Headwind additive for autothrottle use.

[King on a Wing]

Wondering,what is the headwind component additive on the Vref for an approach with autothrottles on.Say an autoland,or,for that matter even a manual landing with autothrottles on.
Normal sop's dictate Vref+5 kts.But what if the wind is say 20 kts headwind gusting to 30 kts.Still the same.....???!? :confused:
Cheers...

[extreme P]

+5 on the Boeing.

[King on a Wing]

Buzzzzzzzzzzzz......wrong.....!

[FCS Explorer]

During autothrottle operation, when on final approach in landing configuration,
it is not recommended to set the A/T command speed to allow for wind or gust
corrections. Through airspeed and acceleration sensing, the A/T corrects for
normal wind gusts. Higher command speed settings result in excessive
approach speeds. The A/T approach speed setting is Vref +5.

[frogone]

Half the headwind, plus all of the gust. In your case 20/2=10 + 10 (Difference between 30-20 is the gust) there for 10+10= 20 so your fly speed would be Vref + 20. Some operators use a correction up to a maximum of 15 knots. Sometimes your fly speed might take you close to the flap limiting speed then in this case it's the flap limit minus 5 knots.

I can only speak for my type. So other guys might have different take on it, but the general principal is usually the same.

IR

[RobinR200]

That's teh formula with A/T off or in ARM.

With A/T operative its Vref+5.

Boeing

[King on a Wing]

@Robin & FCS.....what about airplanes that land with autothrottles on but autopilot off for manual landings.As in the 777 maybe.Does the Vref+5 rule still apply? And why is that?
As far as I know, the autothrottle on a large aircraft is FAR slower and more sluggish to react to a change in speed than a pilot controlling the speed on manual thrust.So why the lower additive on automatics than on manuals?The same holds true for an increase in speed....slow on auto than on manual.
Still wondering why...
Thx for all the replies so far.

[Mach E Avelli]

FCS Explorer is not expressing a pet theory - he is quoting verbatim the Boeing 737 Flight Crew Training Manual. Vref + 5 is the number for that type - applicable to all series fitted with autothrottle from the -200 ADV to the latest.
Note the use of the word 'recommended'.
Only once have I seen a statement in a company operating manual that quite clearly said that they had developed some differing procedures as a result of extensive operational experience, that their procedures were FAA approved, and that they accepted responsibility for the consequences of having changed the procedures. In the absence of that statement, I have always assumed that if a company issues crews with the Boeing FCTM, the intent is for it to over ride the SOP in much the same way as the AFM does.

[King on a Wing]

Mach.....not so for the 777's.Landing manually with autothrottle on is NORMAL procedure there.The 737 is a whole different ballgame...autopilot and autothrottle are extremely degraded with comparison to the 777's..

[BuzzBox]

Autothrust is recommended for all approaches and landings in the B777.

From the B777 FCTM:

"When using the auto-throttle, position command speed to VREF + 5 kt. Sufficient wind and gust protection is available with the auto-throttle engaged because the auto-throttle is designed to adjust thrust rapidly when the airspeed drops below command speed while reducing thrust slowly when the airspeed exceeds command speed. In turbulence, the result is that average thrust is higher than necessary to maintain command speed. This results in an average speed exceeding command speed."

[411A]

As far as I know, the autothrottle on a large aircraft is FAR slower and more sluggish to react to a change in speed than a pilot controlling the speed on manual thrust.

The 'as far as I know' part, is the problem.

It appears that you might be just a tad misinformed.
On the Lockheed tri-motor, just as one example, the autothrottles are very fast acting so...Vref+5 is quite satisfactory.
And, as this is a rather older design, I'm quite sure newer (especially Boeing) types would be just as good, or better.

