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AeroTech
19th May 2012, 19:19
Hi,

a) Why you add wind additives such headwind and gust to Vref?

b) Why you don’t subtract/add wind additives such tailwind and crosswind to Vref?

It seems Boeing recommends bleeding off headwind additives approaching touchdown.
c) How you can bleed off wind headwind additives approaching touchdown?

d) How you can bleed off other wind additives/subtractives (tailwind, crosswind)?
I mean by subtractive the opposite of additive (if this word exists in English, if not add it to Pprune Dictionary :))

These questions concern all aircraft manufacturers (Boeing, Airbus, Bombardier, Embraer...etc)

Feedback appreciated

bucket_and_spade
19th May 2012, 19:39
Done to death already. Run a search.

Pub User
19th May 2012, 20:07
b) Why you don’t subtract/add wind additives such tailwind and crosswind to Vref?

Is this a pilot asking this?

PT6A
19th May 2012, 21:02
Question 1. Why do we have a groundspeed mini function in the first place?

Answer: To enable the aircraft to make an approach at the minimum safe approach speed.

Question 2. What is 'groundspeed mini' anyway?

Answer: If you consider an approach in a conventional aircraft, we will all accept that the aircraft groundspeed is the difference between the TAS and the headwind component of wind. (For all practical purposes, TAS = IAS/CAS at the low levels and speeds associated with nearly every approach). If there is a gust of wind, due to the inertia of the aircraft, the goundspeed stays constant (in an instantaneous sense) but there is an instantaneous drop/rise in IAS. Over a period of several seconds, the groundspeed eventually settles to a lower level (assuming an increase in headwind component) and the IAS settles back to its original level before the gust. If that gust then disappears completely, groundspeed instantaneously becomes IAS (ie TAS) until the same settling process occurs as described previously. If that original gust was substantial (say 25kts+) and the loss of gust is equally substantial, a situation can arise whereby the aircraft is encroaching into the stall regime and at the very least may experience a sifnificant, and potentially dangerous, loss of speed/lift. In a conventional aircraft this potential problem is overcome by adding up to 15kts, typically, onto your approach speed in gusty or crosswind conditions. In an Airbus the problem is handled by working out the minimum groundspeed that is acceptable for a given wind condition and ensuring the aircraft never drops below that value. This ensures that regardless of gusts the aircraft is guaranteed a safe flying speed. This minimum groundspeed is known as 'groundspeed mini' or 'gs mini'. Easy!

Question 3: What do I need to know about 'gusts of wind'?

Answer - What we conventionally think of as gusts and what Airbus calls a gust are 2 different things! A 'conventional' pilot thinks about a gust of wind as being an unanticipated and rapid change in speed or direction of a volume of air. (There are no doubt better definitions but I think you get my drift!) The Airbus, being a dull machine, has a different way of assessing a 'gust'. On the PERF App Page, one of the programmable fields is for wind - that wind is known as the 'Tower Wind'. Although you do not see it, a computer takes that wind and resolves it into a headwind component relative to the programmed runway. The Tower Wind is used to provide a datum setting of guaranteed minimum wind that may be safely assumed to always be there (which is why Airbus insists on entering the wind without the gust component). In addition, the IRS's are always calculating a w/v which is displayed to the pilot on the ND. That wind is resolved into a headwind component by one of the computers. Although that value is never formally displayed, it is easily calculated by taking the groundspeed from the TAS on the ND. The aircraft then takes that value and compares it to the headwind component of the Tower Wind (wind in the Perf App page) - known as the ʻTower Head Wind Componentʼ or THWC. The difference between the two values is taken as the 'gust' - ie the 'unanticipated' wind component. The calculation assumes that the THWC is a minimum of 10 kts so if the Tower wind is say 260/6 the calculation will assume it is 260/10.

Question 4: What does the Airbus do with that calculated gust of wind?

