What is the use of rwy length Beyond glideslope ?
 

RWY LENGTH - BEYOND glide slope

As per Jeppesen, this is definition of this distance : When applicable, the distance from a point abeam the glide slope transmitter to the roll-out end of the rwy is shown. For PAR, the distance is from the GS interception with the runway.

When is it used? I haven't yet come across any usage of it

LDA must be greater than LDR and includes the airborne part as you cross the threshold to the touchdown zone. This will assume max braking. Assuming you touchdown at the GS intersection you can now see what is the available braking distance and select the braking technique as appropriate to meet the turnoff or the end of the runway. It might be less than max.

Something we always double check and include in the descent/approach briefing.

It depends on how your manuals and/or performance calculator are set up...

Many performance guides cite total runway used for stopping, including air distance (from runway threshold to landing point). Some of the backup charts, especially for non-normal configurations, MAY show actual stopping distance from the landing point only. The latter situation is where the Length Beyond GlideSlope is useful.

Something we always double check and include in the descent/approach briefing.

Yes: but the question is WHY?

(Minor pedant mode)
Do not forget that certificated performance starts from 50ft, not the ‘threshold’, and that the point of touch down is only a calculated position determined from flight tests, which is unlikely to be achieved in operations.

Many older ‘Actual’ data formats, published as guidance by manufacturers, used a fixed a distance of 1000 ft - 1250 ft beyond the threshold (from 50ft) depending on aircraft type. More recent actual performance formats (OLD/FOLD) provide a realistically achievable air and stopping distance according to conditions.
Relating landing distance to the threshold is only valid if the aircraft crosses the threshold at 50ft, - a variable.
Relating ground roll distance to the glideslope origin is meaningless as this depends on where the aircraft touches down (variable) and also that the stopping distance depends on how the retarding devices are used, runway condition, etc.
The electronic or visual glideslope origin is not associated with either certificated or actual data other than by relating it to the threashold crossing height.

RAT; ‘Max braking’ ... for the actual runway condition. ;)
…”see what is the available braking distance”…, but if you adjust the breaking technique, how then will you know what the most appropriate technique for the distance will be. :uhoh:

‘double check’ … Why^2

The following paragraph, extracted from a Transport Canada special condition, may give a clue:
My understanding of this requirement was that it is intended to cover any airports where the guidance, for whatever reason, does not bring you to a "normal" position on the runway, but rather some other nominal location. As a consequence, distance required data based on a "normal" threshold crossing height, an allocation of air distance and ground roll onto the LDA might not work. In such a case, you'd use the data required by this para in combination with the "distance beyond glideslope" to work out whether you ought to be landing or not.

The data mentioned in the first post potentially describes a non-standard relationship between the glideslope intercept point on the runway and the threshold, meaning the normal distances might not work. the data required in quoted para would allow the use of the available data for a proper landing assessment.

MFS, #7 interesting.
The exception for LCY is probably based on the use of a 35ft threshold crossing height and steep approach glideslope. Also the old runway had a fixed distance marker along the runway to indicate ‘the last point of touchdown’. However, I do not know if this still applies with the extended runway, or how such a marker might be interpreted for the wide range of aircraft types now using the airport. Even so a fixed marker may not guarantee that the rollout distance to stop is within the remaining runway length, for the conditions.

Your interpretation of the requirement for a non-standard runway would require the crew to add the air distance to that available beyond the glideslope origin. This would provide an estimate of the landing distance available which could be compared with the distance required. However, the (allocated) air distance would still be an estimate, with no relationship with the certificated distance and little with actual distances unless specifically published.
I wonder how the operational regulatory authority might interpret the landing distance requirements vs the description (exemption?) in the airfield requirements.

Hmm, that's not the way i was thinking of it.

What TC are asking is that you relate the amount of runway needed after the glideslope point on the runway. (I think)

If you imagine a "standard" runway and glideslope configuration, then the perf data in the AFM is based on that. So we can take the air distance (from the threshold crossing point) and the ground distance (from the touchdown point, naturally) and add them together, and compare them directly to the amount of runway from the threshold, and we're comparing two sets of distances both references to the threshold. So, apples vs apples, everything works out.

Now suppose that the glideslope intercept point is 1000ft FURTHER down the runway. In effect, the aircraft approach path slides 1000ft downrange, as does the touchdown point, the stop point, everything. So i can't use the normal landing distance required and compare to the LDA, because there's a bunch of LDA 'behind me" by design.

And I can't just provide a crew with the ground roll, because the aircraft doesn't touch down at the glideslope intercept point - by the nature of a flare, the air distance extends further than that. (Only an "unflared" landing would hit - literally - at that point).

But what i can do is...
If a "normal" runway configuration has the glideslope intercept point say 1000ft down from the threshold, and that was the configuration where I developed my performance data, then I can take the 1000ft as an offset and correct all my landing data by that 1000ft.

So if for a 'normal' configured airport I have 4500ft LDR, which means I need 4500ft LDA, that means I also need 3500ft past the "1000ft nominal intercept". So if I move the G/S intercept up or down the runway, provided i fly the G/S and do the same flare, I will ALWAYS need 3500ft past that point. So as long as I know I have that 3500ft of runway after the GS intercept, I am "good".

MFS, Hm Hmm.
We are considering the same issue, but perhaps from an engineering (theory) view vs my piloting (practical) view.

What factors are published for the crew to use (in flight); are they advisory or certification data, would such a calculation be acceptable to a regulator (ops or airfield) - before or after an overrun.
Assuming the use of OLD, where landing air distance would not be known, then ‘length beyond glideslope’ is of little (no) value.
‘Lawyers are always found in the long grass off the end of runways’.

Do Jepp airfield charts publish the values of distances for LDA and ‘length beyond glideslope’ together on the same chart?

Just to muddy the waters, I recall a terminology of “distance beyond glideslope” vice ‘runway’ which I understood included the geometric distance before the glideslope origin, thus would be equivalent to height over threshold and thus LDA, but see #23 below which is a less risky interpretation.
I wonder if the published term used in #1 is a corruption of this; differences in publishing, FAA culture …. Tedium of common definitions … anyone with an ICAO definition?

Previous discussion; http://www.pprune.org/tech-log/49840...available.html
Note #23

Runway length beyond threshold is of use unless you are doing CAT3/auto land. That is the time you will use runway length beyond glisdeslope.

As long as I am doing Cat 1 Cat 2 app followed by manual landing ill use runway length beyond threshold. Only when I am doing CAT3 auto land I'll use runway length beyond glide slope. Once you break off at minimums you follow PAPI or visual perception & Not exactly your gliderslope all the way.