2 Questions. Vmca & Vmcg
 

1. Why do they always ask in a interview how will Vmca change with weight and altitude etc. But Vmca is at fix conditions so how can it change ??? surely only Vmc can change. Doesnt matter what change Vmca will stay the same. Is that correct ?

2. Is it correct to say Vmcg is always more that Vmca due to the fact that you cannot bank 5 degrees on the ground ??

Please help !

control speeds
 

1. Vmcg & Vmca are affected by changes in altitude/temperature as these affect the amount of thrust being prodused by the engines. The less the thrust being produced, the lower the control speed.

2. Vmcg is a higher speed than Vmca as when on the ground the aircraft rotates around its main undercarriage which is aft of the cg. This gives a shorter rudder arm (flight mechanics), it is therefore required that Vmcg is higher to allow directional control to be maintained.

Hope that helps!

Thank you, Answer number 2 aswer my question. And you are most probably correct about Vmca and Vmcg that can change. As far as I understand it Vmca is at a set condition. 8 factors according to the definition. And a red line on most light a/c

By "set conditions" i assume you mean:-

1) Max available TO power or thrust,
2) Aircraft trimmed for TO,
3) Most unfavourable CG position,
4) max sea level TO weight,
5) Aircraft in most critical TO configiration with the gear up,
6) The aircraft airborne and ground effects negligble,
7) for props - the inoperative engine prop windmilling or feathered.

These criteria are used to determine Vmca.

Yes thats it. So if that is the conditions will it then be correct to say it cant change. Only VMC can ?

Happydays, VMCA (Minimum control speed airbourne) does change dependant on, most importantly, altitude and temperature. Bear in mind that two of the most essential conditions used to determine VMCA are FULL thrust on the remaining engine(s) and FULL rudder deflection to control the asymetric condition.

To keep it simple, an engine is producing less thrust when firewalled at 35000' than it is at 5000' (due to, amongst other things, a reduction in air density) resulting in a decrease in the minimum control speed. So, whilst full thrust is applied in both instances, external conditions dictate that the actual value of the thrust being produced is less at altitude. In effect, the rudder is more effective and is able to control the aircraft to a lower speed.

The conditions used to determine VMCA are set, but the nature of the conditions change with variations in things such as altitude, temperature, density etc.

Hope this helps.
GD.

Good grief.

This, again.

There is NO REASON why Vmca or Vmcg should be a higher speed. Either is possible; we have aircraft built and certified to the exact same regulations which have one higher sometimes, or the other.

The aerodynamic condition of the aircraft is different; rudder power at alpha ~0 on the ground can be VERY different to that at high AoAs and low speeds in flight.

FAR 25.149 and equivalent JAR give the precise requirements, and the Flight Test Guides (such as AC25-7A) provide more background on how to test and analyse the results.

And to address the "is VMCA fixed" question.

It is permissible to schedule VMCA (and VMCG) with altitude and temperature, to reflect the reduction in engine thrust that results, so as to avoid excessive penalties to hot/high operations.

But it IS true that VMCA is "set" in the sense that once determined for a give alt/temp, it is then applied for all instances of that temp/alt. That is, heavy aircraft must use the VMC(A/G) applicable to a light aircraft; you can't schedule with aircraft weight.

If the nature of the conditions change dont you call it Vmc then, and not Vmca anymore ? What does Vmc mean then ?

No.

If you look at, for example, FAR 25.149, the terms used are actually:

VMC
VMCG
VMCL
VMCL-2

The term VMCA actually doesn't occur.

In fact, the term VMCA is how people usually refer to VMC in the context of FAR 25 (for example, we state VMCA in our certification reports, yet there's really no such thing in regs, to be pedantic); bear in mind that VMCL and VMCL-2 are later additions, and so it didn't use to be necessary to clarify "which VMC"


You need to distinguish between "certification Vmcx" and "actual Vmcx" - for EACH of the minimum control speeds (a, g or l) there is a certification level and a real level. "VMC" is just a shorthand for "VMCA" and doesn't signify real versus certified.

And, as our esteemed moderate always reminds us, the real value can sometimes be HIGHER than the certified one.

From a pilot's point of view (i.e. oversimplisitc!!), I've only ever understood the following:

VMCG to be the minumum control speed ON THE GROUND
VMCA to be the miminum control speed 'airbourne', although I agree this term is a little vague.
VMCL to be the minimum control speed in the landing configuration, which, although technically still airbourne, is likely to be a different (lesser) speed than VMCA, with high lift and drag devices out and a significantly augmented wing shape.

