Landing distance question
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Landing distance question
Hello folks,
i have a quick question concerning landing distance. Im actually a little bit confused.
Let´s take a runway which shows on the approach chart 2500mX60. So we want to land there and im getting a landing distance required of 2050m. Well, definition of landing distance in my opinion is: 50ft over threshold, vref, max manual braking , isa , etc (actually conditions are not important for my question) so i brief, we have a 2500m long runway we need about 2000m so we can afford a little flare right?
So the discussion was: the 2500m with no displaced threshold is the LDA, landing distance available. So we fly over the threshold at 50ft and then it starts right? My team member said that thats not correct, in fact we land at the 1000ft point so actually we lose roughly 300m at the landing. is that correct? in case of displaced threshold, well yes but without a displaced it counts right?
i have a quick question concerning landing distance. Im actually a little bit confused.
Let´s take a runway which shows on the approach chart 2500mX60. So we want to land there and im getting a landing distance required of 2050m. Well, definition of landing distance in my opinion is: 50ft over threshold, vref, max manual braking , isa , etc (actually conditions are not important for my question) so i brief, we have a 2500m long runway we need about 2000m so we can afford a little flare right?
So the discussion was: the 2500m with no displaced threshold is the LDA, landing distance available. So we fly over the threshold at 50ft and then it starts right? My team member said that thats not correct, in fact we land at the 1000ft point so actually we lose roughly 300m at the landing. is that correct? in case of displaced threshold, well yes but without a displaced it counts right?
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Probably 50 feet over threshold ON GLIDESLOPE, which if you tracked directly into the ground would solidly deliver you about ~500 feet (~150m) from the threshold, but since you're gonna flare, you'll actually leave the GS behind you which consumes some pavement.
Sorry for run-on sentence. Hope that helps.
Sorry for run-on sentence. Hope that helps.
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The test data normally is analysed as three separate segments
(a) steady approach
(b) flare
(c) landing roll.
Final data will be figured from 50ft to a stop with the tightest flare the OEM can get away with ...
Keep in mind that the
(a) TPs will discard untidy test card points
(b) the aerodynamicists doing the data reduction will get rid of some more
(c) the OEM finally negotiates an outcome with the Regulator
Anyone who thinks that the line pilot can replicate the published TP raw data is a comedian. Occasionally one might come close but, on most occasions, the line effort will be found wanting.
Hence we have a landing distance factor to provide a reasonable amount of fat. Historically the factors used appear to be reasonable as we don't have aeroplanes running off the end day in, day out.
If we find ourselves landing on a runway the distance of which is longer than the factored distance required ?
Can we go for a nicer touch down ? Of course.
Should we do so ? Generally not. Reasons are consistency and the (often) significant distance which can be squandered trying for a nice touch down.
14000 ft available in a B737 ? maybe one relaxes.
6000 ft ? bit silly to relax, I suggest.
(a) steady approach
(b) flare
(c) landing roll.
Final data will be figured from 50ft to a stop with the tightest flare the OEM can get away with ...
Keep in mind that the
(a) TPs will discard untidy test card points
(b) the aerodynamicists doing the data reduction will get rid of some more
(c) the OEM finally negotiates an outcome with the Regulator
Anyone who thinks that the line pilot can replicate the published TP raw data is a comedian. Occasionally one might come close but, on most occasions, the line effort will be found wanting.
Hence we have a landing distance factor to provide a reasonable amount of fat. Historically the factors used appear to be reasonable as we don't have aeroplanes running off the end day in, day out.
If we find ourselves landing on a runway the distance of which is longer than the factored distance required ?
Can we go for a nicer touch down ? Of course.
Should we do so ? Generally not. Reasons are consistency and the (often) significant distance which can be squandered trying for a nice touch down.
14000 ft available in a B737 ? maybe one relaxes.
6000 ft ? bit silly to relax, I suggest.
Speedwinner, the discussion depends on how the LDR is defined.
The AFM usually contains the un-factored certificated landing performance: – 50ft, Vref, etc, etc.
Landing distance required (LDR) for dispatch is usually defined by operational requirements and includes a factored increment based on the AFM. This data is the distance from the threshold (assumed to be 50ft) until the aircraft stops, plus the required safety margin, e.g. 1.67.
Most manufactures publish un-factored ‘actual’ landing distance; some operators may use their own data. This distance may (more often does) depend on reverse thrust, which the AFM / dispatch do not, and may cover a range of runway braking conditions, which again may not be the same as for dispatch. These data are ‘pared to the bone’ in making several unrealistic assumptions, e.g. 1000ft touchdown.
