technical questions

[dynamite dean]

Could please someone help me....

How is ranged increased in a headwind?

What is transport wander for an uncorrected gyro?

what is the realtionship between Vmcg and V1?

Tell me about ETOPS?

What is higher Vmcg or Vmca on a 747 -400 why?

What happens to stall speed at hiogh altitudes?

I know I don't ask a lot but I am stuck on these it is for a good cause believe me!

:D :D :D

[Mad (Flt) Scientist]

Afraid I don't have the background to answer all your questions, but here goes for those I can:

what is the realtionship between Vmcg and V1?

According to FAR 25.107a(1)and(2), V1 may not be less than Vmcg plus a margin to account for pilot recognition time. So, crudely, Vmcg sets a min value for V1.

But V1 may also be set by other criteria - V2min, for example. So the relationship between V1 and Vmcg may be very tenuous for some aircraft. Typically V1 may be Vmcg dependent at light weights, but unlikely to be so at heavy weights for the same aircraft.

What happens to stall speed at hiogh altitudes?

At higher altitudes the Mach number at the stall increases, which generally acts to cause a lower stall angle-of-attack, and so increase the stall speed. This is most pronounced for wings with 'hard' leading edges - no slats. Although there are often altitude restrictions on slat extension anyway.

How is ranged increased in a headwind?

ummm - it isn't?

[FlyingForFun]

I'll see if I can add any answers to Mad Scientists:

How is ranged increased in a headwind?
Range can be very slightly increased by flying at a higher speed, therefore reducing the amount of time you're flying for, and reducing the amount time the headwind can affect you.

What is transport wander for an uncorrected gyro?
Can't figure out how to explain this without lots of diagrams and things. Basically, as you move a gyro across the earth, it continues to point the same way in space, but the way it points relative to North will change.

Tell me about ETOPS?
I haven't got to this chapter yet, so I don't know the details. But the idea is that, without ETOPS, you're not allowed to be more than a certain distance from a diversion airfield in a twin-engined aircraft. So flight across the Atlantic, for example, would have to be in an aircraft with 3 or more engines (or take a long route). ETOPS enables an operator to operate twin engined aircraft on these routes, once the authority is happy that the operator's procedures are sufficiently safe.

FFF
-------------

[lightoutandarmed]

dynamite dean,

see if you can find the book "Ace the technical pilot interview" by gary v. bristow, mcgraw hill isbn 0-07-139609-8
all those questions and more are answered.
btw...those questions look like they came straight out of the cx reviewer!:D

goodluck!

[Chryse]

Dynamite D:

Most of our fellow ppruners have answered your questions. If I may I would like to add...

How is range increased in a headwind ?

Take the worst case, one in which the headwind is equal to cruise speed. The only way to increase range would be to increase speed. Therefore, flying at an increased speed in a headwind improves range.

Which is higher Vmcg or Vmca on a B747 ?

Vmca is always higher that Vmcg and this fact is not airplane related. Actually the only factor which affects Vmca and Vmcg is Air Density and no other variable. Example: In High Air Density the engine produces higher thrust therefore the loss of a critical engine causes more asymmetry requiring higher speeds to maintain control. Therefore both speeds are lower in Low Density Air and vice versa.

What happens to stall speed at high altitudes?

If you have had the chance to see any jet QRH performance page indicating Vref speeds for different flap settings, you will notice that no correction is provided for altitude. Vref is determined from 1.3 Vso and the absence of any altitude correction implies that INDICATED stall speed is not affected by altitude. For every AOA there is a corresponding IAS. In addition, the coefficient of Lift does not vary with altitude for the same IAS. Since we as pilots have only the IAS Indicator, Stick Shaker and maybe the AOA sensor, the calculation of TAS Stall speed is not critical. However, you should know that TAS Stall speed will vary with altitude.

If these questions are in prep for an interview good luck.

Chryse

[FlyingForFun]

Chryse,

You said:

Vmca is always higher that Vmcg
Why is this? I would assume it's to do with the stabilising effect of the friction from the gear? Makes sense, it's just that I've never heard this before.

Also, if this is the case, would it not be true that tail-draggers would have a lower Vmca than Vmcg, since the friction from the main wheels will de-stabilise the aircraft on the ground? Not very relevant to modern aviation, since there aren't too many tail-dragger twins still flying - I'm just curious.

