Landing Performance

[Pugachev Cobra]

Hello PPRuNers, this should be a quick one!

I'm having trouble understanding the difference between theory and reality of a landing, as per FAR 25 requirements go.

The aircraft is certified for crossing the runway threshold at 50ft, at Vref, on a 3º slope, and to TOUCHDOWN the main landing gear at the 1000ft markings (the aiming point markings), and then comes the 60% of the dry runway distance.

Now, the reality practical day to day technique that I see, and I just read a 737 FCTM to don't get the technique wrong, is that, this is not what happens.

Generally, the aircraft comes on a 3º glidepath (let's assume an ILS glideslope for being precise), and at landing configuration, descends at Vap (not Vref, more so if you're on autothrottle or the like) until you're about 30ft of height, initiate the flare, which should be no more than 2º - 3º of pitch, and ideally you retard the thrust levers and they should reach idle when you finish the flare, and just wait for the plane to touchdown.

So I'm having trouble understanding, if you use Vap all the way down until you retard the levers, you're not crossing 50ft at Vref, and in a 3º glideslope/path that directs you to the 1000ft mark, the only way of touching down in it is with an almost hard landing, since you're in a +- 700ft/min descent rate established on the glideslope. If you don't want to deviate from the glideslope, nor some EGPWS glideslope warning, you need to maintain it, and when initiating the flare, you will arrest the descent rate enough for the aircraft to touchdown at least some distance far from the 1000ft mark.

What I see used in practice is, that around 200ft of height, pilots generally start to aim between the 500ft and 1000ft runway markings, so that when they flare, they 'float' only enough to touchdown at the 1000ft mark. But by doing that, they're deviating from the glideslope, and it seems to be the so called 'dive down' that we must not do. On very short runways they aim at the 500ft marking, striving to touchdown at the first inch of the 1000ft mark.

So, my guess is, for certification, the test pilot generally crashlands the plane at a high vertical velocity to demonstrate the stopping distance and that it didn't fell off to pieces, or I'm missing something very important, which is the reason for this post.

So, blast away your views, and thank you in advance!

[aerobat77]

So I'm having trouble understanding, if you use Vap all the way down until you retard the levers, you're not crossing 50ft at Vref, and in a 3º glideslope/path that directs you to the 1000ft mark, the only way of touching down in it is with an almost hard landing, since you're in a +- 700ft/min descent rate established on the glideslope.

hi !

the Vapp vs Vref is for wind component correction , the descend rate on a given glide path as well the flaring distance to hit the tdz are purely related on ground speed so adding some airspeed to the vref will not make you go faster in regards to groundspeed when you have a head wind.

when it comes to flare you should aim for a smooth touchdown without a exessive flare and balloning over the runway. depending on the aircraft type balloning may even result in a tail strike and for sure it extends the landing distance.

cheers !

[mutt]

So, my guess is, for certification, the test pilot generally crashlands the plane at a high vertical velocity to demonstrate the stopping distance and that it didn't fell off to pieces

§ 25.473 Landing load conditions and assumptions.

(a) For the landing conditions specified in §25.479 to §25.485 the airplane is assumed to contact the ground—

(1) In the attitudes defined in §25.479 and §25.481;

(2) With a limit descent velocity of 10 fps at the design landing weight (the maximum weight for landing conditions at maximum descent velocity); and

(3) With a limit descent velocity of 6 fps at the design take-off weight (the maximum weight for landing conditions at a reduced descent velocity).

(4) The prescribed descent velocities may be modified if it is shown that the airplane has design features that make it impossible to develop these velocities.

Mutt

[flyburg]

Hello PC,

The requirements you rever to are for dispatch purposes. Only to asses whether it is legal to dispatch to an aerodrome! Quit arbitrary taken, hence the 60% factor. The actual landing distances, to be determined in actual flight are different and there is no requirement to land within 60% of the runway length, only to accomplish a save landing.

Hope this answers your question

[john_tullamarine]

These sort of questions come up regularly and are good for discussion. Unfortunately, we appear still to have some very adventurous folk out there in non-sked land.

The aircraft is certified for crossing the runway threshold at 50ft, at Vref, on a 3º slope, and to TOUCHDOWN the main landing gear at the 1000ft markings (the aiming point markings), and then comes the 60% of the dry runway distance.

