Approach Speed vs Command Speed (737 NG)
 

Dear Instructors & Examiner,

I'm just refer to B737NG FCTM page 1.13 regarding Landing Speed, it says :

"The minimum command speed setting with autothrottle disconnected if Vref+5knots, The gust correction should be maintained to touchdown while the steady headwind correction should be bled off as the airplane approaches touchdown"

Does it mean, you set the command speed to Vref+5 kts if you plan to land with autothrottle disconnected, and if you have gust correction you set the command speed to Vref+gust correction because you will touchdown with this speed, but if you don't have gust correction only headwind correction, you may set command speed at Vref+5 kts and fly the approach speed which is higher than Vreft+5 kts, and bled off this approach speed to Vref+5 kts as the airplane approaches touchdown.

Anyone have better knowledge please welcome to PM me or inform me as I need this clearify badlyhttp://images.ibsrv.net/ibsrv/res/sr...s/badteeth.gif I don't mind if my understanding is wrong:ugh: but I need the information.

Thank you so much, and Happy flighthttp://images.ibsrv.net/ibsrv/res/sr...ilies/cool.gif

Dunno about others, we do set Vref plus additions with a mininum of +5 when flying manual thrust.

I'm looking at the table that's just above your quote, and that should provide some more insight.

Of course my last airline you just told the other guy, "I'm gonna fly a little fast" and he knew you were correcting for the gusts. At my current airline there's no provision for that sort of thing.

I don't know why we don't just add Gust to the REF and get a "REFgust" and then add the wind on top of that for approach speed (eg REF30 = 133. REFgust = 140. REFgust+5 = 145 for command speed for wind 10 gust 20)

in FACT is,

You have just quoted a small section of the section on setting the command speed bug. You need to go back to the top of the section titled "Landing" & read the lot again - carefully.

To simplify -

*When using autothrottle to touchdown set the command speed bug to Vref +5.

*When planning to disconnect the autothrottle at any stage prior to touchdown, set the command speed bug as per the following -
1. Add 1/2 the steady headwind component + all of the gust to Vref (maximum Vref +20) & set that speed.
2. The minimum speed to be set on the command speed bug is Vref +5
3. The maximum speed to be set on the command speed bug is Vref +20
4. The steady headwind additive should be bleed off just prior to touchdown.
5. The gust additive should be maintained to touchdown

Examples -

1. Vref 120kts, Runway 18, wind 180/10. Bug set at 125, touchdown at 125

2. Vref 120kts, Rwy 18, wind 180/20G30. Bug set at 140 (steady headwind = 20, 1/2 of 20 = 10. Gust = 10, so 10 + 10 = additive 20), touchdown at 130 (bleed off the steady headwind additive of 10kts & just carry the gust additive of 10kts to touchdown).

There are examples given in the table above the section of the FCTM that you quoted.

In a lot of situations you will be touching down below the bugged speed. This is normal. However, you shouldn't be touching down below Vref +5. You also shouldn't be flying an approach above the bugged speed or touching down above Vref + the gust additive.

Hope that helps.

- have to disagree there, oak - you should aim to be at Vref(+) at 50' but bleed off to something like Vref-5 at touchdown.

Partially true BOAC. But, as is often the case, things aren't so simple. I was trying to keep it simple & my statement that you shouldn't be touching down below Vref +5 was technically incorrect - sorry.

The following is basicly what the FCTM says (in my words) -

*If you are carrying a gust correction, touchdown should occur at Vref + the gust additive.

*If there is no gust additive touchdown should occur between Vref & Vref -5. This is stated in the 'Landing' chapter of the FCTM.

*Touchdown should occur at no less that Vref -5. To use Boeing's words - touchdown at lower speeds seriously reduces aft fuselage/runway clearance.

The speed will start to bleed off as the power is reduced during the flare. With the A/T engaged, the thrust reduction starts at 27', if memory serves me correct. With A/T not engaged, the FCTM states that the thrust levers should be smoothly reduced towards idle at the commencement of the flare, aiming to have the thrust levers reach the idle stops at the same time as main wheel touchdown.

