B747-200: Vref, V2 and Vmcg

[forsyh]

I quote from one of my books:

"Vref is included on the takeoff data card as a reference for computing enroute climb speeds. Vref varies directly with gross weight and simplifies the additives for climb speeds while V2 is influenced by Vmcg at low gross weights and would require a wider range of additives for the various speeds."

Can someone please explain how "V2 is influenced by Vmcg at low gross weights and would require a wider range of additives for the various speeds".

Thanks.

[18-Wheeler]

At light weights the 747 will have the V speeds below Vmcg & Vmca, so we use a minimum set of figures regardless of the weight.

[john_tullamarine]

Without knowing the source of your information, it is difficult to know just where the author is coming from. Possibly the text meant to say Vmca rather than Vmcg and is just a typo. Alternatively, Vmcg-limited V1 could lead to a similar incidental effect on V2 at low weights.


However, it is useful to review the generic story from time to time as there are some traps here for young players which can bite hard and Mutt and I like to run it past the new people in the forum every now and again.

If I may talk twins, which is what I normally play with (only the factors change .. not the basic story).......

V2 normally is scheduled as the minimum permissible V2 to keep the takeoff distances down.

The certification rules require, for the configuration, that minV2 (or V2min - take your pick) be not less than

(a) 1.2 x Vs for the weight

(b) 1.1 x Vmca

Typically, for any aircraft,

(a) at lower weights, the Vmca requirement will become limiting. By this one means that 1.1Vmca is greater than 1.2Vs. In this region, V2 doesn't vary with weight.

If Vmca is very low, this effect may not be seen .. the Citation, if I recall correctly, is an example. However, I would be very surprised if a wing-mounted four-motored bird did not exhibit the effect.

(b) as the weight increases Vs increases and, at some point, 1.2Vs increases above 1.1Vmca ..... now V2 increases as weight increases.

To check this out, have a look at your standard speed schedule with weight. At low weights, V2 is constant and doesn't vary with weight .. you are Vmca-limited. As the weight increases, you get to a point where V2 starts to increase as weight increases.... you are stall-limited.

A consideration which often is overlooked in training is that the handling problems with a failure, say, halfway through the rotation flare are VERY critical if V2 is Vmca-limited and, depending on Type yaw-roll coupling characteristics, you might need a lapful of wheel rotation to control bank ... Vmca is very bank-dependent ... if you don't control bank, Vmca increases rapidly due to slip ... and you SUDDENLY find yourself in the middle of a Vmca departure ... crash, burn, die. Well worth investigating this in the sim ... just a confidence and co-ordination training exercise ... but to experience it in the aircraft for the first time is not a good thing. Those who haven't played with it might be non-believers ... those who have will smile knowingly ....

As we back up a little on the takeoff, VR is relevant. In essence, VR is a convenient speed chosen so that the OEI rotation will result in somewhere near V2 at screen height. VR will sit somewhere between V1 and V2.

Backing up further, we have to concern ourselves with V1. V1 is limited on the high side by VR and on the low side by Vmcg.

If Vmg is sufficiently high, then minV1 may be high enough at low weights to require that VR be increased which, in turn, might require that V2 be increased. (This may be what the original text was referring to).

If the aircraft is typical ..., in the same way that the speed schedule shows a constant Vmca-limited V2 at low weights, there will be a constant Vmcg-limited V1 at low weights. So, even if Vmca were not going to be a problem, in the scenario described in the previous paragraph, we could see the Vmcg-limited V1 imposing an incidental non-weight-related limitation on V2 at low weights.

As with Vmca-limited V2, there is a trap associated with Vmcg-limited V1 takeoffs.

For the US case, certification Vmcg is determined for NIL WIND. However real world actual Vmcg is highly crosswind dependent. Typically, the effect of crosswind might be to increase the real world Vmcg by half the crosswind for a twin, to more than the entire crosswind for a four-motored bird. So, if you are scheduling a Vmcg-limited V1 takeoff in a strong crosswind ... then you may be between a rock and a hard place.

If the speeds are limited severely by runway distances, then I guess you are stuck with it .. either you go and risk losing the aircraft with a critical failure (and, again, if you have a good sim model, the sim exercise is useful training to hammer this point home), or you delay until the wind abates.

If you are just doing a ferry flight, say, and you have a long runway, do consider increasing the speed schedules in accordance with the OM tables, to a point where the actual V1 is increased sufficiently above Vmcg-limited V1 to take account of the real world increase in Vmcg due to the actual crosswind. So long as you stick with a published speed schedule appropriate to a weight not greater than the RTOW for the runway, then you shouldn't have a problem. Certainly, if you are scheduling a Vmcg-limited V1 with an aft CG, in a strong crosswind, and you lose the upwind engine at or near to V1, then you WILL be in for an interesting ride, especially if there is much in the way of a V1 to VR split scheduled for the takeoff.

One should keep in mind that real world (as opposed to certification) Vmcg and Vmca are very CG dependent.

[Mad (Flt) Scientist]

Just to add a minor detail to the very detailed post bu John T. above.

If the low speed V2 is being constrained by Vmca then it should be as near perfectly constant as your charts will permit, because it is being directly limited by 1.1Vmca.

if you are, instead, Vmcg limited at light weight then you should have the unexpected effect of decreasing V2 with increasing weight. This is because it is V1 which is being constrained by the Vmcg limit, and the V1-V2 difference or "speed spread" will actually be greater at lighter weights, due to the increased acceleration at those lighter weights.

As an aside, I've seen charts where V2 at min TOW was actually higher than at max TOW. That is most decidedly NOT a good thing, but it's theoretically possible if you follow the regs.

So if you look at your V2 data in detail you may e able to determine whether you are Vmca or Vmcg limited at light weight.

[john_tullamarine]

MFS,

Just to clear my mind on a couple of things ...


(a) if you are, instead, Vmcg limited at light weight then you should have the unexpected effect of decreasing V2 with increasing weight

I presume that you are referring to the achieved speed being above book speed here. If you mean otherwise, perhaps you could amplify the point ?


(b) I've seen charts where V2 at min TOW was actually higher than at max TOW

I've not come across this ... again, perhaps you could amplify the point ? I presume that you are referring to scheduled V2 here ?

[maxalt]

You guys seem pretty knowledgeable on performance. Would you have a go at explaining balanced field length theory for me?

[mutt]

The minimum field length where an engine failure was assumed to occur at a speed such that the distance to continue the takeoff and climb to a stipulated height was equal to the distance required to stop

But somehow i guess that you knew that already, so whats the catch???

Mutt

[Mad (Flt) Scientist]

JT

I was referring to the AFM speeds. Perhaps an example would be the best way.