Better to think along the following lines ..

(a) the certification flght test work confirms aerodynamic model predictions and provides the final model for developing AFM performance scheduling data.

(b) the presumption is that the aircraft will pass through a nominal 50ft screen height at a speed and on a slope suitable for the Type. The AFM will contain a description of the technique inherent in the AFM figures.

(c) touchdown cannot be at the aiming point for the reasons indicated by Mutt for a stable approach and must be somewhat further in.

(d) ducking under in the final approach phase is stupid in the extreme as it deliberately introduces a destabilisation into the approach at a critical stage .. plus .. it's not nice to drag the boundary fence wires into the touchdown zone.

(e) the TP will be doing his/her best to get "good" marketing data and will direct "not so good" runs to be discarded

(f) the aerodynamicists analysing the data set will, likewise, discard a few which "aren't quite nice"

(g) end result is that the TP cannot routinely and absolutely achieve the AFM basic data ... keeping in mind that the TP's technique will be a tad more savage than what one sees on the line .. the line pilot has no chance of doing so. It is just as well that we rarely see ourselves faced with seriously minimum runway lengths in the order of unfactored distances.

(h) he/she who routinely tries to achieve basic unfactored distances derived from AFM data .. is stupid and culpable .. no other word for it.

(i) now, whether one "needs" the full Part 25 factors is a moot point but they have stood the test of quite some time. However, one does need some pad to provide a reasonable probability of stopping on the seal.

(j) have an overrun without the factors and you might have some difficult explaining to do at the enquiry and in court during the actions for damages.

in practice is, that around 200ft of height, pilots generally start to aim between the 500ft and 1000ft runway markings, so that when they flare, they 'float' only enough to touchdown at the 1000ft mark.

That's called ducking under and is not sensible in the extreme, especially in other than very good met conditions.

for certification, the test pilot generally crashlands the plane at a high vertical velocity

Not reallly the case .. the TP endeavours to get the aircraft onto the ground in a reasonably repeatable fashion while not exceeding the structural limit descent rate and seeking to achieve the minimum total distance.

Vapp vs Vref is for wind component correction

The AFM will prescribe a technique which usually includes a small margin above the minimum Vref for nil wind conditions. In conditions of steady and gusty winds, additional margins will be prescribed with the intention of trading off some of the factor pad to improve final approach safety

so adding some airspeed to the vref will not make you go faster in regards to groundspeed when you have a head wind.

But keep in mind that the distance required is based on 50% of that headwind so your argument is a bit full of holes.

when it comes to flare you should aim for a smooth touchdown

I suggest that the aim is NOT for a smooth touchdown but, rather, a firm touchdown consistent with minimising the flare distance. Indeed a very smooth touchdown is not a good practice as it generally involves needless float .. although it is very pleasing to one's ego.

Sometimes one gets a touchdown so smooth that one isn't able to determine just where the touchdown occurs .. no sound, no vibration associated with the touchdown. I've only had this happen twice and both occasions were very unpleasant .. especially in the 722 where I knew that, if I were still in the air, .. everyone was going to know all about it in the next few seconds. Mind you, when the girls asked "who did that ?" after the flight .. I was only too modestly willing to take all the (quite unjustified) credit for it.

The requirements you rever to are for dispatch purposes. .. Quit arbitrary taken, hence the 60% factor.

I suggest that that is a foolish view to take. The driver for routine landing command decisions should be probability of a safe landing and eating significantly into the certification factors is not the best way to achieve that desire.

there is no requirement to land within 60% of the runway length

but one had better have a very good reason not to have done so if it all turns to custard ... noting that there is no requirement to achieve the landing within that distance .. only that the factor should be present .. the actual landing is expected to end up somewhere within the pad section of the scheduled distance.

only to accomplish a save landing.

.. and just how do you propose to do that in a reliable and predictable manner if you abandon your distance factors ?

[aerobat77]

hi john !

so adding some airspeed to the vref will not make you go faster in regards to groundspeed when you have a head wind.

