If you’re on the GS and within the parameters it is deemed to be reliable then it should be fine? GS is a physical projected beam at a height above the ground. If you’re on the glide and you’re happy with the reliability of its identification then all should be good.

BARO corrections work both ways hot/cold to MOCA, MFRA, MSA, Platforms. Of course cold the more significant one

had an interesting TCAS event in to a US Airfield whilst on the glide. Hot temps ISA +15 territory. Airfield under the ILS has traffic departing to maintain an altitude below the ILS. We saw a -700ft Sep on the ND, ATC claimed it was more.

throws up the question that in warm altitudes an aircraft on the GS (fixed height above the ground) may not have separation to an aircraft maintaining an altitude below (in hot conditions).

[Bullethead]

Hey Twang,

Is the approach chart you are using for an ILS approach only for both ILS and LLZ approaches? If it is for a LLZ approach as well then don't the hard altitudes on the chart only apply to the LLZ approach and not the ILS?

Cheers,
BH.

[sudden twang]

BH
yes it’s also the LOC chart.


Thank you all for your inputs I’ll try to summarise:

You should be at or above the minimum altitude at the end of the Star which in this particular case is also the IAF.

On a hot day you’ll have a fly down on the GS and a fly up on the GS when it’s below ISA.

You should not rely on the GS for terrain clearance outside of 10 nm unless the chart states differently.

Many will follow the GS outside this distance.

IMHO you should cross other altitudes before 10 nm / FAP above the minimum charted altitudes.

Following the GS before the FAP on a hot day will have you below these altitudes indicated on the altimeter.

Assuming the GS is accurate outside 10 nm and you happen to be in the middle of the band width you will always be at the same vertical distance amsl if on the GS no matter what the temperature.

On a hot day you will have slightly increased terrain clearance if you adhere to the altitudes.

Following the GS on a hot day could mean infringing uncontrolled VFR airspace as the instance given a few posts ago.

Having to correct to the GS from above inside the FAP/ 10 nm will cause a potential increase in workload.

Your clearance will vary around the world.
“ clear ILS “
” once established on the LOC descend ILS”
” Cleared ILS XX cross xxx above xxxx”

Cheers all 👍

[Voidhawk]

You should not rely on the GS for terrain clearance outside of 10 nm unless the chart states differently.

Many will follow the GS outside this distance.

IMHO you should cross other altitudes before 10 nm / FAP above the minimum charted altitudes.
You must not rely on the GS for terrain clearance outside of the Final Approach Segment. The GS (ILS) obstacle assessment surfaces only extend to the FAP or FAF, ICAO Doc 8168, Vol II (and also slightly into the intermediate segment), so the GS is not checked for obstacles or terrain outside of the final segment.

You can follow the GS outside of the final segment, but not use it to go below any charted minimum altitudes.

[Capn Bloggs]

Having to correct to the GS from above inside the FAP/ 10 nm will cause a potential increase in workload.
Let me correct that for you:

"Having to correct to the GS from above inside the FAP/ 10 nm will cause an potential increase in workload." :ok:

[FlightDetent]

Fine discussion.

[TeeS]

It is worth noting that the obstacle assessment surface (OAS) method of calculating DA/DH for an ILS works completely differently to the obstacle clearance altitude (OCA) calculation used in a 2-D approach.

With the OCA, no obstacles may infringe the surfaces: however, with the OAS you expect obstacles to infringe the various surfaces of the OAS (ignoring the missed approach surfaces for now!). The highest obstacle that infringes the surfaces is then used to calculate the DA/DH by adding the relevant height loss margin for the applicable aircraft category, to the elevation/height of the obstacle.

The issue now is that if the controlling obstacle is early in the final approach segment then it must be high (otherwise it would not infringe) and therefore the DA/DH will be unacceptably high. Options at this point include increasing the glidepath angle above 3 degrees (favoured choice) or (if the obstacle is close to the final approach point) PANS-OPS currently allows the designer to introduce a final approach fix which, if the obstacle is below a 15% plane starting at the FAF earliest fix tolerance area at the intermediate segment OCA minus the intermediate MOC then it can be ignored. That is great; however, now that you have a FAF you can't start your descent from the intermediate OCA unless you are at the final approach fix (otherwise you reduce your clearance from the ignored obstacle!) so on an ISA + day, you will not be able to intercept the glideslope from below!

Typically the designer would use the 'Collision Risk Model' (CRM) to analyse the obstacle environment but that involves the casting of bones and use of magic!!

Cheers
TeeS