So 50' is perhaps a little high.

Also, the indications are that you can expect a 5 to 10 kt reduction in speed during the flare maneuver. So perhaps it would have been more accurate to have stated that if you are carrying Vref + 5, you maintain that until the commencement of the flare.

I must say that in my experience most people carry too much speed, too far into the flare - particularly with flap 40. So BOAC's technique has merit. However, if you use the correct speed, get the thrust off at the correct point in the flare & then actually flare the aircraft, it will land quite nicely - main gear first, rather than in a 3-point attitude where the nose gear touches down at the same time or immediately after main gear touchdown.

However, if you have misjudged the height of the flare don't keep flaring to get a smooth touchdown or you risk a tail strike or even going off the far end. Just put it on.

Oakape is pretty much preaching verbatim from the Book of Boeing.

If you're adding gust factor, Boeing says land a Vref+Gust Factor with the steady state bled off.

Thanks ImbracableCrunk, yes I am.

That being said, I have found the technique to work quite well out there on the line. Boeing do seem to know how to fly the aircraft they build.

Assuming a still air situation (no headwind or gust factor required):

The Boeing FCTM guidance on touchdown speed is often sadly overlooked in favour of one to one instructor's technique. When Pilot Monitoring I see the majority of touchdowns occurring at the nominated approach speed (Vref+5 in still air) which is 5 to 10 knots too fast. This is usually due to nervousness that stems from that old fable of "it falls out of the sky if you go below Vref." A very small reduction of thrust (3% N1 depending on landing mass usually works for me) just prior to the 50 feet point will allow crossing of the screen height at Vref; that thrust setting can then remain unadjusted until the end of the flare (ie nose raised to a suitable touchdown pitch angle) at which point thrust is immediately reduced to flight idle allowing wheels to contact at Vref-0 to Vref-5, as BOAC correctly defines.

To nail these speeds proficiently requires some discipline in the PF to constantly check the speed tape against the outside visual picture in the last 100 feet and make tiny adjustments to thrust.

Has anyone been taught the novice's technique of.....

For Flaps 30: CHECK and CHOP (check the nose attitude is raised, then chop the thrust levers to idle)

For Flaps 40: CHOP and CHECK (chop the thrust levers, then raise the nose attitude)

....this worked as a good starting method for me when new to type. I think the different method for Flaps 40 stems from a greater air cushioning effect produced by the flaps allowing more time to place the aeroplane on the runway, but I'm no aerodynamicist.

In fact, I don't even know if aerodynamicist is a word.



For a different can of worms, I never cease to be horrified by that cancer of a technique that thrust is used to correct for glideslope. Why do so many medium jet trainers get away with teaching that???

Well, guys and girls, my 'Book of Boeing' is (possibly now amended?) FCT 737 Oct 31 2007, but I guess a/c still land the same way, and it does in fact match exactly the 'traditional' teachings of aviation for years.

What does your say about 'Normal Touchdown Attitude'? Mine says

"With proper airspeed control and thrust management, touchdown occurs at no less than VREF-5"

You have to ask why does Boeing recommend (among other things) an additive of half the steady HW component. A reply I had from Boeing over 25 years ago was that it was based on many years of jet transport experience. That is all it said. The FCTM for the 737-300 says "...while the steady HW correction should be bled off approaching touchdown". There is no definition of "approaching touch-down".

I may be wrong, but the clue when to start bleeding off the HW component additive may be found in Volume One Supplementary Procedures Adverse Weather, sub heading Approach and Landing," which discusses ice formation observed on airplane surfaces with the advice to add ten knots to the normal Vref. It goes on to say "this ensures maneuvring capability".

A Note states the "combined airspeed corrections for ice formations, steady wind and gust should not exceed a maximum of 20 knots". We know that, because speeds above Vref +20 risk an over-run on a limiting length runway - hence the GA requirement in terms of stable approach criteria if speed exceeds Vref +20 knots.

