VRef speeds and half the HW comp.

[Hudson]

This subject was discussed widely some months earlier under Tech Log and finally faded into the sunset unresolved as far I was concerned. On reflection this would have been a performance engineer's area. Here goes:

Since time immemorial, Boeing have recommended a speed additive of half the steady headwind component plus all the gust factor (up to a combined total of 20 knots) to be added to the Vref speed for landing. The HW additive to be bled back approaching touch-down, while the all the gust factor is retained.

Forget the gust factor for the time being - my question is what is the origin of half the steady headwind component policy?

It is obviously (?) associated with wind gradient of sorts. Normally the free stream wind starts around 2000 feet above the ground and below this the wind eases off due to ground friction.

If for example the full headwind is 30 knots, then following the Boeing rule, 15 knots is added to Vref.

At some point or other this additive is deliberately bled off such that the aircraft arrives over the threshold at Vref.

You cannot bleed off 15 knots solely by the actual flare manoeuvre - so at what altitude should you deliberately aim to start bleeding back this 15 knots additive, yet still maintain the requirements of a stable approach below 500 feet? In other words maintaining the correct speed for the conditions.

I understand that for purposes of certification landing length performance, only half the full headwind component is assumed and twice the tailwind component.

That said, is there any relationship therefore between this certification conservatism of half the steady headwind - and the Boeing policy of adding half the HW component to Vref? If so, why?

If the landing is made using autothrottle, Boeing require no additives to Ref apart from the usual five knots to the approach speed - it too being bled off in theory approaching touchdown.

And how is "approaching touchdown" defined? Is it somewhere on late final (say) 500 ft agl - or is it the period between actual flare manoeuvre and actual touchdown (around 2-3 seconds?).

Sure, autothrottle response is fast, but from what I have seen, the human hand moves twice as fast - so why no additives with autothrottle and possibly significant speed additives required up to 20 knots simply because the pilot is manually using the throttles?

Why is half the steady headwind component added to Vref and not (say) one third - or even two thirds the steady HW component.

[alf5071h]

Hudson, I fear that you are asking manufacturers to disclose the black art of their judgement. Having devilled with the dark side on more than one occasion I offer the following view that is by no means definitive, and I doubt that I will answer your questions.
The manufacturer is boxed in by several hard certification constraints that within them hide assumptions on what statistically the crew will do in order to achieve a safe landing i.e.
Landing (stopping) performance is based a range of threshold speeds (Vat) at 50 ft between +15 and -5 (some -zero), and factored landing distance.
The aircraft has sufficient control for a safe landing over a similar range of speeds including a decelerating landing from Vat-5 (tail scrape); also for some aircraft Vat +15 to touchdown (nose wheel first).
Then variability cuts in;
For aircraft with good gust response characteristics - they do not loose lift quickly or change pitch / trim, then some manufacturers will only consider gusting winds. Thus, the additive is one half or one third of the gust, the choice of magnitude depending on manufacturer and aircraft type – the engine response, lift characteristics from prop wash, jet recirculation, ground effect, etc.
For aircraft with poorer gust response then it is more common to add a factored steady wind, but some manufacturers even add a further additional wind gust factor.
For more fun then mix in icing limitations if applicable.

The change in speed additive with auto throttle depends on the design of the system and how it responds to both steady wind shear (with decreasing altitude) and wind gusts (turbulence). Some modern systems mix airspeed and angle of attack to improve performance; others also consider ground speed, thus ‘sense’ the wind. During the flare auto throttle thrust is normally reduced by a schedule with altitude, but again this can be modified by GS; whereas the pilot can modulate the point of retardation, the rate, and even reapply thrust. The certification for auto systems only considers the simple rule.

Numerical and statistical wind, windshear, and gust profiles are given in JAR-AWO, but unless you are going to challenge an aircraft’s certification don’t even think of referring to them.

I would interpret ‘approaching touchdown’ as quite late during the approach i.e. from 100 ft start the speed bleed off just in time to meet the Vat constraints at 50 ft. Therefore, for an aircraft that normally looses 7 kt in the flare below 50 ft, it is reasonable to bleed the speed off from Vat +15 when below 100 ft.
Now this of course could make a mockery of the stabilised approach concept, but most manufacturers would argue that stability for a safe approach is required earlier in the approach in order to get the aircraft near Vat and 50 ft at the threshold.

The best advice for speed bleed off is to use judgement based on training, in conjunction with knowledge of the basis of landing performance and the aircraft limitations. Also, read the small print in the aircraft’s performance manual / AFM. Often there are additional restrictions for higher speed landings i.e add 2% landing distance for every 1 kt above Vat +15.