Assume an aircraft with TOW from 40,000lbs to 80,000lbs.
Vmca of 90kts
Vmcg of 90 kts
Vs from 80kts at 40,000lbs to 94kts at 80,000lbs.
Vmu sufficiently far below Vs that we can ignore it.

Then the 1.2Vs lower limit to V2 will be 96kts at 40,000lbs and 113kts at MTOW.

Looking at the Vmc-driven limits, the Vmca limit will be 1.1*90kts=99kts. If this were the only limit we would have a constant V2 of 99kts from 40,000lbs up to 61,000lbs or so, then increasing with stall speed up to 113kts at 80,000lbs.

But, the V1mcg limit must also be considered. Since V1 may not be less than Vef which may not be less than Vmcg (and in fact one must also add the acceleration for recognition time too), for the case shown we may have a V1 of not less than 90kts (and probably a little more, let us assume 2kts of accel, giving a V1mcg of 92kts). FAR25.107(a)

FAR25.107(e)91)(i) states that VR may not be less than V1 - assume a V1=Vr schedule.

FAR25.107(c)(2) then states that V2 may not be lower than VR "plus the speed increment attained before reaching a height of 35ft...".
We refer to this increment (at Canadair at least) as the "speed spread" and since the aircraft is often constrained by pitch rate considerations there is a finite time required. At lighter weights this means that the higher T/W accelerates the aircraft more than at heavier weights, and so the "speed spread" is greater at light weights.
One might have a situation where the speed spread was as much as 10kts, say, at the lightest weight, dropping more or less proportionally with weight.
For the numbers used here that would give a V2 at 40,000lbs of 102kts due to Vmcg limits, dropping to 97kts at MTOW say.

The combined effect of all those limits might be (apologies, I'd love to insert a sketch here, but...)

40,000lbs : V2=102kts (V1mcg limit)
45,000lbs : V2=100kts (V1mcg limit)
50,000lbs : V2=99kts (V1mcg and 1.1Vmca limits coincident)
60,000lbs : V2=99kts (1.1 Vmca limit)
61,000lbs and above : 1.2 Vs limit (from 99 to 136kts)


I haven't double checked those numbers, I will do so later - it's Saturday night!

Regarding the V2 at min being higher than at max. Yes, it was the scheduled speed - it was a consequence of unreasonably high Vmc's (both 'a' and 'g'). But it would have been very hard to respect the procedures and not get those speeds in practice. (It would also have been very bizarre)

[john_tullamarine]

MFS,

Thanks for that, mate ... I am now briefed ... the tale was pretty much as I had presumed. I have a Challenger AFM tucked away somewhere ... and will have a looksee when I get a few minutes to spare to dig it out of archives.

Interestingly, with a min weight takeoff's typically 15 kt V2 overrun for the 732 for instance, most people still take a bit of repetitive practice to get on top of the Vmca departure problem as the bird corkscrews itself around the runway ... sometimes I wish I could mount a video on the coaming to record the looks of horror and shock on the pilot's face during the first couple of runs ... as the aircraft rolls through the vertical .. which is about where I normally freeze the box. Seriously, though, I think that this exercise is greatly underrated and that far too many pilots never get to experience, and get on top of, the extremely critical handling of a min weight/min speed takeoff failure.

On another tack .. it is probably useful to put a little bit of explanation to any statements which are out of the ordinary for the interest of those readers who don't have the specific tech background.

I guess if you join up with Mutt and me in the sandpit ... we can style ourselves as the Three Muttsketeers ? ... ah well, I thought it was funny ...


.. anyway, Seasons Greetings to the Canadian engineering fraternity ....

[mutt]

we can style ourselves as the Three Muttsketeers

groan................................. :):)


Mutt.

[RatherBeFlying]

Now I really know why several bods were upgraded to Business Class in a lightly loaded 732 until the seatbelt sign came off;)

Many thanks to the 3Ms and a happy holiday.

[john_tullamarine]

....Mutt, mate ... it wasn't THAT bad .....

[maxalt]

mutt it isn't a trick question (blanced field length) just looking for a better way to explain the benefits of using it to some newbies.
Go on, indulge me.:)

[Mad (Flt) Scientist]

JT

I shall try to be less obscure in future - although its hard to know where explanations of egg-sucking technique begin sometimes - it's easy to forget how obscure some stuff is when it's your bread and butter.

I must be hungry, that's two allusions to food in one paragraph!

I don't think you'll see too much in a Challenger AFM - I think the Vmc's are sufficiently low there that they don't affect v2 (although that's from memory, and I don't really 'know' the Challenger). it does affect some of our larger aircraft though, for sure.

Best wishes to you too for the hols - I'm sure you'll enjoy things in moderation of course.....:) groan :)

[john_tullamarine]

MFS, mate ... you, Mutt and I just HAVE to have a Guinness session ... the puns will fly thick and furious .....

My concern with amplification is that many of the PPRuNe fraternity use tech log as a resource ... what the specialists (in any field) take as a lemma, others might find totally confusing without some explanation to fill in the holes .... I know that I have learnt a great deal from several of the forums over the time that I have been a sandpit player .... and it is terribly frustrating if you only get half of the story ...

MaxAlt,

BFL gives a cheap and dirty quick way to get the RTOW figures done ... and, for convenience, is often referred to in documentation. By using BFL, you don't have to run as many calculations to figure the runway limit weight. It has no inherent technical advantage (Mutt might disagree as his mob use it for other reasons) other than general use and increased simplicity. On many, if not all occasions, a BFL-limited RTOW results in a slightly less than maximum RTOW for the runway and conditions on the day .... with wet or contaminated runways, or with obstacles and clearway, often the commercial needs dictate the use of an unbalanced takeoff.

Also, for many runways, the numbers may not be so critical as to warrant the additional number crunching to allow for the actual TODA, TORA, and ASDA figures.

[maxalt]

Thanks JT, that helps a lot.

The scant info contained in some very old company documentation gave simply the following statement:

Balanced Field Length - The Condition Where V1 Is Selected To Make The TODR Equal To ASDR

It Allows Maximum Performance Benefit To be Gained From any Clearway Associated With the Runway

On thinking about this definition I believe it's wrong, and it leaves a lot to be explained anyhow. Do you agree with the statement?

[mutt]

J_T,

From an operational point of view, we dont have the time nor expertise to do AFM graph calculations prior to takeoff. We therefore use Balance Field Limits on Boeing aircraft with published weights and QRH speeds/FMC speeds, corrections for runway contamination, MEL/CDLs etc are then applied to these speeds.

On the A300 we use optimized v-speeds, while these can offer better weights, however its fun trying to explain why you get a 30 kt V1 increase for a 3,000 kgs weight increase.

Maxalt,

Balanced field length is the ideal situation where the maximum takeoff weight is possible for a given runway length.

