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Vref & landing

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Vref & landing

Old 16th Nov 2008, 23:35
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Originally Posted by TeeEmm
I presume you mean half of the steady state headwind component. That is straight from Boeing.
Yes … that’s what I meant … sorry if that was confusing.
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A/R
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Old 17th Nov 2008, 06:50
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(the “aim” point – that point that doesn’t move up or down in the windscreen) is a point on the runway surface about 2/3 of the way between the threshold and the fixed distance markers
Not sure what you mean by "fixed distance markers". Do you mean the aiming point markings painted on (catII/III) runways?
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Old 17th Nov 2008, 07:50
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Vocabulary approach/landing speeds

Boeing says Vref is equal to 1.3 Vs (for the landing flaps configuration).
Those of you adepts of D.P.Davies "Handling the big jets" will also recall his Vat definition, as "speed at threshold"...
At PanAm, we called that speed "Vth" or "vee-thresh".
Vref+5 seems to be the norm of the industry, as the target speed for landing.
Plus winds additives, as described above, not to exceed 20 knots.
And J.T. mentioned carrying Vref+10 for the 727-200. I did that too.
xxx

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Old 17th Nov 2008, 08:07
  #44 (permalink)  
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Originally Posted by GSPOT
Framer is instructed to be at the Threshold at 50ft at Vref +5
- and that, in my book, is correct. If the approach is correctly flown, it matters not too much what speed the a/c is flying BEFORE touchdown as it is only the energy AT touchdown that affects landing roll, and therefore that should be correct, I.E. reducing to Vref in the flare and Vref-5 (or as directed) at touchdown.

That will go 99.9% of the way to guaranteeing a safe stop, although as someone else has pointed out, your 'limiting' runway generally allows a 15kt overspeed on those figures. NB NOT a 15kt 'target', just a buffer!
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Old 17th Nov 2008, 18:12
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framer, et el “… to confirm that the 'Target Threshold Speed' is indeed Vref+5 on a nice day, and not Vref.” (#37)

In my experience (several operators and manufacturers), ‘Target Threshold Speed’ (TTS) can be used in different ways. Generally, TTS is Vref+5, but this could be a corruption / extension of the ‘calm conditions approach speed’, another ‘target’ speed. Note that the UK AIC (#34) avoids confusion (contention), by using 'basic reference approach speed' (Vref).

The safety view (certification - ignoring add-ons), requires the aircraft to cross the threshold at Vref.
Where manufacturers recommend an approach at Vref+5, then they may have assumed that either the 5kts is bleed off before the threshold, or lost during the landing flare, i.e. a particular technique, not applicable to all aircraft. It might be difficult for a manufacturer to argue otherwise if the aircraft is to retain its certificated landing distances.
If manufacturers do not make these assumptions or have some other justification, then more often than not, the crew’s responsibility for a safe landing is increased by having to consider their ‘calm air’ 5 kt addition in their landing assessment – user beware.

In practical terms for the majority of everyday operations this is not a problem (excess runway length), but occasionally in limiting conditions this small loss of ‘distance’ might be overlooked, which increases the risk of an overrun. (General assumption that +5kts is worth 250ft, but with an extended flare this could be 1250ft; ref AC91-71).
Some regulators/operators accept the status quo without further consideration; the extra 5kts is part of the ‘normal variability in operations’. However, operators should define the non-normal conditions by specifying an acceptable approach and touchdown (i.e. stabilised approach, touchdown position, use of brakes, etc), which IMHO also requires pilots to understand the factors effecting the risks as the conditions change.

BOAC, associating the ‘buffer’ (actually a safety factor in the certificated landing distance) with threshold speed can be misleading. The ‘buffer’ can be associated with any of the ‘normal’ variations in operation, but not all at the same time! A limiting landing cannot tolerate multiple errors, i.e. speed error, height error, touchdown position, spoiler/brake/reverse operation, and runway condition, the latter being one of the great unknowns in aviation.

BelArgUSA “Vref+5 seems to be the norm of the industry, as the target speed for landing. Plus winds additives, as described above, ....”
The UK AIC stresses that any wind addition is added to Vref, not the industry norm (Vref+5).

The short summary is we should follow the advice in the manuals, but use all available information in our landing considerations.
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Old 17th Nov 2008, 23:49
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Originally Posted by Cecco
Not sure what you mean by "fixed distance markers". Do you mean the aiming point markings painted on (catII/III) runways?
Yes, sorry. Very old habits die very hard (apparently) ... and my excuse is that I’ve been told I was around when dirt was invented.

