Airbus ILS course
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Hi michelda,
I think the tolerance check is the last opportunity to confirm the correct ILS QDM is in the box (it's too late to do anything about it by this stage except GA - but I think the "logic" *1 is:
FCOM 1.22.30 P50:
ILS APP Mode Arming Conditions ... "The aircraft is above 400 feet RA," etc.
FCOM 1.22.30 P 53.
"Align sub-mode.
Align is a sub-mode of LAND also known as "decrab" that lines up the aircraft's axis with the ILS course at approximately 30 feet. It is not displayed to the crew."
On TriStars - it was quoted as being acceptable if ILS QDM was within 6 degs of Runway QDM - but I cannot find any Airbus equivalent limit published.
*1 = oxymoron.
I think the tolerance check is the last opportunity to confirm the correct ILS QDM is in the box (it's too late to do anything about it by this stage except GA - but I think the "logic" *1 is:
FCOM 1.22.30 P50:
ILS APP Mode Arming Conditions ... "The aircraft is above 400 feet RA," etc.
FCOM 1.22.30 P 53.
"Align sub-mode.
Align is a sub-mode of LAND also known as "decrab" that lines up the aircraft's axis with the ILS course at approximately 30 feet. It is not displayed to the crew."
On TriStars - it was quoted as being acceptable if ILS QDM was within 6 degs of Runway QDM - but I cannot find any Airbus equivalent limit published.
*1 = oxymoron.
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rudderrudderrat:
Where was this quoted? It certainly wasn't a procedure at my airline. Rather, an autoland below specified visibility minimums was limited to a crosswind component of 10 knots. That took care of things nicely without having to observe a compass heading that might not be all that accurate.
I did quite a few 1011 autolands in clear weather with a 25 knot crosswind. It did just fine. But, the margins in that strong of a wind were outside the statistical envelope for a low-visibility autoland, one where the runway might not be seen until after the main landing gear had touched down.
On TriStars - it was quoted as being acceptable if ILS QDM was within 6 degs of Runway QDM - but I cannot find any Airbus equivalent limit published.
I did quite a few 1011 autolands in clear weather with a 25 knot crosswind. It did just fine. But, the margins in that strong of a wind were outside the statistical envelope for a low-visibility autoland, one where the runway might not be seen until after the main landing gear had touched down.
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in LAND mode LOC and GS are locked. Anything more than 1/4 dot (LOC) or 1 dot (GS) will give you trouble when passing 200', so you check that it is locked within these margines.
1 dot LOC = 0.8 degrees
1 dot GS = 0.4 degrees
1 dot LOC = 0.8 degrees
1 dot GS = 0.4 degrees
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Hi Michelda,
I found this in my Instructor support manual:
"Incorrect ILS CRS (Δ 5° with runway CRS) leads to incorrect autopilot DECRAB.
Check ILS CRS at 350 ft. If incorrect, continue approach down to CAT II minima (AP must be set OFF at 80 ft latest)."
Kind Regards,
Martin Dahlerup
I found this in my Instructor support manual:
"Incorrect ILS CRS (Δ 5° with runway CRS) leads to incorrect autopilot DECRAB.
Check ILS CRS at 350 ft. If incorrect, continue approach down to CAT II minima (AP must be set OFF at 80 ft latest)."
Kind Regards,
Martin Dahlerup
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I made some calculations to relate drift angle to groundspeed.
5 deg drift angle corresponds to crosswind well below the 20 k limit for autoland. At a high ground speed of 160 k, 5 deg would correspond to just 13.3 k CW. To reach 20 knots crosswind and 5 deg drift, the GS would have to be 240 knots.! At a "normal" groundspeed of say 145 knots, 5 deg would correspond to 12 knots.
The 5 deg limit seems to be advisory in nature? the extract from Instructor Manual is
"Some requirements are specific to CATIII with NO DH, because an Autoland is compulsory. These are not
necessarily monitored and thus not indicated.
• Incorrect ILS CRS (Δ 5° with runway CRS) leads to incorrect autopilot DECRAB."
The FCOM extract (PRO-NOR-SRP-01-70 P 14/32) merely calls for the PF to "check ILS Course", and does not actually ask you to DO anything about it. Nor does it refer to a limit.
I found a limit in the AFM saying "rudder travel limiter required to be serviceable for autoland with crosswind over 12 knots".
If everyone who flew a Cat III approach and saw a drift angle over 5 deg (how many of us have looked?) went around, there would be a lot fewer landings. I guess that the Instructor Manual is advisory in nature. The decrab not being completed would mean that the aircraft would touchdown with some drift still on. This would mean a landing gear structural limit coming into question. Does anyone have an idea what that is? In terms of drift angle or crosswind limit for landing without decrab, possibly?
5 deg drift angle corresponds to crosswind well below the 20 k limit for autoland. At a high ground speed of 160 k, 5 deg would correspond to just 13.3 k CW. To reach 20 knots crosswind and 5 deg drift, the GS would have to be 240 knots.! At a "normal" groundspeed of say 145 knots, 5 deg would correspond to 12 knots.
The 5 deg limit seems to be advisory in nature? the extract from Instructor Manual is
"Some requirements are specific to CATIII with NO DH, because an Autoland is compulsory. These are not
necessarily monitored and thus not indicated.
• Incorrect ILS CRS (Δ 5° with runway CRS) leads to incorrect autopilot DECRAB."
The FCOM extract (PRO-NOR-SRP-01-70 P 14/32) merely calls for the PF to "check ILS Course", and does not actually ask you to DO anything about it. Nor does it refer to a limit.