But keep in mind that the distance required is based on 50% of that headwind so your argument is a bit full of holes.


but adding just 50% of the headwind component makes you go actually slower over ground ( comparing to vref no wind ) and reduces the disctance to the tdz on a steady glide .

when it comes to flare you should aim for a smooth touchdown

I suggest that the aim is NOT for a smooth touchdown but, rather, a firm touchdown consistent with minimising the flare distance. Indeed a very smooth touchdown is not a good practice as it generally involves needless float .. although it is very pleasing to one's ego.

i meant with it you should aim for a manouvre where you in one sentence come down to ground, minimize descend rate at the last feet of height and neither hit hard nor stop descend fully , maybe climb again one feet, trying to correct for a smooth touchdown and so floating over the runway. you aim for a firm touchdown only on wet runways to minimize risk of hydroplaning. too firm may also result in bouncing, especially when not having autospoilers.

but i agree that in doubt a firm touchdown is better that floating over the runway- on some airplanes both excessive floating as well bouncing may also result in a tailstrike.

best regards !

[misd-agin]

Did the OP have a question in it?

Test guys generate numbers and then the regulators increase those numbers by X% for line operations.

Following the GS electronically, or with PAPI/VASI with your eyes, to a 1500' aimpoint doesn't equate to a 1500' touchdown. Eye height, pitch attitude, distance from pilot's eye to main gear, etc, results in a touchdown point that is different(short) from the aimpoint.

[safetypee]

aerobat77, re headwind speed addition. What you state is ‘texturaly’ correct, but operationally of little value for maintaining the required (assumed) level of safety.
If you base the landing performance on the reported head wind, i.e.100% wind, then adding 50% of that wind speed to Vref, results in the aircraft having a higher groundspeed than assumed in the landing calculation. This requires more landing distance, which against a planned full-wind distance represents a reduced margin of safety.
Landing distance calculations should be adjusted to account for the higher Vref.

The UK AIP / AIC has a useful reference: Landing performance of large transport aeroplanes. (www.ead.eurocontrol.int/eadbasic/pamslight-EC3807BC691CFF8019155B6BD6A3A85D/7FE5QZZF3FXUS/EN/AIC/P/014-2006/EG_Circ_2006_P_014_en_2006-02-02.pdf)

So too AC 91-71 Runway Overrun Prevention. (www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/0052F2A2A00D91B28625738E0071E44C?OpenDocument&Highlight=ac%2091-79)

[Pugachev Cobra]

John, always appreciate your answers!

but one had better have a very good reason not to have done so if it all turns to custard ... noting that there is no requirement to achieve the landing within that distance .. only that the factor should be present .. the actual landing is expected to end up somewhere within the pad section of the scheduled distance.

So, if I understand correctly, the pad you mention is that, in line operations (and not a prescribed factor) we should use some distance that is between the 60% and the runway end?

As for misd-agin, perhaps I wasn't clear in not putting a question mark.

My question is, do we as pilots have to cross the runway threshold at Vref? Is that mandatory?

I mean, I've just read safetypee links on certification and what john said makes sense, even though they always mention crossing the threshold at Vref, period.

What I know it's common practice about the wind correction for the Vap, is a minimum of 5kts, so Vref + 5kts = Vap, always, unless the headwind component is greater than 10kts.

The speed correction should be half the headwind component, and also we should add the gust factor. But the Vap should not be more than Vref + 20kts.

With these in mind, and these corrections with at least + 5kts, should we reduce from Vap from Vref somewhere before crossing the threshold, or just use this wind correction, for a good performance's sake.

And yes, ducking under is a concern, which is one of the reasons of this post.

I've seen a video where I assume it's a test pilot landing, because of the abrupt flare very near the ground. I begin to question if the certification is just so that the manufacturer is demonstrating that the airplane can perform adequately, but as there is almost no margins involved, real world landings would never perform as the certification landing requirement.

I'm gonna look for more of these videos.

I just wanted to clear that. I was assuming that all pilots should perform exactly, or better, as the requirement dictates.

It seems to be just the opposite?

Maybe landing performance requirements are not minimum limits, but maximum conservative limits that it's difficult to duplicate, but since there is 40% of a runway to be used, actual operations don't neet to do it? If so, it would be the opposite of takeoff performance where everything's assumed to fail in a worst case scenario, but normally everything performs much better than the requirements.

[safetypee]

PC, attempting to answer some of the questions.

