Primarily for general aviation discussion, but other aviation topics are also welcome.
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By Iceman
FLYER Club Member  FLYER Club Member
#1890024
Surely, the reason for the offset ILS at Lydd is to keep it clear of D141.

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Iceman 8)
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By chevvron
#1890053
No; as the localiser is not on the runway centreline, there is a 5 deg offset with the DME zero range at the runway 21 threshold.
The LLZ final approach course is 207 deg M whereas the runway QDM is 212 deg M due to the LLZ being positioned some distance to the left of FAT.
What I'm trying to say is, the offset is beause the LLZ cannot be positioned in line because of its location presumably because the owners decided to cut costs by installing on an already prepared and stable surface as opposed to expensively excavating new foundations; Danger Activity can be 'managed' by LoAs as with other DAs.
By Ibra
#1890059
It does not explain why RNP21 has an offset as well? (maybe for easy approval they went for overlay of existing ILS21 final & missed?)

Although RNP21 is not even an LNAV overlay of LOC, it has greater offset to D141 than ILS21: it's designed with legacy lateral guidance for RNP APCH with 0.6nm margins rather than using Localizer Performance with angular guidance (e.g. LPV or new LNAV with SBAS TSO146 box)

As @Iceman said, I think it's DA141 that prevents the straight-in, the fire range is prohibited area when active and definitely needs a serious LOA :thumleft:

There are not that many GPS offset approaches (except overlay on offset LOC, overlay on some legacy VOR/NDB non-collocated nav aids with huge offset junk, or "pure cloudbreaks" and VFR bellow MDH due to terrain), the majority of RNP GPS approaches are simple IFR straight-in guidance down to 0nm of the runway threshold...
By Ibra
#1890087
RNP approaches, especially using GPS, can be designed with no dependency to existing ground aids: RNP21 with GPS should not require any LLZ antenna at all (near FAT or away should not matter), the only technical reasons why one can't do GPS IFR straight-in is airspace (what prevents having them in UK) & terrain (what prevents having them in Switzerland)

Lydd DME/VOR is likely required as backup for those flying RNP21 approach with FMS and no GPS, still does not explain their choice of an offset RNP unless VOR/DME signals are dodgy along coastal line? at least it's the reason why NDB is 100% over land

The only suspect for RNP21 offset is D141 :wink: by comparison RNP03 to opposite runway cuts an IFR straight-in through DA/RA but it's unusable when DA044 is active...better have something that works all the times even with a funky offset than nothing at all form time to time :thumleft:
By AlanM
#1890099
Ibra wrote:still does not explain their choice of an offset RNP unless VOR/DME signals are dodgy along coastal line? at least it's the reason why NDB is 100% over land


:?: :shock: :?:

Wow
By Bathman
#1890101
What instrument vapproachrs did it have when it first opened in the fifties?

Did the now closed runways have instrument approaches to them?
By Ibra
#1890109
AlanM wrote:
Ibra wrote:still does not explain their choice of an offset RNP unless VOR/DME signals are dodgy along coastal line? at least it's the reason why NDB is 100% over land


:?: :shock: :?:

Wow


Haha I was just guessing any other reason why RNP21 with DME/INS will have an offset than the obvious D141, please disregard :lol:
By AlanM
#1890113
Ibra wrote:
AlanM wrote:
Ibra wrote:still does not explain their choice of an offset RNP unless VOR/DME signals are dodgy along coastal line? at least it's the reason why NDB is 100% over land


:?: :shock: :?:

Wow


Haha I was just guessing any other reason why RNP21 with DME/INS will have an offset than the obvious D141, please disregard :lol:


Disregarded and Struck from my memory…… :P
By Uaglio
#1890127
RNP approaches, especially using GPS, can be designed with no dependency to existing ground aids:

Is it not the case that when RNP approaches were first approved by the UK CAA , the associated Safety Case specified that there had to be a nearby navaid with which pilots could conduct a gross error check prior to commencing the approach ? if that happens in real life...... Operationally the IAP may not depend on the navaid, but they did feature in the approval process
By Ibra
#1890144
Uaglio wrote:Is it not the case that when RNP approaches were first approved by the UK CAA , the associated Safety Case specified that there had to be a nearby navaid with which pilots could conduct a gross error check prior to commencing the approach ? if that happens in real life...... Operationally the IAP may not depend on the navaid, but they did feature in the approval process


For private operators, the regulators were aware of their limitation vis a vis loss of GPS (it's the only way to comply with RNAV1 and RNP APCH), for these guys there has been always a requirement in NCO to plan for conventional extraction to MSA on missed or departure (NavAids or Radar) and to plan conventional approach at destination or alternate, my understanding this will get replaced in 2022Q2 where conventional planning will get removed but RNP minima in planning will get beefed up (like VMC++) unless there is a backup like ILS is selected...of course not a reason for GPS offset, with straight-in you have better minima on GPS besides you cab see the runway like a fairy and land on it like a princess :wink:

As of now there is no reason to have any sort of NavAids at aerodrome to be able to operate RNP and usually those from nearby ADs their signal is lost bellow 500ft agl (anyone who has flown cloudbreaks on BNN or BPK would know about this :lol: )

Indeed, initially UK CAA went for something really bold like requiring NDB to go missed from GPS approach :eye:

PS: that is doom scenario for GPS loss, anyone who flies IMC in UK should know how to climb to MSA without any GPS guidance, people without PBN do it all day along with barely a wet compass before picking a VOR radial, when was the last time someone got offred conventional NavAids SID departure or STAR arrival in UK? I was given Brecons departure from Cardiff once (two arrivals to Heathrow & Stansted before going to Denham or Cambridge), most of the time people will magically disappear from the runway and move to above MSA following crystal balls in IMC to then fly to DET or CPT, direct some IAF or go on radar vectors in airspace :lol:
By Atcomgr
#1890342
chevvron, all the reasons you have given for the Lydd IAP’s to be offset are not correct. Not one of them.

chevvron wrote:No; as the localiser is not on the runway centreline, there is a 5 deg offset with the DME zero range at the runway 21 threshold.
The LLZ final approach course is 207 deg M whereas the runway QDM is 212 deg M due to the LLZ being positioned some distance to the left of FAT.
What I'm trying to say is, the offset is beause the LLZ cannot be positioned in line because of its location presumably because the owners decided to cut costs by installing on an already prepared and stable surface as opposed to expensively excavating new foundations; Danger Activity can be 'managed' by LoAs as with other DAs.
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By Atcomgr
#1890343
Absolute rubbish.

chevvron wrote:I believe it was acquired from Manston after that airfield closed (hopefully to be re-opened next year) so maybe there's some reason the CAA don't allow them to use it.
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By Ibra
#1890365
Atcomgr wrote:chevvron, all the reasons you have given for the Lydd IAP’s to be offset are not correct. Not one of them.


What is the reason for offset in the ILS @Atcomgr ? D141 or something else?