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GAMEOVER
8th Oct 2010, 22:56
Hi.:rolleyes:

I know that

V1 must be equal or greater than Vmcg, and can not be greater than Vr or Vmbe
Vr must be equal or greater than Vmca
V2 must be equal or greater than Vmca, and VsBut, I donīt know, Why?

I would be gratefull if someone could explain it, easily please.

Thanks a lot.

Pugachev Cobra
9th Oct 2010, 00:49
"V1 must be equal or greater than Vmcg, and can not be greater than Vr or Vmbe"

When the pilot recognizes and initiates the action to abort the takeoff, he must be able to control the aircraft by aerodynamic means (the rudder), for certification purposes. Vmcg is the minimum airspeed which you can do that. If V1 was lower than Vmcg, the pilot wouldn't be able to control it on the ground by aerodynamic means.

Example: Engine fails, assymetric thrust veers off the aircraft, pilots reacts and abort the take off while maintaing the aircraft on the runway using the rudder.

It cannot be greater than Vr simply because, it would be a lot harder to abort the takeoff after you have initiated rotating the aircraft. And Vmbe, being maximum brake energy speed, means the maximum airspeed which you can use maximum braking without putting it on fire, in simple terms.

So, if V1 was greater than Vmbe, the pilot could choose to abort the takeoff and when he did apply the brakes, the wheels would get on fire.

"Vr must be equal or greater than Vmca"

Actually VR must be equal or greater than 1.05 of the Vmca.

The rotation speed must be such that, after an engine fails and the pilot elects to continue the takeoff (it happened at or after V1), you need to be able to control the aircraft while airborne. In other words, when you rotate, just before lifting off, you must be able to control it airborne, and maybe, if needed, bank in the direction of the working engine for a maximum of 5 degrees, to compensate for the assymetric thrust.

"V2 must be equal or greater than Vmca, and Vs"

Actually, V2 must be equal or greater than 1.10 Vmca and/or 1.20 of Vs1. The greater value applies.

In other words, when you reach V2 climbing speed at 35 feet above the runway/clearway, you have a nice cozy margin of aerodynamic control, and above stalling speed at the current takeoff configuration.

All that is for certification purposes based on the performance specified in FAR Part 25, and all other countries that follow (not implying that FAR was the first).

Hope it was clear enough, I could've directed you to a site/pdf, but I wanted to brush up my memory.

GAMEOVER
9th Oct 2010, 09:21
Dear Pugachev Cobra,

Thank you very much.

I friend of mine was asked during an interview about these speeds, and during the speech she couldīt be able to give good reasons, some explanations about the matter. We are not native.

We have books obviously, but in an interview they expect you talk to them in a coloquial way as you gentelment has done.

Thanks a lot for your colaboration.

B.R:)

If anyone wants to continue participating about the title..., feel free please. Thanks.

aerobat77
9th Oct 2010, 17:52
"V1 must be equal or greater than Vmcg, and can not be greater than Vr or Vmbe"
"Example: Engine fails, assymetric thrust veers off the aircraft, pilots reacts and abort the take off while maintaing the aircraft on the runway using the rudder."


i think you gave here the wrong example related to the topic why v1 must be grater/equal to vmcg .

at or above V1 we have to CONTINUE when an engine fails. the vmcg is the minimum speed where the aerodynamic rudder can keep the aircraft straight when the other engine runs full rated thrust.

so, the pilot must not react to abort but must continue. and so- when the v1 ( so the speed where we have to continue) is below vmcg it would be impossible to continue since the plane would veer off even at full rudder input. when belov V1 the decision is made to stop and the other engine retarded the vmcg is in every case irrelevant .

regarding the 1.1-1.2 vs at v2 :

1.2vs at two engines, 1.15at three 1.1at four and more.

the vmcg as well vmca on a four engine plane is related to the failure of the "worst" engine regarding assymetric thrust - the outer engine.

i learned it thatlike on my Bae146 time.

the other comments of you seem to be fully correct for me .

best regards

john_tullamarine
9th Oct 2010, 22:45
vmcg is the minimum speed where the aerodynamic rudder can keep the aircraft straight when the other engine runs full rated thrust.

