VMCG Testing

[idg]

Gents,
I work for an outfit that has operated the A330-300 for many years. Just recently we have changed our performance planning system from AI Octopus to an LPC (computer). The result is that we will be using very much lower speeds almost all the time. The change is in the order of 10-15 knots (from typically 137, 137, 140 to something like 126, 128, 135).

I know from watching other Prune threads that VMCG 'grows' with a crosswind and I am concerned that we are now operating all the time much closer to our limits than before. Indeed our Boeing 747 manuals talk about this very problem.

Vmcg for the -300 is 122 knots but our sim is based on a -200 and has a Vmcg of 112 knots. I think therefore that we are 'flattering' ourselves into believing this is a satisfactory situation! A contributing factor here is that we are required to do the V1 cut in 150M viz for LVP certification and many crews are right on the runway edge when rotating. Doing it in CAVOK makes it much more controlled.

I have access to the sim and have tried to do engine failures at these limiting speeds (and even 10 knots lower) but the results are inconclusive.

So how do you go about establishing Vmcg during flight test and am I barking up the wrong tree with a problem that is more evident on quads than twins? Is there a time factor built into the certification that I'm missing? Other threads mentioned 5 seconds but this seems too long to me! How much would the CoG (being at the mid point) shift the actual Vmcg vs fully aft?

Humble apologies for the long post and looking forward to being enlightened!

[Tester07]

Some very interesting questions!

I cannot enlighten you regarding operating procedures, but I can tell you about testing.

Although of course you are correct that the ability to control the aircraft under such circumstances varies with crosswind, as far as certification is concerned Vmcg does not.

Vmcg testing is always conducted under conditions of less than 5 knots. There is no time delay built into the test, and so reaction times will typically be of the order of half a second. Testing is always conducted at aft CG (worst case).

Therefore the Vmcg quoted will theoretically be for the 'worst case', but does not take into account any crosswind, I guess the theory being that in the unlikely event that you have an engine failure at this speed it is too difficult to account for the fact that you may also have a crosswind.

There are certainly some within the test establishments who have felt uncomfortable with this, but as far as I know these are still the rules for certification.

Hope this enlightens at least a little bit!

[idg]

Tester
Thanks a lot...very much as I thought.

Follow on question if I may then...how much difference would the aft CoG make 10%, 20% etc and how much would the Vmcg vary with crosswind....wind *0.7 = increment?

Perhaps I'm asking how long a piece of string is!! :ooh:

[sycamore]

idg,personally speaking, I would be inclined to fire off an email to the FltTestDept at Airbus Toulouse with all relevant info and ask them for a definitive answer; not that you won`t get help on here, but it does appear there is a fundamental flaw somewhere, and it may well be `flattering to deceive`, with all those risks. It may be you have been operating using flawed info up to now, but much better to get the correct info from the `mouche du cheval` and consider it over a coffee in an armchair, than `on a dark wet night with 30kts across !`:uhoh:

Ed. Alternatively/additional to , you should contact your Certification Flt Test dept as well.This may seem as if you are `going behind backs`, depending where you are on the Company ladder ; personally ,a piece of authoritative paper is better than` your hat, my office, no coffee`, remembering that all rugs are attached to a big spring if you have to see the CP.I would hope you get the right answers..

[BelArgUSA]

Gentlemen -
xxx
Regarding the Vmcg and its relation to minimum V1... I constantly run across conflicting numbers on V speed tables for a given type airplane, depending on the certification of that airplane... that is - US/FAA FAR 25 or UK/CAA...
xxx
I fly and instruct in 747-200s... occasionally the 747-300s
Frequently instructing performance subjects in classrooms, I often lecture that the UK/CAA assumes a crosswind (was it 7 kts or 10 kts crosswind...?) for a more conservative (higher) VmcG (I love that...) since the US/FAA certification is not using any form of crosswind factor...
xxx
Some 2 years ago, I was performing certification flights for a new AOC with a 747-267B equipped with RB211-524D4 engines (53,000 lbs max trust)...
I normally fly 747-287B airplanes, equipped with JT9D-7Q engines (same thrust, 53,000 lbs)...
And a few years ago, I flew a 747-341B we had leased (CF6-50E engines which are nearly same, 52,600 lbs thrust)...
xxx
I compared the data of the 3 types, and although the power of the 3 types of engines is very close, the 747-267B (ex Cathay, ex Virgin) V1 and VmcG tables showed differences at times up to 10 knots (VmcG)... then I further investigated their AFM and AOMs, and sure enough, all differences came between different certification standards. The manuals, all printed by Boeing, mention either "FAA" or "CAA" depending which of the airplanes.
xxx
A real conflict here... I mentioned that fact to the head of my DGAC here, but his aviation background was Mirage fighters... so he said "yes, I will advise you" - I never heard anything again...
xxx
Personally, when taking off with nice weather, I make my mental VmcG and minimum V1 with FAA figures and standards... and nasty weather or crosswind, I do it CAA style... Just it is not easy to convey all that in a classroom...
xxx
:)
Happy contrails
idg - I think your 747-200 simulator (112 VmcG) is obviously JT9D-7A power, while your 747-300s (122 Vmcg) is based on more powerful engines. With equal power, there is really no difference between 200s and 300s...