Answer: It simply adds the rest of that ʻgustʼ onto the calculated approach speed (VAPP) on the PERF App page. That is then displayed to the pilot as the VAPP TARGET, which is the magenta triangle approach speed we all know and love on the PFD.

Question 5. How many possible approach speeds does the Airbus calculate and what one does it use?

Answer: The Airbus actually calculates 4 possible approach speeds but only displays the highest one to the pilot as the magenta speed triangle (VAPP TARGET). That also becomes the autothrust speed target. Two of those speeds do not consider groundspeed mini and 2 do use it. The first 2 are straightforward and are calculated from the following equation:

VAPP = Max (VLS + 5, VLS + 1/3 THWC [to max of 15kt]) The important thing about VAPP is that it is known beforehand as it appears on the PERF App page. It is the highest of VLS + 5 or VLS +1/3 of the THWC (limited to a max of 15 knots). Say for example VLS is 125 kts, and the reported wind is 260/50 on runway 26 (ie all headwind) then VAPP would be 140 kts as the max value of the tower headwind component would be 15kts. VLS + 5 would only be 130 kts so the higher value would be displayed on both the PERF App page and on the magenta triangle speed bug on the PFD.

In equation terms, groundspeed mini is described as follows:

GSmini = VAPP – THWC or VAPP – 10 [If Tower tailwind or THWC < 10] This leads us to the calculation of the next 2 possible speeds, both of which consider groundspeed mini. The equation is as follows:

VAPP TARGET = Max (VAPP, GSmini + Current HWC)

As an example:

VLS = 120kts Tower Wind 260/27 1/3THWC = 9.0 R/W Dirn 260 VAPP = 129 kts Current Wind 260/35 Current HWC = 35.0kts x-wind = 0 GS Mini 102kts

Groundspeed Mini Explanation

VAPP-derived speeds:

VLS + 5 = 125kts VLS + 1/3 THWC (max of 15kt) = 129kts

Groundspeed mini-derived speeds:

VLS + 5 -max(THWC,10) + Current HWC = 133kts VLS +min(1/3THWC, 15) - max(THWC, 10) + Current HWC = 137kts

Therefore, VAPP TGT = 137 (which is displayed on the PFD)

Question 6: In general terms then, what is the rough rule of thumb about the expected approach speed?

Answer: The magenta bug speed will always be VAPP from the PERF App page, plus any ʻgustʼ along the runway axis.

Question 7: Does it matter what wind I write in the Perf App page?

Answer: If the wind is 10kts or less you can write anything you like and it will have no effect whatsoever on the final approach speed. So, for example, if landing on runway 26 you can write 080/10 and the approach speed will still be VLS + 5. Once the wind is greater than 10kts what you write does affect VAPP TGT (ie the magenta bug speed).

Question 8: What is the effect of increasing the Tower Wind on VAPP target?

Answer: It is the exact opposite effect many people imagine. If I am approaching runway 31 and the instantaneous wind is 310/35 but the Tower Wind in the PERF APP page is 310/8 the ʻgustʼ is taken as 25kts (the calculation always assumes a minimum headwind of 10kts). That would be added to the VAPP of say 135 knots to give 160kts magenta bug speed. If I now write 310/15 in the Perf App page as the Tower wind that will have the effect of reducing the approach speed because the gust is now only 20kts. That would be added onto VAPP of 135kts to make 160kts. You can try this for yourself and see it instantly work. So in general terms, reducing the Tower Wind increases the approach speed and vice versa. Therefore it is important to put in the steady state wind and not the max gust because by so doing you can erode the protection the function is trying to provide. Putting in a very high wind at the last minute will instantaneously decrease the approach speed bug.

Question 9: When does the groundspeed mini function cause problems and what can I do about it?

