VMCG Testing
 

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!

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!

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:

Vmcg
 

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..

Conflicting data for Vmcg
 

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...

@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.

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:

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.

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...

Vmcg testing
 

Vmcg is discussed in the FAA AC 25-7A

http://www.airweb.faa.gov/Regulatory...A?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_Gu...e?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.

Tester 07
 

PM for you....

TT

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

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.

Practical facts about VmcG
 

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

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 ....)

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

G

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.

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"

Vmcg discussion
 

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.

@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:

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.