[Mad (Flt) Scientist]

@BelArg

I believe the OP is referring to an A330-300 sim, not a B747-300

UK CAA BCARs was a 7kt adverse Xwind requirement.

And 1:1 ratio between Xwind and Vmcg increase isn't a bad rule of thumb.

But, to be honest, the Vmcg test is such a test pilot "party trick" - no "average pilot reaction time" involved there - that I doubt ANY line pilot will keep his aircraft on the tarmac if he gets the critical case. Remember, the TP/crew are BRIEFED for the test - yet when we do calculations involving pilot reaction times for "normal failures" we use anywhere between 1 and 3 seconds. Vmcg is SO sensitive to pilot reaction that realistic reaction times make the values obtained for cert pretty much a theoretical number, nothing more or less.

[idg]

Gents (there's an assumption!!)
Many thanks for your inputs one and all. This is as I supected and I will now go into the sim and try again with full aft CoG etc. In addition I will ask the learned folk @ AI.

I was refferring to an Airbus, but the comments on the 747s are pertinent as we also operate that type and the FCTM says that the x-wind to Vmcg increment is more like 2:3knts. ie 3 knots increase for every 2 knots x-wind. I know BA used to use this factor (or something similar) on the 747 to increase V1 if necessary but I don't believe that is done now on the -400.

I will report back with more info when I can! :ok:

[john_tullamarine]

For crosswind .. DC9 around 0.5 kt/kt (provided by the OEM for some specific testing we were doing) up to something in excess of 1 kt/kt for 747... and I echo MFS' thoughts .. things happen quickly and, for the crosswind case it is all too late at the time for the pilot to start thinking about the niceties .. One notes that, in general, the typical takeoff CG is a stabilising influence.

My concern has been the situation where the operator trains for high V1 (typically a routinely scheduled overspeed takeoff scenario) .. but permits low weight ferries at min V1. In respect of one operator for whom I was doing some sim instructor training, I posed this problem as an "at the end extra" for a couple of the management pilots .. with some wide-eyed results .... not sure that it produced any changes in training or policy .. but one can only try.

[LOMCEVAK]

One other consideration is that Vmcg is determined using aerodynamic control only. Usually, you will not lose nose wheel steering following an engine failure, and directional control will be easier with NWS than just using rudder. Therefore, a straightforward engine cut in the simulator at Vmcg may not seem to be too much of a problem. Now try it with the NWS inoperative...

[No_7DAD]

Vmcg is discussed in the FAA AC 25-7A

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/4144773715D01E9A86256B9F0072F02A?OpenDocument

One observation about laptop performance systems is that they sometimes utilize "look-up" tables vice the perfromance graphs found in AFMs. Not sure that is the case here. Also, the rationale for the performance numbers is sometimes based on additional considerations beside the actual test data. For instance, a control system limitation, hydraulics, etc.

The other factor for consideration is performance requirement in place at the time of certification. This information can be determined from the Certification Basis in place at the time of certification. AC 25-7A is current information. The Type Certificate Data Sheet will document the certification basis for any aircraft. http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/MainFrame?OpenFrameSet

I agree about seeking help from Airbus. And, simulators do not necessarily model runway dynamics as well as one might like. If you review the FAR Part 60, you will see that the simulators are centered on training (procedural) vice an engineering simulator at the manufacturer.
Hope this helps.

[Trumpet_trousers]

PM for you....