Cheers,

FFF
-------------

[john_tullamarine]

Chryse,

Leaving the headwind thing to one side .... for the benefit of all, could we get you to substantiate some of your comments, please ? A few authoritative references would be nice ...

[mustafagander]

On the B747 series, Vmcg is higher than Vmca by about 5 or 6 knots.

It's a certification thing. Remember "3 to 5 to the live" when you did your first twin rating???

The advantage gained by the bank angle/sideslip ought to mean that most aircraft Vmca figures are less than their Vmcg.

[Mad (Flt) Scientist]

[QUOTE] If you have had the chance to see any jet
QRH performance page indicating Vref speeds for
different flap settings, you will notice that no
correction is provided for altitude. Vref is
determined from 1.3 Vso and the absence of any
altitude correction implies that INDICATED
stall speed is not affected by altitude. For every AOA there is a corresponding IAS. In
addition, the coefficient of Lift does not vary with altitude for the same IAS. Since we as
pilots have only the IAS Indicator, Stick Shaker and maybe the AOA sensor, the calculation
of TAS Stall speed is not critical. However, you should know that TAS Stall speed will vary
with altitude. [\quote]

Sorry, but you cannot derive information about the
aerodynamic behaviour of an aircraft from the
QRH. The aerodynamics determines what goes in the
QRH, but you can't go backwards, and you
certainly should not generalise from a QRH to the
underlying theory.

Stall speed is quite definitely affected by
altitude - whether IAS, CAS, EAS or TAS. The Mach
number effect is real, and is presented in the
stall speed charts of our aircraft (we quote
stall speeds for SL and 15,000ft)

If a single Vref has been chosen this is because
the plane is likely only certificated for
landings at low altitudes, and the variation in
stall speed is small. The Vref will have been
then defined by the highest stall speed in the
altitude band. It *could* have been defined as a
function of altitude, but it was probably only a
knot or so variation so the manufacturer decide
to swallow the effect in the interest of
simplifying the charts.

[Chryse]

John T. and other fellow ppruners,

Did not mean to cause ripples here, after all I thought it was a discussion forum. Anyways heres some supporting documentation and Authoritative References....

Regarding Range and Headwind - refer to Aerodynamics for Naval Aviators by H.H. Hurt Jr. (ISBN 1-56027-140-X) page 170 second para from the top left.

Regarding Vmca and Vmcg: I agree that I could be wrong about Vmca and Vmcg on a B747. I was writing in reference to an airplane performance graph depicting the various speeds in a timeline. In the graph (generic airplane) Vmcg was depicted before Vmca. Vmca, from my basic performance classes, is the threshold speed for getting airborne, so should follow Vmcg, which is the threshold speed for continuation of takeoff. If Ive stated something wrong please let me know why? The rest of the info on Vmca and Vmcg I provided is listed in Aircraft Performance Theory for Pilots by P.J. Swatton 2000 (ISBN 0632-05569-3) pg 138 third para from the top.

Regarding Stall Speed at high altitude: The formula for stall speed is given by Vs (ktas) = Sqrt (295 L/qSClmax) where q is the density ratio. However since the speed is given in Vs (ktas), this part of the formula can be rewritten as Vs(ktas) = (SMOE)*(EAS) where SMOE = 1/sqrt(density ratio). When both equations are simplified the density ratios at altitude cancel out leaving only EAS. Position error also cancel each other. Refer to Classnotes for Basic Aerodynamics by Norbert R. Kluga 1991 page 124 from Embry-Riddle Aeronautical University and Flight Technique Analysis for Professional Pilots by Les Kumpula page 30 also from Embry-Riddle Aeronautical University.



Cheers

[RadarContact]

How is range increased in a headwind ?

Take the worst case, one in which the headwind is equal to cruise speed. The only way to increase range would be to increase speed.

Actually, reducing speed to minimum clean might do the trick. Of course, you'd be landing in Moscow instead of New York...
:D

[mustafagander]

Don't know about threshold speeds???

Vmca - Minimum Control Speed in the Air

Vmcg - Minimum Control Speed on the Ground.

