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Landing distance available

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Landing distance available

Old 19th Oct 2012, 09:58
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BBK
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Landing distance available

This is probably a geeky question but I've yet to get a concise definition regarding LDAs as stated in the Jeppesen manuals. None the wiser even after consulting the grey info books.

Here's an example:LHR 27L and 27R shown as full length ie no displaced threshold. However, 27R gives 2 values whereas 27L only "beyond g/s". I don't have the chart in front of me but I jotted down the numbers.

27R distance available: 12,743 ft
27R glideslope : 11,586 ft

27L only gives a g/s value of 10,905 ft although runway geometry appears the same ie no displaced threshold.

My two questions are: why the difference here at LHR and when, in general, would you use the glideslope figure/
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Old 19th Oct 2012, 12:17
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If only a "beyond g/s"-figure is given, then LDA = full length displayed on the map. If a "from thr"-figure is given, then a displaced threshold exists.

The "beyond g/s"-figure is probably a GA heritage - not used for anything....
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Old 19th Oct 2012, 13:32
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9.G
 
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BBK, beyond the GS is dictating for landing performance calculation when given. This is the point where your wheels are gonna touch the RWY thus deceleration begins.
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Old 19th Oct 2012, 13:42
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....but LDR from your AFM is given from 50' (apart from certain steep approach perf charts or corrected for runway upslope in some fashion) - and that's your target THR crossing height. So LDA is measured from the threshold.

Your deceleration will often begin well before touching down, as you typically will have some Vref+ - corrections applied for wind or similar. These need to be washed off to cross threshold at 50' and at Vref, with further deceleration from 50' till touchdown.

So - the beyond g/s-figure is not used for any practical application on the line (ie excluding test-pilot- and runway design-stuff).
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Old 19th Oct 2012, 14:20
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EC you, in LHR 27R your TCH is 58 ft to start with. LDA beyond the glide is equal to the TDZ aiming point which you would normally be for after visual transition. Basically is roughly 1000 ft mark into TDZ. If you wanna disregard 1000 ft of the left behind RWY,well, the choice is yours. What distance will you take if you conduct auto land?

Last edited by 9.G; 19th Oct 2012 at 14:21.
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Old 19th Oct 2012, 19:15
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9.G,

Would use the LDR table from the AFM with any corrections applicable for autoland

Still - Landing Distance Required is not the same as Ground Roll Required. Normally, with a TCH of 50 ft ATDz and a 3 deg glide, the LDR will be - as you correctly point out - 1000 ft longer than the GRR, if such a term even exists - haven't dealt with ground roll since SEPs. Even an puddlejumper like the ATR doesn't have it

Still - nothing unsafe in doing it the way you suggest, you have 1000 ft to spare - on a marginal runway might be able to drag less mass in, but hey - better out to that side than the other
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Old 19th Oct 2012, 19:43
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Excerpt from ICAO definitions published in Jeppesen:
LANDING DISTANCE AVAILABLE (LDA) (ICAO)
The length of runway which is declared available and suitable for the ground run of an airplane landing.
As you can see it's a very real and unambiguous term and it exists.
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Old 19th Oct 2012, 20:01
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Agreed - but that doesn't mean the 1000 ft in front of the TDZ are not suitable for the ground run of an aeroplane. If you land short, you're still on a load bearing surface of just the same quality as that of the TDZ and beyond.

The definition of Ground Run in performance is a different kettle of fish and involves the distance travelled by the aircraft from main wheel touchdown till the aircraft comes to a complete stop, assuming maximum breaking effort. Landing distance is the distance travelled from a screen height (typically 50 ft) till the aircraft comes to a complete stop.

Still agree with you - all of the LDA must be suitable for the ground run. However, using standard screen heights and path, your ground run will only start at the TDZ. If you look at the AFM tables for LDR, you will see one of the conditions being a screen height of 50 ft., signifying that the AFM figures include the distance travelled from 50 ft to the TDZ.
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Old 20th Oct 2012, 09:05
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Agreed - but that doesn't mean the 1000 ft in front of the TDZ are not suitable for the ground run of an aeroplane. If you land short, you're still on a load bearing surface of just the same quality as that of the TDZ and beyond.
Of course it doesn't but would you land short or are you supposed to to aim for touch down point after visual transition? All the RWY left behind bears no relevance and it can be 1000 or 3000 ft. It's gone and is not part of the equation. Certification data have got nothing to do with a landing at a particular airdrome as the TCH can vary. The problematic of application of ALD used for certification on daily basis is well known and is addressed by both airbus and boeing, by issuing more realistic operational landing distances. However the original question is when would you use GS figure and the answer is whenever it's published and ILS or PAR is followed.
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Old 20th Oct 2012, 11:00
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Or never if you work according to EU-OPS. Subpart G (Performance Class A) doesn't mention any beyond GS figure and all landing performance calculations (at least two every flight) use a landing distance with a minimum TCH of 50ft which can be lowered to not below 35ft for steep approaches of 4,5 or more.

Of course it can be different in other parts of the world, but EU-OPS does not allow the use of a beyond GS figure.
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Old 20th Oct 2012, 12:08
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Denti, Dutch folks are part of EASA, aren't they. They seem to have a slightly different view on EU OPS https://www.vnv-dalpa.nl/commissiepub/4450 Luckily LHR example was used so I'll refrain from commenting as it's obvious.

Last edited by 9.G; 20th Oct 2012 at 12:10.
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Old 20th Oct 2012, 12:57
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Nice report from the dutch colleagues, however not a regulatory text and there is no indication that a beyond GS figure is used in any performance calculation.