TT

[Genghis the Engineer]

One other consideration is that Vmcg is determined using aerodynamic control only. Usually, you will not lose nose wheel steering following an engine failure, and directional control will be easier with NWS than just using rudder. Therefore, a straightforward engine cut in the simulator at Vmcg may not seem to be too much of a problem. Now try it with the NWS inoperative...

Out of interest LOMCEVAK, do you have any idea why the part 25 standards (and presumably 00-970) prohibit use of nosewheel steering in establishing Vmcg? I'd guess it's inherent conservatism, but I don't think I've ever seen any justification for it?

G

[Two_Squirrels]

I believe it is to take account of wet and icy runways where nosewheel steeering may not be as effective, or indeed for cases where the NWS may be inoperative. I think for older aircraft NWS may be used, but it then becomes a GO/NO GO item.

[BelArgUSA]

Gentlemen -
xxx
Many of you mention the use of nose wheel steering to assist directional control at speeds close to VmcG... As pilot, I have a very poor opinion about the effectiveness of nose wheel steering, except... at taxi speeds...
xxx
One day, we were nº 2 for takeoff behind a Saudia 747-168B in Jeddah. That plane lined-up on 34L in front of us, and, when cleared for takeoff, they applied power. The aircraft did a violent "snap" ground loop to the right, nearly some 60º (?) to its right, and we did observe the nose wheel steering being operated in futile attempts to control their violent ground loop. Obviously, their nº 4 engine failed to spool-up, finally, taxied back to the ramp.
xxx
In my classroom lectures and briefings, I caution all trainees that nose wheel steering is as effective as a piece of wet Kleenex tissue with airplanes. My personal handling of the steering on takeoff with the 747 (as I did same with 707, 727 and DC8s) is strictly aerodynamic controls as soon as the aircraft moves at some 20 or 30 Kts, hands off tiller, and on the control wheel. Aerodynamic steering (rudder surface) is quite satifactory as early as some 50 or 60 Kts is attained.
xxx
I like to remind that PanAm (my airline alma mater) had disconnected the nose wheel steering linkage from the rudder pedals on their 747s, with the philosophy that since the 707 had no rudder nose wheel steering, it would not be required on the 747 as well...
xxx
I really dislike to see the pilots "pushing" the wheel forward during takeoff rolls as a poor pilot technique, bearing in mind that elevators down increase drag, in any way elevators down have no effect at low speed, and at high speed, put strain on the nose wheel strut. I personally recommend pilots to maintain the wheel in a "neutral" level position, to achieve less drag.
xxx
For those of you who are knowledgeable about the little Learjet, I used to be a flight instructor for type rating in these aircraft, to supplement my meager F/O salary in airline service. The Lear 24 had a VmcG of 93 Kts, and our normal "training" V speeds V1-VR-V2 were around 120-132-132, yet when we trained for engine failures (yes, we did that for real, IN the aircraft, not a simulator), it took FULL RUDDER deflection when one engine was retarded to idle thrust.
xxx
Nowadays, in 747 simulators, I often take advantage of available time to practice engine "failures" myself, near V1 speed. Generally it is no problem for control at heavy weights and high speeds, but it is not the case if practiced at light weights, and low V1 speeds, selecting 20 kts crosswind, and a full aft CG and V1 is equal to VmcG, using max EPR (or N1) - Instructors, if you wish to "bust" a snotty captain in a simulator, just try that recipe...
xxx
:)
Happy contrails

[Mad (Flt) Scientist]

Out of interest LOMCEVAK, do you have any idea why the part 25 standards (and presumably 00-970) prohibit use of nosewheel steering in establishing Vmcg? I'd guess it's inherent conservatism, but I don't think I've ever seen any justification for it?


NWS isn't taken credit for because the nosewheel may not be on the ground!

Consider a case where V1=Vr and V1 is at the limit for Vmcg.

If my Vmcg is predictaed on NWS use, what happens if I have an engine fail at V1+1kt? I've already started to rotate, but I'm still "on the runway" for another couple of seconds at least. But I don't have the use of NWS, because the nosewheel isn't in contact. So I'm actually below the "real" Vmcg for the configuration I'm in, even though I'm above the "cert" Vmcg.

Safest to assume the nosewheel won't be there to assist me.

(Of course, one could also ask whether a Vmcg determined at near-zero AoA is valid for the case where AoA is close to the lift-off attitude. Perhaps best not to pursue that line of thought too far ....)

[Genghis the Engineer]

Ah, thank you MFS - it all makes so much sense when you put it that way!