Answer: The function causes problems typically at 1500ʼ above the runway on a very windy day when the wind can be enormous compared to the Tower Wind. If for example on RW 08 with a VAPP of 125kts and the Tower Wind is 080/15 but the instantaneous wind is 080/70 (as can happen) then 55 knots can be added to VAPP making VAPP TARGET 180kts. This can be above the flap limiting speed for Config Full (177kts) and give an enormously high approach speed. However as you approach the ground that speed will progressively decrease as the headwind component (and ʻgustʼ) decreases. There are 2Groundspeed Mini Explanation

ways to overcome this. One is to enter an artificially high Tower Wind and thereby reduce the ʻgustʼ and subsequent VAPP TARGET or the more common method is to immediately select a speed (say 160kts) and wait for the gust to die down. As soon as it has done so, you manage the speed again and the VAPP TARGET will be sensible. Dead easy!

Question 10. Why do we activate the secondary runway on a circling approach?

Answer: This is because the groundspeed mini calculation will see any wind over 10kts from the reciprocal direction as only 10kts. Therefore it will make the approach speed VLS + 5 which removes all the gust protection that should be there. If you activate the correct runway then the headwind components are resolved in the correct direction and any genuine ʻgustʼ is taken into account during the VAPP TARGET calculation.

Question 11. Is GS Mini not potentially dangerous on short runways?

Answer: No! The whole point of GS Mini is to provide the lowest possible safe approach speed. It assumes that the ʻTower Windʼ is always there and is not a gust. By definition a gust is temporary and therefore if a gust appears it will be added onto the final approach speed but the groundspeed will still be the same as if the gust was not there. Therefore no extra landing distance will be required even if it is a high approach speed. The key thing is that the correct wind should be entered on the PERF App page – as long as you do that then you will not have any snags.

sky-738
20th May 2012, 02:54
agree with above

Capn Bloggs
20th May 2012, 12:13
I'm glad I don't fly one of those. My whole FCOM is as long as that post from PT6A! :p

Centaurus
20th May 2012, 12:20
It seems Boeing recommends bleeding off headwind additives approaching touchdown.
c) How you can bleed off wind headwind additives approaching touchdown?


Easier said than done with the common result being most pilots ignore bleeding off the additives (for half the steady headwind component). In turn and especially in conjunction with the Boeing recommended policy of adding all the gust, this has most pilots coming over the fence with excess speed. If the aircraft is allowed to float as a result of this excess speed over the basic Vref, then the aircraft will land long. Depending on runway length in excess of minimum required this may not matter.

But if the runway is wet for example and thus less braking efficiency, there is a risk of going off the end. This risk is increased significantly where company SOP requires minimum use of reverse thrust for various reasons. Often those reasons do not stand up to commonsense argument.

Where Boeing recommend bleeding off the half the steady headwind additive it gives no guidance at what height to start the bleeding off of airspeed.

The reason for adding half the HW component stems from the fact that wind gradient caused by the slowing of wind near the ground starts from about 1500-2000 ft above which is what is called free-stream wind. In other words below this height the wind slows due to ground friction. By slowly bleeding the airspeed it accounts for the drop in wind speed due ground friction. Not very well explained I know.

Boeing give a hint under the adverse weather section of the FCOM when landing with ice on the airframe and recommend adding 10 knots to Vref plus other additives already applied - up to a max additive of 20 knots. Boeing goes on to say "bleeding off the 10 knots (ice additive) below 200 ft. This would suggest that the deliberate bleeding off the half HW additives should start at 200 ft. It is a tenuous example, but Boeing tend to be vague in such areas.

Pub User
20th May 2012, 13:00
I agree that Boeing's advice is often vague, but not in this instance.

From the 737 FCTM:

If the autothrottle is disengaged, or is planned to be disengaged prior to landing, maintain VREF plus the wind additive until beginning the flare. The steady headwind correction is bled off during the flare, however the gust correction is maintained to touchdown.

john_tullamarine
20th May 2012, 14:43
There are several long-winded threads on this topic in the archives. This is one such animal (http://www.pprune.org/tech-log/391078-headwind-additive-autothrottle-use-2.html).

