Landing Performance on a sloping Runway

[rudderrudderrat]

Hi Jwscud,

He felt I was "high", despite me touching down abeam the PAPIs.

There are a couple of reasons your captain might have said that:
1) Airbus visual illusion awareness, page 6 "An uphill slope creates an illusion of being too high (impression of a steep glide path);" or

2) He may of been mindful of the landing distance available. e.g. If you look at CDG RW27L Difference between RWY 27L LDA (3,600m) and Distance beyond Glide Slope (3,256) = 344m. If you needed that extra 44m landing distance, due to aircraft performance, then you would have to plan to cross the landing threshold at 50 feet.

[cosmo kramer]

If you look at LGW 26L / 08R, LGWrunway there are two landing distances published for each runway. They depend on whether you follow the glide path to touch down, or deliberately fly below the glide path to cross the landing thresh hold at 50 feet on a 3 degree profile.

Long ago since using Jeppesen. Are you sure about this? A reference would be nice.

As far as I remember "beyond the glide slope", is simply a measurement of the distance from the glide slope antenna to the runway end. Which fit very well with the the antenna, in your example, is almost exactly 1000 feet down the runway or 50 feet crossing altitude of the threshold, being on the 3 deg glide slope.

[rudderrudderrat]

Hi cosmo kramer,

Please see https://www.vnv.nl/commissiepub/4450
"As discussed previously manufacturer data is normally based on a threshold crossing height of 50 ft and a touchdown point that may not be achievable in line operations."

The picture shows the increased Landing Distance by being 100 ft over the threshold = 300m.

All of our performance data is based on LDA (i.e. assumed from being 50 ft above the landing threshold.)
Please See Alicante ALC Difference between RWY 10 LDA (3,000m) and Distance beyond Glide Slope (2,650m) = 350m.
& CDG CDG Difference between RWY 27L LDA (3,600m) and Distance beyond Glide Slope (3,256) = 344m.
& Madrid MAD Difference between RWY 32R LDA (3,000m) and Distance beyond Glide Slope (2,722m) = 278m.

It would be interesting to learn from the OP what the difference in the published "LDA" and "Distance beyond the Glide Slope" was for his 2,000m airport.

[cosmo kramer]

It's NOT a correct calculation you are using:

50 feet height = 1000 feet or 300 meter
100 feet height = 2000 feet, 600 meter or 0.3 nm.

All in round numbers of course.

Height (feet) = tan(glide path angle) x distance (has to be same unit, feet too).

Unless you can provide a reference FROM JEPPESEN (don't have Jeppesen available to look it up), I'll maintain that "Beyond the glide slope", is just a measurement of the distance from the antenna to the end of the runway (landing roll available, so to speak).

"As discussed previously manufacturer data is normally based on a threshold crossing height of 50 ft and a touchdown point that may not be achievable in line operations."

Normally = unsloped (flat) runway. And 3 degs glide slope. Steeper glide and upslope you pass the threshold higher, but still touchdown 300 meter down the runway (assuming no flare). Touchdown point decides your landing distance, not threshold crossing height.

[cosmo kramer]

All those airports you mention are completely normal 50 feet crossing height (on the glide, give or take a few feet). Seriously, I'm speechless if you are of this belief and practice this in daily ops!!!

The picture, I believe you are referring to, shows an EXTRA distance of 300 meter for crossing the threshold 50 feet too high (= 100 feet high, on a flat unsloped runway).

50 feet crossing height = 300 meter to touchdown, 300 meter for flare = 600 meter.
100 feet crossing height = 600 meter to touchdown, 300 meter for flare = 900 meter = an extra 300 meter.

[rudderrudderrat]

Hi ,

I'll maintain that "Beyond the glide slope", is just a measurement of the distance from the antenna to the end of the runway (landing roll available, so to speak).

Correct.

Steeper glide and upslope you pass the threshold higher, but still touchdown 300 meter down the runway (assuming no flare).

Not so, other wise all the above quoted examples would have a difference of 300m between the published LDA and the "Beyond the glide slope difference".
Please see MRS.pdf for Runway 31R. The difference between LDA and "Beyond the glide slope" is only 262m. (Due crossing the threshold at 50 feet but on 4 deg glide slope)

There is a spread from 262m (MRS 31R) to 344m (CDG 27L) of 82m.
The point I am trying to make (unsuccessfully it appears) is that our performance calculations are based on LDA, but the "landing roll available" (for braking etc.) can differ by around 82m from the LDAs published for different Runways.

[PEI_3721]

cosmo, rudderrat, the simple position ‘beyond the glideslope’ may not be that easy; the ILS GS normally involves a reflected beam off the ground some distance in front of the antenna. This distance may vary with antenna/beam type, height, and offset.