G

[Tester07]

I am more inclined to believe the icy/wet runway theory for discounting nosewheel steering.

I would imagine that any transport aircraft pilot will continue with the take-off once 'rotate' has been called and rotation has commenced.

[Mad (Flt) Scientist]

Of course you'll continue after V1/rotate.

Which means you'll have max thrust on the working engines - precisely the Vmcg scenario. Vmcg is irrelevant once the decision to abort is taken - which is why it sets the lower limit for a safe speed for a continued takeoff OEI.

The wet & slippery runway argument is basically nonsense (and I say that even though I've seen it used in our cert reports too) - unless you somehow magically eliminate the sideforce generated by the mainwheels, you will NOT get the same behaviour "dry/NWS off" as you will "wet"

[galaxy flyer]

I'm putting this topic in for a Pprune Lifetime Achievement Award" A month hardly goes by without a Vmcg wind-up. It has to be the least understood perf topic out there, and I think, the reason-performance is SOOOO shabbily trained everywhere. It is the Friday afternoon, post-lunch, subject on the last day. Manuals (ours included, MfS) have no detail and no one is interested in them anyway.

NOT to belittle what is an important and should interesting topic.

MfS, I'm flogging our products in a very nice locale, so back to the pool and the scenery thereby.

[Mad (Flt) Scientist]

@g_F :envy

With further regard to the "wet runway" argument for NWS off...

This is the wording of FAR 25.149(e) as introduced in amdt 25-42, in 1978:

(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 recover control of the airplane with the use of primary aerodynamic controls alone (without the use of nose-wheel steering) to enable the takeoff to be safely continued using normal piloting skill and rudder control forces not exceeding 150 pounds.

Yet in 1978 there was no consideration for wet runways in Part 25 - wet runway performance was only specified in Part 25 at Amdt 25-92, in 1998. If one were concerned enough about wet runway effects on Vmcg, surely you'd ALSO impose wet requirements for braking, for example. The reality is that there's no logical link between the NWS requirement and the state of the runway.

[No_7DAD]

From the preamble to Amdt. 25-42 to Part 25 reads in part:

Several commentators recommended that the proposal be revised to allow the use of nose wheel steering in the determination of VMCG under Sec. 25.149(e), if control is through the rudder pedals and the demonstration is made on a wet runway. The FAA does not agree. The effectiveness of nose wheel steering depends to a large degree on runway friction characteristics and the load on the nose wheel. Certification tests on a wet runway would not cover the more extreme slippery runway conditions or all possible variations in takeoff conditions and techniques likely to occur in service. The FAA therefore believes that VMCG should be determined without the use of nose wheel steering, as stated in proposed Sec. 25.149(e).

FYI -- Explanations to changes to FARs are normally found in the preambles. The preambles can be found at the same link posted above.

[tribo]

New serious incident report august 2007 (http://www.aibn.no/items/2045/144/6466135812/G-CRPH.pdf)

During the application of take off power, there was an asymmetric build up of engine thrust causing the left engine to lag the right engine. This caused a yawing moment that resulted in a loss of directional control. The aircraft yawed approximately 40° and departed the partially snow covered runway in spite of the crew selecting engine idle, applying nose wheel steering and braking. The aircraft continued to move forward at a slow speed off the paved area and onto an area of snow-covered soft ground. The nose wheel created a large furrow as the aircraft came to a stop in snow and soil at an angle of approximately 40° to the runway centre line. The tail and the nose of the aircraft were 12 m and 35 m from the runway edge respectively. The distance from the runway centre line to the edge was 22.5 m.
Damage to the aircraft was limited to a punctured left nose wheel tyre, a separated and deformed left nose wheel hubcap and a broken nose leg taxi light.

[moch330]

Hi friends,
I am new here, and read this interesting conversation on Vmcg and V1 started by IDG.
1.If you look at the factors affecting Vmcg using Vmcg tables, the values are put against two variables that is Temp and Press Altitude. These 2 variables only change the thrust and Vmcg is directly proportional to Thurst. More thrust more the Vmcg and vice a versa. Xwind has no affect on Vmcg.
2. Vmcg testing: xwind of 7kts from the worse direction is considered. Pilot should be able to control using aerodynamic controls only. Initial force applied could be 180lbs, but later the force on leg should not be more than 150lbs. The aircraft should not drift more than 30 ft from centre line + certain restriction on change of heading that I dont remember offhand. A lot more variables are used to determine like bleeds off takeoff, Minimum weight and Max aft cg. So enough built-in safety.