Hence Vmcg is the minimum speed from which the pilot will be able to maintain directional control to continue the take off, all other performance factors being satisfied. Quite obviously below Vmcg you must stop or go off the runway.

There are, of course, various configurations defined for the testing. JT and Mutt can, I'm sure, expand on this.

[Chryse]

mustafagander,

Vmcg is defined as the minimum CAS at which when the critical engine is suddenly made inoperative during the take-off run it is possible to maintain control of the airplane using primary aerodynamic controls alone (no tiller control - nosewheel steering) to enable the takeoff to be safely continued using normal piloting skills. This includes:
a. the use of primary aerodynamic controls only (rudder only) since on the ground
b. Rudder pedal force not exceeding 150 f/lbs
c. assuming 7 knts of crosswind from the inop engine side
d. without reducing thrust on the live engine, and
e. landing gear remaining extended.
In addition, the path of the aeroplane at which engine failure occurs to the point at which recovery to a direction parallel to the centre-line of the runway is attained may not deviate by more than 30 ft laterally from the centre-line at any point. JAR25.149(E).

Vmca is the minimum CAS at which when the critical engine is suddenly made inop, it is possible to maintain directional control of the aircraft with:
a. Rudder pedal force not exceeding 150 f/lbs
b. Bank not exceeding 5 degrees
c. Directional Change not exceeding 20 degrees
d. Without reducing thrust on the live engine
e. A/c airborne with little or no ground effect. It may not exceed 1.2Vs JAR 25.149(B) and (D).

Since V1 can never be less than Vmcg, and Vr cannot be less than V1 or 105%Vmca (among other things) it is possible to have Vmcg higher than Vmca in theory, but it should not be more than by a few knots. If anyone knows of such cases and why/ how, I am always eager to learn.

Cheers

[Mad (Flt) Scientist]

Chryse
Regarding Stall Speed at high altitude: The formula for stall speed is given by Vs (ktas) = Sqrt (295 L/qSClmax) where q is the density ratio. However since the speed is given in Vs (ktas), this part of the formula can be rewritten as Vs(ktas) = (SMOE)*(EAS) where SMOE = 1/sqrt(density ratio). When both equations are simplified the density ratios at altitude cancel out leaving only EAS. Position error also cancel each other. Refer to Classnotes for Basic Aerodynamics by Norbert R. Kluga 1991 page 124 from Embry-Riddle Aeronautical University and Flight Technique Analysis for Professional Pilots by Les Kumpula page 30 also from Embry-Riddle Aeronautical University.

Your equations are valid only if you assume that CLmax does not vary.

In practice what happens is that CLmax is a function of Mach number, and so altitude for a given speed. This results in a lower CLmax at higher altitudes and hence higher stall speeds. I'm afraid you are not accounting for all the factors with those equations.

[john_tullamarine]

Chryse,

No problem with your basic explanations.

However, it is endemic in most areas of study, not only flying, that different explanations are pitched at different target audiences and, in consequence, varying levels of simplification/approximation/assumption are used appropriate to the reasonable needs of the target audience. So, for instance, a new PPL student needs a story rather different to that which would be appropriate to a post-graduate level. You will notice that several of the posts make reference to the Mach No. variations (which appear in numerous texts), but not the Reynolds No. variations (which tend to hide in aerodynamics texts), when discussing indicated speed at the stall .. same sort of thing.

One needs to be a little wary when quoting design standards (Part 25 and the like) as the "real" words of interpretation are in the explanatory documentation used by certification practictioners. One ought to be very wary of consulting design standards in respect of a particular aircraft unless the relevant (ie not the current) standard is obtained. These animals are an evolving thing and the differences in requirements over time vary quite significantly.

Vmcg/Vmca

There is no necessarily hard and fast relationship between the Vmcg and Vmca numbers of which I am aware... the point I was trying to make is along the lines that I can't see any reason why one need be higher than the other. Just trying to highlight the dangers in making definitive statements and/or drawing a general conclusion from an inadequate range of specific examples. Perhaps some of our experienced FT brethren might be able to add comment ?