But - there is a second Note and that says "To prevent increased landing distance due to high airspeed, bleed off airspeed in excess of VREF +5 knots + gust correction when below 200 feet AGL. Maintain the gust correction to touchdown". .

This suggests to me that the Boeing term "approaching touchdown" can arguably be defined as below 200 ft - and NOT as some people interpret as during the flare manoeuvre.

The HW additives are to account for the steadily decreasing wind experienced on short final which is due to wind surface friction. The wind at five feet above the ground may be 5 knots and at 25 feet perhaps 8 knots and 50 feet maybe 12 knots and so on, until reaching a steady wind unaffected by surface friction which is known as Free Stream wind and which occurres around 2000 feet AGL.

Over-runs are usually caused by excess speed on final approach up to touch down well down the runway. It is my experience that most pilots are reluctant to deliberately bleed off the HW additive and instead assume the flare will do it for them. Well it doesn't always happen that way.

If Boeing state in the FCTM that 200 feet is their idea of the starting point when to commence a deliberate bleed back of airspeed in excess of VREF + 5knots + gust correction, then how else can the original statement of FCTM General information page 1.14 (in my copy of the B737-300 FCTM) that gives the words "steady HW correction should be bled off approaching touchdown?" be interpreted. To me it is black and white. At 200 feet you commence speed reduction in order to meet the Boeing advice of "bleeding off the steady HW additive.

Of course the pilot should brief his intention otherwise he can expect screams of "SPEED SPEED SPEED" from a thoroughly alarmed but not very clued up copilot.:ok:

That's the minimum, of course. With a gust factor added, you would keep that to touchdown. Book of Boeing 1:13. Amen. (Oct 08 version.)

This all backs up my idea that we should have a Vref30+gust factor = VrefGust. Then, BOAC, you can still touchdown at VrefGust-5. http://images.ibsrv.net/ibsrv/res/sr...ies/thumbs.gif

Dear all, thank you for all the feedback I'm really appreciate that, going back to my question really is about the command speed setting (the bug), consider there is no gust correction only head wind component.

Example : the Vref is 130 kts and the head wind component is 8 kts

FCTM :
then you set on the MCP Vref+5 which 135, you fly with A/P & A/T disconnected manually at 138 kts (theoritically) 3 kts above the bug:ok:

FCTM :
when approaching to touchdown then you bled off the 3 kts, so as to touch down with Vref+5

Centaurus :
So, would it be correct (procedure) to set the command speed to Vref+5 on the MCP :confused:

in FACT is,

I don't know where you get the 3kts from in your post & no, you don't fly bug speed +3 (or anything else for that matter!).

Half the headwind is your example is 4kts. So you would expect to set 130(Vref) + 4, or 134kts. However the minimum to be set is Vref + 5, so you must set 135kts. You then fly that speed.

In short -

*You add the correction to Vref - not to Vref +5.

*The minimum correction is 5kts & the maximum correction is 20kts, regardless if the actual correction turns out to be less or more than these amounts.

*You then fly the bugged speed, not a speed above it. The bugged speed has the corrections built in & there is no need to carry any more.

Oakape,

I'm sorry my mistake not to explain :ugh:, I mean here is the head wind is 16 kts, so take half of it 8 kts, than you add this to your Vref that comes to 138 kts to fly until approaching touch down, than you touch down with Vref+5 (on the bug), otherwise what is Boeing means by :

bled off to what speed..??:confused:

would it be correct if you have steady strongth head wind (no gust at all) just to set Vref+5 on the bug than fly manually with the Vref+ head wind correction, and what is the different between command speed and approach speed....??

what should I set on the MCP, if for example I have 10 kts head wind correction and 10 kts gust correction, and fly mannually, so I have to set on the MCP Vref+20 kts (in this example 150 kts) then approaching touch down to bled off the steady wind correction to 140 kts at 50 ft AGL, looking at the figur on speed tape, while command speed bug give better indication:ok: if you set to 140 kts to maintain until touch down, my apologize this only my understanding from FCTM and my poor english (not my mother language), I'm new in the cockpit;)

in FACT is,

No poblems.

