V1 Versus Vmcg

[Springbok260]

Hi there everyone, I'm in the process of studying for my Atp's and in my notes on flight planning the book suggests that when your working out take-off speeds that if your speeds land in the shaded area you can use them as long as your V1 speed isn't lower than Vmcg.

However I thought that if your speeds were in the shaded area that you'd have to re-calculate your speeds using a higher assumed weight?

If you could please shed some light on whether the book is correct or not thanks.

[welliewanger]

I think nobody's answered your question as we have no idea what "the shaded area" is.
As for V1 / Vmcg:
V1>Vmcg and Vmca

[peter knapp]

Its very simple.
If in the shaded area compare V1 with VMCG. If V1 is less than VMCG you cannot take off!!

[Alex Whittingham]

If you take a tabled V1 of 127KT and apply the maximum negative corrections for wind and slope it would come down to 120KT. The highest possible value of VMCG in the table is 118KT + 2KT for packs off = 120KT. Thus tabled V1s of 127KT or greater cannot be at risk from VMCG.

The shaded area shows you where tabled V1s are 126KT or less, and therefore at risk from VMCG. If your V1 is in the shaded area it just means you have to check your VMCG, it may still not be limiting. As Peter says, if you check it and find that VMCG is greater than V1 you cannot take off at that weight.

Being outside the shaded area means you don't have to check VMCG, it cannot affect you.

[Keith.Williams.]

If in the shaded area compare V1 with VMCG. If V1 is less than VMCG you cannot take off!!
Yesterday 10:23

if you check it and find that VMCG is greater than V1 you cannot take off at that weight.

I do not believe that the situation as clear cut as that, but if it is then what exactly can we do about it?

If we assume that we are already at the FLTOM, then increasing weight to increase V1 will make us overweight for the field lengths.

Looking at the tables it looks like we couuld wait until the temperature or pressure altitude increase to move us to the right, to give a greater V1. But again if we were already filed limited then this is not an option.

The only other options appear to be to wait for a strong headwind come salong, or else arrange road transport to get teh aircraft out?


But instruction vii states: "Compare V1 with minimum Mmcg. If it is less make V1 = min Vmcg."

Now this poses a problem in that if the aircraft is at its FLTOM, then increasing V1 to Vmcg will risk an overrun in the event of an RTO.

Instruction viii then states: "Check TORA exceeds TORR. If it does not, the take-off weight must be reduced."

This appears to provide a bit of reassurance regarding the risk of an overrun, but there is still room for doubt, because Figure 4.4 which will be used to ensure that TORA is greater than TORR does not include any arbitrary increase in V1.

And if we are already at the bottom row of the tables where the weights are lowest (40000 kg) how exactly can be do the calulations for a lower weight?

I am intrigued by the term "Assumed Higher Weight" in the OP.

Could it be that SpringboK 260 is refering to a system whereby we assume that the weight is higher than it actuially is, in order to get V1 greater than Vmcg? Provided we then check that the TORA exceeds the TORR for that assumed weight, then the above risks will be eliminated.

How exactly does your "Assumed Weight" system work Springbok?

[Alex Whittingham]

Hi Keith,

I had not assumed that we were operating at FLLTOM, as there were no indications that we were. VMCG is unlikely to be limiting (assuming a well designed aircraft) at anything other than very light weights. I suppose you could arrive at a very light FLLTOM by operating from a very short runway, in which case you would have to look for more headwind or at least turn the packs on.

I think we might be working from different editions of CAP 698, my version (July 2006 http://www.caa.co.uk/docs/33/CAP698.pdf) has the following instructions:

2.5.2 The Calculation of V Speeds
To calculate the V speeds use the tables (at Figure 4.8 or 4.9 as appropriate) in the following manner:
a) Enter the density sub-graph (below) with pressure altitude and ambient
temperature to determine which of the columns of the tables should be used.
b) Select the tables appropriate to the flap setting from Figure 4.8 or 4.9.
c) Enter the V speed tables at the actual take-off mass. Extract V1, VR and V2.
d) If it is necessary to correct V1 for slope and/or wind component, enter the table at the top of Figure 4.8 or 4.9, as appropriate, at the actual take-off mass and interpolate the correction necessary.
e) Apply the corrections to V1.
f) Use the sub-table below Figure 4.8 or 4.9, as appropriate, to determine the VMCG. Enter the left column at the ambient temperature and then proceed right along the row to the appropriate aerodrome pressure altitude (interpolating if necessary). Extract VMCG.
g) Compare V1 with VMCG. If V1 is less than VMCG, take-off is not permitted.
h) Check TORA exceeds TORR. If it does not, the take-off mass must be reduced.

and instruction (g), rather than your (vii) is significantly different.

Springbok may correct me but I'm interpreting his statement as 'I'm not FLL limited and I can't operate at this silly light weight because of VMCG so I'm going to have to tanker some fuel to increase the TOM and get a higher V1'.

There are, of course, other techniques available if you're not FLL limited such as choosing a higher V1 if a range of speeds is available. I don't think that the CAP offers this as an option. This is probably in keeping with modern operations, I certainly wouldn't like to do a range of V1s calculation on a 20 minute turn around.