This means that the aircraft can accelerate to Vef, suffer an engine failure and continue the takeoff reaching 35 feet by the end of the runway, or the aircraft can transition into a stopping configuration and stop by the end of the runway.

Unbalanced field length may also result from runway slope and wind corrections, and the use of clearways and stopways.

By definition a clearway doesn't have to support the weight of the aircraft, therefore if you are allowing the accelerate go portion of your takeoff to be over the clearway, you are left with an unbalanced situation. I would therefore say that the statement which you were given is incorrect.


It Allows Maximum Performance Benefit To be Gained From any Clearway Associated With the

Runway

Mutt.

The statements in italics are taken directly from Boeing documentation, which i can send to you if required.

[john_tullamarine]

I have to concur with Mutt, the statement to which maxalt refers misses some important points of the runway limited takeoff calculation -

(a) most runways don't have much in the way of clearway declared and, in any case, even if a lot of clearway is declared, a takeoff for a particular Type is limited in the amount of clearway which can be used due to TORR considerations - recall that at least half (US rules) the airborne distance to screen has to be over the TORA, which is normally threshold to threshold... not much point running off the end of the hard bit before getting airborne ...

(b) BFL is a simplified approach to the sums, mainly of use for typically level-ish runways without any clearway declared or where a quick, manually-derived, answer is needed and we are prepared to forgo any benefit to be derived by considering the small margin of TODA over TORA. Consider that many flight manuals provide a BFL subset of charts which is significantly easier and quicker to use than the detailed charts. In this case the declared TODA should include the stopway (typically 60m or so) or remaining strip length and the declared TORA usually is not going to be limiting (ignoring the strange climb antics of helicopters). There will be little advantage in playing with all the charts as the end result will be much the same weight.

As Mutt indicates, there is a very important advantage for an operator with lots of runways to look after in that simplified rules can be generated for flight crew use (correction data and the like) ... in my view, this is a very valid consideration and the main reason that many people emphasise the BFL approach.

The Boeing BFL quote referred to has to be read with these points in mind... if we talk about "runway", specifically, we mean "no clearway" and I suspect that the Boeing statement is looking at the barebones runway .. ie no stopway either.

But it is important that pilots realise BFL is just a constraint on the more general takeoff weight analysis and, if you need the last kilo RTOW on the day, then the generalised analysis is the better procedural way to go. As Mutt suggests, this can become quite an involved, pain in the neck, process when done manually. However, if it be simulated on the trusty PC, then it becomes a doddle ... working up the simulation is quite straightforward but takes a lot of time. When the work has been done, the production of optimised RTOW tables is quick and straightforward ...

Mutt, I presume that the 30 kt delta on the A300 relates to an obstacle limited takeoff from a long runway ? I have no background with the Type but is that a typical sort of variation ?


Now that, as so often happens, this thread is totally off the original question .... I wonder where it will head off next ?

[mutt]

Now that, as so often happens, this thread is totally off the original question .... I wonder where it will head off next ?

Well then i think that its a perfect time to wish you a very Merry Christmas and to thank you for all the effort that you have put into the Tech Forum over the last year.

Mutt.

[LOMCEVAK]

It is interesting that in all of this discussion no-one has mentioned OEI climb gradient! Remember that, essentially, V2 is the minimum speed at which a specified gross climb gradient (OEI) can be achieved (2.4% 2-engined aircraft, 2.7% 3-engined, 3.0% 4-engined). The 1.2 x Vs and 1.1 x Vmca are caps below which V2 cannot go.

A lot of the discussion above can be simplified by saying that Vr cannot be less than Vmcg. When V1 = Vr = Vmcg, V2 is the speed which is achieved at screen height following rotation at the normal rate for the aircraft type.

The case in point here, the B742, does reach this situation at light TOW, although the Boeing take-off data is a little confusing as the shaded part (low TOW) of the T/O speeds table just notes that this area is "minimum control speed limited". As there is no promulgated Vmca-1, this must be a Vmcg limiting case. As has been stated above, what actually happens to V2 will depend on the variation of Vmcg with AUW and the acceleration from Vr to screen height. However, do not try to read too deeply into the B742 table; I have never quite figured out quite what is happening! One thing to be wary of is that if your company SOP is to set the ASI bugs for departure based on V2, they will be set artificially high. In such light TOW cases it is worth calculating the approach Vref for the AUW as this is the true speed on which flap retraction should be based (ie stall margin). Therefore, if you end up with a loss of thrust and high drag after T/O, you are aware of the true min speed for flap retraction should you really need it.

With regard to Vmcg and crosswind, in the B742 you need a speed greater than Vmcg by approximately an extra 1.3KIAS per kt of crosswind in order to maintain control to the same criteria (max 30 ft displacement from centreline, max 150 lbs rudder force). Note that the B742 was originally certificated in the UK to BCARs which assumed a 7 kt crosswind, and some operators may still use these figures. Crosswind is ignored in FAR and JAR 25 on a statistical basis related to the probability levels of losing an engine in an adverse limiting crosswind!!! I agree with the above comment that the best you can do is to use the scheduled V1 for RTOW although knowledge of V1 max (ie Vstop) would help even more. Note also that Vmcg is based on aerodynamic control alone so the NWS will help for real.

[john_tullamarine]

Best I leave 742 comment to Mutt if that be appropriate.

If the takeoff is seriously Vmcg-limited, then one is likely to be (near) aft CG. In this case, especially in the wet case, NWS is not likely to be of much use at all .. it is for this reason that the certification process doesn't consider the potential benefit of NWS.

Did anyone miss the reference to 1.3kt/kt ?

An uncorrected min V1, aft CG, max thrust takeoff with a critical (upwind) failure in a significant crosswind leaves the crew between a very big rock and an equally big hard place .... and looking very, very seriously at the grass ....

[fdr]

What continues to amaze me is the lack of interest shown by the operators, pilots, regulatory authorities, and manufacturers in the failure of the aircraft to achieve the peformance specified in the certified takeoff data.

refer www.airliners.com for great photos of failure to achieve the performance (4 really neat photos at last count).

The maths required to quickly scare yourself is not highbrow, but appears to beyond the capacity or interest of the aforementioned parties in general.

The discussion at:
http://www.geocities.com/profemery/aviation/takeoff_distance.html

is marked by concern and lack of understanding of the reuirements specified by FAR/JAR25 and other state criteria.

notwithstanding the above, the evidence of compliance or not resides within the grasp, and observation of the pilot. The authorities could readily externally monitor compliance but fail to do so.