The ‘pad’ should be considered a safety margin which provides some ability to minimize the risk from variations in normal operation. It does not ensure risk free operation, e.g. you cannot expect the margin to cope with several extreme variations (fast, high, and long) all at once, nor significant variations in surface conditions or weather (crosswind), especially if wet.
The Captain has responsibility for the safety margin. In EU operations s/he has to ensure a safe operation (pre landing EU-OPS 1 ref …?).
A margin of 60% landing distance is required for dispatch. A Capt would have to justify how an operation would be safe if less than that margin were chosen. It might be justified in abnormal circumstances, but in normal operations, less so.

The landing performance is based on crossing the threshold at Vref; pilots should aim for that speed, but as above there will be ‘small’ variations. If other operational decisions require a higher approach speed such that Vref will not be achieved, then the landing distance required should be adjusted to maintain the safety margin.
Wind and gust additions are usually according to manufacturers’ recommendations, use those, and consider adjusting the landing distance.

There should be no need to duck under the glideslope (nor any thought of doing so), as this reduces other safety margins. The aim is for a stabilized approach, but some manufacturers and operators allow a small speed reduction when approaching the threshold to achieve the target threshold speed, i.e. when a safe approach is assured.

Certification landings do not involve exceptional high rates of descent or difficult maneuvers. Manufacturers are required to recommend a normal landing technique which does “not require exceptional piloting skill or alertness, nor involves excessive vertical acceleration, tendency to bounce”, etc, (CS.25.125). Thus manufacturers may be limited in their methods of obtaining performance data, but it is most unlikely that normal operations will be able to reproduce an equivalent distance, either in flaring and touchdown, and perhaps also in braking.

The 60% safety margin should be considered a normal minimum, particularly on wet runways where there can be many more unknowns and greater variability in the conditions. Accident statistics and some technical reports suggest that a larger factor than currently used is required in adverse conditions in order to maintain the same level of safety. Note that in some operations on contaminated runways there may be little or no margin.

Do not equate landing distance safety margins with any other operational safety factors, particularly for takeoff where there may be no margin; – JT will give you full Chapter and Verse on those issues – search for relevant threads.

[john_tullamarine]

Stirred up some thought .. good to see.


adding just 50% of the headwind component makes you go actually slower over ground ( comparing to vref no wind ) and reduces the disctance to the tdz on a steady glide .

I think you might be missing the point a tad. By including some wind in the calculations (whatever factor) the result is that the landing distance required is reduced. Certainly, for a given airspeed, the groundspeed will reduce as well but the concern is that the scheduled distance is less and, for a limiting runway you have less seal to play with ..

and neither hit hard nor stop descend fully , maybe climb again one feet, trying to correct for a smooth touchdown and so floating over the runway.

If your technique and finesse is of such a high order, you might be one of these exceptional folk who can flare onto the ground every time. As I indicated, as an example, I have managed only two landings where I didn't know I was on the ground .. others, I acknowledge, are rather more skilled than I.

However you might like to style it, the aim is to get the aircraft onto the ground as soon as is feasibly practical without shaking out everyone's false teeth. Clearly, on a runway much longer than required, one can play about a little and aim for a smoother touchdown. However, the problem here is that one develops undesirable habits which might bite one on the occasional limiting runway ?

Philosophically, for a dry runway, one endeavours to flare onto the ground or, at the least, flare and then put the aircraft onto the ground without worrying too much about finesse in the touchdown. I was fortunate in that I flew little aeroplanes (737/727) and, generally, had plenty of runway.

Of course we all like to get a nice, smooth touchdown. Just be careful that you don't fall into bad habits on the usual long runways and then get burnt when you have a short one ...

Landing distance calculations should be adjusted to account for the higher Vref.

.. consistent with whatever guidance the AFM provides .. Keep in mind that the landing distance factor is there to cover a host of minor errors and problems .. including a modest overspeed into the flare.

in line operations (and not a prescribed factor) we should use some distance that is between the 60% and the runway end?

No, I don't suggest that at all.

(a) in normal operations, one should ALWAYS have the scheduled distance available - preflight planning or inflight management. The QRH provides emergency data .. it doesn't mandate that you put yourself into a situation where you commit to landing with only such emergency distances available.

(b) in an emergency, the commander may exercise the relevant regulatory authority to vary the rules to suit the needs of the occasion.