Point of style, if I may.

(a) the aircraft won't keep straight - physical impossibility

(b) the requirement is that it be capable of not exceeding more than the prescribed permitted centreline deviation.

(c) although a given aircraft may be able to do so, this does not even require that the aircraft be capable of regaining the centreline - so long as the speed schedule permits it to be off the ground within the deviation permitted - this may constrain V1/Vr so that the deviation by Vlof is not excessive.

JIC
9th Oct 2010, 23:22
John very interesting. Never heard about those requirements.

Does it say in the CS-25?

Pugachev Cobra
10th Oct 2010, 14:40
Thank you aerobat77, it makes much more sense!

From the book I read about performance, it wasn't really clear about Vmcg being related to continuing takeoff, even though it briefly described the other engines being at takeoff thrust.

Maybe it should be more clearer, I don't know.

Anyway, now it makes more sense, thanks again for correcting me.

About John's answer, I'm not reading part 25 right now to confirm, but from a performance pdf class, the test pilot must keep the aircraft within 30 feet of the runway centerline, or 25 feet for pre amendment 42 aircraft.

Correct?

aerobat77
10th Oct 2010, 17:55
you,re welcome pugachev!

the crucial key of Vmcg is like said the situation when we have to continue the take off with one engine inop and so assymetric thrust.

of course even the best pilot has a reaction time in slamming the rudder and the plane will durning this time try to veer off.

not sure if there are regulations how much deviation is approved- it basicly has to be demonstarted that above Vmcg a directional control on the described situation is possible.

when ( like john said) this:

"(c) although a given aircraft may be able to do so, this does not even require that the aircraft be capable of regaining the centreline - so long as the speed schedule permits it to be off the ground within the deviation permitted - this may constrain V1/Vr so that the deviation by Vlof is not excessive. "

is true,

we would have to consider even runway width , and i must say i personally never saw a chart or calculation where Vmcg is dependant on runway width.

i think here we would start hairsplitting.

averybody of us commercial drivers knows that a flameout at just V1 is a serious eye opener, especially in twin engine aircrafts, and the real deal then is to keep the ship on runway and finally to take off and land again, whatever deviation , whatever runway width. the first thing after landing maybe the change of underwear clothes. :\

best regards !

HazelNuts39
10th Oct 2010, 19:40
not sure if there are regulations how much deviation is approvedRef. CS 25.149(e)(in part):In the determination of VMCG, assuming that the path of the aeroplane accelerating with all engines operating is along the centreline of the runway, its path from the point at which the critical engine is made inoperative to the point at which recovery to a direction parallel to the centreline is completed, may not deviate more than 9.1 m (30 ft) laterally from the centreline at any point.we would have to consider even runway width , and i must say i personally never saw a chart or calculation where Vmcg is dependant on runway width.I believe the situation is (just quoting from memory) that ICAO standards define minimum widths for certain categories of runways. Operation into narrower runways requires special approval, in the course of which Vmcg is redefined for the particular runway.

regards,
HN39

john_tullamarine
10th Oct 2010, 21:17
we would have to consider even runway width

ah .. we OUGHT to consider runway width. Quite some years ago ICAO made such a recommendation. However, as far as I know, only Australia took it up and made a requirement for runway width testing in some cases

i think here we would start hairsplitting.

not at all. For any aircraft looking at a failure around Vmcg, directional control things can happen VERY quickly and, in the last few knots approaching Vmcg from above during the testing, the ramp up of deviation is quite rapid with speed .. nothing at all relaxed and gentlemanly. Having watched this from the runway head for several different Types I can make a categoric statement that it is interesting to see the aircraft in the camera viewfinder simply go out of field either left or right ....

averybody of us commercial drivers knows that a flameout at just V1 is a serious eye opener

for V1 well above Vmcg, generally a walk in the park. For min V1 schedules down around Vmcg, quite an eye opener.

whatever deviation , whatever runway width

if you reconsider that observation .. clearly, if the deviation puts the aircraft into the grass then you might just have some problems ?