As demonstrated by the last post, things can all get very detailed very quickly on this subject :confused:

MINIMUM VR

Altitude: Sea Level and below
OAT WEIGHT (kg)
(°C) 24000 26000 28000 30000 32000 34000 36000 38000
10 98 96 95 95 96 96 96 97
15 98 95 95 95 95 96 96 96
20 98 95 95 95 95 96 96 96
25 98 95 95 95 95 96 96 96
30 98 95 95 95 95 95 96 96
35 94 92 92 92 93 93 93 94
40 90 89 90 90 90 90 91 91
45 87 87 87 87 87 88 88 88
50 84 84 84 84 85 85 85 86


From an AOM near me.

Although it's min VR in this manual in the FAA version the tables are VMCA and a knot or two lower.

So it looks like you can factor weight into the mix.

Those don't look like any kind of Vmca to me.

If you notice, for some cases the speed increases with increasing weight, for others it decreases. If it was Vmca scheduled with weight it would be decreasing as the weight increases. It's a very odd behaviour if it is Vmca. In fact, sometimes the minimum speed occurs at a mid weight. I'm confused.

You couldn't share the identity of the type concerned, perhaps? It might explain what's happening.

Looking at Ac25-7A, it's fairly clear that altitude and temperature are the only variables allowed for scheduling Vmca:

Thanks for all the info, but I am still not convinced Vmca can change. If it can change by weight the definition will not include "Max all up weight" as one of the factors. Why do we do a Vmc demonstration in a twin aircraft then ? We might as well then do a Vmca demonstration. The Vmc demonstration is to show that the speed that you loose control is actually diffirent from Vmca due to the fact that the "set conditions" is now changing, is that not correct then ? That is also why Vmc is ALWAYS less than Vmca.

The "max all up weight" in the definition is wrong.

If you check the Flight Test Guide for FAR 25 (AC25-7A) it says:

Additionally, the same document states (in the introduction to the minimum control speed section:

(Those are on about page 91 of the copy I have, by the way)

VMC and VMCA are used interchangeably in the FAR 25 world.

Ahem.

FAR 23.149 - MTOW is correct. For part 23 aircraft. Probably what n90bar and others are quoting.

But now I've learned something. Light aircraft interview - VMCA doesn't change with weight (legally speaking). Airline interview - VMCA may change with weight, in accordance with ...

Regards,
O8

VMCA
Engine: CF34-8E5A1 – T/O-1
Altitude: Sea Level and below
OAT WEIGHT (lb)
(°C) 47000 50000 55000 60000 65000 70000 75000 80000
10 97 95 92 89 86 83 80 76
15 97 95 92 89 86 83 79 76
20 97 95 92 89 86 83 79 76
25 97 95 92 89 86 83 79 76
30 97 95 92 89 86 82 79 76
35 93 91 88 85 81 78 74 71
40 89 87 84 80 77 73 69 65
45 85 83 79 76 72 68 64 60
50 80 78 74 71 67 63 58 55


these figures are from the FAA version.
ERJ 170-100

The EASA manual has min vr which is not always limited by vmca.

Actually, Part 23 is better worded, because it avoids the nonsense wording about MTOW. The most unfavourable weight for VMC is the lightest weight, unless your weight /cg envelope is ridiculously skewed, such that the most aft cg is much further aft than the light weight case, and even in such a case you'd struggle to convinvce the regulators that anything except lightest weight and furthest aft in combination be tested.


Interesting that those data are for the E170. I note its for T/O-1 thrust, which I assume is a flex/derate setting. I seem to recall hearing that Embraer was using a variable rating system on the engines, such that they were adjusting the engine power for the conditions required, and then scheduling VMCs for THAT thrust.

So, technically, they are not scheduling VMC with weight. They're scheduling thrust with weight (probably to achieve a fixed or near-fixed level of performance, which means a roughly constant T/W) and then using the lower thrusts which naturally occur at lower weights to schedule VMC accordingly. That would explain why the values vary slightly up and down for the same OAT - it's not a perfect balance, sometimes the thrust reduction is big enough to outweigh the light weight effect, sometimes not.

Is there a corresponding table for the full rated thrust case, or is it a single value?

oops, sorry, I didnt look closely at the numbers, just the model, and assumed it was the same data. it's not.
scratches head

Thrust on the EJETS
 

On the Ejets the engine thrust is rated as TO1 TO2 and TO3. These are fixed ratings. They also use ATTCS and you can derate using Assumed Temp method up to 25% of the rated thrust.

In the engine failure case the FADEC will set TOX reserve thrust on the running engine. TO1 reserve and TO2 reserve are the same thrust and share the same VMCG/VMCA (V1min/VRmin) data. TO3 reserve is less and has lower VMCG/A speeds.

Its not possible to select higher than TO3 reserve until you are out of the take off phase to ensure you respect the lower VMC used in the TO calc.

But as you can see from the data VMCA(and vmcg) is scheduled according to alt, temp and weight.