Although this performance can be used for pre-landing calculations according to local operational regulations, it is now generally accepted that like the un-factored AFM data, the distances cannot be achieved in everyday operation.
The FAA recommends a minimum addition of 15%, which even then might just be a better ‘actual’ if everything works; this may be the operator’s LDR.
EASA (EU-OPS 1) requires a check to ensure a safe landing, which many operators interpret as a fully factored margin equivalent to dispatch and only reduce that in emergency conditions.
Currently the industry is trying to standardise the inflight-use data to provide a more realistic 'actual' distance – Operational Landing Distance, OLD. This makes more realistic assumptions about touchdown speed and distance amongst other apsects.
Airbus have started to use OLD, and often with the minimum addition of 15% publish Factored OLD, FOLD.
Thus which distance are you considering in your debate?
Perhaps your debate is more about not knowing what the LDA is based on?
The AFM usually contains the un-factored certificated landing performance: – 50ft, Vref, etc, etc.
Landing distance required (LDR) for dispatch is usually defined by operational requirements and includes a factored increment based on the AFM. This data is the distance from the threshold (assumed to be 50ft) until the aircraft stops, plus the required safety margin, e.g. 1.67.
Most manufactures publish un-factored ‘actual’ landing distance; some operators may use their own data. This distance may (more often does) depend on reverse thrust, which the AFM / dispatch do not, and may cover a range of runway braking conditions, which again may not be the same as for dispatch. These data are ‘pared to the bone’ in making several unrealistic assumptions, e.g. 1000ft touchdown.
Although this performance can be used for pre-landing calculations according to local operational regulations, it is now generally accepted that like the un-factored AFM data, the distances cannot be achieved in everyday operation.
The FAA recommends a minimum addition of 15%, which even then might just be a better ‘actual’ if everything works; this may be the operator’s LDR.
EASA (EU-OPS 1) requires a check to ensure a safe landing, which many operators interpret as a fully factored margin equivalent to dispatch and only reduce that in emergency conditions.
Currently the industry is trying to standardise the inflight-use data to provide a more realistic 'actual' distance – Operational Landing Distance, OLD. This makes more realistic assumptions about touchdown speed and distance amongst other apsects.
Airbus have started to use OLD, and often with the minimum addition of 15% publish Factored OLD, FOLD.
Thus which distance are you considering in your debate?
Perhaps your debate is more about not knowing what the LDA is based on?
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Landing Distance: "what" is being compared to "what" ?
@Speedwinner,
At the expense of repeating some of what the other excellent posts have said, here's another detailed look. Also, your understanding is largely correct, but here's a structured way to look at, and analyse the issue:
Landing distance calculations come into the picture at 2 stages - the Planning stage, and the Enroute/Destination/Diversion Landing stage.
1. Planning stage: the FLL, Field Length Llimit, landing weight, is derived by taking the destination LDA and finding the weight at which the 'actual' maximum performance Landing Distance factored 60% ( i.e. divide by 0.6 or multiply by 1.67, then termed as the LDR, Ldg Dist Required) results in LDR = LDA.
For a short RW at destination, this could in turn affect the RTOW, Regulated Take Off Weight at the departure point if TOW minus predicted Fuel Burn > FLL Landing Wt.
2. Inflight stages:
After take-off,
(i) if all goes as planned, we have an ample margin already in place as described in 1 above. At the planned destination, this would take care of issues such as variations in piloting technique - "so we can afford a little flare right?", use of brakes - variations in weather to a reasonable extent, aircraft snags resulting in small increases in RW length requirements etc. this is why LDR is not generally worked out before landing, except perhaps to help with 'minimum RW occupancy' criteria to facilitate exiting at a particular RW exit.
(ii) unanticipated conditions, in case of diversion Enroute, landing weight and/or aircraft snag issues could arise, or even at destination, an increased LDR could arise due to tailwinds, aircraft system failure etc. and/or reduced LDA for any reason such as RW closure/change etc. in this case we must work out the LDR.
Having stated that, a few comments on the issues you raise:
A. The easiest first - a Threshold is a threshold, whether displaced or not it stands for the same thing, treat it with equal respect. You have to be 50ft, nominal, crossing the threshold. The Landing Distance from your tables will be from 50ft down as you have already mentioned so this must be compared to the "distance beyond threshold" whatever way that is depicted, in your chart.