A simple case in point: refer criteria for 4 engine aircraft.
requirement is to achieve in normal service 35' at 100/115% (87%) of TODA, so 13% of TODA remains when aircraft achieves 35'. before that, the aircraft must achieve Vr, rotate to liftoff attitude and then climb from liftoff to 35'. training manuals will provide basic timing, as does flight data. these things take a finite period at a given velocity, and therefore cover a given distance. Simplistically, it takes 5-6 seconds at ~90 msec to liftoff post Vr call (450-540mtrs), and 2-3 sec to achieve 35' (180-270mtrs). So, minimum distance is 630mtrs to about 810mtrs before the 87% TODA point, the aircraft must achieve Vr, at heavy weights. Heavy weights for the current aircraft need BIG runways, and therefore have also lots of TODA after the 87%TODA point is achieved. Criteria limits clearway incorporated into TODA to 0.5 X distance from liftoff to 35'... crosscheck for above...)

So...

big airfield: 4000mtrs, what do we get...

13%=520mtrs... distance remaining to end of TODA when we should be at 35'. Stated another way, we travel an additional 520mtrs after achieving 35' before reaching the end of TODA. During this period we climb in second segment ( assuming we didn't hit anything) and average some climb rate X time to traverse 520 mtrs, at V2+10 etc. That definitely gets us above 35' at the end of the TODA.... like about 160'+ etc. ( getting a queasy feeling yet?)

Of the 13%, the 520 mtrs above, part of that can be over the clearway, (0.5 x distance liftoff to 35') which gives from above 70-135mtrs. (AAC data actually gives 180mtrs interestingly, ie very long time to 35' from liftoff). From this we can calculate that the 35' must be achieved some distance from the end of the runway while we use the clearway beyond runway end. Using maximum clearway from AAC of 180mtrs, not from the above calculation, this would occur some 340mtrs from end of runway. That is 35' is qchieved some 340mtrs before end of runway. Great!

To achieve that though we got to Vr sometime before that, ie some 7 to 9 seconds earlier we got to Vr. Simply, to underestimate the answer just use your Vr speed in M/sec X time to achieve 35', and U get an answer like 615mtrs to 790mtrs from this calculation.

Add the above together:
using AAC distance for liftoff to 35' (only for increasing useable clearway, but using minimum distance from data to 35')
minimum: 615+340= 955mtrs
maximum : 790+340= 1130mtrs.

So what?

This indicates where U should be when U get the Vr call at heavy weights, and what sort of height U should be when U get to end of TODA. Simplistically, having a coaming cuttoff of the endzone markers or centreline lights at 3000' to run is approximate limit case. A worked example for a given Jet is such that the limit occurs 2.3 seconds before coaming cuttoff of the 3000' markers.

Only problem is above URL, photos, personal experience and observations are inconsistent with above on many occasions.

Implications are only this, refusal point is not where it was supposed to be. Stopping may be problematic.

Go case is compromised, how much? Don't know, I am not paid to answer that, the regulators, and manufacturers are...

What do pilots do about it? evidently, nothing. Begs some questions regarding ethics. Don't expect your company to thank you for any observations on the above. Don't expect the regulators to do their job either.

Want to be really annoyed? Look in your flight manual, the one by the manufacturer, and have a look at the basis for the performance data. Consider that against the advise given in training, company policy and experience on how U conduct a takeoff. Read very carefully the wording, and where the wording changes slightly obfuscating the matter.

Personally, the risk is not the issue to me personally, but we sell the passengers a contract with every ticket, that we collectively will provide safe passage compliant with the law. Do we achieve that by silence of our observations.









;)

[john_tullamarine]

fdr,

Good post ... but, may I offer a few comments ? .... (and, as the hyperlink to airliners.com is to the homepage ... could you edit your post to give the full URL so that we can find the pix which you would have us see, please ? .. thanks in anticipation ...)

(a) you seem to presume that the takeoff is ALWAYS AEO TOD limited ie there will be a 15 percent pad. This usually will be the case for low weight, low altitude, low OAT, takeoffs ie where the OEI performance is comparatively good and the AEO factor results in the AEO case being the more limiting for the AFM schedules. More generally, though, in real world commercial operations, the OEI case will be the more limiting and the 15 percent pad associated with the AEO case becomes irrelevant. More importantly, the heavyweight takeoff aircraft on a limiting runway is going to be VERY close to the end of the TORA (runway) when it finally drags itself into the sky ..... seeing you mentioned Professor Emery's website ... do have a look at the picture of CX taking off (http://www.geocities.com/profemery/aviation.html) halfway down the page .....

Then again,

(i) sometimes you might be WAT or ASDR-limited and the porridge pot becomes a little stickier if a pilot is of a mind to do some sums ...

(ii) often an aircraft will have a nasty little first segment due to the time it takes for the wheels to tuck themselves away .. with little or next to no climb at all in the OEI case ..... and this might extend to a comparatively significant distance past the end of the runway .... the ops engineers take it into account but, often, pilots don't give it a second thought ...

(b) contributors may not realise that Professor Emery's hyperlink (as listed in the previous post) was back to a PPRuNe thread ? Emery is a regular visitor to PPRuNe and greatly do we value his occasional contributions in the discipline of airport engineering. The educational value of PPRuNe to industry participants is one of the benefits of this site which Danny et al continue to pursue ... to the benefit of all who play here in the sandpit.

(c) the regulatory standards built into the AFM should be viewed as reference data in that, if the takeoff is conducted in conditions and using techniques equivalent to the certification procedures used during the aircraft's development, then the distances achieved will be fairly close to the AFM data. Unfortunately we work in the REAL world which often is considerably at variance with the idealised certification world .. so, on occasion, the prudent operator/crew puts a few extra cushions on the seat for comfort.

That application of the Standards do, on the very great majority of occasions, result in aircraft achieving flight ... suggests that ICAO and the various State implementations of ICAO requirements have got it reasonably right ....

Most people would be aware that, as the technological capabilities improve, the Standards are revised to take advantage of such capabilities by mandating their direct or indirect incorporation in aircraft design and manufacture.

However, woe betide the operator or pilot who routinely ignores the differences between the certification and real worlds of operation ..... Generally, the pilot is not equipped with the tools to apply commercially acceptable margins for real world variance from the certification assumptions .. but the responsible operator, through the use of in-house or subcontracted operations engineers, does the work and presents the results for the use of crews and dispatch personnel.

(d) the TORR limitation (clearway restriction) is occasionally, but not always, relevant to the particular takeoff. Boeing's suggestion for one Type (and I guess that the other Boeing and Airbus Types are similar) is that, by the time you are within 2000ft of the runway head in adverse conditions such as windshear ... then it is time to rotate, regardless of other considerations, if you are to have a reasonable chance of getting airborne before the end of the runway ...

(e) so far as V1 is concerned ... the waters are easily muddied in the real world case as the certification conditions are not always easily replicated ... In the case of an ASD-limited takeoff with a failure very close to V1 and a subsequent rejection ... ? .. rather you than me ...