I am, indeed, referring to the aiming point markings, which used to be called “fixed distance markers” (somewhere, long ago) and are normally painted approximately 1000 feet from the threshold on instrument marked runways.
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Old 18th Nov 2008, 20:51
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Thoughts about landing performance...

While we are all busy going through the alphabet, with FMS, SOP and CRM...
Any other letters of the alphabet...? How about time to think landing performance.
We all know from initial training that our airplanes are supposed to be landed at Vref.
We know that we land at a Vref that is 1.3 x Vs... and the 20 knots additive.
And we studied that our planes landing performance is based on 60% of the runway bla bla bla...
xxx
Reading our friend IGh here above, he reminds us of the spooling of engines.
Good point.
In many types of airplanes I flew, we had two flaps settings.
The 727 could be landed with 30 or 40 flaps, the 707 did 40 or 50 flaps.
And my old 747 can land with 25 or 30 flaps.
xxx
Not much difference in stall speed between the two... some 5 knots.
The big difference is drag, not lift. The difference is power on the engine.
With extra drag, we keep our engines to higher power. Great for spool up.
Sure it burns fuel, sure it makes noise, but maybe is extra safety, no...?
xxx
I recall the UAL Salt Lake City 727-122 accident, mid-1960s...
Captain approached fast, then went to full idle to reduce speed.
Then when he realized he would not reach the runway he added power.
Too late. The engines did not have time to spool up. He was a former DC6 captain.
In a prop airplane, you want power now, you get it. Not in a jet.
xxx
Then there are the gentlemen here who are shy of "extras" over Vref...
Vref+20 is all Boeing says to add for wind components factor. Fine, no objections.
A little hypothetical landing performance exercise here...
Just for classroom and play with your brains.
xxx
Suppose you land a 747-200 on a long 11,000 feet runway (as usual).
Average passenger weight - say 230 tonnes - your Vref is 134 KIAS.
If landing in a stiff wind, your Vref could be 134+20 = 154 KIAS. Still ok.
So, you good CRM and SOP boys would no squeal a word.
Do you really know landing performance, let us see.
xxx
Suppose same conditions, but the wind is a "very stiff" 60 knots.
Why would I possibly carry Vref+30 then... too fast...?
No, gentlemen. I looked at my landing tables.
I see I could land a heavy 747 at 285 tonnes on this runway (I am only at 230 tonnes)
The Vref then would be 155 KIAS... and I would add 20 knots = 175 KIAS.
xxx
Bottom line is this. Nothing wrong about landing at say Vref+30 (or 40).
That would be Vref+30 = 134+30 = 164 KIAS (I could do as much as... 175)
I consider this... Hopefully dry runway, and still being stopped on 60% of the runway.
Maybe even 70%, as I am not that good, I tend to flare for smooth landing.
Go ahead with your objections - I am retiring and know what I say and do.
xxx
Maybe if the L-1011 (Delta 191) in Dallas would have done so, they might still be here.
And approve.
Extra speed is like extra money in the bank, within reason, and knowing performance.

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Old 19th Nov 2008, 00:22
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Poor spool-up response is generally overrated as a safety issue; the often quoted 6-7 sec is probably the 8 sec certification requirement to achieve ‘landing-climb’ thrust (usually TOGA) from a low power setting (CS 25.119).
Adequate thrust response is available during the approach and landing. Most approaches can be controlled with changes of only 2-4% N1, and this should be sufficient to prevent a sinker during the flare.
One of the tail strike problems is that the sinker may not be recognised; another is the habit of relinquishing control of thrust during the flare – selecting low thrust/idle early or asking the other pilot to control thrust so that a two handed landing flare might be ‘finessed’. These actions may result in the handling pilot attempting to control the combination of speed/touchdown position during a ‘sinker’ with pitch only, usually resulting in a high nose attitude, whereas a small thrust increase might soften the arrival.

Re “Nothing wrong about landing at say Vref+30 (or 40).”
What an appalling statement given to number of fatalities in overrun accidents this year.
Touching down with excess airspeed in many aircraft may prevent the main wheel air/ground changeover working, thus preventing deployment of lift dump/spoiler, thrust reverse, and probably operation of antiskid/brakes. In these circumstances the “knowledge of performance, within reason, or experience – what you say and do” will not prevent an overrun.
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Old 19th Nov 2008, 00:54
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Re “Nothing wrong about landing at say Vref+30 (or 40).”
yeah that doesn't sit right with me either. It's not the total energy the a/c has that worries me, it's the aerodynamic side of the equation, the lift being generated, weight on wheels etc.
It has made me think though BelArgUSA, thankyou for that.
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Old 19th Nov 2008, 07:10
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PEI 3721 -
xxx
Do YOU know how the "air/ground changeover" works in a 747...?
Apparently, you don't...
Also, re-read carefully the presentation of problem... 11,000 ft, dry, etc.
So (1) lack of system knowledge and (2) cannot read and understand.
xxx