The command speed bug is set to the target speed you will be flying on the approach for both manual flying & with the autopilot engaged. So you set the speed you want to maintain - in your 1st example, 138kts.

In this case 130kts (Vref), as your headwind correction was 8kts.

No, the bug is set to the speed you wish to fly. You shouldn't be flying a speed above the speed you have set on the bug.

Approach speed is the speed you have decided is appropriate to fly the aircraft at, for the given conditions on the day. Basicaly, it is an adjustment for wind to the Vref calculated for the current weight of the aircraft.

Command speed is the speed you have set on the bug. On approach it is the same speed as the approach speed. With the autothrottle engaged & the appropriate mode selected, this is the speed that the autothrottle will try to maintain during the approach. We set it for manual approaches as well in order to keep manual & automatic approaches as similar as possible & also to have the target speed that we are attempting to fly shown on the ASI.

You are correct in having 150kts set on the bug. I can see what you mean by having 140kts set, however this means that you will be trying to fly the entire approach at a speed higher than the bug & will have no immediate reference when you check the ASI of where you are in relation to your approach speed. Dropping below the bug speed approaching the flare is of less consequence, as you spend more time looking outside & you are reducing speed anyway. Vref is still bugged as well, so you have an indication of minimum speed. As mentioned previously, the ideal touchdown speed is Vref to Vref -5, so you have a little lattitude in the final few feet to touchdown.

You have to find the balance between reducing the speed too soon & carrying too much speed into the flare, resulting in floating & perhaps going off the end on a short runway. Most people seem to be more afraid of getting too slow & end up carrying too much speed too far into the flare. We have seen the possible consequences of that (along with unstable approaches) in a number of accidents over the years.

Hope this helps.

Oakape,

Thank's Oak.. I do understand what you means, and again thank's for your explaination, it is a great help for my flying career, and untill next question again ;) and you have a safe flight.

in FACT is

While your SOP tends to contradict the Boeing 737 FCTM advice I can understand why you prefer to stick to one command speed setting whether for manual or autothrottle.

Does that mean you approach at Vref + five knots for all final approaches (autothrottle engaged or not engaged) regardless of HW comp and gusts? The problem with deviating from manufacturer FCTM recommended practice is that it requires a risk management approach which needs to be cleared by Boeing to cover your backside from legal attack if something should go wrong.

Centaurus

Sorry, but you have misunderstood my statement.

I am talking here about the action of the setting of the command speed bug, not the particular speed it is to be set to. I was not saying to set Vref +5 for both automatic & manual approaches, which is incorrect. Actually, that is what in FACT is was suggesting & I was trying to explain why it isn't done that way & why it is done the way it is.


Perhaps I wasn't clear enough or perhaps you should read the thread more carefully. Talk about communication issues & CRM - a perfect example:ok:

- I have to award round 1 to Centaurus here:8

It was
"The command speed bug is set to the target speed you will be flying on the approach for both manual flying & with the autopilot engaged. So you set the speed you want to maintain - in your 1st example, 138kts"
which contradicts the Boeing 'advice' and indeed highlights the dilemma that advice is known to cause.

I NEVER bug Vref+5 with autothrottle in anything above a 'zephyr day' since the sometimes wild attempts by the a/t to maintain its 'programmed' margin are often too much to tolerate (for a sensitive soul like me). In fact I normally prefer manual throttle in gusty/windy days which makes the bug setting EASY.

Boeing left us with the "what do I do with the bug when I disconnect the a/t" puzzle - I need, say, Vref+10 but I am bugged at Vref+5.