I get your point about the 'assumed higher weight'. It would certainly work.

[Keith.Williams.]

Hi Alex,

I had both the 1999 version and the 2006 version on my bookshelf. But I picked up the 1999 version.

But that amendment raises the question of why the statement was changed from "Compare V1 with minimum Mmcg. If it is less make V1 = min Vmcg."
to "If V1 is less than VMCG, take-off is not permitted."

I recall that a CQB question on this subject appeared, and the staff answer was "Take-off is not permitted." If I remember correctly this question raised objections because it did not specify that the aircraft was at its FLTOM. So there was nothing to preclude increasing V1 to equal Vmcg.

I suspect that the amendment to the CAP was in response to these appeals. But didn't any of the people involved ask the obvious questions of "So what will the pilot do?" "Will he/she really increase the mass rather than simply assuming a higher mass?"

As you say pilot doing 20 minute turn-rounds cannot be expected to work out a range of V1s. But can they really be expected to wait until the wind pick up?

It would be interesting to hear what current line pilots do when V1 is less than Vmcg.

[Denti]

We used to have a rule to adjust V1 to VMCg if it was below. In the NG it is not a case that i've seen ever happen as the VMCg is usually below 100kts at a sufficient low derate and the lowest V1 i have seen so far was 104kts.

Nowadays the boeing software takes care of the whole issue and we only get the three speeds, but no clue as to how close we are to VMCg, only that we know that it allways checks V1 against VMCg and keeps us safe. The company therefore removed the VMCg tables from the QRH as we do not need it anymore.

Just one thing. VMCg is only fixed for a thrust setting, since thrust settings are not fixed though it is another value that can be played with.

[Springbok260]

Hi everyone!

Thanks so much for the response tomy question it has really cleared up some things:). When I asked if we can assume a higher weight for take-off I was refering to the same graph you use to attain your V1,Vr,V2 speeds and if your speed were to fall in the shaded area you would simply stay in the same column and just assume a heavier weight by going up the column until your speeds were no longer in the shaded area and then select those speeds provided you weren't limited in other areas such as field length.

Thanks again.

[john_tullamarine]

at anything other than very light weights

At least one 737 operator in RSA had this situation some years ago when I was working over there but they weren't training for it. Led to some interesting eye-opening in the sim when I starting introducing the problem in free time.

The company therefore removed the VMCg tables from the QRH as we do not need it anymore.

Don't like that. So your company doesn't have any interest in the interplay of Vmcg and crosswind ?

VMCg is only fixed for a thrust setting, since thrust settings are not fixed though it is another value that can be played with.

Now that's just plain dangerous thinking. Vmcg is fixed for the rated thrust setting. When you flex (I'm presuming that you are so referring) the Vmcg stays the same. When you derate, the rated thrust and Vmcg reduce. Beyond that you have no option to play with the numbers.

just assume a heavier weight by going up the column until your speeds were no longer in the shaded area and then select those speeds provided you weren't limited in other areas such as field length.

For a typical RTOW chart (I presume this is what you are looking at), and in the absence of any specific prohibition, that should do the trick. Functionally all you are doing is generating an overspeed takeoff. Regarding field length, you will be covered by the weight delta.

[Denti]

The company therefore removed the VMCg tables from the QRH as we do not need it anymore.

Don't like that. So your company doesn't have any interest in the interplay of Vmcg and crosswind ?


I don't like it either to be honest. However as we have to stick to the performance program figures anyway there is not much we could do in any case. On the classic VMCg was pretty high compared to the NG though, so the issue is basicly non existant anymore. Even on the short one (700) we achieve VMCg values well below 100kts (yes, i have the old tables stashed away for my own usage, and there is allways the AFM).

VMCg is only fixed for a thrust setting, since thrust settings are not fixed though it is another value that can be played with.

Now that's just plain dangerous thinking. Vmcg is fixed for the rated thrust setting. When you flex (I'm presuming that you are so referring) the Vmcg stays the same. When you derate, the rated thrust and Vmcg reduce. Beyond that you have no option to play with the numbers.


Quite right, but it is not a fixed value for all operation anymore as we do have the freedom to use different thrust settings and use that freedom nearly every take off. In fact i used full rated thrust only once or twice since changing onto the NG, every other take off was derated in some way or another (and "flexed" too of course).

All in all take-off performance is not one-dimensional, we have so many different things we can change that there are allways several solutions for a given problem. We have 5 different take off flap settings, we have 3 different thrust rating, we can use ATM additionally, we can change seamlessly into improved climb speeds increasing our Vr/V2 by up to 35kts, or choose not to do so, use tailwind up to 15kts. Boeing made the 737 a pretty versatile beast.

For the given problem, VMCg too high, the software would most probably simply reduce the thrust setting by a notch and with that the VMCg, given that there are no other restrictions.

[Keith.Williams.]

Many students complain that the material in the JAR ATPL syllabus is not relevant to the real world. I believe that Aircraft Performance is one of those areas where it could and should be very relevant indeed.