For those interested in the sort of boundary layer under consideration, the standard equation used for steady wind variation (without obstructions to do silly things) is the standard 1/7th power law used for flight test and often graphed in the AFM performance section.

v1/v1 = (h1/h2)^(1/7)

A quiick plot of speed by height will show where the consideration becomes important - around the 300ft mark.

The reason one is exhorted to bleed off this additive is that the profile is reasonably predictable. Gusts, on the other hand, are rather random and there is no basis for trying to fiddle with them in the middle of late final and the flare to touchdown .. the better risk management approach is to use the 1.67 factor as a risk mitigator for the higher touchdown speed.

alf5071h
20th May 2012, 15:56
JT, “.. the better risk management approach is to use the 1.67 factor as a risk mitigator for the higher touchdown speed.”
Agreed in principle, but the landing distance safety factor (concept) is only intended to provide some buffer for every-day normal variability in landings and may not be sufficient if one or more variables are in error in addition to a wind additive approach speed ( Large Transport Aeroplanes - Landing Performance (www.ead.eurocontrol.int/eadbasic/pamslight-EFEEB9AA018E1010EE17C973765EA138/7FE5QZZF3FXUS/EN/AIC/P/014-2006/EG_Circ_2006_P_014_en_2006-02-02.pdf)).

In addition, strong winds / gusts are often associated with wet runways; and although the ‘wet’ landing distance factors are higher, the actual safety margin may not be as good as on a dry runway. In rare conditions there may be no margin at all, thus the landing techniques, accuracy, and use of retarding devices requires careful thought and briefing. Crosswinds should also be considered as they can affect landing distance required, particularly in conjunction with fast approach/touchdown speeds.

Centaurus “but Boeing tend to be vague in such areas” (#7).
Yes; not only vague but they have got their ‘unmentionables’ in a twist. On one hand they support industry standards for stabilised approaches, yet suggest a late deceleration; elsewhere their recommended max approach speed is above the industry accepted maximum of ref+15 and there is little guidance on the subsequent landing techniques and distance. Their problem appears to be a need to encourage safety, yet protect their landing performance. Perhaps different parts of the organisation?
The result is similar to other advice on approach speed / landing performance – manufacturers and regulators; in that operators have to be aware of the extensive range of issues, the limitations of information, and the assumptions made about these operations, and thus safety depends extensively on the crew decisions and actions during approach and landing; it is not assured by safety margins.

john_tullamarine
20th May 2012, 20:52
Agreed in principle

Indeed and agreed. However, at the end of the day, life is an exercise in risk assessment and management - flying aeroplanes is, I suggest, no different.

The initial risk is not getting to the runway.

The subsequent risk is not stopping on the runway.

The older standards envisaged a maximum 20kt approach speed exceedance and, on the basis of simple mechanics this accounts for something in the order of 35-40 percent worry on the distance. Given that 1.43 worked reasonably fine for alternates and we normally have 1.67 or higher for wet conditions, carrying a modest margin appears to be a reasonable balance to me.

Centaurus
21st May 2012, 12:22
The older standards envisaged a maximum 20kt approach speed exceedance

I vaguely recall reading an incident report about a German registered 737-200 that over-ran the runway. Seems he had the full 20 knots additive to his Vref for Flaps 40 and touched down with all the additives. Now this is where I get a bit vague but I am sure the investigation came up with the fact that at Vref 40 plus 20 knots touchdown speed, the lift was equal to the weight and the 737 would not "sit" down.

I know that spoilers must come into the story somewhere but maybe they did not actuate because of no weight on the wheels for that few critical seconds of skimming along the runway?

Maybe that is why the max additives are 20 knots?

john_tullamarine
21st May 2012, 16:44
the 737 would not "sit" down.