I'm sorry guys, but you 'bush lawyers' are half the reason there is so much confusion with what is required & what is not!:ugh:

seems to be the problem here but frankly, it's not that difficult! And what exactly is it about my statement that contradicts the Boeing 'advice'?

Let me put it as simply as I can -

*The 'target approach speed' for autothrottle engaged is Vref +5.
*The 'target approach speed' for manual thrust is Vref + wind correction, with a minium correction of 5kts & a maximum of 20kts.
*The command speed bug is set to the 'target approach speed' on both autothrottle engaged approaches & for manual thrust approaches.
*If you disconnect the autothrottle during an approach, you should technically reset the command bug speed to Vref + wind additives.

'Target approach speed' is my term & I have used it to refer to the speed you have calculated that the approach should be flown at, either Vref +5 for autothrottle engaged approaches or Vref + wind additives for manual thrust approaches.

As for -

This is outside the recommendations of the FCTM.

However, I do agree that the speed keeping ability of the autothrottle & sometimes large thrust lever movement can leave a lot to be desired. You may prefer to go outside the FCTM recommendations in order to deal with this issue, but I prefer to set the command speed bug as per the FCTM & simply over-ride the autothrottle manually when required.

- yes, simpler and correcter- which bit is that? The FCTM actually says that UNLESS you are going to TOUCHDOWN with the A/T engaged, you should bug Vref+additives, not Vref+5 and 'tweak it' when you disconnect. (1.16) so
is only correct for an unplanned A/T disengagement.

A Boeing 737 Memorandom from about 15 years ago advised against forcing the autothrottles against their clutch motors where the autothrottles are attempting to maintain the MCP speed. I am a bit vague but the figure given by Boeing was something like any more than 6 lbs manual pressure against the throttle levers while the autothrottle system is trying to do its thing, is likely to cause eventual damage to the clutch motors.

This would suggest that if you are unhappy with the performance the autothrottles are trying to give you in certain weather conditions, then it is better to completely disengage the autothrottle system and use manual throttle control, rather than "fight" the automatic system? Seems logical to me, anyway

I agree with BOAC in regards to the REF+5 and A/T: that is only for autolands. If you're planning an autoland, bug REF+5. Otherwise, bug the REF plus winds, etc.

I C - absolutely, but the imponderable with the 'Boeing' teaching was that if you bugged your 'chosen' Vapp with A/T engaged in gusty/windy conditions, the kit would probably be flying you faster than bug and when you do 'disconnect' you would be faced with a power reduction to achieve bugged speed, hence my choice of manual thrust and fly what I want.

Oh yes, as 'C' says, do NOT over-ride the A/Throttle except in an emergency. Like any 'automatics' - if they are not doing what you need, take them out, don't fight them. NOT good teaching!

That should read "1.5 lbs of force - not 6 lbs.

Another Boeing originated document here: Inadvertent Thrust lever Interference. The autothrottle system is designed to provide over-ride capability. This is acoomplished by a force sensitive switch in the autothrottle mechanism. During the takeoff prior to THR HLD, a pilot following the thrust lever motion with his hand can inadvertently apply sufficient force to de-clutch the autothrottle servo-motor drive. This may temporarily stop the thrust lever motion, possibly resulting in an undershoot or overshoot of target N1."

Oakape, just read back the FCTM word by word to understand it correctly,

on page 1.12 FCTM October 31,2008 :

When using autothrottle, position command speed to Vref+5 knots.
Sufficient wind and gust protection is available with autothrottle engaged because the autothrottle 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 average thrust is higher than necessary to maintain command speed. This result in an average speed exceeding command speed.

Just imagine if you can work like an autothrottle than you can set the command speed to Vref+5kts, am I correct..???