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Old 19th Nov 2008, 23:24
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Originally Posted by BelArgUSA
Bottom line is this. Nothing wrong about landing at say Vref+30 (or 40). That would be Vref+30 = 134+30 = 164 KIAS (I could do as much as... 175) I consider this... Hopefully dry runway, and still being stopped on 60% of the runway. Maybe even 70%, as I am not that good, I tend to flare for smooth landing. Go ahead with your objections - I am retiring and know what I say and do.
I understand your comparisons of landing weights. I also recognize that you can land on a given runway at some maximum landing weight, with whatever VREF speed that weight would require – then add the appropriate additions for winds and gusts – giving you a “maximum condition” for that particular runway. And I appreciate the fact that if you land at a landing weight less than this “maximum” landing weight, with identical wind conditions you shouldn’t have a problem in getting the airplane stopped once on the ground, even if you flew the approach at the speed that would have been required for that higher, “maximum” gross weight. But that philosophy presumes that you’re going to put the airplane on the ground at the point it should be on the ground – and very often that is not accomplished precisely because of the extra airspeed the guy driving has elected to carry. A given speed in a lighter aircraft is not the same as that same speed in a heavier aircraft. As you undoubtedly know, the handling characteristics and performance are somewhat different. It is also true that if you don’t get the airplane on the ground, it will eat up distance a lot faster by staying in the air – and a faster than required approach is much more likely to result in a circumstance where unnecessarily overflying concrete will occur. Sure, you can plant the airplane at a speed that is a lot higher than what would have been normal, but, you have to know what to do and when to do it. (Personally, I prefer to train the pilot to be able to recognize how to do this – and the procedure I outlined for flare and landing earlier in this thread will allow you to do that.) However, if a pilot is not familiar with the handling characteristic differences presented by flying faster than what the weight requires, and does not know what to do about it when getting down to the runway, the probability goes up rather dramatically that a problem will occur.

Can a pilot do what you have outlined? Absolutely. But because not everyone is going to recognize the performance and handling characteristic differences, I would not recommend anyone doing anything differently than what they’ve been trained to do – compute the required approach speed based on the existing landing weight; fly the approach with the appropriate wind adjustments; reach the threshold with the appropriate airspeed (that means reducing the airspeed an amount equal to the amount added for the steady-state headwind component – keeping any additive for gusts); continue to descend to the runway, flare to reach the level flight attitude with the main wheels just off the runway, take no more than 3 seconds to get the throttles to idle while you continue to descend those last couple of feet – keeping that level flight attitude constant as you descend those last couple of feet; put the mains on the runway while still in that level flight attitude; and quickly fly the nose to the surface.

Please understand … I’m not at all suggesting that you cannot, or should not, fly your airplane the way you fly your airplane. I’m just saying that the technique you’ve outlined is not something everyone should try.

Originally Posted by BelArgUSA
Maybe if the L-1011 (Delta 191) in Dallas would have done so, they might still be here. And approve.
Sorry. On this one I’m going to throw the flag. That was a microburst windshear. It is highly unlikely that even had the flight crew carried an unrealistically high approach speed, the conclusion would have been anything different than the tragedy that occurred. And I certainly don’t want to indicate to some of the younger, more eager, perhaps a bit too self-confident aviators who frequent this forum that windshear is “no big deal,” just add a big handful of knots and press on. That is a sure recipe for disaster that should not stand without a significant and very vocal challenge!
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Old 19th Nov 2008, 23:36
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What does the autopilot do during the autoland - vref at 50, reduce idle or touchdown at vref?
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Old 20th Nov 2008, 07:58
  #53 (permalink)  
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Although I generally have my eyes closed and hands clasped in front of me in a prayer-like manner, I would say the 737 can land you as low as Vref-10 depending on what happens during the 'flare'. If I were controlling throttle manually I would aim for Vref-5
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Old 20th Nov 2008, 20:39
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IGh Re adequate thrust response on the approach (#55).
IIRC this is the basis of being able to fly stabilised approach - the basic control of the aircraft. The magnitude of the thrust change (N1, EPR, etc) and engine response will depend on the aircraft type, but neither the thrust change nor the response time will be ‘excessive’. Excessive is a relative term, but this covered by the certification of the aircraft i.e. the aircraft can be flown/landed “without undue skill” – satisfactory handling qualities, etc, etc. (CS 25. AC 25-7 “flight test guide for certification of transport category airplanes”)