However, the commander ought to consider very carefully the wisdom or otherwise of reducing available factors below what might reasonably be available to him/her.

For instance, inflight one has some emergency necessitating an urgent landing. If one has NO alternative, one will take one's chances with whatever only runway one has, as a general rule.

If, on the other hand, one has several runways available then, as PART of the emergency planning management, one should consider the various runways - strengths and weaknesses - as part of the overall strategy with the intention to end up with the best compromise practicable to achieve the most favourable outcome.

do we as pilots have to cross the runway threshold at Vref? Is that mandatory?

No, not necessarily. One should be aiming to operate in accordance with the AFM protocols and company SOP. Generally, this will prescribe a modest margin above Vref.

The speed correction should be

The actual recommendation will vary a little from one OEM to another. That cited follows the Boeing line

should we reduce from Vap from Vref somewhere before crossing the threshold

We should aim to follow whatever AFM and SOP requirements are prescribed in line operations.

question if the certification is just so that the manufacturer is demonstrating that the airplane can perform adequately

Performance landing trials are for the purpose of getting numbers for the certification process. The average line pilot will NOT reproduce the TP's efforts.

I was assuming that all pilots should perform exactly, or better, as the requirement dictates.

The TP provides the absolute brick wall data - it is foolish, in the extreme, for the line pilot to presume that he/she will be able to achieve raw test data results derived from the factored AFM data on any given occasion.

The certification process puts a factored pad in for the rest of us average pilot folk. Our responsibility is to do our best to fly in a manner which is compatible with AFM and SOP requirements and recommendations.

The 60% safety margin should be considered a normal minimum,

concur - my bolding. Note that, depending on the source, AFM landing data may be factored or unfactored and the use needs to account for that.

particularly on wet runways where there can be many more unknowns and greater variability in the conditions.

Generally, and traditionally, wet runways attracted an additional 15% on top of the normal factor. This has been superseded in many cases by contaminated surface protocols

[aerobat77]

If you base the landing performance on the reported head wind, i.e.100% wind, then adding 50% of that wind speed to Vref, results in the aircraft having a higher groundspeed than assumed in the landing calculation. This requires more landing distance, which against a planned full-wind distance represents a reduced margin of safety.
Landing distance calculations should be adjusted to account for the higher Vref.

thats right - when you PLAN the landing distance taking into account the wind you reduce margins by this method. but i must say i never had a situation where the decision if a landing is possible or not ( due to rw lenght) was made with taking the assumed wind into account. especially not at scheduled operations .

i referred to a planning with zero wind and vref on a given runway and adding the half head wind to actual conditions. this will reduce the point of touch down ( the same glide path) as well the actuall overall landing distance vs planned landing distance.

I think you might be missing the point a tad. By including some wind in the calculations (whatever factor) the result is that the landing distance required is reduced. Certainly, for a given airspeed, the groundspeed will reduce as well but the concern is that the scheduled distance is less and, for a limiting runway you have less seal to play with .

the same like written above- you are right when you have to hope for a headwind to make a landing on a given ruway lenght legally possible. but i never saw this on scheduled operations on a fixed route.

If your technique and finesse is of such a high order, you might be one of these exceptional folk who can flare onto the ground every time

lol, no, no extraordinaire aviatior behind the aerobat ;) thats kind of touch down is just what you aim for. i would say i am able to manage good landings, the technique depends much on the type, but of course one is more firm than the other. and when you see you tend to float away you just set her on the ground- firm or not . beautiful landings are possible with the bae146 when retarding power early since this plane is a glider and flares in, on a turboprop its the opposite- pulling back to idle on a cheyenne at 50 ft over the threshold may even result in a crash since she virtually stops flying at power idle and the wing is not that forgiving like e.g in a king air and you may stall and complete your last feets ballistic - that would be a truly firm touchdown.

[john_tullamarine]

you are right when you have to hope for a headwind to make a landing on a given ruway lenght legally possible. but i never saw this on scheduled operations on a fixed route

General convention is to plan on nil wind. However, the actual landing is predicated on the reported (or forecast) wind.

