737 Wind increment to Vref

[FlapsOne]

Why do we apply a wind increment to approach speed in a steady wind?

I can understand a gust factor but what is the point if the wind is steady?

[Checkboard]

The ICAO rule for quoting winds on the ATIS is:-

If the swing in wind direction is 60 deg or less, quote the mean direction. If the variation in wind speed between minimum and maximum is 10kt or less, quote the mean speed.

Say the wind is 260-320deg/10 to 15kt, gusting 20 kt. In ICAO it would be quoted on the ATIS as "Wind 290 degrees,15 knots."

A feature of the ICAO way is that a maximum component may be greater than the windspeed quoted (because the windspeed is an average.) EG, for a RWY 36:-

"Wind 290 degrees, 15 knots, crosswind mean 15 knots, maximum 20 knots. Downwind maximum 4 knots." i.e. ATS quote a wind speed of 15 knots, with a maximum crosswind of 20 knots!

So your "Steady" wind on the ATIS can include quite a variation, which is covered by the wind additives to the approach speed.

[Centaurus]

My understanding of the half-the-steady -H/W component additive is this: The free-stream wind (that is the wind not affected by ground friction) starts around 2000 feet agl. Below that height, the wind strength gradually reduces until reaching ground level.
Assuming a wind gradient reduction of say 15 knots between 2000 ft and surface, then if you let the aircraft fly itself without any power adjustment down the slope, in theory the IAS would gradually bleed itself back by 15 knots. The Boeing recommendation is to add half the headwind component to Vref and to bleed off this additive approaching touch-down so as to cross the threshold on Vref.

But few operators attempt to bleed off the additive, with the result that the aircraft invariably arrives fast over the fence. In my view this is one contributory cause of over-run accidents more usually on wet runways. With auto-throttle engaged, Boeing do not require the additives (H/W), as it is assumed that the automatics will quickly correct any speed loss due gradient.

But the rub is when the throttles are used manually, and the half-the- HW component is added to Vref. So, again in theory, the speed should be allowed to slowly reduce on final from 2000 ft downwards until crossing the fence at Vref. But we all know that if say 15 knots is added due HW component, there would be blue murder called by the PNF as he watched the IAS start to reduce from the Vref plus additive, on the way down from the free stream height of 2000 ft to the threshold height.

Some say that you will lose the additive as you flare for landing. That is not necessarily true because you simply cannot wash of 15 knots of additive at the flare. Keeping in mind that the aim is to have Vref crossing the threshold - not at the actual flare. We are talking about headwind component additives - not gust factors. That is an entirely different story.

The Boeing manual,(at least for the 767 and 737 anyway)states you get rid of the half-the-steady HW component approaching touch-down. Boeing do not define the specific position on approach which is termed "approaching touch-down" and that is why the term is so vague and means different things to different pilots. Does it mean passing a point in time such as the outer-marker? Does it mean passing through the free-stream wind aloft at 2000 ft? Does it mean coming over the fence at 35-50 feet?
Until Boeing or whoever clarify the term, there will always be different opinions on when exactly to commence a deliberate speed bleed off of the half-the-steady HW component during a manual-throttle approach.
And too fast over the fence speeds will continue.

[FlapsOne]

Interesting replies, thanks.

My query stems from the fact that I have never had to do this, in a steady (or reported steady) wind, with another aircraft type. Only a gust factor. I fully appreciate the variations allowed in a 'steady' wind.

So why Boeing?

What's the Airbus view on this? :confused:

[john_tullamarine]

I suggest that the rationale may be somewhat simpler than CB's comment and rather similar to Centaurus' suggestions.

The discussion below is my own long held view as to the thinking behind approach additives - one would need someone from Boeing Aerodynamics/Flight Test/Certification to resolve the specific question with authority.

Like the boundary layer associated with flow around an aeroplane, a steady wind near the ground (assuming no obstacles) reduces as the aircraft approaches the flare. For flight test and certification purposes, this is normally modelled using the "one-seventh" power law - if your AFM has a wind graph which varies with height above the runway then this model is most likely what is built into the picture. The practical thickness of this "boundary layer" is, however, considerably less than 2000 feet.

By allowing a modest additive (half the steady headwind component) to the basic AFM Vref we try to provide for this wind component decay, considering that, on most occasions, we have no idea at what height the (tower) reported wind was measured.

The thinking is a bit like that used when talking about undershoot shear - one is trying to preserve a high probability of not finding the airspeed too low entering the flare. As the wind profile is reasonably predictable for a steady wind, and the tower wind height is not likely to be less than around 30 feet, it is appropriate that this speed additive be bled off as the aircraft enters the flare and touchdown. The whole thing is a bit waffly, so it is not critical to worry too much about whether we start the bleed reduction here or there - more important to hold a reasonable margin on until the aircraft is approaching the runway. Perhaps coming over the fence is a reasonable point to think about permitting a speed bleed.

