View Full Version : Reducing headwind additives on final approach

3rd Feb 2002, 16:25
B737 on final at 2000 ft - auto-throttle disconnected. Runway 27 - wind 270/30 knots. Boeing say add half the headwind component to Vref and to bleed back the additive approaching touch-down so as to cross fence at Vref.

Met people advise that the free stream wind starts above 3000 feet and that below that altitude the wind gradually reduces due friction effect with ground. . .Question: When does the pilot deliberately start reducing the IAS half headwind component additive in order to cross fence on Vref as per Boeing FCTM?. . . .My guess is around 1500 feet, so that there is a gradual bleeding off of the 15 knots (of half the HW component) by the time you arrive at Vref at the fence. Certainly in a steady wind condition you cannot expect to suddenly get rid of the 15 knots in the flare.

Any thoughts on this? Reason I ask is that I have seen countless occasions when the pilot fails to get rid of the additive and crosses the fence far too fast, eventually landing long. I suggest that not getting rid of the half HW component particularly if landing on wet runway, is contributory cause of over-runs on limiting length runways.

4th Feb 2002, 02:35
Can't speek for the 737 but on the DC-9 we add to Vref the greater of; all the gust, half the steady state winds greater than 20k, or 5 knot. In any case don't exceed the orginal Vref + 20 knots. The head wind componet is compansated for in the landing distance chart. We carry this speed to the flare.

I find if you accuratly aim for touch down spot, don't carry power though the flare (reduce power at the correct point)and don't try for a "greaser" you'll do just fine.

4th Feb 2002, 05:45
May I offer the following thoughts. Reference to an earlier thread ("737 Wind increment to Vref" on Tech Log, around OCT01) may be useful.

. .(a) the empirical profile equation normally used for AFM and certification purposes is

wc1/wc2 = (h1/h2)^(1/7).

This equation more or less fits the typically observed lower level wind profile where there are no obstructions of any note, ie for a largish, flattish sort of area.

In particular, it is used to correct the reported tower (flight test) wind (which will relate to the height of the anemometer) back to whatever the AFM data is (to be) based on, typically 50 feet, so that the distance calculations from the AFM have some repeatable basis.

(b) the AFM scheduled distance data is based on

(i) maximum effort landing factored by 1.67

(ii) 0.5 (1.5) of the advertised headwind (tailwind). . . .so there is a degree of conservatism to start with.

It was long ago considered appropriate in turboprop operations that a maximum additive of 15-20 kt would provide a reasonable protection against overrunning the scheduled distance and this was stated as such in some manuals.

For approach speeds of around 100 kt an increase of 20 kt would result in a notional distance increase of around 45-50 percent. Probably the 15-20 kt wisdom figure could be increased sensibly to account for the higher approach speeds on jets (eg at 140 kt an increase of 30 kt results in a similar reduction in basic margins... however, that has not occurred and the earlier recommendation continues.

In addition, it is generally common to land with LDA exceeding AFM minimum scheduled distance (minimum distance required, if you like) so, for many landings there is plenty of fat, assuming that the pilot constrains problems associated with floating during the flare.

. .To get a feel for the variation in wind component, consider the following sample data. Although I have extrapolated to 1000 ft for the exercise, I suspect that this is not appropriate in the real world and is included only to emphasise that the main concern is in that final 100-200 feet or so of the landing approach.

Anemometer height 50 ft. .Anemometer H/W 40 kt . .Ht (ft) 5 10 20 30 40 50 75 100 200 500 1000. .W/C (kt) 29 32 35 37 39 40 42 44 49 56 61

(apologies for the formatting ... I have no idea how to get it to accept what I am trying to send .. )

So, for instance, the postulated wind reduction between 50 ft and 10 ft (40 ft change) is around 8 kt while on the high side an 8 kt change fits about 150 ft height change - hence the main concern is around the vicinity of coming over the fence.

For the pilotage point of view, the concern comes down to balancing

(a) imprecise knowledge of the actual wind profile for THIS particular line operation landing especially when one considers that the real world generally has lots of wind-affecting bumps and lumps.... against

(b) a need to guard against overrunning the available runway.

As to the Boeing advice (which is not startlingly different to that offered by other manufacturers), they needed to give some imprecise guidance for a very imprecise topic. Just where does one start the speed bleed ? ... I suggest wherever you think is appropriate considering the specific conditions pertaining to the particular landing .. the aim, typically, is to constrain the approach speed to remain above Vref plus a bit but not above around Vref + 15-20 kts as the aircraft transits the landing flare .. I don't get too fussed if I end up a little faster than I would have liked (ie aimed for) in the flare .. BUT .. for me

(a) I use auto and manual brake as seems appropriate for the specific conditions and LDA.

(b) I don't tolerate aiming long.

(c) I don't tolerate anything more than a sensibly minimal float .. if the float looks like presenting a problem .. either put it on the ground or execute a missed approach.

Especially in less than ideal conditions, I am far more concerned about running out of speed in the very late landing approach (is it obvious that I have embarrassed myself in the 727-200 from time to time ?) than overrunning the runway so I try to maintain the approach speed down to a low height, say, 50 feet.