I find that a little difficult to contemplate unless the pilot gets into a float and deliberately lets the thing sit there rather than putting it onto the ground when necessary. Finesse is fine and we all like a greaser but sometimes it just is not going to work out to our advantage and we need to make it do the right thing.

maybe they did not actuate because of no weight on the wheels for that few critical seconds of skimming along the runway?

which is why you and I will go for the lever to make the boards come up.

Having said that, I recall my one and only "real" greaser on the 722 .. a very uncomfortable and unpleasant feeling not knowing if the tyres were on the ground or a bit above (no shudder or squeak .. it just skimmed onto the runway) .. if the latter, of course, then EVERYONE was going to know very soon thereafter. Mind you, although it was a pure fluke I had no qualms about taking credit for it when the head lady came up after we had parked and wanted to know who had done the thing.

Maybe that is why the max additives are 20 knots?

Going back to the BCARs (and my memory is a tad faded on the detail now) there was a 20kt maximum on Vat. Never found the rational basis for it but a look at the simple dynamics suggests that it was a reasonable balance between final approach comfort and stopping. It should go without saying that the aeroplane has to be put on the ground without any attention to finesse, especially in wet conditions and reverse and brakes are applied vigorously until the stopping state is comfortable.

JammedStab
22nd May 2012, 00:33
There is no guidance in the Boeing manuals regarding the impact the gust correction, or any other increase in touchdown speed, will have on overall landing distance.

FlightPathOBN
22nd May 2012, 00:58
or you could just be like some operators and cross the threshold at 150 kts in a 737-700/800's and have to brake so hard you pull up pieces of the asphalt....
(JT..check some of the FAS at YMML)

Capn Bloggs
22nd May 2012, 01:06
There is no guidance in the Boeing manuals regarding the impact the gust correction, or any other increase in touchdown speed, will have on overall landing distance.

The generic 737 NG QRH I have (2009 version), has, in the PI section, Landing Distance adjustments for speed above Vref eg Autobrake 2: 540ft per 10KIAS above Vref.

FWIW, my "Boeing" ;) QRH shows:

"DRY RUNWAY ESTIMATED LANDING DISTANCE (METERS); 40/EXT

VREF: Valid from 1 knot to 20 knot above Vref.

METERS PER KIAS ABOVE VREF: +6"

Now I'm confused...

de facto
22nd May 2012, 02:00
Vref+5 instead of Vref at threshold is 95m extra landing distance in my qrh so i guess vref+10 would be cumulative 190m.
Extra speed means extra chance to float hence even extra distance,i think 80m/sec.
So if a 10 kts gust correction is flown until landing and the negative gust never appears then landing distance +190m.
If the neg gust happens just as one touches down,vref is the speed hence no extra distance.
Simply a trade of possible extra landing distance against a possible underspeed at touchdown and all problems related to it.

AeroTech
22nd May 2012, 03:07
Thank you for your feedback.

Factoring landing distance didn’t prevent the occurrence of overrun so far. Wind additives might be a possible cause of overrun (among other causes of overrun). It seems the only way to bleed off aircraft speed is to reduce the thrust (at least for jet aircraft).

If you have to bleed off aircraft speed (i.e. reduce the thrust: for example from approach idle to flight idle) during the flare or at very low altitude as recommended by your company or Boeing, you will loss altitude in case of Go-around because of low acceleration/spool up time of the engine (flight idle thrust setting). There is a risk of collision with the ground.
As a layman and in my humble opinion I think there is conflict between bleeding off wind additives (reducing thrust) and performing Go-around.
a) What do you think?

b) Is it easier (efficient) to bleed off wind additives in prop aircraft?

c) Is it possible to bleed off wind additives in jet aircraft using thrust reversers during the flare or before the flare?
(Assuming it is possible to use thrust reversers in flight as allowed in few aircraft).

Feedback appreciated.

john_tullamarine
22nd May 2012, 07:30
and have to brake so hard ..

I think I definitely qualify as an olde phart past his use by date ... I can only recall really significant braking during really significant need occasions.