B. A simplistic look at margins on 'actual' Scheduled Landing Distace - once airborne, variations are catered for by the regulations by requiring 15% to be added to the actual maximum performance distances demonstrated by the manufacturer.
One manufacturer assumes 50ft over the threshold followed by a flare (technique also described in their documentation) resulting in touchdown between 6 to 8 seconds later. Depending on aircraft type, it is quite normal, to specify a touchdown point range for larger and longer-body jets - 1000 to 2000ft, or for eg for the B747 it started at 1500ft in the age of the 3 bar vasi.
Geometrically a 3 deg angle corresponds to a 5.2% gradient which corresponds to 961ft horizontally forwards for every 50ft of height change, so that's where a 'no-flare' would land us.
Note that 15% of a 6000ft RW is 900ft, so a prolonged flare, for instance, could eat away a substantial part of the TD Point variation mentioned by the manufacturer.
C. PIC discretion can and must always be invoked when the variable factors cannot be enumerated with any degree of confidence - if the assumptions made by taking data directly from Landing Distance charts do not satisfy the real world conditions, certainly add a further arbitrary but satisfactory margin but remember, you must be able to defend such position taken by an argument that shows sound professional judgement was used.
Hope that helped.
At the expense of repeating some of what the other excellent posts have said, here's another detailed look. Also, your understanding is largely correct, but here's a structured way to look at, and analyse the issue:
Landing distance calculations come into the picture at 2 stages - the Planning stage, and the Enroute/Destination/Diversion Landing stage.
1. Planning stage: the FLL, Field Length Llimit, landing weight, is derived by taking the destination LDA and finding the weight at which the 'actual' maximum performance Landing Distance factored 60% ( i.e. divide by 0.6 or multiply by 1.67, then termed as the LDR, Ldg Dist Required) results in LDR = LDA.
For a short RW at destination, this could in turn affect the RTOW, Regulated Take Off Weight at the departure point if TOW minus predicted Fuel Burn > FLL Landing Wt.
2. Inflight stages:
After take-off,
(i) if all goes as planned, we have an ample margin already in place as described in 1 above. At the planned destination, this would take care of issues such as variations in piloting technique - "so we can afford a little flare right?", use of brakes - variations in weather to a reasonable extent, aircraft snags resulting in small increases in RW length requirements etc. this is why LDR is not generally worked out before landing, except perhaps to help with 'minimum RW occupancy' criteria to facilitate exiting at a particular RW exit.
(ii) unanticipated conditions, in case of diversion Enroute, landing weight and/or aircraft snag issues could arise, or even at destination, an increased LDR could arise due to tailwinds, aircraft system failure etc. and/or reduced LDA for any reason such as RW closure/change etc. in this case we must work out the LDR.
Having stated that, a few comments on the issues you raise:
A. The easiest first - a Threshold is a threshold, whether displaced or not it stands for the same thing, treat it with equal respect. You have to be 50ft, nominal, crossing the threshold. The Landing Distance from your tables will be from 50ft down as you have already mentioned so this must be compared to the "distance beyond threshold" whatever way that is depicted, in your chart.
B. A simplistic look at margins on 'actual' Scheduled Landing Distace - once airborne, variations are catered for by the regulations by requiring 15% to be added to the actual maximum performance distances demonstrated by the manufacturer.
One manufacturer assumes 50ft over the threshold followed by a flare (technique also described in their documentation) resulting in touchdown between 6 to 8 seconds later. Depending on aircraft type, it is quite normal, to specify a touchdown point range for larger and longer-body jets - 1000 to 2000ft, or for eg for the B747 it started at 1500ft in the age of the 3 bar vasi.
Geometrically a 3 deg angle corresponds to a 5.2% gradient which corresponds to 961ft horizontally forwards for every 50ft of height change, so that's where a 'no-flare' would land us.
Note that 15% of a 6000ft RW is 900ft, so a prolonged flare, for instance, could eat away a substantial part of the TD Point variation mentioned by the manufacturer.
C. PIC discretion can and must always be invoked when the variable factors cannot be enumerated with any degree of confidence - if the assumptions made by taking data directly from Landing Distance charts do not satisfy the real world conditions, certainly add a further arbitrary but satisfactory margin but remember, you must be able to defend such position taken by an argument that shows sound professional judgement was used.
Hope that helped.
Last edited by catpinsan; 30th Jul 2013 at 22:54.