The TC circular focuses on the energy of the aircraft and not the characteristics of the engine response. It specifically defines low energy state as being where “thrust has stabilized in the idle range”, not that the engines might not spool-up quickly. The other low energy aspects are that the airspeed is decreasing and the aircraft is at low altitude.
The low airspeed reduces the option to ‘trade’ speed for increased altitude (reducing the descent rate in the flare), nor can the altitude be traded -nose down- for speed, i.e. the aircraft is low and slow in the flare.
In these circumstances, thrust is the only means of adding energy. A meaningful change (not TOGA) may take 2-3 sec to take effect, and for an average flare time of 7 sec this might not be sufficient to correct the low energy situation.
I do not read the TC comment about ground contact as relating to a tail strike or the severity of contact, only that the aircraft may touch the runway during a late go around from low airspeed. I recall other advice that in such situations it may be necessary to accelerate to the GA (approach climb) speed (~Vref) before attempting the climb away.
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Old 22nd Nov 2008, 22:43
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I have read, with interest, IGh’s “Examplar for PEI” in which he has pointed out several areas where the FAA (specifically the, Flight Standards offices – the HQ in Washington, and one he referred to as the SW Region) and how those offices have either incompletely addressed the issue of “stabilized approaches,” or have, if I understand his meaning, differing opinions on what that means, or at least, how it is to be achieved. He also has indicated his belief that the US NTSB erred in their investigation of the AA2065 tail strike at Denver in 2006, citing the Chairman’s comment about the board agreeing that the AA’s definition of a stabilized approach not being in “compliance” with FAA Order 8400.10. I’m not sure how that should warrant a criticism of the NTSB, but that’s a discussion for a later time. IGh has also, and very correctly, pointed out that there was a problem in the management of some activities in the SW Region. Since that time, the FAA has apparently taken significant steps to correct the lack of oversight that had apparently existed, according to DFW area newspapers and according to non-attributable confirmations in Washington, by removing and replacing the top two position holders with personnel from other areas within the FAA.

Additionally, some of you may or may not be aware that the FAA Flight Standards HQ office has been engaged for quite a while in updating and modernizing the regulations under which most passengers in the US are carried. You would note that the material that IGh referenced are “orders” and “advisory circulars,” and, as any astute observer or participant in aviation within the US would immediately recognize, the only thing the FAA can enforce are regulations. All the rest of it – including “orders” and “advisory material,” of all types – are recommendations that, while the FAA may believe important, are not requirements that anyone must follow.

You should also know, that, because the US is a nation of laws, it is imperative that the laws that will ultimately govern what happens be subjected to the scrutiny of anyone who may have an interest. It is this process that has lengthened the time that the Flight Standards office has taken to complete this project. Obviously, when anyone attempts to write a “regulation” or a “rule” that is to be followed by everyone, the task of the rule writer becomes more than a bit challenging … as this industry has long ago departed from the idea that a “one-size-fits-all” approach to something like this is not an easy thing to achieve. Also, the speed with which new technology is entering all facets our lives, it is a huge force making its way into the inner most workings of aviation. We are learning more each day about the way people learn to do things … we learn more about how simulation may be used to assist in that learning … we are able to determine, with much greater accuracy today, the root cause of incidents and accidents. And, because all of this is terribly important to the safe operation of aircraft, it is imperative that this material be taken into consideration when the rules and regulations governing the operation of aircraft are levied on those who make their livings in this industry. The rather archaic method in which the US regulatory functions have been mired was beginning to be seen as an “un-indicted co-conspirator,” so to speak, in obstructing efforts to have rules and regulations stay abreast of the situations currently existing in this fast-paced industry.

The FAA, to their credit, I believe, has opted to try a new approach to incorporating into their rules and regulations, a process that sets out requirements in a regulation (a regulation being enforceable) but in a manner that will allow that regulation to be modified rather quickly, when necessary (e.g., learning something new about how pilots learn, or learning something new about why certain accidents occur, or to keep pace with developing technology in simulation, etc.) – while still providing all of the “safe-guards” of having the general public have an opportunity to read and comment on the proposed rules prior to their going into effect. This new approach has been successfully employed in a recent rule on the evaluation of simulators, and, I understand, will be employed in the new rules governing cockpit crewmember training. As a part of that new approach, there is considerable undercurrent in the industry about what it says and what it doesn’t say … but one of the things that it will allegedly contain is a bit more specificity for a “stabilized approach.”