3. V1mcg. We all know that V1 cannot be lower than Vmcg. So the lowest V1 is always higher to cater for the max x-wind component allowed for the aircraft.

IDG has noted that using LPC the speeds have gone down quite a lot. The operation/performance engineering department of the airlines can choose between three V1. Minimum V1, Maximum V1 and Optimum V1. May be IDGs airline has chosen a V1 that is lower than Optimum V1. Minimum V1 gives better safety margin and Maximum gives better takeoff performance. In my airliner this kind of revision has taken place many a times.

Thanks for bearing such a long one.

Moch
A330-200

[Airborne Viking]

And why does an aft CG have any influence on the calculation of Vmcg? Remember according to rules, no nosewheel steering!
In real life NWS would make a difference of course. But as soon as you try to counteract, you will start skidding, and forget about traction and directional control. Even more valid on contaminated runways of course!

[jb5000]

Aft C of G = closer to the rudder, so with a constant aerodynamic force created, the turning moment to keep the aircraft straight will be less.

You therefore need a higher airspeed to increase the turning moment of the rudder to counteract the yaw from the live engine, so Vmcg increases.

[Airborne Viking]

Wouldn't that apply to the turning moment of the remaining engine(s) as well?
Less abrupt yaw, - better rudder control, if you (read me!) are a little slow applying rudder?
Basically, I would think, the yaw moment (force X arm), with assymetric thrust is not dependent upon CG, and the yaw moment of the rudder would only be dependent upon deflection and speed? Aft CG would only result in a slower yaw (tail sideways) acceleration?
Wow, this was complicated, and it's getting worse!

[mutt]

Its always nice to see interesting threads start again... :)

Xwind has no affect on Vmcg.Yes it does.

Vmcg testing: xwind of 7kts from the worse direction is consideredPlease quote the "present day" regulation showing this is the case.

So the lowest V1 is always higher to cater for the max x-wind component allowed for the aircraft.Not correct.

between three V1You forgot about balanced V1.

Good Night.....will expand on these points when sober... :)

Mutt

[Pugilistic Animus]

Here it is right from the Administrator [page 145]


http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgAdvisoryCircular.nsf/0/C2614E27B49BF38686256BA300696689?OpenDocument

[Mad (Flt) Scientist]

For those as easily confused as I was, I believe the post above means you to look at p145 (actually 146 I think :E) as numbered on the pages. Which is p118 per the pdf file numbering.

If you look at the 145th page of the pdf file, you'll get a couple of graphs relating stall speed and CL to various factors, which certainly threw me until I realised the pdf pages don't line up with the document pages....

[Mad (Flt) Scientist]

@Airborne Viking

Wouldn't that apply to the turning moment of the remaining engine(s) as well?
Less abrupt yaw, - better rudder control, if you (read me!) are a little slow applying rudder?
Basically, I would think, the yaw moment (force X arm), with assymetric thrust is not dependent upon CG, and the yaw moment of the rudder would only be dependent upon deflection and speed? Aft CG would only result in a slower yaw (tail sideways) acceleration?
Wow, this was complicated, and it's getting worse!

Nope.

The moment due to the engine is basically net thrust of the asymmetric live engine * the distance from centreline - which is essentially independent of forward or aft cg.

The rudder moment is taken about the cg, so must be affected by fore/aft cg movement. So the rudder is less effective as the cg goes aft.

There are secondary effects (the aircraft will be less directionally stiff at aft cg, and likely have more weight on mainwheels, both of which will have a small effect on the motion) but the big effect is aft cg = less effective rudder control, so harder to fight the engine yawing moment.

[Mad (Flt) Scientist]

@ Mutt

Xwind has no affect on Vmcg.

If I were being incredibly pedantic I'd agree with the statement. Vmcg is defined, for certification, for a specific (zero) xwind. So, technically, the declared Vmcg doesn't change with Xwind.

But, the real speed at which the equivalent conditions of Vmcg are met certainly does vary with Xwind, which is what you are saying, and I'll heartily agree with that; only a bloody idiot would go do Vmcg testing in any kind of significant Xwind.

[john_tullamarine]

.. which brings us to the main concerns ..

(a) Vmcg is one of those certification beasties which have their purpose in life but shouldn't be nipping at the heels of line pilots ... unfortunately, Vmcg is one which does do just that in some circumstances... which is why Mutt, MFS, I and others like to see it come up periodically in the sandpit .. although more frequently in Tech Log than FT.