Far more important is your suggestion that the values are only dependent on density... while the certification figures are very tightly controlled it is necessary that the pilot-in-the-wild be aware of a variety of factors which, most definitely, do affect the real values which are going to bite the pilot on the day. Consider

(a) CG

(b) crosswind for Vmcg (as an aside, the 7 kt derives from ancient UK practice .. be aware that US practice is to determine Vmcg for nil wind .. and the actual limitation on the day is VERY wind dependent. Also, depending on the vintage of design standard being used, the centreline deviation may relate to different limits)

(c) bank angle for Vmca (very dependent)

...etc

Indicated Stall Speed

Putting to one side the argument that stall is driven by incidence, not speed, indicated stall speed will vary with altitude. What you have missed, I suspect, is the often/usually ignored variation in CLmax due to variations in other parameters such as Mach and Reynolds Nos. which, themselves, vary with level. CLmax reduces at higher values of Hp and the KIAS increases slightly. Reference to any standard aerodynamics text will bring you up to speed on this matter. It is, however, generally not addressed to any extent, if at all, in pilot texts.

[Chryse]

John T. and Mad F. S.,

You both are correct regarding the Reynold no./Mach effect on Clmax. An oversight on my part.

Thanks for the info and a pot of ale for you sir.

Cheers

Chryse

[Capt Pit Bull]

I don'y suppose that question is about *gliding* range is it?

If so the answer is be heavier.

CPB

[FlyingForFun]

CPB,

Correct me if I'm wrong, but I don't believe that gliding range varies with weight, so long as you fly the best glide speed for that weight.

As weight goes up, best glide speed goes up. Rate of descent also goes up by the same percentage, resulting in a glide of exactly the same distance, but taking less time.

However, manufacturers generally only quote best glide speed for maximum all-up weight. So, if you fly the speed in the manual, then you are correct - the heavier you are, the closer the quoted best glide speed is to the actual best glide speed, and so you'll be able to glide further if you fly the book numbers.

At least, that's how I understand it.....

That aside, as with powered flight, you'll get better range by increasing your speed slightly in a headwind, and decreasing it slightly in a tailwind, to maximise or minimise the effect of the wind, but this effect is negligable.

FFF
---------------

[Capt Pit Bull]

Flying for fun,

I was referring to the effect that you mention in your final paragraph. Whether its a negligable effect or not is obviously a function of proportions. In any event negligable effects are often the nub of many an exam question....

CPB

[rjer]

can someone clarify exactly why Vmca is lower than Vmcg in the 747?

thank you

[SR71]

Regarding Stall Speed at high altitude: The formula for stall speed is given by Vs (ktas) = Sqrt (295 L/qSClmax) where q is the density ratio. However since the speed is given in Vs (ktas), this part of the formula can be rewritten as Vs(ktas) = (SMOE)*(EAS) where SMOE = 1/sqrt(density ratio). When both equations are simplified the density ratios at altitude cancel out leaving only EAS. Position error also cancel each other. Refer to Classnotes for Basic Aerodynamics by Norbert R. Kluga 1991 page 124 from Embry-Riddle Aeronautical University and Flight Technique Analysis for Professional Pilots by Les Kumpula page 30 also from Embry-Riddle Aeronautical University.

Stall.....a viscous phenomenon.

Just to be pedantic, the above is essentially wrong because you've made an incompressible flow assumption to ascertain the second flow equation (and forgotten about the compressibility correction), which will obviously not show a Mach number depedence!

Its fine for M<=0.3.

A better idea of the aerofoil C_p valid up to M~=0.7 would be to apply a Prandtl Glauert correction, whereupon

C_p = C_p(INC)/sqrt(1-M^2)

where

C_p(INC) is the incompressible flow pressure coefficient.

Above that, there are other pressure coefficient corrections, such as those given by von Karman & Tsien.

Bear in mind that the aerofoil lift is merely the integral of the pressure over the geometry.

The Mach number dependence in C_L(max) is obviously related to the, initially, advantageous presence of shock waves in the flow, which increase C_L(max) until the point of drag divergence, whereupon shock induced flow seperation means you suffer a collapse of lift.

So true stall speed is obviously a function of altitude i.e., density, (or alternatively remember that CAS is EAS plus a compressibility correction), but perhaps less/more intuitively (depending on how your mind works), a function of the similarity parameters, Mach Number and Reynolds Number.