[cosmo kramer]

In my opinion it's all extremely theoretical what you say, borderline gibberish.

The point I am trying to make (unsuccessfully it appears) is that our performance calculations are based on LDA, but the "landing roll available" (for braking etc.) can differ by around 82m from the LDAs published for different Runways.

"Beyond the glide slope" (from memory) is just an information to you where the glide slope antenna is placed. Nothing more and nothing less. Someone with a Jeppesen may provide an exact definition. Nothing to do with LDA or LRA.

I agree there is a variation of a few meters, where the glideslope antenna is placed. It probably has to do with obstruction areas, aircraft categories and type expected to use the approach etc. Anyway, 344 meter equals a height above threshold of 59 feet (assuming 3 degs glide). Tell me how much do you need to go below the glide slope (dots) to be 9 feet lower 344 meters from the antenna? But more important: Who even flies the glide at that height? You should be looking at the big white squares on the runway (aim point markings).

I agree that if making an auto land, the aircraft may be 9 feet too high in CDG (3600 meter runway). In my EFB there is a selection for auto land which will cause an additive to the LDR, probably to cover such minor discrepancies and system performance.
Further, if your performance calculations to another airport are that tight, that 82 meters means the difference of going of the runway or not, maybe you should consider a diversion (lousy visibility and 82 meter margin at max braking is not a cocktail I would recommend, even if factored).

Anyway it has nothing to do with the point or what the poster was asking. If there is an upslope in the touchdown zone it normal to pass the threshold higher than 50 feet. I.e. if threshold is at 230 feet and touchdown at 240 feet, passing over the threshold at 60 feet is normal, and does not affect your landing calculations (because you are touching down on the runway where you are supposed to).

[Gysbreght]

Originally Posted by cosmo cramer

Touchdown point decides your landing distance, not threshold crossing height.

To a first approximation (no flare) that is correct (*). However, the flare will take the airplane above the glide path, and the touchdown point will be beyond the aiming point. How much depends on the length of the flare. For a given amount of "gee" in the flare arc, the length of the flare increases with the change of rate-of-descent (relative to the runway surface) between the start of the flare and touchdown, i.e. with the angle between the glide path and the runway.

The AFM landing distance is measured from "a point 50 ft above the landing surface" for standard temperature. It is corrected for altitude and wind, but not for runway slope so it is sufficiently conservative for uphill sloping runways to absorb some increase of the flare distance.

Therefore the pilot of the OP did nothing wrong, and the captain had no reason to reprimand him or to call for a go-around. But suppose the captain said something like: "Look here, your landing was good, but it would have been even better if you had managed to cross the threshold below 50 ft" - would you disagree with him?

For downhill sloping runways the AFM landing distance is unconservative. Would it be wrong to use a glide path angle of -4 degrees when approaching a runway with 1.7% downslope?

(*) You seem to refer to 'touchdown point' when you really mean 'aiming point', i.e. the rectangular runway markings at the intersection of the glide path and the runway.

[cosmo kramer]

But suppose the captain said something like: "Look here, your landing was good, but it would have been even better if you had managed to cross the threshold below 50 ft" - would you disagree with him?

If you are asking me, then yes I would disagree. First, you are eating away on one margin (obstacle clearance), to increase another that needs no increasing (landing distance). Second, you change your profile in the last seconds of the approach, which may lead up to a long flare = nothing gained.

Boeing directly advices against doing so, as quoted earlier:

FCTM:
When visual contact with the runway is established, maintain the glide path to the flare. Do not descend below the glide path.

For downhill sloping runways the AFM landing distance is unconservative. Would it be wrong to use a glide path angle of -4 degrees when approaching a runway with 1.7% downslope?

Any slope affecting the ground roll would need to have appropriate corrections applied to the total LDR. A slope in prior to the touchdown point (normally upslope) doesn't affect the LDR, as the aircraft still doesn't have contact with the runway - pretty logical I'd say.

[Kefuddle]

Therefore the pilot of the OP did nothing wrong, and the captain had no reason to reprimand him or to call for a go-around. But suppose the captain said something like: "Look here, your landing was good, but it would have been even better if you had managed to cross the threshold below 50 ft" - would you disagree with him?

I would heartily disagree with him. A 3 degree glide path to a 1% upslope would be 70' above the threshold for a 1,000' aiming point. If you try and make 50' over the threshold I reckon the only practical option is to increase the RoD of descent below 200' which isn't really buying into the spirit of a stabilized approach.

[Gysbreght]

If I may ask yet another ignorant question - if there is a PAPI/VASIS on a downsloping runway, would it be located 1000 ft from the threshold, pointing up at 3 degrees to the horizon, thus providing less than 50 ft threshold crossing height?