[King on a Wing]

411.....thx
Unfortunately,I've heard u mention the "lockheed tri motor" about 5000 times in ur six odd thousand posts.Wonder why.Maybe cause that's ur last airplane flown? I appreciate ur emotions,but not ur knowledge.
That said,I of course am well aware of what the B777 fctm says Buzzbox.I was very categorically asking for personal opinions here.And not quotes from boring self centered hardcopies that boeing publishes to cover its back side! Well,if they said fly Vref in microburst conditions,just to sell their new airplanes,would u do that.I certainly wouldn't.
They also said the GE 115 would have less than 6 engine failures per million cycles so that they could sell the same (and yes there is a covert handshake between Boeing and GE).But we now know that not to be true.
So whats your personal take on this command speed.And believe me,i'm looking for an experienced personal viewpoint.The books are for all the 6.2 billion of us to see.
The reasoning behind those books however,are for but 6 odd good ones to understand and challenge.Am looking for those 6 thankfully.....
Thanks.
@411-you are an extremely experienced aviator.I acknowledge and salute that.Pls don't take my remarks incorrectly.I am just looking for genuine opinion...
Cause i'm just NOT convinced...
And when i said 'as far as i know'....it meant just that.Not meant to be mis quoted,rather understood.And i DO know quite a bit...:)

[BOAC]

KoaW - in my 20 years or so on the 737 I have never found Vref+5 to work in anything other than a steady 10 kts down the runway. The 737 A/T is notoriously poor at maintaining a 'suitable' speed, and there is also the problem, when/if you disengage it on the approach with Vref+5 set, of having to then increase the 'dialled' speed to cope with headwind/gusts. In my experience most folk dial the 'corrected speed' into the window and the A/T has more or less coped with that. Only problem there is if you ever land with the A/T engaged (non-autoland) you could be outside the 'margin' allowed for landing perf. On CAT II/IIIs the wind limits are such that the problem does not normally arise.

I have generally bugged 'what I want' with A/T engaged and if it is flying me too fast, taken it out. The key with all things 'automatic' on a/c is to see if they are doing what you want, and if not, take them out (or is that modern-day heresy.....)

[King on a Wing]

I agree with your thought BOAC.
Unfortunately the 737 was NOT designed to be flown manually with the A/T engaged..(FMS not in sync with the A/T etc etc).Thus the obvious issues of 'dialling up' the approach speed on disconnecting A/T for landing.However,larger jets ARE designed to be flown manually WITH the A/T engaged and ON.All the way to reverse thrust.Wherein the A/T automatically disconnects.But is still armed.
Therefore doesn't the 737 logic apply on these larger jets?
That is.....fly the higher speed upto touchdown?
Or are we presuming an extremely responsive A/T all the way to touchdown...?? Which obviously comes with its cons of HUGE pitch changes very close to the ground in gusting conditions.Which gets me to the start of another thread......."Would you rather fly a gusty approach on autothrust or manual thrust"....:p
Well.....thats for another decade.....

[Denti]

On CAT II/IIIs the wind limits are such that the problem does not normally arise.

They are lower, but not all that much lower to be honest, head and cross 25, tail 10. Crosswind limit for take off (yes, i know, a different flight phase) on a wet runway is 25 as well. Some of those limits might be company reduced though or just lower because its for NGs, in the classics we flew manual landings with up to 40kts cross, but that isnt allowed with our NGs anymore, just 33 on dry runways.

[BuzzBox]

I was very categorically asking for personal opinions here.And not quotes from boring self centered hardcopies that boeing publishes to cover its back side!

Perhaps if you'd asked for a personal opinion in the first place (which you did NOT), you might have got one.

Or are we presuming an extremely responsive A/T all the way to touchdown...?? Which obviously comes with its cons of HUGE pitch changes very close to the ground in gusting conditions.

Thrust changes do NOT cause pitch changes in the 777, due to the FBW.

"Would you rather fly a gusty approach on autothrust or manual thrust"

I am more than happy to leave the autothrust engaged when landing the 777 in gusty conditions, it does an excellent job. In an extreme gust the autothrust can always be overridden if it doesn't react quickly enough.

Well,if they said fly Vref in microburst conditions,just to sell their new airplanes,would u do that.I certainly wouldn't.

What a stupid response. I won't even dignify it with an answer. May I suggest you stop attacking all those who have posted quite reasonable answers to your original question.