I found a limit in the AFM saying "rudder travel limiter required to be serviceable for autoland with crosswind over 12 knots".
If everyone who flew a Cat III approach and saw a drift angle over 5 deg (how many of us have looked?) went around, there would be a lot fewer landings. I guess that the Instructor Manual is advisory in nature. The decrab not being completed would mean that the aircraft would touchdown with some drift still on. This would mean a landing gear structural limit coming into question. Does anyone have an idea what that is? In terms of drift angle or crosswind limit for landing without decrab, possibly?
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A320 A/p limits
On a CAT 2 approach, incorrect inbound course at Land Green.
FCOM note stipulates that AP should be disconnected by 50ft(ie before Flare where the aircraft would be aligned to the incorrect runway QDM).
Question I have is that if it's going to be a premeditated manual landing from 350ft why isn't the normal 80ft Auto Flight limit applied?
FCOM note stipulates that AP should be disconnected by 50ft(ie before Flare where the aircraft would be aligned to the incorrect runway QDM).
Question I have is that if it's going to be a premeditated manual landing from 350ft why isn't the normal 80ft Auto Flight limit applied?
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Salamander, I am under the impression that you think we have to check the drift at 350 feet. Am I correct?
Anyway, I always wondered why we have to check the course at 350 feet (busy time!). Why not before the approach? It is not going to change unless you change runways. Or am I missing something?
Anyway, I always wondered why we have to check the course at 350 feet (busy time!). Why not before the approach? It is not going to change unless you change runways. Or am I missing something?
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About AP disconnection, ie aborting the autolanding. This is required only if the red AUTOLAND light comes on. This does not happen regardless of the strength of the crosswind. Drift angle is not ever monitored, only beam drift or loss of signal. And below 100 ft, even a single autopilot failure will not change the landing status and an autolanding would be executed without any capability downgrade being indicated.
At 350 ft we ARE required to check the drift angle (refer Task Sharing, "check ILS Course"). This is not the ILS Final Approach Track which is fixed, but the actual aircraft course to keep you tracking the ILS beam. This may not correspond EXACTLY to drift angle, because the aircraft might possibly be correcting for a (small) beam deviation caused by changing winds. Also, this drift is only indicative of the crosswind at landing, because the winds would change as we go down.
Thanks a lot, Clonecity and PENKO.
At 350 ft we ARE required to check the drift angle (refer Task Sharing, "check ILS Course"). This is not the ILS Final Approach Track which is fixed, but the actual aircraft course to keep you tracking the ILS beam. This may not correspond EXACTLY to drift angle, because the aircraft might possibly be correcting for a (small) beam deviation caused by changing winds. Also, this drift is only indicative of the crosswind at landing, because the winds would change as we go down.
Thanks a lot, Clonecity and PENKO.
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Sorry salamander, but check ILS-course means what it says: check the course of the ILS in the FMGC. How do you interpret that as check drift angle? The only action the FCOM describes at 350 feet is: 'incorrect selected course at 350ft revert to CATII minima-disconnect ap at 50FT at the latest' or 'check ILS course on PFD'.
Course, not drift. This has, as far as my knowledge goes, nothing to do with the 20kt crosswind limit!
Course, not drift. This has, as far as my knowledge goes, nothing to do with the 20kt crosswind limit!
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one needs to check the drift angle between ILS FC and aircraft's kinetic vector e.g track as it's simple logic. If you have the correct DA then de-crab isn't a issue in case the aircraft believes to have it's vector aligned with CL but's not you end up like SQ in MUC. Aircraft will try to to align it's ground track with the CL and guess where you'll end up with the ground track diverging more than 5 degrees.
Last edited by 9.G; 2nd Mar 2012 at 16:00.
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have you ever thought that it simply means:
- ILS course? A/C course? are they the same? If so... then the A/C decrab algorithm will work... If not? well... it will not decrab normally, then maybe you should go with Max CAT II manual landing... or GO Around...
The FMGS is supposed to update your BIAS as soon as a LOC signal is caught, but even so, if you're on the LOC beam, with the correct ILS course set, but your real course is +7º, then I think your IRS just got busted in att mode.
Will you really continue APP for landing, flare and roll-out when something's not smelling right (lets say... with 150m visibility?)
- ILS course? A/C course? are they the same? If so... then the A/C decrab algorithm will work... If not? well... it will not decrab normally, then maybe you should go with Max CAT II manual landing... or GO Around...
The FMGS is supposed to update your BIAS as soon as a LOC signal is caught, but even so, if you're on the LOC beam, with the correct ILS course set, but your real course is +7º, then I think your IRS just got busted in att mode.
Will you really continue APP for landing, flare and roll-out when something's not smelling right (lets say... with 150m visibility?)
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QDM v QFU
The correct Q code for runway heading is QFU. Period! (ICAO Doc. 8400 or, ACP 131E)
ILS QDM? How about LOC's QDM, to be correct.
So, it should read: a check of the LOC's QDM against the (runway) QFU.
ILS QDM? How about LOC's QDM, to be correct.
So, it should read: a check of the LOC's QDM against the (runway) QFU.
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PENKO, does you company SOP forbids you to auto land on RWY with LOC offset? I'd be extremely surprised if it did. Neither there's such limitation in the Airbus AFM. No one in the right state of mind would ever do it. It's self explanatory mate. Aircraft's ground track and LOC CL must be coinciding otherwise you go mow the lawn. Whether it's aircraft's internally produced failure or LOC offset on the ground makes no difference the result is the same you become a gardener.