[Kefuddle]

There certainly seems to be some airfields with threshold crossing altitudes below 50'. As far as I can determine according to Annex 14, the maximum slope in the first 25% of the runway (essentially the touchdown zone) is 0.8%. This could mean a threshold height of as little as 30' for a 3 degree slope, which seems a bit extreme. This also makes the maximum crossing alt 66', so disregard the 1% example!!

However, from what I can determine the design of a non-precision approach, the glidepath is worked back from 50' above the threshold, so threshold crossing heights other than 50' seem to apply only to precision approaches.

[rudderrudderrat]

Hi Gysbreght,

if there is a PAPI/VASIS on a downsloping runway, would it be located 1000 ft from the threshold, pointing up at 3 degrees to the horizon, thus providing less than 50 ft threshold crossing height?

If you look at the ALC link I posted #23, the runway has a down slope of 98 feet in 3,000m (1%). The ILS glide slope transmitter is located 350m from runway threshold (difference between LDA (3,000m) and "distance beyond glide slope" (2,650m)).

The 3 deg glide slope would be 18.3m (60 feet) above the threshold on a level runway. Alicante's runway threshold is 11 feet higher than the glide slope transmitter, so the threshold crossing height is 49 feet.

The published LDA (no inset landing threshold) is 3,000 m, but the effective landing distance is 50m shorter (due to the down slope).
The converse would be true if you landed on the reciprocal runway and crossed the threshold at 50 feet.

Maybe that was the point the OP's Captain was trying to make?

[Gysbreght]

Hi rudderrudderrat,

Thanks for your reply which was most helpful. In your ALC example the treshold height of 50 ft and the glideslope of 3 degrees result in the glideslope/runway intersection being further from the threshold on a downslope runway.

Originally Posted by rrr

The converse would be true if you landed on the reciprocal runway and crossed the threshold at 50 feet.

That's logical, isn't it? Therefore it is perhaps worth repeating:

Originally Posted by rrr

It would be interesting to learn from the OP what the difference in the published "LDA" and "Distance beyond the Glide Slope" was for his 2,000m airport.

[Jwscud]

Runway length 2004m

Beyond GS 1704m.

The ILS is also of poor quality, meaning the glide slope is untrustworthy below around 500' afe and does not keep you on a straight 3° path but shallows then steepens slightly which does not help a stable approach if you are following the GP.

[rudderrudderrat]

Hi Jwscud,

This is a link to an accident report of a TriStar which ran off the end of Leeds Bradford airport. http://www.aaib.gov.uk/cms_resources...7%20G-BBAI.pdf

Landing distance calculations is not an exact science (too many variables) so there is a big scatter in achieved results. Sometime you may wish you had that extra 50 m of runway in front of you rather than behind you.

Please would you tell us which was the airport in post#1?

[Gysbreght]

Hi rudderrudderrat,

thanks for the link to an interesting report. I think the following (from para. 1.10.4 of the report) is relevant to our discussion:

The relevant CAA recommendation was that PAPIs should be sited to give a wheel height over the landing threshold of 30 feet for the most demanding aircraft that used the aerodrome.

It would be of interest to know if that recommendation is still valid and also what practices apply in other parts of the world. Questions with regard to the OP:
a. How does the eye-to-wheel height of the OP's aircraft compare to the "most demanding aircraft that use(s) the aerodrome"?
b. Are pilots expected to make adjustment for their eye-to-wheel height when using PAPIs?

[cosmo kramer]

Landing distance calculations is not an exact science (too many variables) so there is a big scatter in achieved results. Sometime you may wish you had that extra 50 m of runway in front of you rather than behind you.

I wholeheartedly disagree.

One thing is the dispatch criteria, where the distance is factored with 1.67 (dry), 1.92 (wet) or for contaminated highest of 1.15 x actual contaminated of wet, which ever is higher. A 50 meter margin may be ok, since you already have a margin in the calculations (but be careful with wet contaminants, as they will generally be limited by 1.15 and thus have the lowest margin).

If your inflight calculations (ACTUAL unfactored landing distance) are so tight, that you believe you need to do "tricks" to obtain an extra 50 meter, and if you did a calculation for dispatch and it was ok, the only possible explanation that your inflight calculations may show such a discrepancy, is that during the flight the runway unexpectedly became contaminated/wet (and was not forecasted for you to consider during dispatch). In that case I would either recommend to hold for the runway to be cleared or a diversion. If your company choses to operate into such a short runway, for the type you are flying, I am sure they also accepted the higher risk of diversions in such cases.

[cosmo kramer]

...and as a note to the above. Obviously you wouldn't dispatch to a runway expecting it to be contaminated. You would expect it to be cleared and wet, when arriving. And bring the necessary extra fuel, to be able to hold waiting for it to be cleared (if possible), or divert.