For a gust, on the other hand, there is an element of randomness, so it is appropriate to carry the full gust value. Further, due to the randomness consideration, it is appropriate to maintain the gust additive through to touchdown.

To guard against touching down with too much speed for comfort (keeping in mind that a constant stopping effort landing roll has something to do with speed squared) it is appropriate to put a cap on how much of an additive one carries - Boeing opts for 20 knots maximum. Provided the pilot controls approach, speed, and float within reasonable margins, the normal 1.67 distance factor provides sufficient fat to allow for the modest increase associated with approach speed additives. In the case of wet runways, the normal distance penalties (typically 15 percent) provide an additional acceptable balance for speed additives.

This seems to sit well with the Boeing recommendation for the 737 (FCT 737 (TM) Chapter 1)

".. add one half of the tower reported steady headwind component plus the full gust increment above the steady wind to the reference speed .... The gust correction should be maintained to touchdown while the steady headwind component may be bled off as the airplane approaches touchdown"

Furthermore, as Mr Boeing thinks that his autothrottle is much better than you or I, the recommendation is that the wind additives be applied only for manual throttle operation. The autothrottle is presumed to be able to catch any wind variations.

Any other thoughts .... ?

[Checkboard]

I thought of the friction layer effects, however the reported wind from the tower isn't the wind at 2000 feet. Makes that argument superfluous.

[john_tullamarine]

CB,

2000 ft has no relevance to the near surface drop off in free stream wind velocity. The effect is only noticeable at heights sensibly relevant to the landing. Centaurus, while confusing two different effects is, I suggest, bang on target.

CB, you might like to play with the model generally used to get a feel for the effect.

speed(1)/speed(2) = (height(1)/height(2))^(1/7)

As a for instance ...for those of you who flew F27 in Oz, you would recall the crosswind limit of 22 knots. The RLD AFM limit was, if my memory serves me, 30 knots measured at 50 feet (and that document contained a very nice picture of the above equation). The then DCA, when the aircraft was first introduced, decided that this fire-breathing monster was too much of a handful for mere mortals so the relevant flight standards chap in Melbourne HO (whom many of the older brethren will recall with mixed feelings - including Centaurus - however I must admit that I learnt quite a few things from him in my early years) decided that a limit related to a lower height would be more appropriate. The formula will show that the height chosen approximated the height of a hand held anemometer.

How do I know this ? In another life, I argued until I was blue in the face to have this unnecessary conservatism removed (in fact, Centaurus will recall this as we were, at the time, working for the same operator). The aforementioned chap, however, was not to be denied his view .. so I lost ... and Centaurus continued to have to observe a 22 knot limit instead of the 30 knots which Fokker intended.

[ 08 October 2001: Message edited by: john_tullamarine ]

[olivasnooze]

Because of all of the above I prefer the refernce groundspeed method.

[FlapsOne]

Geeeeeeeeeeee!!

..and I just thought I'd missed something obvious!!

Thanks again for the replies.

BTW.....any Airbus thoughts?

[Hot End]

I knew I'd been thinking about the wrong things down final all these years :D I always thought the reason was a much simpler. Since the reported wind may or may not be there, you take an average of it to add to the aircraft energy. Hence the half, as in the kinetic emergy formula.

I dunno????????

[Old Aero Guy]

Mr. tullamarine's discussion has got it right. Couldn't have said it better myself.

[john_tullamarine]

Guys,

I think that we ought not to lose sight of the basic objective - getting the bird safely onto the ground.

On the one hand we don't want to get sweaty palms during the flare and subsequent crash because the aeroplane was allowed to get a bit too slow.

On the other we don't want to get palpitations as we go off the far end because the aeroplane was allowed to get a bit too fast.

The Vref additives as recommended by Boeing (and I guess that Airbus has a similar story) are just an attempt to provide sensible guidance to help us keep sensible boundaries on how we might try to achieve the goal of a sweat free landing.


OldAeroGuy,

I suspect that Mutt and I will enjoy your commentaries on these sort of subjects. I see that you do not have an email contact. Perhaps you might like to email me (and perhaps Mutt as well) for the purpose of establishing a comms link ?

[ 12 October 2001: Message edited by: john_tullamarine ]

[crossfeedclosed]

john-tullamarine is spot on in this. However, Mr Boeing states that one should not add the gust factor when using his autothrottle for the approach. Methinks he has far too high an opinion of his autothrottle's capability which reacts at snail speed. :eek:

[john_tullamarine]

In respect of the 737, around which the discussion has gravitated, Boeing's recommendation is that the approach be conducted at Vref+5 if using autothrottles - ie no wind correction need be applied at all. (ref FCTM, pp1.22, dated 01OCT97)

[Denti]

Just a question, does anybody land with autothrottle engaged and autopilot disengaged in the 737? We have to left the AT at ARM, but it's normal SOP to deselect SPEED if flying manually.