I have learned from someone who claims to know a lot (and has always been relatively accurate in his comments) that the material on stabilized approaches will look like the following:
A pilot flying a stabilized approach must …
1. Have the aircraft in the desired configuration for landing with the engines spooled and stable.
2. Maintain a constant pitch attitude.
3. Maintain a constant heading (within ±10°) or maintain electronic navigation indication with no more than one-quarter scale deviation vertically and laterally
4. Maintain a constant airspeed within +5 and -0 knots.
5. Maintain a constant rate of descent (not to exceed 2,000 fpm below 2,000 ft AGL or 1,000 fpm below 1,000 ft AGL).
6. Keep the aircraft trimmed.
7. Maintain altitude at MDA, when reached within +50 to -0 ft.
8. For constant angle non-XLS approaches (where “XLS” means either ILS or MLS), execute a missed approach when reaching the MDA, DA, or DH (as appropriate – and that is purportedly to mean, if the runway is not in sight or cannot be reached safely).
9. Except where the required visual references for the runway are distinctly visible and identifiable, going below the MDA, DA, or DH prior to the initiation of the missed approach procedure, is unsatisfactory performance.

To maintain my own reputation (such as it is anyway) I can’t guarantee that the above is an accurate listing of what the FAA is proposing, but it does offer an interesting series of steps which we all could, and probably should, contemplate.
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Old 23rd Nov 2008, 15:50
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Dear AirRabbit,

I read your article on final approach / flare...it's great. On your scale C150 to 747 I'm definately at the low hours C150 end. However I'm slightly confused...a not unusual situation. Two points
a) I've got three seconds to retard the throttle and remove any crab. Does the three seconds start when I start to initiate the flare or when I achieve the level flight attiude?

b)Is the gust factor that's added the total gust or the component along the runway?

Apologises if this is all blindingly obvious.
TIM
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Old 23rd Nov 2008, 16:16
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If you have been in icing conditions for an extended period of time, and at 50ft you fly below Vref, you will drop like stone. The stalling caractristics are based on a clean wing. The 1.30 and (1.23 in fly by wire airplanes) stalling margins will not protect you. I am of the opinion to fly at Vref + all the corrections(wind and gust) and bleed the speed off during the flare and touch down at Vref +the gust factor.
The advantage of reducing the landing distance, by flying below Vref is outweight by the chances of tail strike and reducing the climb performence in case of abort landing.
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Old 23rd Nov 2008, 21:54
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I read your article on final approach / flare...it's great. On your scale C150 to 747 I'm definately at the low hours C150 end. However I'm slightly confused...a not unusual situation.
Dear RansS9,

If you are a low time pilot just do exactly as you are being taught by your instructor. Don't muck around with final aproach speeds etc. Wait until you have 1000hrs then revisit the idea.(Unless Airrabbit is your instructor of course)
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Old 24th Nov 2008, 01:44
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tournesol Re – “If you have been in icing conditions for an extended period of time, and at 50ft you fly below Vref, you will drop like stone”.
Your explanation perhaps states the obvious.

Certification requirements cover these points, and require manufacturers to determine any change in stall speed and publish procedures relating to this.
CS 25, AMC 25.21,
4.4.2. c. Changes in operating speeds due to flight in icing conditions.
6.8 Landing (CS 25.125). The effect of landing speed increase on the landing distance may be determined by a suitable analysis.


The AFM or operating manual should have information on the approach and landing speeds (or additions) to be used in icing conditions and the effect on landing distance required.
Some manufacturers have specific speed additives for approaches made in severe icing conditions (where the de/anti icing system can be overwhelmed); again, landing distance corrections are required.

Re “I am of the opinion to fly at Vref + all the corrections (wind and gust) and bleed the speed off during the flare and touch down at Vref +the gust factor.”

This belief might result in an overrun, as all of your excesses will reduce the landing distance safety margin. I suggest that you study the references above and the guidance in ‘Managing Threats and Errors During Approach and Landing’.
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Old 24th Nov 2008, 04:50
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[QUOTE][Re “I am of the opinion to fly at Vref + all the corrections (wind and gust) and bleed the speed off during the flare and touch down at Vref +the gust factor.”
/QUOTE]

Safetypee

This is the same advice I have in both our company ops manual and the Boeing FCTM. There is of course the proviso that you have added a maximum of +15 or +20 to Vref depending upon type but that aside I think it is correct.
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