[aerobat77]

General convention is to plan on nil wind. However, the actual landing is predicated on the reported (or forecast) wind.

thats correct, but in this case i cannot follow you gents in regard to reduced margins adding 50% wind to the vref on actual condintions and planned with zero wind for a given runway.

to keep it simple : having an Vref of 100 kt IAS and zero wind gives you a GS of 100 kt on final ( neglegating the IAS-TAS on that small altitudes) . this will result on a given runway xx landing distance and xx margins to runway end.

having 10 kt headwind and adding 50% of it to the Vref will result in Vapp of 105 kt IAS and just 95 kt GS and this will reduce landing distance and improve the margins- the runway has the same lengtht with or without wind .

so you a ) go slower above the ground ab b) have bigger margins for landing on actual vs planned conditions since its common knowledge that headwind reduces landing distance.

what i am missing ?

best regards !

[john_tullamarine]

i cannot follow you gents in regard to reduced margins adding 50% wind to the vref on actual condintions and planned with zero wind for a given runway.

(a) we aren't adding the wind to the Vref. Generally, there will be a wind additive for steady wind (which is expected to be bled off as the aircraft descends through the lower boundary layer) and gusts (which is kept on throughout due to the random nature of gusts) with a total maximum additive (typically 20kt) to make sure that there is some sensible balance between handling and stopping ..

(b) the wind goes into the distance sums using 50% of the headwind (150% tailwind). It follows that the distance calculated for wind conditions is a little more conservative than for nil wind

(c) for a headwind, the distance required reduces. Main concern here is that one should make sure that the distance available is not squandered by excessive speed, height on the approach, floating, looking for that elusive smooth touchdown to impress the hosties, etc.

the runway has the same lengtht with or without wind

Quite true. However, for the discussions, we generally tend to be talking about the hypothetical limiting runway (ie nothing extra above the AFM requirement). In this case the required length for the headwind case reduces so that we might find ourselves attempting to land on a shorter runway which will require greater attention to getting it right ...

[safetypee]

aerobat77, Re “… what am I missing.” It’s more general than that, like what we – many pilots in the industry are missing.

Wind speed is a significant factor in landing distance because the effect of wind may not be a 1:1 relationship with distance (GS) as you imply. Thus, one of the reasons that only 50% of the headwind can be claimed as benefit, but 150% of tailwind has to be accounted for as a hazard.
An example of some of the aspects see - AC 91-71 Runway Overrun Prevention. (www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/0052F2A2A00D91B28625738E0071E44C?OpenDocument&Highlight=ac%2091-79) Considering the effect of high airspeed on distance (table 2), the addition is 30 ft/kt; however the effect of high airspeed on the tendency to float is 250ft/kt. Furthermore, these distances might be additive. Thus it is important to consider the landing distance increase when making approach/threshold speed additives.

There are similar considerations for tailwinds except the effect is much more – hence 150%. Note the significant number of overrun accidents that have involved a tailwind.

The point about commercial operations is interesting. Many pilots use the dispatch planning data for arrival data – weight, etc; - all in limits – a standard operation.
However, if this mindset is carried over to tailwind operations without checking the distance required, then a rapidly decreasing safety margin can be missed. Landing distance should always be checked (not just weight); this also provides a guide of how hard you need to brake as well as the accuracy required in the landing speed and position.
Note that EASA has identified the ‘failure to calculate landing performance’ as a safety factor in their recent analysis, also see – Lessons from Landing Excursions. (http://dl.dropbox.com/u/6109264/Study.pdf)

P.S. you mentioned the BAe146 (#12) - IIRC the manufacturer’s landing tests were based on a 5 deg approach with a target touchdown rate of 6 ft/sec to determine the airborne portion of landing distance. At least five pilots had to provide data to enable a suitable average; in addition a UK CAA pilot flew some check landings, and many more landings were observed by CAA flight test engineers. Some ground roll / braking distance measurements were included in these tests, but additional data was gained from full braked landings on different runway surfaces in conjunction the anti-skid tests which also included checks in damp / wet conditions.

[john_tullamarine]

Wind speed is a significant factor in landing distance because the effect of wind may not be a 1:1 relationship with distance (GS) as you imply

Stretching the memory way back to simple mechanics in secondary physics courses, most of us can recall something along the lines of

s = (v^2 - u^2)/2a

which, for a first approximation consideration of a landing (and quite reasonable for the ground roll bit) simplifies to

s = v^2/2a

where

s = distance gone
v = initial speed
a = constant acceleration (deceleration)

ie to a reasonable approximation, distance varies with (ground) speed squared. Hence the very real concern with getting everything right for tailwind landings ....