(f) I trust that your level of cynicism is not quite so high as your post suggests ? .. but, then again, perhaps the smiley at the bottom indicates that you are, like most of us, a pragmatist ... :D

[fdr]

DEAR JOHN,

oh how I hate to write.....

I am an enforced pragmatist. Stood up once for my beliefs on the side of angels, and found the whole system stinks. The good news is I saw the light onrushing, and took an enforced career opportunity...

The discussion indicates certainly the performance of all engine case AEO, as guess what, that is what the crew generally get to see.

My career change came about when asking how an aircraft would get to 35' at end of TODA one engine out, when it only got to 12' on all engines, recorded. The casual position of the system regarding such events is quite stunning, and therefore forces one to become a pragmatist.

The matter remains however, I see on various occasions:

CX B747F takeoff 07R VHHH (the nu one...) still with wheels on ground approaching end of runway, looked at Jepps, plane within 1000' of DER.

EVA 3 holer taking off on 21R BKK, dissapearing out over the night, I'm at cargo apron looking SE, and plane still on ground.

United B744 as per CX photo.

Camera shots of another B744 running past this point at night, more than 87% down TODA still firmly attached to ground. Sadly night photo, just blur of lights where they should not be. (didn't hear of any emergencies on any of these)

The great thing about most takeoffs are we have all engines running...which only means we are one turbine disk occlusion away from proving physics. ( bird strike/compressor stall is probably gonna be OK so long as N1 keeps a spinnin...

plant yourself 87% down side of big runways where limit departures happen. watch aircraft taxi past where they are supposed to be at 35' if they didn't blow a motor. (don't bother with 2 holers, they aren't any fun at all to watch, but much more comforting to fly)

:rolleyes:

[Alex Whittingham]

.. it is for this reason that the certification process doesn't consider the potential benefit of NWS.

Can you reference this JT? I have other sources suggesting that it is because the Vmc figures must be correct for both wet and dry cases and NWS would be ineffective in the wet. Alternative explanations are that on some older types the NWS, when used at speed, would break or that the pilot already has both hands occupied at this stage of flight, one on the stick, one on the throttles.

I can't find any authoritative reference.

[john_tullamarine]

fdr,

As to standing up for a point .. regardless of whether it be right or wrong ... be prepared to be shot down if the aim is to reduce payload beyond that permitted by regulation. That view may be cynical but it is realistic. An individual operator has to make a commercial/risk decision regarding whether, and to what extent, any additional conservatism might be appropriate.

There is no simple answer .. no matter how you look at it .. and the world, unfortunately, is not an idealised place ... and, no matter how much we may not like it .. the bean counters rule the world these days ...

AFM data is

(a) performance model predicted data, which is

(b) validated during certification testing,

and has not a lot to do with real world observations unless both (aircraft/environment) conditions and (pilot) techniques are very similar to those which went into the numbers in the first place. It would be inappropriately idealistic to presume that the manufacturer might contemplate building into these data any conservatism not required by the rulebook ....

If you saw (as we all do routinely) any particular takeoff which had AEO liftoff very close to the end of the runway (or low achieved screen), this could be due to any of a host of reasons .. some of which would include

(a) engine(s) down on predicted thrust, either due to the engine(s) or, more likely, crew operation

(b) line up distance squandered prior to spinup

(c) spinup not in accordance with AFM techniques

(d) dragging brakes

(e) substantial error in the loading sums versus the loading ... a common tale .. errors in empty weight, errors associated with the use of standard weights, loading scale errors in freight operations .. etc., etc....

(f) late and/or slow rotation resulting in longer flare distance and speeds higher than AFM-predicted ... generally the case.

(g) for 4-motor birds, the AEO performance is going to be less spectacular than for 3-motors .. which is less than for 2-motors .. as the OEI case for continued takeoff generally sets the level of the bar ...

The latter points constitute the most likely story. Keep in mind that flight test techniques may tend to be somewhat more aggressive than what one sees routinely on the line.

If you feel the need to work on the presumption that the routine AEO case should see 35ft with a 15 percent pad, then do ensure that this relates to the TODA and not the TORA ... which could be considerably less.

The fact that we don't see routine AEO overruns or terrain collisions suggests that the risk levels associated with the AEO problem you identify do not present an unacceptable situation .... ?


Alex,

You might have put me on the spot if the aim is to find a reference on which I can hang my particular words ... although I suspect that we are saying the same thing ? ... the worst case is going to be the aft cg wet runway .. so that is what has to be addressed ... ? in any case, at aft cg, the steering is not going to be particularly effective even on a dry runway if the yawing moment results in lateral skidding due to the reduced vertical load on the tyre. This discussion, of course, could go around in circles for a long time ..

Useful references -

(a) FAR 25.149(e) "VMCG, the minimum control speed on the ground, is the calibrated airspeed during the takeoff run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane using the rudder control alone (without the use of nosewheel steering) ....."

(b) AC 25-7A 23(3)

(iv) Control of the airplane should be accomplished by use of the rudder only.....

(vi) VMCG testing should be conducted at aft c.g. and with the nose wheel free to caster, to minimize
the stabilizing effect of the nose gear. If the nose wheel does not caster freely, the test may be conducted with
enough nose up elevator applied to lift the nose wheel off the runway.

(vii) For airplanes with certification bases prior to Amendment 25-42, VMCG values may be
demonstrated with nose wheel rudder pedal steering operative for dispatch on wet runways. The test should be
conducted on an actual wet runway. The test(s) should include engine failure at or near a minimum VEF associated
with minimum VR to demonstrate adequate controllability during rotation, liftoff, and the initial climbout. The
VMCG values obtained by this method are applicable for wet or dry runways only, not for icy runways.

.. or have I just fogged it up a bit more ?

[fdr]

John,

points agreed.

One however assumes that the data is valid from the start, and that raises issues.

when it can be indicated that the line up was correct, wind, temp weight etc were valid, and the data shows gross errors in outcome, the veracity of the performance data becomes an issue.

when the manufacturer indicates the basis of performance is on methodology physically not allowed on the aircraft in operation, then one should be allowed to be skeptical. Particularly when no allowance is made for actual method of takeoff being totally diferent to that from which the performance figures are derived.

The problem I see is that over the top of basic errors are the factors you indicate, that certainly don't help matters further.

As to the smoking evidence, there are cases that have been put down to pilot error where either takeoff performance was marginal engine out, or the crew rejected and overran which are consistent with this topic.

The truth is out there, at 87% TODA.... watch fat albert taxi past.
:)

[mutt]

when the manufacturer indicates the basis of performance is on methodology physically not allowed on the aircraft in operation, then one should be allowed to be skeptical. Particularly when no allowance is made for actual method of takeoff being totally diferent to that from which the performance figures are derived.