Reynolds number is essentially a ratio expressing the relative importance of the inertial and viscous forces of a flow.

As Reynolds number tends to infinity, the viscous effects, become less and less important to consider.

Whereupon, one may conclude that stall being a viscous phenomenon, will display some dependence on Reynolds number. A good illustration of this dependence can be found in Fundamentals of Flight by Richard Shevell, Pg 248. As it suggests, in flight tests, Reynolds numbers are often far higher than those found in tunnel testing, whereupon the demonstrated C_L(max)'s are somewhat higher.

The simple expanation for this is that the inertial effects in the flow are better able to combat the adverse pressure gradient and further delay separation at a given constant angle of attack.

For those interested, Fundamentals of Flight by Richard Shevell also gives some graphs of the compressibility correction factor, F, on Pg 106/107 used in determining EAS's.

My $0.02

:ok:

[SR71]

rjer,

can someone clarify exactly why Vmca is lower than Vmcg in the 747?

You might like to check the following thread:
V_mca versus V_mcg Discussion (http://www.pprune.org/forums/showthread.php?s=&threadid=136625&highlight=Vmca+Vmcg)

:ok:

[rjer]

thank you.
this forum has a lot of information that I haven't even tapped yet..

[squalo]

in regards to VMCG vs VMCA on the B744, here is the way it was explained to me during an interview prep course: (i am in no way and aerodynamics expert, but it made sense to me)

VMCG on the 744 (and some other A/C) is higher than VMCA because:

VMCG has to do with controllability on the ground - therefore the effective rudder moment is between the tail and the main gear.

VMCA = in the air, so you have an effective rudder moment between the tail and your CofG.

because the rudder moment between the tail and CofG is longer than that between the tail and the main gear on the ground, your rudder will be more effective in the air, therefore VMCA will be lower than VMCG.

does that sounds right?

[mutt]

Remember that for the VMCA you have the ability to bank the aircraft to aid recovering control.

AFAIK there isnt a relationship between VMCG and VMCA, so you really cant compare them.

Mutt.

[Tonic Please]

What is EAS? Have not come across that one before.

Dan

[Mad (Flt) Scientist]

Vmca and Vmcg

There is no fixed and direct relationship between these because they occur under different aerodynamic conditions with no single, simple, factor dominating. The only true answer to your question, without delving into the minatuae of Boeing's aerodynamic and systems data, is "because it is".

What some airlines like to use as interview questions, they are welcome to use. But the fact that the interviewer expects you to say the moon is made of blue cheese, and everyone is therefore coached to say the moon is made of blue cheese, in no way relates to the composition of lunar rocks. For that I'd want to talk to one (or several) lunar "geologists".

EAS is "Equivalent Air Speed" and is commonly used for e.g. loads calculations, where the fact that the kinetic pressure is the same at all altitudes for the same EAS is very convenient.

[idg]

A question for this learned thread and an observation.

Squalo,

If the Cof G were closer than the main gear would the a/c not fall on it's tail? Or is this what you were trying to imply was the reason for VMCG being higher than VMCA? I have always understood (mainly from what I have learned in these Tech Log threads!) that the two values are essentially totally different.

I was under the impression that JAR had harmonised with FAA on the X-wind value for VMCG on modern a/c ie: A320 onwards. I was aware that the L1011 was certified by the CAA with 7 knots across but that the FAA used 0 knts. Can someone clarify this please?

Thanks idg.

[Mad (Flt) Scientist]

Vmcg is indeed now harmonised with a zero Xwind requirement. Can't comment on the particulars for any given cert without digging, though.

[john_tullamarine]

BCARs used 7 kt for Vmcg determination .. which led to a few nuisance recertification issues when US aircraft were to be put on the Brit register.

[Old Smokey]

Tonic Please,

You asked " What is EAS ?. Mad (Flt) Scientist addressed it correctly but rather fleetingly. One of my pet "hobbyhorses" is that most pilots don't have a real appreciation of the importance of EAS, due to no fault of their own.

EAS = Equivalent Air Speed, Ve. It is the speed that pilots want to see, and need to see, but cannot because CAS, which pilots do see, is affected by compressibility. CAS is corrected for compressibility at ISA Sea Level, where CAS = EAS. At higher levels, relative to the increasing Mach Number, Indicated Airspeed lies to the pilot indicating more than he/she really has, and airspeed is "money in the bank".The higher you fly, the greater becomes the error.