[ 04 February 2002: Message edited by: john_tullamarine ]</p>

4th Feb 2002, 08:37
Have found that in large aircraft such as the TriStar, ref+10 works quite well for most all occasions, except in the -500 series, where +5 is better due to the higher computed Vref speed.. .Have watched many pilots try for a greaser, only to float half-way down the runway....not good.. .Get it on (and keep it on) at the aiming point for best results.

4th Feb 2002, 16:13
What I am unable to explain to myself is that no wind additives are needed if the auto-throttle is used ? - except that Boeing recommend adding five knots to Vref in the 737.. .There is a significant difference between adding up to 20 knots max additive to Vref for a non-auto throttle approach yet only a five knot additive to Vref is required for an auto throttle approach.

Does that mean that landing in a gusty max limit crosswind plus a significant headwind additive one should always use the autothrottle in order to keep the approach speed at Vref plus five?. .Certainly less chance of over-run on a limiting wet runway I suppose.

In the simulator it seems almost impossible to see anyone arriving at the flare at good old Vref. Invariably crews seem apprehensive of Vref even though it gives ample fat above the stall.. .Is it because the simulator runways lengths are mostly around 7-10,000 ft and who gives a damn if the aircraft floats half way down the runway as long as it is a greaser.

4th Feb 2002, 18:30
Don't know about the 737....but in the TriStar, using autothrottle during gusty conditions, you often see ref+20 at 400 feet, slowly bled off closer to the runway.

4th Feb 2002, 19:14
I suspect that Boeing's (and Airbus') attitude is that the autothrottle does it (maintaining a given airspeed) better than the human pilot, therefore no need to pad the autothrottle approach beyond a small amount.

There ought not to be too much concern over a modest speed margin, especially considering that the normal 1.67 pad is then further padded by 1.15 (typically) for a wet (not flooded or slush contaminated) runway... provided that the touchdown is well controlled and brake usage planning is sensibly related to the situation.

[ 04 February 2002: Message edited by: john_tullamarine ]</p>

4th Feb 2002, 23:56
I noticed that statement as well. I do not go along with it, and do in fact put in an additive for gusty wind conditions when A/T are engaged. I do not use autothrottles all the way to touchdown, and I can always put it down near my target location. The speed might be greater than Vref, but well within published parameters.

&gt;What I am unable to explain to myself is that no &gt;wind additives are needed if the auto-throttle &gt;is used ? - except that Boeing recommend adding &gt;There is a significant difference between adding &gt;up to 20 knots max additive to Vref for a non-&gt;auto throttle approach yet only a five knot &gt;additive to Vref is required for an auto &gt;throttle approach

7th Feb 2002, 03:17
I have not flown many large jets, besides an A300 sim, but I would have thought that there is less chace of overshooting using Vref + 20 with a 30kt headwind than just Vref in calm conditions. Just a thought.

7th Feb 2002, 05:46

Minor point overlooked in your post. Ignoring the wind component factoring built into the charts, we are considering limiting cases, or near so. With a 30 kt headwind, the runway required becomes somewhat shorter so the problem doesn't really go away that simply ....

7th Feb 2002, 09:47
I'm just a small driver and have been following the above with great interest, but I am curious of one thing.

I can understand the fact that in gusty conditions carrying extra speed above Vref is needed, but i thought that this speed was carried all the way through to touchdown as it was required to give an extra buffer near the ground. To me this is when you would be most likely to need it - near the ground, loss of speed, less time to spool up etc etc.

I guess what I'm really asking is why do you carry it when you have height available and then slowly get rid of it the lower you get to the ground. If you don't need it during the last 500 - 1000' and when flaring, then why carry it at all? <img src="confused.gif" border="0">

Thanks in advance for an explanation.


7th Feb 2002, 11:02

The standard practice to which Centaurus refers in his opening post relates to the Boeing 737 recommendation to carry

(a) half the steady headwind component which is bleed off somewhere approaching the flare on the basis that this reflects the ground "boundary layer" velocity profile. .(b) the full gust value which is carried through the flare to protect against the random nature of gusts. .(c) a maximum additive of Vref+20

7th Feb 2002, 12:19
Thanks for the explanation JT. It starts to make more sense now! With regard to the second point,ie taking gusts into account is it 1/2 of the gust that is added or the full amount??

eg: 1/2A + 1/2B = Vref amount &lt;20kts

7th Feb 2002, 15:04
All the gust as the random nature of gusts could well see the aircraft experiencing wind speeds anywhere within the gust range.

7th Feb 2002, 15:19
Fully understood. Thankyou for the information and patience JT.

7th Feb 2002, 15:53
100% with JT there.

The problem with the autothrottle approach is that as JT says, it (Boeing, anyway) adjusts for wind effect by flying above 'bug', but what people are not aware of is that when you disconnect the a/t, the bug setting is then incorrect as it does not have the 'extra' margin. It should then be adjusted to the target Vref or you finish-up 'ignoring' it which is not the taught technique. Winding the bug up to target with a/t engaged merely has the a/t fly even faster!

As JT says, there is no easy answer to what to bug. Each case is different. EG R26 LGW with a south-westerly gusting wind finds me keeping the excess on till quite late!