Would you mind expanding on this, what "methodologies" are used in certification which cant be used in line flying?

Thanks

Mutt.

[Alex Whittingham]

Thanks JT, I'm familiar with all those references.

My interest stems from an exam question which states, as the correct answer, that NSW is not taken into account because VMCG must be valid for both wet and dry conditions. I know it is not the whole truth, and you clearly agree, but I have never been able to find a reference to support my arguement!

[quid]

jt-

quote-(a) you seem to presume that the takeoff is ALWAYS AEO TOD limited ie there will be a 15 percent pad. This usually will be the case for low weight, low altitude, low OAT, takeoffs ie where the OEI performance is comparatively good and the AEO factor results in the AEO case being the more limiting for the AFM schedules. More generally, though, in real world commercial operations, the OEI case will be the more limiting and the 15 percent pad associated with the AEO case becomes irrelevant. More importantly, the heavyweight takeoff aircraft on a limiting runway is going to be VERY close to the end of the TORA (runway) when it finally drags itself into the sky ....

I've had the long held belief (and I don't remember where I learned it) that a 4 engined jet transport was NEVER balanced field limited, but rather always limited by the 115% AEO requirement, and the 2 engine a/c is limited by stop/go and virtually never by 115%. There may be exceptions to many rules, but in general is this belief correct?......or are there enough exceptions that I must change my memory?

...assuming, of course that the t/o is not limited by another factor such as brake energy, tire speed, etc........

[mutt]

I've had the long held belief (and I don't remember where I learned it) that a 4 engined jet transport was NEVER balanced field limited, but rather always limited by the 115% AEO requirement, and the 2 engine a/c is limited by stop/go and virtually never by 115%. There may be exceptions to many rules, but in general is this belief correct?......or are there enough exceptions that I must change my memory?

Boeing also believe in this theory.

Mutt. :)

[john_tullamarine]

fdr,

An aerodynamic performance model is only as good as the effort which went into its development and validation .. this latter normally being a combination of wind tunnel and flight test activities.

If the data has been developed on the cheap, as it were, then one normally sees an appropriate level of conservatism. This might be the case where a particular aircraft needs a performance chart to cover a modification in the field and the local PEs do the job .. depending on the needs of the customer, cost generally dictates that a conservative approach might be the way to go.

On the other hand, for a major project line, ie the larger manufacturers' various aircraft models, one would expect to see a LOT of money get thrown at the performance estimation and validation work, as this has a big bearing on the commercial success of the project. But don't expect an ounce of needless (ie not required by the rules) conservatism .. the manufacturer is out to get the best data for commercial purposes that is feasible from an engineering point of view .. keep in mind that it has to get through the certification invigilation process and that is often an exercise in heated discussion.

If one cannot presume that the AFM data is reasonably able to be re-validated by test (ignoring the usual minor test point skewing which attends being a little selective about which test points are rejected during the test and, subsequently, the data analysis programs), then this probably amounts to suggestions of conspiracy and fraudulent activity. While mistakes and poor judgement can be found in the history of aircraft certification, I think that wilful and deliberate misdeeds are rather uncommon.

The AFM provides the bottom line requirements to meet the airworthiness Design Standards. In some cases additional levels of conservatism are mandated by operational rules. There is nothing to prevent an operator making a commercial risk assessment to add further levels of conservatism (other than commercial pressures, of course).

But none of this suggests that the AFM data is going to be faithfully reproduced day-in, day-out in the real world .. it just doesn't work that way ... unless the conditions and techniques are near identical, then there is no reason to expect the outcomes to be the same .. is that not a reasonable logic ?

And if one is going down a path of the "actual method of takeoff being totally different to that from which the performance figures are derived" then that is a problem belonging to the crew on the day .. nothing to do with the OEM I would suggest ... and the real world problems which Mutt and I push in these threads is not the province of the OEM or the airworthiness rulemaker due to the variability of the beast ... it is up to the operational rulemaker and the operators to put a finger in the wind and determine where their levels of regulatory and commercial risk comfort might lie ...

As to your "smoking evidence", of course this is the case ... if the aircraft is heavy (ie a routine commercial operation), then small real world variance from the certification assumptions makes a very critical change in the game .. but it is not an OEM or airworthiness problem .. rather an operational regulatory and commercial risk assessment problem.

Or perhaps .. your mind is made up and discussion in not going to change things as you see them ?

Quite obviously you are referring to specific investigations, events or accidents .. like Mutt, I would be very interested in the details as there is always something to be learnt along the way.


Alex,

Probably more light might be thrown on the subject were some of our appropriate specialist flight test and certification colleagues wade into the discussion as they would have the more detailed knowledge and may be able to point to specific documents of which I have no knowledge.

However, much in the rulebook is amplified in the design and certification processes and practices .. in the same way as precedent plays a major part in law.

The usual philosophy is that one ought to design and build to suit sensible engineering and commercial constraints while making sure that one doesn't miss passing the rulebook tests .. or, at least, negotiating a concession against the odd rule which provides a stumbling block.

When it comes to the rules, the explanatory detail in the ACs is essential as is that in many of the other documents which are not in general circulation. Hence the often made suggestion that one ought not to read the rulebook and try to make determinations on this and that matter on that sole basis .. only leads to confusion.

All I can suggest is that the logical intent of defining Vmcg is to "put a line in the sand" and that it would make little sense to use other than a reasonably conservative set of boundary conditions in that determination ... hence CG, thrust, and the other factors called up in the rules. It would not be useful to try and schedule a great variety of data to cover variations in many of the parameters. Unless the scheduled Vmcg covers wet and dry, then you are straight away into the arena of more cost and more data for a not very general benefit .. on most occasions, Vmcg considerations are not limiting.

If one is going to look at an aft limit cg case, then the fact of the matter is that nosewheel loading, and hence steering capability, is fairly minimal. If the added problem of wet runway is to be addressed then the problem increases .. is there any other practical way of addressing it other than by requiring the removal of nosewheel steering from the cooking pot ?

Again, there is nothing to stop an operator from adopting additional levels of conservatism .. as you would be aware, Mutt and I have a jaundiced view about blindly using min weight/speed schedules in conditions of strong crosswind if there is spare runway available to trade our way out of a potential Vmcg embarrassment ...

When you find a specific reference to support your concern, do let the rest of us know ...


Quid,

You answer your question indirectly in your final comment.

There is a set of cases to be considered in determining the limiting takeoff weight on the day. Each has to be addressed and the one yielding the lowest weight (OK, mass for the purists) gets the nod.