The accident statistics suggest that both judgement and knowledge is in short supply; thus, the key points are to aim to touchdown at the correct speed, in the correct position and then use all retarding devices immediately at their maximum setting. If the approach is unstabilised or the speed high at the threshold, then Go Around.
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Unless specifically authorized everything else is forbidden.

[Hudson]

alf507h. Thank you very much for your reply. I have printed a copy and will carefully re-read it again over a nice cup of tea.
Some months ago I brought up the question of interpretation of speed additives with a God like senior check captain of a major airline.

His replied that he expected his pilots to take all the additives into the touch-down - despite the Boeing advice to bleed off the half-the-steady HW component.
His reason was that all the company landings were on rarely less than 10,000 ft runways so the wheel brakes took care of the additives. Not quite the thoughtful answer to my question that I had expected- but nevertheless, pragmatic.

My reason for trying to pin down the question of bleeding off additives, was because in my previous job we operated 737's into short runways (varying from 5000ft to 5600 ft) at night, and these became critical in wet weather. You simply could not afford to be liberal with additives.

[alf5071h]

Hudson, I fear that your ‘God like’ senior check captain is constructing a latent safety hazard that awaits the unwary or unfortunate aviator. As you realise there will come the day in your operation, or for a pilot who retains this poor procedural guidance in another operation, where the aircraft weight / runway length, conditions, etc in conjunction with a speed addition will result in an overrun.

I recall some interesting data from an old BA study (SESMA) on landing operations, that showed the point of touchdown was proportionate to the runway length i.e short runway, short touchdown. Similarly, speed control was more accurate on the shorter lengths; unfortunately, this data was not correlated with wind or gusts. Thus while there are no surprises here - the data represents commonsense, it is thus the more surprising that a senior management pilot is so lacking in this department. If required I can provide contact details for the chief test pilot / safety pilot for manufacturer A or B so that appropriate communication may be exchanged. This may also provide an accurate answer to your question for your aircraft type.

This problem is like many others in the industry that are due to the lack of knowledge of the certification requirements, particularly about what is assumed of the operating crew. I lay most of this complaint against the authorities, particularly where there is a major discontinuity between aircraft and operational certification departments. With greater knowledge, I hope that there would be less adding of belt and braces or making rules up because it seems a good idea.

Information on Airbus speed additions in the following link (page113), but they are very type specific; beware large file size 7.9 mb Getting to grips with Aircraft Performance (http://www.nathangb.com/wingfiles/files/GTG_perfo.pdf)
The site page is here: Wing Files (http://www.nathangb.com/wingfiles.htm) where you can also see information on approach and landing accidents.
ALF

[BOAC]

most manufacturers would argue that stability for a safe approach is required earlier in the approach in order to get the aircraft near Vat and 50 ft at the threshold - most companies too. I am sure this is the point of a 'stabilised' approach. In BA, the last 'decision' gate is 500ft AT, with the proviso that Vref+15 is the 'limit' at touchdown for 'limiting' runways. I have used Vref+25 into LGW R26 in conditions of high windshear (long live 'Sir Freddie'!) with no problems. I tend to 'bleed' the excess as I see the conditions. Once I have a landing surface below me I am somewhat more relaxed:D

The auto/manual argument is an interesting one: I have NEVER had an answer from the 'knowledgeable' (ie trainers!):

Boac is barrelling down the approach, auto-throttle engaged, runway wind +20 gusting 10-30. Boac would like Vref + 30 (incidentally, I believe half the wind plus all the gusts is an OLD aviation rule, not a 'Boeing'?). Being a good lad, he bugs Vref+5 as per Boeing, thinking Vref+15 would be acceptable if runway is limiting. The A/T makes valiant (if uncomfortable!) effort at controlling at a safe speed. At some height he bravely disconnects the A/T :eek: since leaving Boeing's A/T engaged TOO long would be exciting!

Dilemma! He is supposed (and trained) to fly 'bug' speed. Does he re-bug to Vref+15 or should he have selected Vref+15 with A/T?

[alf5071h]

BOAC, a word of caution on Vref +15 at touch down; I understand that the performance certification assumes Vref +15 as the max speed at the threshold; perhaps this is what you meant.

Also, to remind us all that landings at Vref +15 (from the threshold) do not guarantee stopping in a declared (limiting) distance, particularly where the performance margins are degraded (brake / tyre wear / operational technique) or runway conditions are less than ideal (wet – verging on contaminated / at near max wt). Most of the assumed margins can be quickly eroded by crew judgement (or lack of judgement) e.g long / fast landing, failure to apply full brake, misjudging a ‘wet with water patches’ runway as a wet runway, and assuming dry performance always works on a wet grooved runway. As ever, it is safer to arrive at the correct place at the correct speed.