I don't claim to be an expert on this or any other subject, but this does not prevent me from recognising lack of expertise in others. Unfortunately most of the "experts" who control the JAR ATPL syllabus actually know and understand very little about the subjects. When faced with problems they tend to take very short-sighted actions without conducting any meaningful research. The question of what to do when V1 is less than Vmcg is one example of this problem.

The performance manuals provided for use in the exams originally stated "Increase V1 to Vmcg." But they did not discuss the hazards involved in this, nor how to avoid these hazards. The manuals were then amended to state "If V1 is less than Vmcg then take-off is not permitted." The amended manual did not go on to discuss what action should then be taken.

The obvious solution at that stage would have been for the "JAR experts" to carry out some research to find out what actually happens in the real world. Or at least in that part of the real world which still uses paper performance data. Unfortunately obvious solutions often aren't at all obvious to those involved.

I was hoping that some current line pilots would add their own experiences to this thread.

[Alex Whittingham]

The B737-300 Performance manual says the following:V1(MCG)
Regulations prohibit scheduling takeoff with a V1 less than minimum V1 for control on the ground, V1(MCG). Therefore compare the adjusted V1 to the V1(MCG). To find V1(MCG) enter the V1(MCG) table with the airport pressure altitude and actual OAT. If applicable, add the adjustments shown below the table. If the adjusted V1 is less than V1(MCG), set V1 equal to V1(MCG). If VR is less than V1(MCG), set VR equal to V1(MCG), and determine a new V2 by adding the difference between the normal VR and V1(MCG) to the normal V2. No takeoff weight adjustment is necessary provided that the actual field length exceeds the minimum field length shown in the Takeoff Field Limit chart.

Although I am far from current, even when I was flying it was very unusual to hit a VMCG limit. It would only happen on very short runways when your FLLTOM was neccessarily light. In part, this was exacerbated by the old UK certification requirements which set VMCG some 10KT higher than FARs and JARs would do.

I would be surprised if an airline would schedule any aircraft certificated under JARS or FARs into a VMCG limited field, they certainly would not do it without providing clear guidance to the crew.

[Keith.Williams.]

I don't think that it will happen very often either, even in the case of a diversion.

But if we take the instructions as printed in the (improved?) CAP 698, I have this vision of the Captain announcing "We regret to announce that the aeroplane cannot take-off today, because it is not heavy enough".

[Denti]

Yup, have the same excerpt in my old -300/500 manuals, we had the same basicly for the NG but with the introduction of software based performance calculation it vanished.

As i said, if a lower thrust rating is available that could solve that problem if it exists nowadays in the first place. VMCg is very very low for the 737 NG and not too much of an issue in the classic, although it sometimes very rarely became one.

V1 is an adjustable value, in part VMCg is as well and of course Vr/V2 have a very broad band of possible values. That might not be part of the JAR training material, but it was actually partly expressed in our pre-JAR material issued by Lufthansa. Interesting is the boeing issued training material for narrow runway operation which adresses VMCg concerns, especially in the combination of narrow runway and crosswind.

[john_tullamarine]

All in all take-off performance is not one-dimensional ..

.. of course, until one comes to the Board of Inquiry/Coronial/Royal Commission/whatever ... when the legal folks will make it very much so. Not a problem, I guess, provided that one has a good story and that story is based soundly on certification and operating regulation ?

even when I was flying it was very unusual to hit a VMCG limit.

Indeed. Most pilots rarely come across the problem which is one reason for its not figuring much in the accident record. Minimum weight, minimum speed schedules is where one might see the problem, depending on Type characteristics.

In part, this was exacerbated by the old UK certification requirements which set VMCG some 10KT higher than FARs and JARs would do.

For most who might be confused by this statement, the old BCARs figured Vmcg for a 7kt crosswind (if my memory serves me correctly) while everyone else uses nil wind. I would expect that the delta should vary depending on number of engines etc.

I would be surprised if an airline would schedule any aircraft certificated under JARS or FARs into a VMCG limited field

An interesting point of view. I think it more likely that such will be of little concern to the commercial driver .. if the aircraft is able to get in/out and the aerodrome meets the various aerodrome standards then, guess what ?

they certainly would not do it without providing clear guidance to the crew.

It is on this point that many operators fall down miserably. Removing all reference to specific Vmcg data is a salient example

Interesting is the boeing issued training material for narrow runway operation which adresses VMCg concerns, especially in the combination of narrow runway and crosswind.

I can't recall reviewing the Boeing presentation(s) but I'm certainly not surprised. The practical limit on runway width generally comes out to be the Vmcg centreline deviation figures under whatever conditions might be built into the operating (as opposed to certification) rules.

It is pertinent that line folks should be aware that crosswind has a VERY real effect on the book Vmcg for the day's takeoff. Typical rules of thumb vary from 0.5kt/kt increase for twins up to something in excess of 1kt/kt for quads. Now, if one happens to be in the (very bad hair day) situation of having a near Vmcg schedule, with a significant crosswind, and one loses one on the "wrong" side, then one is highly likely to have a close encounter of the grass filled kind ....