If, as might well be the situation, a particular aircraft doesn't ever have a problem with one case for the envelope to be scheduled in the AFM, then it may well be ignored in the data presentation ... or the OEM may choose to adopt a simplification in data presentation for whatever reason even if that might involve a conservatism. These thoughts would be relevant in respect of Mutt's comment above ...

For rules of thumb it would be a case of reviewing a sufficient sample of AFM datasets to develop a rational thesis ... I have never done this for such a purpose so I ought not to comment on what you have read elsewhere in the past. Personally I can't see much benefit to be had in rules of thumb for this discipline as there is more than enough variation to be had to complicate the issue on any given point.

This is a little different from getting a feel for a particular Type and knowing intuitively which case is probably going to be limiting in a particular set of circumstances .... If you are talking about 742 as in the thread topic, I have a few AFMs for the model in the cupboard .. but, as Mutt is au fait with the series, his implied answer above saves me the effort of playing with the books to get a feel for the situation in that case ...

I don't think that one is BFL-limited as such .. rather BFL may offer some advantages in calculation effort and data presentation .. if, indeed, the particular AFM provides data to permit both balanced and unbalanced data runs. What are your philosophical thoughts on this point, Mutt ?

[fdr]

John,

I shifted away from the problem as the problem is intractible on the type.

Did a change of type to a BIG twin and found that the manufacturers takeoff data on the BIG twin is based in their own words on STATIC takeoff thrust. Would anyone building, operating, or responsible for PW4090/PW4098 engines or their application to an airframe please show me when that has ever been conducted?, including flight test please. Until that time I reserve my right to be somewhat sceptical of the system we pilots work within(around?)

Look in section 4 page 14 -16 of the AFM produced by the manufacturer. I do not have the resources to make up that...

regards, and safe flying.:D

[john_tullamarine]

Ah ... now we are getting onto rolling versus standing starts ... I have not much concern with rolling takeoffs at high weight due to the comparatively low acceleration prior to achieving takeoff or near takeoff thrust .. but do have reservations for low weight takeoffs from short, limiting runways ...... another thread altogether ....

I don't think that rolling takeoffs is the answer you are looking for to explain the observation. There is still nothing to stop the operator building in a suitable pad to account for any rolling start losses. One longtime international operator did just this sort of thing when they introduced reduced thrust takeoffs years .. decades ... ago ... I don't know what their present attitude is.

The other problem with real BRTs is that they might tear the runway off the ground and strain it through the engine doing a static runup prior to brakes release ..... :)

[olivasnooze]

Wow
All pretty good stuff. Back to the start someone mentioned about V2 increasing at low weights.
The B747sp did. This was because of body angle limits (geometry). The speed achieved at screen height, I think.

[fdr]

"STATIC"

not rolling, not standing, "STATIC"

definition is available from manufacturer.


In case I didn't mention it: "STATIC" rpt "STATIC"

The truth is 87% down TODA: either the planes are nicely airborne ( authorities indicate a failure to achieve this needs to have an explanation, ie weight/wind/temp variance... or the certificate of airworthiness is invalid) or they are taxying by. In the one case I am aware of where this was explored on specific takeoffs, 7 out of 7 werereported to be still on ground at that point, (87% TODA) without cause. Of interest, no authority would act.

recall the CX photo waas of an aircraft running on all engines, had not had a failure. How neat would that takeoff have been with a failure? If that had paying passengers onboard, would they have been receiving the care and due diligence of their contract with the carrier? Would the authorities have been compliant with their duties?

Who out there has been trained for or ever seen a STATIC takeoff other than JT8D engined types?

bye bye now.:)

[john_tullamarine]

.... methinks that you might possibly be a man on a mission or with an agenda ?

(a) you see an important distinction in the definition of "static" .... and it is obvious that you know something which I don't .... as I don't have a relevant AFM or similar, perhaps you can list the definition to which you refer for the benefit of the readership and we can take the discussion from there ?

(b) for interest, by what instrument(s) do which "authorities" require to see routinely achieved levels of takeoff performance with the threat of punitive action if this does not occur ? (noise monitoring runways and CAA style CofA renewal tests aside).

(c) I still fail to see the justification for your unwavering preoccupation with 0.87 TODA unless the particular takeoff is, in fact, AEO TODR limited .. in which case the aircraft should achieve something in that region, AEO and variations of the day and particular hull not considered. Clearly we cannot comment on a particular study relating to variance without details of the relevant report.

(d) re the CX pix (nice shot), one would need to know the circumstances on the day of the photo to make any rational observation.


It might just be that you are trying to infer too much AFM-derived precision in real world operations when that may not be justified.

[Mad (Flt) Scientist]

I'm also curious to know more, because FAR 25 is pretty clear on requiring the OEM to account for thrust variation with, for example speed.

25.101(c) states:
...performance must correspond to the propulsive thrust available under ... the particular flight conditions..."

So unthinking use of static thrust would appear to be a contravention of FAR25 - unless it is shown elsewhere in the cert that this is conservative, of course.

[boofhead]

Re the 747 particularly:

Look in the AFM to see the all engine and three engine takeoff distance and you will see that the all engine case is limiting. This is because the all engine net figures are reduced by 15 percent, while the three engine case is not. The three engine performance degradation is not as much as 15 percent, since it only applies to the continued takeoff beyond V1.

Many inexperienced pilots will be amazed at how close the end of the runway appears to be when the airplane lifts off, but this is immaterial; the time to check the runway remaining is when the airplane reaches V1. The stop case is limiting on all engines. Should the decision be made to reject at V1 then the three engine case benefits from the all engine 15 percent margin also, so in practice the crew have the advantage of reverse as well as the all engine margin and unless there is a brake problem or runway contamination, a stop should not present a problem. Hot brakes, sure. See the Boeing performance tapes.

If the PF waits until he hears the "VR" call before starting the rotation then a lot of runway will disappear before the airplane flies. Starting to reduce the weight on the nosewheel prior to VR will allow a lift off right on or just prior to V2 and make the picture look better. But there is enough performance available for the pilot to pull the airplane off the ground instantly if he feels it necessary. Crossing the end of the runway below 35 feet is not a problem, so long as there is a clearway or stopway.

Boeing recommends the use of symmetrical reverse, but personally I reckon that if a 777 can use reverse on one side a 747 should have no problem with three engines in reverse on takeoff abort or landing. At least initially, but it is my opinion only.

At heavy weights and an aft cg it is not a good idea to do a full power runup prior to takeoff, despite what some company manuals might tell you.

When calculating V1 for contaminated runways, using the V1 for the max weight for those conditions regardless of actual weight and applying the corrections to the higher V1 (then use the lower of the dry V1 or the max V1 with corrections) gives a safer solution. In fact whenever the weight is less than maximum for the runway and conditions, the true V1 is that for the max weight, which means that V1 can be increased to VR in most cases. But do not do this unless you are qualified to calculate these numbers. Knowing that you have a pad should take away some of those jitters and allow you to carry out a reject in a deliberate and positive manner rather than rush it, thinking that you are running out of runway.