Re bug setting. I hope that the time at which any speed additive is reduced in gusty conditions is at an altitude where reference to the approach speed bug is no longer required (predominantly visual flight). Alternatively assuming that you have the luxury of dual bugs, the final speed checks can be made against that value to be achieved at the threshold (set on the second bug); the monitoring pilot also uses this as the final reference.
This could open a debate about minimum approach speeds in turbulence. One of the reasons for using speed additives is to mitigate the effect of airspeed fluctuations. If speed temporarily deviates low, then the limiting condition is the min Vref for the aircraft configuration / wt, as is still the min safe speed for landing; the limit is not the higher bugged ‘target’ threshold speed; this is based on the gust being transient. However if a long term speed error is introduced then the bugged (increased) threshold speed applies in order to retain some gust protection. I see too many pilots using speed additives as absolute values and not as speed targets.

[Hudson]

Alf. Thanks for the offer of manufacturer management pilot contact. I will certainly keep it in mind.

I retain regular email contact with a senior 737 simulator instructor at Seattle and we have thoroughly interesting discussions on such thing as speed additives. He has asked around the Boeing camp on the subject above - and he was quite unable to pin any of his colleagues down on the specifics of wind additives - IAS bleed back and so on. At this stage I think I am flogging a dead horse and boring people to death, unfortunately.

[BOAC]

Caution noted, Alf, hence my use of the word 'limiting'. The day I find 26L at LGW limiting in a 737 I shall quit! :D

Fully agree about the use of excess speed as a 'buffer', and I have many times seen a raised Vapp interpreted as a 'target' which makes life unsettled again. I'm unsure about 'dual bugging' though - I'd much rather see a single setting which one then tries to fly!

Hudson - not dead yet:D
IAS bleed-back for me is, as I said, when I am 'comfortable', taking it to the ground where LDA permits if necessary. 99 times out of 100 you can 'feel' the way the shear is working, and some of those times the additive proves not to be necessary either.

[411A]

When operating TriStar equipment into limiting airfields with shorter runways and gusty wind conditions, I generally used autothrottle, with the speed set at Vref.
On final at 500agl, it was not unusual to see the airspeed at Vref+25, but crossing the threshold, the speed was Vref+5, nearly every time.
In fact, this is a Lockheed recommended procedure...and it worked good.
Of course, the TriStar was designed as a CATIII aircraft from the outset, so the autothrottle was very accurate.

[alf5071h]

411A your post raises an interesting aspect of human nature with respect to the apparent ease, safety, or ‘good’ quality of an operation, particularly during approach and landing

For any given airfield length and set of fixed conditions (wind, weather, aircraft type, weight, etc), the risks of that operation are encompassed (accepted) by the certification rules - it is safe. Thus, why do pilots decide to change an operating procedure because it appears to be better?

For approaches during windy / gusty conditions the industry accepts (in some cases requires) a speed increase as it adds to or maintains the existing margins of a safe operation. Flight in these conditions at normal approach speed may have increased the risk due to reduced stall margin or difficult aircraft handling.

However, for a landing in gusty conditions whilst maintaining the same safety margins, the threshold speed should be the standard value. A balance of risk is required between the speed reduction and the probability of encountering the conditions (time of exposure). In practice, I suspect that for non-limiting runways, the increased risk from landing at higher than normal speeds is hidden by the spare runway length. This complacency is only highlighted in more limiting conditions, shorter runway or wet / contaminated surfaces, but here again some aircraft have ‘emergency reverse’. (How often is emergency reverse used, should it be reported? It could identify an increased risk of operation – FDM / FOQA will tell). The accident statistics highlight the continuing safety problems during landing and those of over-runs.

Why then should a pilot change the operating / landing technique just because the runway is limiting. The aircraft is certificated to operate into that runway with the appropriate level of risk – it is safe. Whereas a change in procedure may increase the overall risk of the landing due to an un-stabilised approach, reduced stall margin or reduced obstacle clearance. Whilst it is quite feasible to approach at Vref +25 and cross the threshold at Vref+5, this operation increases the workload from that normally accepted (assumed by certification), thus the overall risk of the operation increases. Those manufacturers who advise a speed increase during gusty conditions try to balance the risks by their choice of speed additives and change in operating technique. Some add warnings as to the danger of higher than normal threshold speed and others publish additions to the landing distance required.

A possible reason that the Tristar appeared so good and that the manufacturer approved the procedure was that the aircraft used direct lift control, which I understand was also integrated with the autothrottle. However, just because one aircraft / operation is good does not mean that the technique is universally good. All of us need to be aware of using any false premise of an apparent safer operation (“I know better”); if in doubt follow the certification rules and the manufactures advice, and if you don’t know then don’t make it up.