EAS, as Mad (Flt) Scientist stated, is the kinetic value of the airstream expressed in knots. That's what we want to see in flight, but don't. At low altitudes, it really doesn't matter a great deal, e.g. even at 400 Kt CAS at 10,000 feet, the EAS is 393 Kt. If, however, minimum manoeuvre for an aircraft at a particular weight is 250 KIAS at low level, the aircraft would need to be flown at 266 KIAS at 37000 feet to have the same stall protection, as 250 KIAS equates to only 236 Kt EAS at that level - stick shaker time!

For a given weight, the CAS for all of the following situations increases with increasing altitude, whilst EAS remains constant - Stall speed, Holding speed, Best Angle of Climb speed, Maximum Range Cruise Speed, Vmo, Minimum manoeuvre speed, and even V1, Vr, and V2 at relatively low altitudes. All of this, of course, applies in the regimes of flight where airspeed is the primary performance management parameter, above Mcrit, at high altitudes where the effects of Reynolds number and kinematic viscosity come into play, Mach No. or a mix of EAS and Mach No. become more important.

Private Pilots please note - It's been EAS you've been assuming that you have when you calculated TAS using CAS and Density Height, actually this is DAS but carries little error at low speeds and altitudes - horrendous errors at jet type speeds and altitudes prevail, but they could do it with EAS also.

If EAS were introduced as standard cockpit equipment tomorrow, the bulk of Aircraft performance manuals would be significantly reduced. It beats me why "they" didn't introduce it years ago. I'd be genuinely interested to hear if anyone can think of ANY advantage of retaining CAS with all of it's attendant errors.

Vive la revolution!

Old Smokey (aka Old Grumpey)

[SR71]

To add to the debate, I remember the relationships between the speeds via the following:

IAS = Indicated Airspeed
CAS = Calibrated Airspeed = IAS +/- Instrument & Position Error
EAS = CAS - Compressibility Correction
TAS = EAS * Density Correction

EAS is defined as:

V_e = V_0*\sqrt(\rho/\rho_s)

where

V_0 = TAS
\rho = ambient density
\rho_s = sea level density

As one may infer from Old Smokey's post the compressibility correction (typically known as the F factor) that one may apply to CAS to acquire EAS may be thought of as a function of two variables, namely, CAS and altitude.

The derivation is relatively simple and starts from the isentropic flow relationship for pressure:

p_0/p = (1+(\gamma-1)/2*M^2)^(\gamma/(\gamma-1))

If we solve for TAS, V, in terms of the difference between p_0 and p, and substitute for M, bearing in mind we know that:

M = V/(\sqrt(\gamma*p)/\rho))

we find that:

V = a complicated expression

which may be subsequently re-arranged, using the fomula for EAS, to give V_e.

The unknowns in this equation are the static pressure and the difference between the static and total pressure, which is exactly what a pitot-static system measures.

As Old Smokey alludes to, it is standard practise to calibrate the instrument at sea level, whereupon, the complicated expression above is evaluated in terms of sea-level standard pressure. The speed given by such a calculation is the CAS.

It is now clear that, to evaluate the EAS at other altitudes, a correction must be made, and that this correction is the ratio between the equation for V above, evaluated using the actual pressures at the altitude you are interested in and sea level values:

V_e=F*V_cal

where

V_cal = CAS
F = Compressibility Correction factor

I certainly agree that in the days of EADI's and ADC's, whereupon I suppose that at some stage during the path from sensing airspeed to displaying it, a pressure transducer is involved, it would not be hard to apply the relevant correction to the signal.

Maybe there are certification issues involved here involving the nature of standby instrumentation?

If one retains traditional standby instruments, one would have to remember, in the event of a failure of all primary instruments that one was working with CAS again rather than EAS.

But, of course, we seem to work with CAS successfully at the moment...

:)

[mutt]

I could never understand the logic of flight testing VMCG with zero crosswind and then approving the aircraft for operation with a 30 kt crosswind.

Can anyone explain this to me?

Mutt.