Boeing (and the FAA) do not consider the failure of two engines on the same side during takeoff, and it is statistically so remote a possiblility that it can be ignored. But it is an interesting simulator exercise. The Vmca for two engines is around 186 knots (anyone who has a better figure?). This is above the V2 for all engines at max TOW. Although it is not likely to be a real world problem, it is a good idea to use (company permitting) F10 to reduce the drag and maybe allow maintenance of Vmca. If the speed drops below Vmca then thrust will have to be reduced and at more than light weights this will give a negative gradient of climb. The latest Boeing Training Manual shows that tail clearance is not reduced by much if F10 is used, contrary to what had been published before.

Training in performance nowadays is pretty poor. Then again, most training nowadays is the same, with the goal being to provide the pilots with the minimum amount of knowledge needed to do the job. A good pilot will realise this and make an attempt to learn something new every day.

[LOMCEVAK]

Boofhead,

For the B747-200, the 2-engines inoperative Vmca with Flap 1 set is 146 KIAS for PW JT9D-7J engines and 152 KIAS for RR RB211-D4 engines. These are the only figures that I have seen published and are related to the 2-engine inoperative go-around procedure.

Rgds

Lomcevak

[john_tullamarine]

Boofhead,

Could you perhaps revisit your first two paragraphs ? I am not all that sure that I follow or concur with your comments ?

[boofhead]

Maybe I have presented it badly, but I will try again.

Consider two airplanes on takeoff with an engine failure for one of them at V1. Up until the engine quits the performance of both is the same. At V1 the all engine airplane and the three engine one both reject takeoff. The only difference will be the reduced availability of reverse for the three engine airplane, and since reverse is not considered, to all intents and purposes the distance needed to stop will be the same. The ASDR will be the same, except that the all engine case is subject to a 15 percent penalty, ie the required runway (plus stopway) for the all engine case is always longer. The case is similar for the continued takeoff; even though the all engine airplane will perform better than the three engine one, it is rarely more than 15 percent better, since only the time from V1 to the end of the TOD is considered.

In my second para, I was referring to the way many new pilots are surprised at the closeness of the end of the runway, as it approaches at 180 knots or so just prior to liftoff. Many have commented to me that it would be diffcult to stop should a problem develop at that point. But the only time it is relevant to check the runway remaining would be at the V1 speed, since a reject is not guaranteed above that speed. The mindset should change to GO when past V1.

Where have I gone off the rails?

LOM, the Vref with Flap 30 is around 140, so I doubt you would have 146 or 152 with Flap 1. But a good point. Must try it in the sim.

[john_tullamarine]

Boofhead,

It is quite possible that I may have missed something here ...

(a) which design standard imposes the 15 percent pad on AEO ASD ?

(b) a significant difference between AEO and OEI reject is the maximum speed achieved during the acceleration and reject sequence. In the AEO case, the reject will see a higher maximum achieved speed and longer distance to stop .. one of the main hazards in rejecting from high speed for other than a failure on a limiting runway.

(c) while the current rules require the 2 second pad, which provides a bit of a buffer, I would question any suggestion that a reject from V1 on a limiting real world runway involves any sort of guarantee that you will actually get away with it .... especially on a pre-amendment 42 certification aircraft .....

[LOMCEVAK]

Boofhead,

As I said above, the Vmca-2 speeds with flap 1 are promulgated with respect to a 2-engined go around where you would be climbing at Vref + 60 with flap 1. The Boeing procedures do not address any other scenario with 2 engines inoperative and so I suppose that is why no other figures are published i.e. the 2-engined go-around procedure is the only case where you have to worry about Vmca.

Rgds

L

[boofhead]

LOM, my point was that Boeing does not address the possibility of a double engine failure on takeoff or soon after. Such a scenario is considered statistically impossible and is not covered in their manuals.
If the sim is any guide, an outboard engine failure requires full rudder or almost to maintain directional control, although a little bank assists when airborne. The Vmca is not published but is probably close to Vmcg. The loss of two engines on thesame side would exceed the rudder authority of the airplane and it would be likely that the outboard engine would have to be throttled back to maintain directional control. A figure of 186 knots was given to me as the Vmca for this situation and I have no reason to doubt it.
For a two engine go around the thrust is increased on the inboard to full thrust and the outboard only as far as directional control will allow. Acceleration to Flap 1 speed is achieved in the descent and by the time the Flap 1 speed is reached full thrust can be applied to both engines and a climb commenced. I hope the next time I get a chance to experiment in the sim I can resolve this and will report back.
Unless someone out there has already done it?

John..
a. None. My mistake, sorry.
b. Agreed. This does, though, show that the all engine case is still the limiting one so far as the performance charts are concerned.
c. Most Operators recommend that the V1 speed not be used as a decision speed but the speed at which the decision has already been made and the reject commenced. As line pilots we have to assume that the experts have done their job well and that if we follow procedures properly we will not get into trouble. If the runway is wet, the runout area is covered in rubber, there is no stopway or there are close-in obstacles, it would be prudent to reduce the V1, but since this will affect the GO case it should be considered carefully. The contaminated runway speeds can be a guide for this although I do not know the legality of using these speeds on a dry runway.

[john_tullamarine]

Boofhead,

(a) all's well ... just thought that I must have missed something along the way

(b) people need to be aware that the current rules address AEO reject ... and it would be reasonable to presume that this is applied to new TCs ...... however most of the aircraft out there were certificated to earlier standards which DID NOT ..

(c) my point precisely .. the real world boundary conditions are, in general, quite different ... and usually unconservative, with respect to the certification presumptions. The book figures are fine if the conditions and techniques are replicated ... in the real world situation this usually is not the case ... so we need to be a little circumspect about rejects from near V1 speeds from an ASD-limiting runway ....

[mutt]

Boofhead,

If the runway is wet, the runout area is covered in rubber, there is no stopway or there are close-in obstacles, it would be prudent to reduce the V1, but since this will affect the GO case it should be considered carefully. The contaminated runway speeds can be a guide for this although I do not know the legality of using these speeds on a dry runway.

This gives me the impression that you dont reduce the V1 on Wet Runways, where did you get this policy from?


Mutt.

[boofhead]

Company policy and the Boeing manuals spell out what to do in the case of a wet/contaminated runway. My point was that you might be able to reduce the V1 even if the runway is dry, if there is a good reason for doing so. Using the QRH tables will give you a guide, rather than making a random reduction.

[mutt]

What you are looking for is V1min which is easily available from any of the Boeing takeoff software. You just need to change your companies policy.


Mutt