Descent on the ILS
Joined: Nov 1999
Posts: 647
Likes: 19
From: East side of OZ
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.
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.
Thread Starter
Joined: Mar 2006
Posts: 354
Likes: 39
From: s england
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 👍
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 👍
Joined: Dec 1998
Posts: 54
Likes: 0
From: Denmark
You can follow the GS outside of the final segment, but not use it to go below any charted minimum altitudes.
Joined: Mar 2002
Aviation Qualifications: ATPL
Posts: 9,226
Likes: 995
From: Seat 1A
Originally Posted by Twang
Having to correct to the GS from above inside the FAP/ 10 nm will cause a potential increase in workload.
"Having to correct to the GS from above inside the FAP/ 10 nm will cause an
Joined: Aug 2001
Posts: 696
Likes: 10
From: Shropshire
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
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