(b) most of the time Vmcg (and crosswind effects on the "real world" Vmcg) is irrelevant to line operations as it is way, way below the V1 for the day. This brings into play that dreadful animal called line pilot complacency. Added to training bias, this can set up such folk for embarrassment .. easier for the TP who is more prepared to think on the run and abandon a takeoff post-V1 when things are obviously going pear shaped.

(c) if the V1 of the day is not too far above Vmcg, then crosswind needs to be in the line pilot's mind.

(d) for those occasions where one has little option (short runway etc) then it may be reasonable to accept the crosswind risk and go (although perhaps one could consider delaying until the wind abates ?) .. on the real world basis that the risk of a failure at/near a critical stage of the takeoff is pretty low .. and the chances of all the holes lining up (CG etc) are pretty low as well

(e) however, the concern we have relates to the case where options ARE available but the system sets the line pilot up for a fall. Consider the case of a ferry flight at very low TOW. If one is fat, dumb and happy, it is too easy to use a very low speed schedule appropriate to the weight. If the crosswind is a problem .. then it may be quite easy to ramp up the speed schedule to make the directional control problem go away. In the absence of specific Type data, a starting point is to aim for a pad in excess of 0.5 kt/kt for (rear-mounted) twins increasing to 1 kt/kt for quads (and, perhaps, wing-mounted twins ?)...

One would feel an awful goose .. sitting in the mud off to one side ... with the bird all bent and mangled ... knowing that one could have elected to use a V1 many tens of knots higher than the min speed schedule V1 one did, in fact, use ... with spectacularly unsuccessful results ...

Is this a problem we should lose a lot of sleep over ? .. probably not .. but I did some twin training for one largish operator which routinely used very high overspeed takeoff speed schedules (for appropriate reasons) but had a revenue sequence which included a short hop ferry to position the aircraft. So here we had all these fine folk doing all their training at humungous V1s .. but quite frequently doing this short hop ferry with a V1 down in the weeds near Vmcg .. is that not an inadvertant set up ? (As far as I knew, there was no specific training directed to this end of the operational spectrum)

For those command upgrades I put through I wasn't too worried .. they could handle sim exercises at min weight, min speed, aft CG etc .. and track on the centreline using the other end localiser .. but, judging from a few routine renewal checks I got roped into doing .. some of the other folk had the odd problem or two with low speed cuts ... not too hard to get an experienced pilot up to speed with the problem in several practice cuts .. but one really doesn't want a chap (or lady, for that matter) to find out .. on the line .. at V1 .. that his (or her) training missed that small point.

[mutt]

I'm disappointed that Moch330 didnt reply :(

MFS, thanks for not putting on your pedantic hat :):)

In the absence of specific Type data, a starting point is to aim for a pad in excess of 0.5 kt/kt for (rear-mounted) twins increasing to 1 kt/kt for quads (and, perhaps, wing-mounted twins ?)The problem with rules of thumb, is trying to justify them later. I remember when we had this discussion before, I got a PM from someone in the FAA that stated they didn’t consider V1=VMCG with a crosswind to be a problem as you had the nose steering to counter the effect. It’s probably a valid argument on a dry runway, but i would hazard a guess that when the runway is wet/slippery and you are near Vmu, then the nose steering isn’t effective. Unfortunately this message doesn’t appear to be getting distributed to the general population.

The comment from BelArgUSA was extremely fitting.....
Nowadays, in 747 simulators, I often take advantage of available time to practice engine "failures" myself, near V1 speed. Generally it is no problem for control at heavy weights and high speeds, but it is not the case if practiced at light weights, and low V1 speeds, selecting 20 kts crosswind, and a full aft CG and V1 is equal to VmcG, using max EPR (or N1) - Instructors, if you wish to "bust" a snotty captain in a simulator, just try that recipe...

Mutt

[john_tullamarine]

The problem with rules of thumb, is trying to justify them later

True .. but, for the record .. 0.5 kt/kt comes from Douglas for the DC9-33F (I probably still have the fax on file somewhere) and I use that as the bottom figure (the Diesel 9 being held to track on the runway like it's on rails .. which it certainly DOESN'T near Vmcg .. and, probably, I still have the videos to back up that statement .. DC9 exiting the field of view left or right as the case may have been .. with my good mate CJF busily treading the light fantastic on the pedals ..) and the B747 is a bit above 1.0 kt/kt as I recall from some OEM paperwork I read years ago ... hence my rule of thumb range. Is it a guarantee ? .. of course not .. there aren't any in this life.