[Spooky 2]

A real pilot has his/her hands on the thrust levers to beigin with and IF NECESSARY will move them accordingly to the conditions that exist. Otherwise do what Mr. Boeing says in the FCTM and add 5 knots assuming you intend to leave the A/Ts on through touchdown.:ok: (B777)

[Tee Emm]

That's teh formula with A/T off or in ARM.


Boeing advise not to use ARM for landing. FCTM 1.33

[A37575]

Half the headwind, plus all of the gust

While the Boeing FCTM for the 737 states the HW component should be bled off approaching touch-down, it does not define "approaching touch-down". That inevitably leads to numerous personal opinions. Elsewhere the FCTM states that with proper airspeed control and thrust management touch down occurs at no less than Vref minus five knots.

This suggests that if you have (say) a 15 knot half the HW component additive there is no way you can bleed this lot off in the flare. In fact, normal bleed off in the flare is three knots. Taking the argument a step further, I would hazard a guess (from frequent observation) and say the majority of HW component additives using manual throttle, are rarely bled off and are taken right down to the flare - resulting in many instances, a long float touch down.

The reason for the Boeing half the headwind additive is to cater for ground friction slowing the wind down below the free stream flow which starts around 1500-2000 ft agl. This means you should start bleeding away the half headwind component at that height so that in theory you arrive over the fence at Vref. If you do that without telling the PNF first, his screams of "SPEEED" from an uneducated PNF would likely waken the dead.. In which case tell him to read his Boeing FCTM or kindly pull his head in. Good CRM that, as he will undoubtly learn from your superior knowledge..:ok:

[Kilo-club SNA]

If I may just inject someting....

There has been several quotes of the 737 FCTM so I won't add any more (mainly because I don't have it available)

Now, If we are manually man-handling the thrust levers we bug the whole headwind and half the gust and then aim for the bug. Later we bleed of some of that but that has already been discussed.

The autothrottle does things a little bit different (no surprise there, it is a 737 after all) it wants the bug to be set at +5knots but tht doesn't mean that it's aiming for it. Boeing engineerd this thing in the pre-byte (bite?) era and solved the problems by setting different speeds on the forward and retarding motions. The effect of this is that the autothrottle is incapable of maintaining the bug speed in anything else then conditions where the wind is completely stable and critically doesn't change with altitude. So what may appear as a rubbish autothrottle is a design feature so that the actual speed is always higher than the bug speed. the more gust or wind change that is encountered the higher the margin to the bug.

The real dilemma occurs if you intend to disengage the autothrottle at say 500'. If you bug +5 the autotrottle will fly at a speed in excess of it and one might argue that you would have to change the bug setting when you disengage.
If you choose the other option the autotrottle will now fly much to fast (in theory twice as much wind addative as you wanted and bugged) and it will be a handful to get rid of that speed eventually and you could potentially exceed the flap limit.

All above not withstanding the fact that the autothrottle is a not-to-be-trusted-piece-of &^%%^ in most scenarios!

signing off

[Tee Emm]

Now, If we are manually man-handling the thrust levers we bug the whole headwind and half the gust and then aim for the bug. Later we bleed of some of that but that has already been discussed.


Err...slight error there old chap. Try half the HW component and all the gust.

[Kilo-club SNA]

Yes very true! Good thing you knew what I meant!:)

[ranklein]

Guys,

Generally speaking...

Why are we adding speeds for HW and/or gust, but with Tailwind we had nothing if we have a steady wind (no gusts of course)?
Speaking on all kinds of planes, not A/T equipped necessarily.

Thanks!!!

[john_tullamarine]

Why are we adding speeds for HW and/or gust, but with Tailwind we had nothing if we have a steady wind (no gusts of course)?

(a) the presumed friction layer used for certification is a power law (the 1/7th relationship) which describes a commonly observed reduction in steady wind velocity as the height above ground reduces in the absence of obstructions. It is presumed that the steady wind will be reasonably predictable in its behaviour. This relationship typically is the basis for low level wind profiles incorporated into Flight Manual data.