On the routine longish runway it was a matter of pride to delay the brakes (in dry conditions) until just a touch might be needed for the turnoff .. reverse was in and high as soon as the interlocks permitted one to pull the levers back further.

It seems the only way to bleed off aircraft speed is to reduce the thrust

The intention is not so much that the pilot actively sets out to reduce the speed. Rather, if a stable approach is conducted, then, in the latter stages (say, the last 200 feet or so), one probably should notice a modest and progressive reduction as the aircraft transitions the boundary layer. If one rigorously is pushing and pulling the throttles to stay on speed then this shear-related bleed, of course, will be hidden. This is more so the case in gusty conditions. In nicely smooth conditions, where one can maintain a sensibly constant thrust, the effect should be more readily observed.

The speed reduction is the idealised situation and we do the best we can in the particular circumstances. Main aim is to nail the flare and the touchdown point to avoid floating off down along the runway - that is the real hazard facing the pilot.

(i.e. reduce the thrust: for example from approach idle to flight idle) during the flare

... and, if you don't do that routinely, sooner or later you are going to be bitten rather badly on a short runway. Fine to enter the flare with thrust on but it needs to be reduced to closed throttles during the flare transition. It doesn't take too much residual thrust on to see the float take over ..

you will loss altitude in case of Go-around because of low acceleration/spool up time

a missed approach from flare/touchdown is an extremely unusual situation. In general, and especially on a short runway, when the throttles are closed you are either on, or nearly on, the ground.

There is a risk of collision with the ground.

A missed approach from the flare would be expected to result in a brief touchdown and is not uncommon in the case of low vis operations. There ought to be little risk in such an event. Certainly more a case of interest in older aircraft with slow spool up but still it should not be a significant risk other than in the case of a fairly short runway.

I think there is conflict between bleeding off wind additives (reducing thrust) and performing Go-around.

a) What do you think?

of course there is. However, one sets out to balance the risks. If you don't carry some additional speed the approach risk increases and if you do the roll out risk increases. With the ample factors on scheduled landing distance the risk is small PROVIDED that the pilot nails the flare and touchdown.

b) Is it easier (efficient) to bleed off wind additives in prop aircraft?

indeed - propellers have much the same effect as speedbrakes. One's first landing with a feathered outboard engine is illustrative during the flare as the remaining throttles are closed, especially with larger HP engines.

c) Is it possible to bleed off wind additives in jet aircraft using thrust reversers during the flare or before the flare?

generally this would be a guaranteed outcome .. along with a significant probability of breaking the aircraft on touchdown.

(Assuming it is possible to use thrust reversers in flight as allowed in few aircraft).

... but not during late final approach I shouldn't imagine ?

JammedStab
23rd May 2012, 13:00
The generic 737 NG QRH I have (2009 version), has, in the PI section, Landing Distance adjustments for speed above Vref eg Autobrake 2: 540ft per 10KIAS above Vref.

FWIW, my "Boeing" ;) QRH shows:

"DRY RUNWAY ESTIMATED LANDING DISTANCE (METERS); 40/EXT

VREF: Valid from 1 knot to 20 knot above Vref.

METERS PER KIAS ABOVE VREF: +6"

Now I'm confused...

Thanks for the correction...it would appear that the newer types have adjustments for extra speed on approach. Perhaps I will stick with making that earlier statement in reference to the 727.

AeroTech
28th May 2012, 17:56
Thank you for your feedback.

a) On what speed the landing performance computation is based on?
On Vref (Vref+5/-0, Vref+20/15 …etc?) or on touchdown speed?
Extra speed (due to wind additives) + residual thrust when closing the thrust levers (due to longer spool down time) + the ground effect ==> increase the chance to float (due to extra lift). In order to reduce the “floating” you have to reduce the extra lift, thereby you have to reduce the speed (lift is proportional to the square of the speed).

b) Why aircraft manufacturers didn’t use/recommend the use of thrust reversers to reduce the “floating”?
c) Is there a risk for aircraft or engine limitation/problem…etc?

d) Is there a risk of “floating” for prop aircraft during the flare? I am assuming they can use the props as thrust reversers to reduce the speed and the lift during the flare.