[Old Smokey]

Mutt,

The logic you may never find.

The rationale, as I accept it, is that the VMCG is determined at the performance testing stage to meet FAR25 requirements with Nosewheel steering made inoperative in zero (or up to 7 Kt) Xwind and an engine failed.

The crosswind capability is determined at a different (handling qualities) phase of testing with Nosewheel steering operative, also with an engine failed, different FAR (not one that I use regularly, don't have the number, sorry).

Just take a look at the Xwind limit plummet towards zero when Nosewheel Steering Inop is considered from the 'handling qualities' perspective in other 'Non-Normal Procedures'.

The very significant determinant, VMCG, is evaluated with Nosewheel Steering Inop AND an engine failure, but the on-going assumption for day to day operations (at least at the Takeoff phase) does not consider multiple failures, in this case both an engine AND Nosewheel steering, thus allowing normal steering capability to be factored into Xwind limit evaluation.

Good to see you're back from the Emerald Isle Mutt, it was becoming quite boring without you.

[john_tullamarine]

.. also, consider that Vmcg is for fairly critical conditions .. aft CG, no NWS, rated thrust, min weight (generally at very low fuel with min yawing inertia).

In the normal operational environment, we don't go anywhere near the actual Vmcg for the day. Given that the onset of controllability problems generally is quite rapid (in terms of speed deltas), we don't normally need to worry too much.

However, several of us restate the hobby horse, regularly, that one needs to be cautious if the actual conditions tend toward the certification configuration, especially if there is an option to use a higher speed schedule to avoid the problem region. This crops up in the ferry/positioning scenario from a non-critical runway ... if the crosswind is significant, consider using a higher speed schedule (but still within the RTOW data for the runway).

[Mad (Flt) Scientist]

The only "logic" is that we don't have large numbers of aircraft running off the sides of runways when an engine fails; the conditions for Vmcg are arbitrary, and they way we apply that restriction to the performance data is arbitrary. But the end result is an acceptable "level of safety".

It's the same as trying to find an academic reason for using 1.2Vs or 1.13Vsr; there isn't one. It just happens to work out ok most of the time. If we had 12 fingers we might still be using 1.2Vs, just because we'd have picked the digit "2" for the first "decimal" (twelvical??) place; but it's be the equivalent of 1.167Vs in decimal notation. If it works....don't amend it.:)

[john_tullamarine]

.. and, following on from MFS ..

For certification exercises, one needs a boundary condition ("rule") against which to measure compliance.

Many of the "rules" originated in "finger-in-the-wind" professional assessments by the chaps who ran the show in the early days of commercial aviation.

So, for example,

(a) the initial screen height of 50 ft dates back to a demonstration of an early aircraft to the US military. Apparently, the parade ground from which the demonstration flights were conducted was surrounded by a tree line of such approximate height ... seemed a good idea so the FAA ancestor adopted it.

(b) the 70 mph stall speed limit (dutifully carried across to the modern era as 60.8 kt - rounded to 61 kt - in FAR 23.49(c) ) originated in an early engineer's need to have a figure .. consideration of motor vehicles apparently led to the 70 mph figure's being thought to be a reasonable place to start from ....

Vmcg, being an extreme certification boundary, is not often encountered in routine operations, but we need a "rule" against which to show compliance .. the accident record's having shown the definition to be reasonable, it hasn't altered materially.

[mutt]

The only "logic" is that we don't have large numbers of aircraft running off the sides of runways when an engine fails

I personally feel that John_Tullamarine and I have harped on so much about doing cross wind takeoffs with VMCG limited speeds, that people have realized that its much safer to increase the V1, that has had a direct impact on the number of aircraft running off the side of runways :):):):):):)

Mutt.

[Old Smokey]

Mutt, John_T,

Please feel free to harp on, and harp on, and then when you retire, leave a computer virus that keeps on harping on about VMCG. Some things cannot be over-emphasised.

SR71,

You make a good point about standby instruments if we were to introduce EAS as standard. It wouldn't take much of a notice to crews if reverting to CAS on standby instruments. High speed limits would need no notification, CAS is more conservative, for low speed operations a simple "Add 20 Kt to CAS indications above F/L 250 (or something similar) would suffice.

Regards,

Smokey