Vref = 1.3 Vstall at actual configuration and you add another 5 kts, you are still far from stalling the airplane, while you still aimming to stall the airplane at very low altitude (10 to 5 ft AGL)

I asumming, whatever the condition are as long you are landing and set the bug to Vref+5 (consider head wind correction only) and you fly the entire approach with your corrected speed (head wind) you are in safe side as the : "This result in an average speed exceeding command speed"

then you : "bled off this speed as the airplane approaches touchdown".

even the FMC Approach page give you and +5kts as default.

correct me for my low understanding and poor english, thank you:ok:

in FACT is,

read the paragraph after the one you have just quoted. It begins with "If the autothrottle is disengaged, or is planned to be disengaged prior to landing........"

You need to read the entire section entitled 'Landing' in order to understand what Boeing recommends, not just a portion on the section.

BOAC & Centaurus seem to be more up to speed than me, so I will defer any further comment to them.

So by the above statement about pushing the thrust levers, its not wise when going from a stable 40% to takeoff thrust to give them a prod after pushing TOGA? .:confused:

Edited to say:

And how about for a GA, push TOGA and give it a full arm of thrust, does that cause any issues?

Nick - ASSUMING you mean 737???, we have discussed the take-off bit before on PPrune, and there is a somewhat old Boeing Tech paper on this incorrect 'shove them up for TOGA' and the associated dangers (due to the way the A/T actually sets TOGA). As far as g/a is concerned, no harm that I can see, and in fact standard procedure with manual throttle?

Sorry yes,

737-800. I will have to have a look for the paper as it seems to be important. Our full arm of thrust for GA is for both manual thrust of course but also for A/T engauged with the argument that if you always do it you wont forget if you are manual. Any harm in that?

Thanks

There are two issues. If you are flying a manual throttle approach and decide to go-around the only reason you press the TOGA switch is to bring up the FD needles to their GA attitude. Most pilots should be able to perform a perfectly safe hand flown go-around without the urgent need for FD guidance. However, other pilots are so reliant on automatics that a GA without FD guidance is almost akin to a Mayday. The Turkish Airlines B737 crash at Amsterdam was inevitable when the crew seemed mesmerised by the fact the throttles did not appear to work automatically.
If the auto throttle system is operating during an instrument approach, then commonsense would assume you guard the throttles and when TOGA is pressed and instant opening of the throttles did not occur , you would quickly set them at the go-around setting.

Tee Emm,

Thanks for the reply. Obviously we should all be capable of flying raw data under any stage of flight thats the whole point of being a pilot being able to cope when the automatics cannot. My questions was directed more at the effects on the a/t of pushing when its trying to do its job. Our SOP for a GA is to push TOGA (to get the aircraft in GA mode FD's and all that jazz) and push the thrust levers forward towards GA thrust. The PM sets the GA thrust (or monitors the a/t setting) I was just wondering whether doing so with a/t engaged causes an overshoot/undershoot or wears out the a/t motors or something similar?

Nick - certainly unwarranted 'fiddling' (ie pushing) during take-off is known to be potentially hazardous (see Boeing paper). I can see no problem with a similar over or undershoot of N1 in a g/a where performance is slightly less critical. As to any mechanical effects, it is the clutch that is being over-ridden, and I don't think that would cause a problem. I personally go with Tee-emm's last paragraph.

anybody got a link to the Boeing memorandum/Bulletin that's being referred to here

Agree wholeheartedly. There seems to be a fear of flying at Vref. This speed has already plenty of fat built into it by certification rules. With regard to bleeding off the half HW component additive the FCTM says "approaching touch down." This statement frequently leads to a varied interpretation of "approaching touch down" Some assume it means during the flare and float.

The clue is in Supplementary Procedures (Adverse Weather) Approach and Landing where it says if ice formations are observed on the airplane surfaces, add 10 knots to VREF. This ensures maneuvering capability. There is also a Note that says "To prevent increased landing distances due to high airspeed, bleed off airspeed in excess of VREF+5 knots+gust correction when below 200 feet AGL. Maintain the gust correction to touch-down"

This statement suggests that the Boeing definition of "approaching touch-down" is when below 200 feet AGL. It does not mean after you flare.
Being on VREF at the point of flare is perfectly safe.