ICAO standards define minimum widths for certain categories of runways.

there is a schedule of runway width against parameters such as wheel geometry which you should find in your local AIP.

Operation into narrower runways requires special approval

unfortunately, many States apparently come up with rules of thumb for making such authorisations. For some aircraft, testing shows up the problems with this approach. Indeed, simulator testing done in Australia at the time suggested that some larger aircraft probably ought not to be operating on the standard runway widths ...

in the course of which Vmcg is redefined for the particular runway

that doesn't make a great deal of sense to me. Perhaps you can elaborate a little ? The Australian requirement functionally achieved this in an informal manner by limiting the minimum V1, where considered appropriate. However, I have some difficulty seeing this being done empirically.

HazelNuts39
10th Oct 2010, 22:44
The Australian requirement functionally achieved this in an informal manner by limiting the minimum V1, where considered appropriate. However, I have some difficulty seeing this being done empirically.
Ah yes, I remember now. The case I was involved with many years ago was in Australia. The additional requirements for landing were actually more onerous than for takeoff. The takeoff just meant repeating the Vmcg test at a slightly higher speed (if I remember correctly). At the time I was under the impression that 'narrow runways' for Part-25 airplanes was a problem particular for Australia.

regards,
HN39

john_tullamarine
11th Oct 2010, 04:20
The additional requirements for landing were actually more onerous than for takeoff.

Presumably due to the side step manoeuvring which was a consideration in the local requirements ?

The takeoff just meant repeating the Vmcg test at a slightly higher speed

as necessary to retain acceptable deviation control .. but, also, in a manner which addressed wet runway considerations ... ie with NWS disconnected or, as an alternative, with pole back to give negligible NW load

I was under the impression that 'narrow runways' for Part-25 airplanes was a problem particular for Australia.

Not so much a local problem. Australia elected to implement the ICAO recommendation re runway width demonstrations especially after the relevant test folk in Canberra had had a chance to play in some sims and discovered that, just maybe, this was a seriously real consideration to which one ought give some thought ...

No doubt you came in contact with good folk such as Chris Furse et al in the Certification Section within the local Regulator. ..

HazelNuts39
11th Oct 2010, 10:34
Australia elected to implement the ICAO recommendation re runway width demonstrations especially after the relevant test folk in Canberra had had a chance to play in some sims and discovered that, just maybe, this was a seriously real consideration to which one ought give some thought ...I don't remember the 'ICAO recommendation'. Could it be that the 'discovery' of the 'relevant test folk in Canberra' was the origin rather than the result of it?

regards,
HN39

PM See para. 3.2 of the document referenced by Mad (Flt) Scientist in next post. (Thank you MFS).

Mad (Flt) Scientist
11th Oct 2010, 10:42
we would have to consider even runway width

ah .. we OUGHT to consider runway width. Quite some years ago ICAO made such a recommendation. However, as far as I know, only Australia took it up and made a requirement for runway width testing in some cases


Canada does so too, in fact. Certification of Transport Category Aeroplanes On Narrow Runways (http://www.tc.gc.ca/eng/civilaviation/certification/guidance-525-525-014-967.htm)

Basically it says that "standard" cert testing is adequate for operation at and above the ICAO Annex 14 widths, but that if you go below that then extra conditions apply.

decurion
11th Oct 2010, 21:24
Both FAR 25 and CS (JAR) 25 don’t specify the determination of a minimum runway width. ICAO Annex 14 contains International Standards and Recommended practices that prescribe the physical characteristics to be provided at airports. This document provides guidelines to be used for the planning and construction of airports. It does not define limits or regulate the operation of aircraft! In particular, it provides guidelines to the airport planners on the width of the runway to be built considering the characteristics of the aircraft to be operated on it. EU-OPS and FAR 121 do not address runway width either.

For operations on narrow runways, an approval by local authorities could be required. This process is normally done by the operator and the local authority. The aircraft manufacturer is not always involved in this process. Some manufactures give guidance materials on the operations on narrow runways (as defined by ICAO 14 for runway design). One aircraft manufacturer even asked for an official approval from the regulator for the operations on narrow runways.