When you see simplified approximations for distance variations along the lines of so many feet/knot, such figures are based on straight line approximations of the realworld relationships and should only be applied for small speed variations.

[aerobat77]

Considering the effect of high airspeed on distance (table 2), the addition is 30 ft/kt; however the effect of high airspeed on the tendency to float is 250ft/kt. Furthermore, these distances might be additive. Thus it is important to consider the landing distance increase when making approach/threshold speed additives.

thats interesting and from practical point of view not wrong even when i think the official AOM,s do not cover it.

we can say that adding IAS for headwind component improves the risk to float and so by this increases runway lenght needed. thats why a maximum addition is given .

wind is a "strange " thing and may be a pain in the a... when we leave for one moment the paragraphs and performance charts and face real life. hard wind mostly does not come steady. going to slow may result on approach in a hazard when coming to gusts or even microbursts. adding much to vref may results the aircraft is going too fast regarding IAS and simply do not wants to land and floats away.

the regulations say the wind will bled away at tochdown- nice said, but reality may be different. you may encourter severe gusting and sudden direction changes in 3 feet height so you are struggeling to correct as good as possible for a fair and not pretty hard touch and try to keep centreline forgetting that you are floating away.

all that is not covered by any book and it happens fast.

my practise in real heavy and gusting winds is to use as little flaps as possible and to aim to bring the aircraft just down- firm or smooth, who cares.

the me known procedures are to calculate with zero wind and add 50% of the actual wind. when a zero wind landing is possible any headwind should improve margins. but reality is different- thats right.

further it depends on the location and every airport is different- trees, hangars, wind direction , this all affects the gusts, the down and updrafts and much more. i saw 30 kt which were easy and 10 kt which were a real eye opener on flare, no question.

i think when it comes to this theme regulation and standard procedures may differ much from reality, every wind on every airport may be different.

best regards !

[FlightPathOBN]

From a procedure design perspective, the above mentioned numbers are part of a balanced equation for the approach procedure.

The balanced equation, includes the approach at 3 degrees, a 50 foot TCH, and a missed approach surface of 200 ft/nm.

This is the basis for the minima, coupled with obstacles in the approach or missed approach surfaces.

The minima is based on aircraft CAT, which is detailed by the final approach speed. As an example CAT C is 140 kts. While you may agree or disagree on the final approach speed, this is the criteria.

The equation is based in the TCH, the runway slope, and the available landing zone. If you have a 48 foot TCH, that likely means the runway is sloping down, so you dont land long, same...if you have a 54 foot TCH likely means the runway is sloping up, so you dont land short, as it relates to the TCH....

Approach speed is not an option....

[aerobat77]

Approach speed is not an option....

when the aircraft is approved for landing at several flap settings it is an option i would say.

[FlightPathOBN]

Well, hell, you didnt tell me you had on o those new fangled multi-flap setting thangs...and you can land on many flap settins...hot damn...

so since speed aint no issue wit you...which CAT minima do you figurin using on da chart?

or don that she'et make a heap o difference to ya all...

just 'git er down"?

[aerobat77]

you may ask questions- of course always cat A for best possible minimums !:ouch:

seriously... when the aeroplane is approved for many flap settings and gives you tables for each flap setting you have to look at the actual vref at a given configuration , because vref is the thing category depends on .

[FlightPathOBN]

with actual weight, flap settings give VREF

VREF X 1.3 equals final approach speed....but

then there are MET conditions...winds.

perhaps, far down the variable list, VREF is a component of the CAT, but it is but one that trumps...
final approach speed rules.

[Denti]

Didn't you mix up Vs with Vref? Never heard about a factor added to Vref, just a fixed additive (737 for example +5 to +20kts depending on wind/gust).

[FlightPathOBN]

Denti,

Thanks for pointing that out...you are correct VREF is 1.3 times Vso...

then add winds...

[misd-agin]

Vref can be lower than 1.3 Vso.

737-800 Flaps 40 is either 1.23 or 1.25 Vso.

I forget the details but I think the FAA(FAR?) minimum is 1.21 Vso.