They do put a finger on the specific lack of guidance in OPS 1.400, however do note that all landing performance data given by manufacturers include a 50ft TCH in their LDR data for inflight calculation whereas the dispatch data required is specified in SUBPART G and requires a 50ft TCH in any case (plus factoring and no wind) except steep approaches. And of course OPS 1.400 has to be seen in accordance to OPS 1.410 which requires a safe TCH to be passed in landing configuration and attitude. Again, no beyond G/S figure.
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Old 20th Oct 2012, 13:36
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there is no indication that a beyond GS figure is used in any performance calculation.
of course it's not used as it would result in a longer ALD. That's the whole point here. Not much logic is really required to understand the difference between the certification data and real life performance. That's what this is all bout. Not all the GS antennas and PAPI are located exactly 305 m beyond the THR and if located further away it must be taken into account as one is bound to follow the GS and when transitioned to visual continue to aim for the TD point. Following your logic neither there's a mandatory requirement to use beyond THR value. EU OPS ONLY prescribes 50 ft for RLD at the dispatch phase in flight it's a different story altogether. Exactly the same problem is faced with certified actual landing distance used for certification thus slowly but surely operators and manufacturers recognized that those figures are pretty much useless in real life. Guess what, they came up with operational landing distances almost doubling the actual ones. Healthy skepticism will prove useful eventually whereas blindly believing it's all been taken care of might bite one in the a&se.
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Old 20th Oct 2012, 18:45
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landing distance performance in the FCOMs is calculated from 50 ft. It includes the distance covered during flare, and float before touching down.

Even in the piper and cessnas performance is given as that.

So you have to compare that and the distance available from the threshold.

It is true, however, that those extra 8 ft in LHR mean quite a few extra meters in landing distance...
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Old 20th Oct 2012, 19:13
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Alright M2002, now tell me which distance do take in case there's only LDA beyond the GS published? Plenty of those around.
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Old 20th Oct 2012, 19:47
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I'm gonna throw coupla definitions and values at the audience just for sake of understanding how the AFM data is derived:

Touchdown Zone. As referenced in the Air Traffic Rules and Procedures Service (ATP) Practical Test Standards Guide, the touchdown zone is defined as a point 500-3,000 feet beyond the runway threshold not to exceed the first one-third of the runway. This definition is not used in landing distance performance calculations. The touchdown zone for certification may be as short as the point where a 3.5 degrees glidepath passing 50 feet over the landing threshold, intercepts the runway surface, which is 820 feet past the landing threshold.

Landing distances determined during certification tests are aimed at demonstrating the shortest landing distances for a given airplane weight with a test pilot at the controls and are established with full awareness that operational rules for normal operations require the addition of factors to determine minimum operational field lengths. Flight test and data analysis techniques for determining landing distances can result in the use of high touchdown sink rates (as high as 8 feet per second) and approach angles of 3.5 degrees to minimize the airborne portion of the landing distance. Maximum manual braking, initiated as soon as possible after landing, is used in order to minimize the braking portion of the landing distance. Therefore, the landing distances determined under 23.75 and 25.125 are much shorter than the landing distances achieved in normal operations.

Flare with 8 feet per second, really? nearly 500 ft/min think bout it for a sec.

Last edited by 9.G; 20th Oct 2012 at 19:49.
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Old 21st Oct 2012, 02:23
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Empty Cruise, has a good handle on the topic.

Thanks and keep up the good work.
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Old 21st Oct 2012, 10:33
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The one mentioned on the runway layout page, next to the runway!

Our OPS manuals have it written exactly as Denti says.
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Old 21st Oct 2012, 11:55
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well, J.C. you might earn some extra credit with your standard dep. if you advised them to include TCH into the equation. It's even more important when it comes to LAHSO. How bout this one then HNL - Honolulu International Airport | SkyVector 08L with TCH of 80 ft. What's your LDA here? Where do you recon is LAHSO LDA is taken from? Why would jeppesen even bother publishing LDA beyond GS if it wasn't of any operational relevance? I've already mentioned a healthy skepticism before not to mention the number of manual revisions I've seen during the last 2 decades. In the end you're the one making a call so make sure it's right one.
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Old 21st Oct 2012, 12:03
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'Not all the GS antennas and PAPI are located exactly 305 m beyond the THR
No, but regardless of where they are situated, they are required to deliver you on the correct glide slope to a point 15 to 18 m above the threshold (some caveats apply, but that's the gist of it).

If they do that, they can be located in the next county. It won't matter, if they are within specs you'll be at your 15-18 m. Conveniently, this corresponds nicely to the certification specifications used to calculate the published LDR. The additional three meters from 15 m will mean all of 58 additional meters of runway consumed before touchdown.

9.G,
which documents are you referring to in addition to CS25.125 regarding what you can do during certification tests? I read CS25.125 a lot more conservatively regarding how 'creative' you can be. "The landings must be made without excessive vertical acceleration,[...]", "The landings may not require exceptional piloting skill or alertness.", "The brakes may not be used so as to cause excessive wear of brakes or tyres[...]. Interestingly, I can't find a specification of the glide slope angle for certification in CS 25, only in CS 23...

You're talking about "ALD"? That's a creature I'm not familiar with. Clarification?

Has anyone found a definition for "distance beyond the G/S"? Is it beyond the GP mast, beyond point R... for starters? If there's no definition, the figure is utterly useless IMNSHO. The two definitions I mentioned can differ significantly, and you could probably invent a fair few more.
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