Feedback appreciated

alf5071h
28th May 2012, 19:45
Certificated landing performance (AFM) is based on Vref; - CS/FAR 25.125
Operational landing performance (QRH) may be based on a higher speed than Vref; e.g. many manufacturers use Vref + 5 as a baseline, and for autoland. Distance corrections should be applied to the AFM performance for any difference from Vref: – AC 91-79.
In normal operation, residual thrust should be removed during the flare. It will only be a small component of any increased distance (float) in comparison with high airspeed or a tailwind. Lift per se, does not create the ‘float’; the pilot can still land the aircraft at a higher airspeed at the required position, but the higher energy has to be dissipated during the landing rollout.

Thrust reverse is not normally included in AFM performance due to reliability, consistency, and usability issues; - CS 25.125 (c) and (g).
Most manufactures recommend the use of reverse on the ground; however the extent of this is often left to the judgement of the crew based on the conditions. Thus this decision is subject to the variability in human performance.

The problems above could be considerably exacerbated if airborne use of thrust reverse was considered; - CS 25.933 (a) (2), in flight use.

Propellers slipstream may increase lift, but as above, the aircraft can still be controlled to reduce float and land at the required position, albeit at a higher speed. Speed is the dominant issue.
The regulations for propeller driven aircraft differ from jets, but use of ‘reverse’ (discing) in the air involves other risks, and depending on aircraft type, may affect pitch control.

The better defence against landing overruns is to fly speed accurately, avoid tailwinds (higher ground speed), land at the planned position, and select retardation devices promptly. Distance corrections should be made for planned higher speeds, touchdown position, height over threshold, tailwind, and runway condition – wet … contaminated.

CS 25 http://www.easa.eu.int/agency-measures/docs/certification-specifications/CS-25/CS-25%20Amendment%2011.pdf

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

de facto
3rd Jan 2013, 13:12
Boeing give a hint under the adverse weather section of the FCOM when landing with ice on the airframe and recommend adding 10 knots to Vref plus other additives already applied - up to a max additive of 20 knots. Boeing goes on to say "bleeding off the 10 knots (ice additive) below 200 ft. This would suggest that the deliberate bleeding off the half HW additives should start at 200 ft. It is a tenuous example, but Boeing tend to be vague in such areas.

I always reduced the steady headwind approaching 50 ft to a minimum of vref or any necessary increment such vgust of Vref ice.
But I cant find the info in my airline fcom supp info.
Could someone paste and copy please?
Thanks.

Centaurus
4th Jan 2013, 00:58
But I cant find the info in my airline fcom supp info.
Could someone paste and copy please?
Thanks.








Try B737 FCOM Vol 1 Supplementry Procedures Adverse Weather (Approach and Landing) page SP.16.14 June 12, 2009. It states:

"Note: To prevent increased landing distance due to high airspeed, bleed off airspeed in excess of VREF+5 knots + gust correction when below 200 feet AGL. Maintain the gust correction to touchdown."

My copy of the FCOM is out of date, so check with latest revision number.

de facto
4th Jan 2013, 06:14
My airline cold weather supp does not have such note.
If you are comp genius,would you be so kind and show such page here?(copy-paste?).
Thank you.

Denti
4th Jan 2013, 08:41
Interesting, just checked my FCOM (newly released company version valid on january 1st) and the only note referring to cold weather approach and landing is to use normal flaps and speed additives except for non normal situations requiring flaps 15 (not approved as normal landing flap setting) in which case Vref ICE has to be used (Vref 15 +10kts). No other speed additive is mentioned at all.

Could there be an FCOM difference for those planes which have flaps 15 as normal landing flap setting and those that don't?