An interesting presentation on the topic of narrow runways can be found on: http://www.smartcockpit.com/data/pdfs/flightops/flyingtechnique/Narrow_Runway_Operations.pdf.

Airbus also has an interesting presentation on the topic, however this is not publicly available.

johns7022
12th Oct 2010, 01:47
Each successive Vspeed is there to assure safety of flight....

If you pulled up early, in ground effect, say on a soft field take off in a twin, it's possible to lose an engine prior to an airspeed where you have enough control forces to counteract the asymetrical thrust of your last good engine...

So staying on the ground...past VMCG, past V1, Rotating at VR, maintaining V2, then then V2+10...ect....is successively giving you assurances for each stage of the departure that you have enough airflow over control surfaces to maintain directional control, assure obstacle clearance, ect........as opposed the plane going where the one good engine now wants push the plane....

So the concept is simple..how to mitigate all risk out of the flight, adherance to flying properly calculated vspeeds is one of the strategies.

john_tullamarine
12th Oct 2010, 02:32
I don't remember the 'ICAO recommendation'. Could it be that the 'discovery' of the 'relevant test folk in Canberra' was the origin rather than the result of it?

I was involved with some of the early testing from the Industry side of things at the time so I can only go on the advice from my Canberra Regulatory colleagues. Certainly, the tale was that ICAO had made a recommendation that States should test in some circumstances. If you have current ties to CASA, I'm sure that Nick Coulson or Dave Punshon would be able to cite the original paperwork off the top of the head.

Basically it says that "standard" cert testing is adequate for operation at and above the ICAO Annex 14 widths but that if you go below that then extra conditions apply.

That's a moot point. Sim tests at the time suggested that some aircraft might have a problem under reasonably critical conditions even on the standard width requirements. Let's not identify specifics, though, which would create a stand up battle.

One aircraft manufacturer even asked for an official approval from the regulator for the operations on narrow runways.

Although the aircraft didn't have any problems due to the large Vmcg/V1min split, I gather that our original work on the Citation ended up in the OEM flight manuals (for which we never got any royalty but that is another matter)

Each successive Vspeed is there to assure safety of flight

I never like the use of "assure" as it infers a guarantee when none is justified. Rather, we should think in terms of risk control/mitigation and a progressive reduction in risk level.

prior to an airspeed where you have enough control forces to counteract the asymetrical thrust

but we make the presumption that the smart pilot will snap the remaining throttles closed as part of the reject. If he/she doesn't, then a delicate dance amongst the daisies is a reasonable prospect.

how to mitigate all risk out of the flight

that's easy.

Leave the birds in the hangar with the hangar locked to keep folk out (in case a wing falls off and hits one on the head) and the whole surrounded by a tall fenced off area (to guard against the hangar's collapsing and injuring bystanders).

johns7022
12th Oct 2010, 03:34
No guarantees in life...but IF a flight is hell bent on reducing all risk from the flight...then they TRY...so yeah, it's expected that under certain conditions the pilot's do exactly what the book says he should do....when you start making excuses for the pilot being inadequate, under trained, negligent...then why bother....

The object is to mitigate all risk from the flight...those who don't agree can go sell cars, run porn shops, whatever, where professionalism doesn't exist....

john_tullamarine
12th Oct 2010, 03:51
Not trying to start an argument but the philosophy is very important.

No guarantees in life...

Apart from death and, ultimately, that's guaranteed, probably a fair comment. (Taxes, at least for some, appear to be optional)

reducing all risk from the flight (my emphasis)

not possible, but certainly admirable so far as the ideal is concerned

when you start making excuses for the pilot being inadequate, under trained, negligent

all part of the equation .. along with design inadequacy, airspace problems, etc., etc., etc. Sensible risk management has to look, quite critically, at all relevant sources of risk. Sometimes that's embarrassing, costs in resources or whatever .. but that's the way it goes.

...then why bother....

because the alternative (not bothering) is an inevitable increase in risks and increase in the probability of a mishap.