(b) for an headwind, the wind speed is expected to reduce somewhat as the aircraft approaches the runway resulting in an undershoot shear .. hence we carry a margin to give some fat for this. If a tailwind, we would be at double jeopardy if we were to carry a margin as the effect now becomes an overshoot shear situation.

(c) the usual Boeing margin (half the steady headwind) is an attempt to provide some protection against undershoot shear while not exposing the landing to an overly excessive speed entering the flare.

(d) for the gust situation, we presume a randomness about the gust time history so it is more appropriate to allow for all the gust value on the basis that this may appear at any time during the landing.

(e) one recalls that the landing factor is 1.67 dry and this provides a reasonable margin to accommodate a small variation in speeds in the flare. The 20kt limit reflects a reasonable maximum speed delta to fit in with the 1.67 if circumstances on the day conspire to find the aircraft entering the flare with the full additive still present. This has been a long used value both in US and UK certification practices.

An older thread (http://www.pprune.org/tech-log/10456-737-wind-increment-vref.html) looked at these things in some detail and is probably worth a read in the context of this present thread.

[safetypee]

Speed additives during an approach with autothrottle engaged depend on aircraft type / system performance; hence refer to the manufacturer’s advice.

The additives may only be required in particular conditions or at certain stages of the approach. The additives provide additional safety protection, but also require consideration of the landing distances required, which if excessive, present an additional and potentially greater hazard to safe operations.
Speed may be bleed off before the threshold enabling ‘standard’ performance to be used, but any speed change may destabilize the A/T system or trim condition – invalidating autoland performance, or result in higher crew workload, or violate stabilized approach criteria.

Boeing provides guidance for increased landing distances due to speed excess (also see AC 91-79), but these data assume manual thrust reduction. Some A/T’s have a variable retard initiation height or rates-of-closure resulting in different flare distances whilst attempting to give a consistent speed loss (normally 7kts). However, Boeing’s landing performance assumes a constant flare distance of 1000ft for manual landing (1200ft for big jets), which with A/T may not be achieved, or 1500-2500 ft for autoland/HUD which may be more realistic for manual landing with A/T engaged.

Thus, when using A/T during a manual landing consider carefully the implications on landing distance and the actual runway conditions, particularly where gusting / cross winds can also add to the actual stopping distance.

JT respectfully disagree with your para (e).
Whilst landing distance factors provide a safety margin for small variations in normal operations, they are not intended to accommodate ‘deliberate’ speed increments.
See AIC 91_06. (www.nats-uk.ead-it.com/aip/current/aic/pink/EG_Circ_2006_P_014_en.pdf) Note that the ‘15kt margin’ for higher speeds at the threshold refers to the ‘reference’ method of certification which is no longer used; and AFAIK no modern commercial aircraft uses this datum for landing performance.
Also, it should be remembered that the actual safety margin on a wet or contaminated runway is not the same as that for a dry runway even though the certification distance factors are greater. Recent Canadian research suggests distance factors of 2.2 – 2.4 may be required in contaminated conditions to provide the equivalent level of safety as on a dry runway.

“The margin of error for the threshold crossing airspeed is +5/-0 knots. If the pilot has planned to carry additional airspeed beyond the threshold due to gusty surface wind conditions, or other factors, then the effect of this additional airspeed must be included in the actual landing distance”. ( AC 91-71 Runway Overrun Prevention. (www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/0052F2A2A00D91B28625738E0071E44C?OpenDocument&Highlight=ac%2091-79))

[john_tullamarine]

I am quite comfortable that my comments are compatible with both circulars.

Consider that

(a) the Boeing 20kt max additive is for the approach and is not intended to be carried into the flare

(b) the factored W/V built into the AFM numbers provides a significant margin

(c) entry into the flare significantly outside "normal" parameters is adequate justification for a missed approach unless the excess runway is significant.

(d) there is implicit an understanding that the pilot will consider all factors in determining just how critically he/she views the maintenance of profile etc. and, in particular, touchdown zone and retardation effort.

We can, of course, continue to agree to differ.