My concern is that of risk .. why put the aircraft in that sort of difficult speed range .. if you don't need to ? .. and then, given the flexibility to do so .. I'd include a few extras knots for mum and the kids.

they didn’t consider V1=VMCG with a crosswind to be a problem

.. and neither it is .. so long as you don't have an engine failure with the wind from the wrong side. AFAIK, Oz is the only country to have looked at the problem in detail ... I wouldn't give a fig for NWS based on some of the videos I have taken from the upwind threshhold during testing ... did I mention how it's very illuminating not to see an aircraft .. after it's departed the video viewfinder during critical cuts ? .. centreline tracking during a failure is one of those folkloric myths the pilot fraternity likes to perpetuate ...

I often take advantage of available time to practice engine "failures" myself

.. only the one caveat .. depends on the fidelity of the particular simulator in that area of the operational envelope. I recall a 732 box which was dreadfully and unrealistically boring before, but illuminating and electrifying following .. a software upgrade tweak in compliance with the FAA's stance on rudder modelling ...

[Matthew Parsons]

Mad Flight Scientist wrote: The moment due to the engine is basically net thrust of the asymmetric live engine * the distance from centreline - which is essentially independent of forward or aft cg.

The rudder moment is taken about the cg, so must be affected by fore/aft cg movement. So the rudder is less effective as the cg goes aft.

MFS, I have no doubt you're correct about the standard rules of calculating moments within the industry, however moments that are calculated about different datums cannot simply be added together. To use them, you have to convert them to the same data, which would require knowing c of g for the assymetric thrust moment.

The other thing to consider is that the moment itself is not that interesting without knowing the moment of inertia. Of course, moment of inertia is dependant on mass distribution.

I guess the only caveat here is actual experience with this condition. I.e. Does the c of g actually affect Vmcg? Certification people seem to think so. What is the experience of the ppruners who have flown these tests?

Matthew.

[Mad (Flt) Scientist]

Actually, those are moments about the same position, to a first approximation.

If you assume:
That the thrust vector from the live engine is along the centreline; and
The aircraft cg lies on the centreline

Then the moment due to the engine relative to any point on the centreline is the same - it's the net thrust of the engine times the lateral offset of the thrustline from centreline.

Therefore whether the c.g. moves forward or aft does not affect the thrust moment - only lateral motion of the cg would make a difference.


Similarly, since the sideforce generated by the rudder is considered to act purely sideways, the yawing moment due to the rudder does not vary with lateral motion of the cg - all that matters is the distance from the point of action of the force to the cg, meausred at right angles to the force.


Now, I'll grant that in practice the thrust line is often slightly canted in or out, and that the cg is not perfectly on the centreline, and the real direction of the rudder force may even be slightly canted (though by definition that is accounted for in another force component). But these effects are largely secondary to the main effects.


With regard to moments of inertia: if looking at a simple force/moment balance, then inertia terms don't actually matter - rudder moment = engine moment is a calculation that can be done independent of inertia terms. Where inertias come into consideration are dynamic events (and Vmcg can certainly be dynamic in real life). Although Vmcg is often tested at light weights, the actual reg states "most unfavourable weight"*, and AC25-7A suggests "VMCG testing should be conducted at the heaviest weight where VMCG may impact the AFM V1 schedule." My impression has always been that weight/inertia is at best a second order effect on VMCG - while the heavier aircraft is slower to diverge, it's also slower to return to centreline and the two effects are closely cancelling.


* Incidentally, the current FAR 25.149(c) states that VMCL uses " (4) The most favorable weight, or, at the option of the applicant, as a function of weight;" which is certainly an interesting typo!

[chornedsnorkack]

The rudder moment is taken about the cg, so must be affected by fore/aft cg movement. So the rudder is less effective as the cg goes aft.


Where is the pivot point for an aircraft on ground? Is it at CoG, or is it at main wheels (which are on ground and therefore not free to slide sideways)?

If main wheels, then the rudder moment arm should be unaffected by CoG.

Obviously, the "loss" of an outboard engine causes asymmetric weight as well - those high-bypass engines are heavy, so CoG would move sidewards away from the lost engine.