2 Questions. Vmca & Vmcg

[Happydays]

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 !

[n90bar]

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!

[Happydays]

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

[n90bar]

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.

[Happydays]

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

[Go_Detent]

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.

[Mad (Flt) Scientist]

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.

[Mad (Flt) Scientist]

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, no; Vmca is applicable at the MINIMUM flight weight, usually
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.
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.

[Mad (Flt) Scientist]

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.

[Happydays]

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.
GD.

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

[Mad (Flt) Scientist]

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.

[Go_Detent]

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

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:

[FE Hoppy]

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.

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.

[Mad (Flt) Scientist]

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:
If, at the option of the applicant, the AFM value of VMCA is to vary with pressure altitude and temperature, the test day minimum control speed and the corresponding thrust should be used to calculate an equivalent yawing moment coefficient (CN). This CN value may then be used to calculate VMCA as a function of takeoff thrust, thus permitting VMCA to be scheduled as a function of pressure altitude and temperature for takeoff data expansion and presentation in the AFM

[Happydays]

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.

[Mad (Flt) Scientist]

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

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

To determine VMCA, use the configuration specified in § 25.149, except that VMCA is normally determined at minimum weight in order to minimize the stall speed and because static VMCA decreases with increased weight if a 5-degree bank angle is used.

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

Explanation. Section 25.149 defines requirements for minimum control speeds during takeoff climb (VMC), during takeoff ground roll (VMCG), and during approach and landing (VMCL and VMCL-2). The VMC (commonly referred to as VMCA) requirements are specified in §§ 25.149(a), (b), (c) and (d); the VMCG requirements are described in § 25.149(e); and the VMCL and VMCL-2 requirements are covered in §§ 25.149(f), (g) and (h).

(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.

[Oktas8]

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

[FE Hoppy]

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.

[Mad (Flt) Scientist]

Ahem.
FAR 23.149 - MTOW is correct. For part 23 aircraft. Probably what n90bar and others are quoting.
§ 23.149 Minimum control speed.
(b) VMC for takeoff must not exceed 1.2 VS1, where VS1 is determined at the maximum takeoff weight. VMC must be determined with the most unfavorable weight and center of gravity position and with the airplane airborne and the ground effect negligible, for the takeoff configuration(s) with—
....

§ 25.149 Minimum control speed
....
(c) VMC may not exceed 1.13 VSR with—
....
(4) The maximum sea level takeoff weight (or any lesser weight necessary to show VMC);
...
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

[FE Hoppy]

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.

[Mad (Flt) Scientist]

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

I have to agree it sure looks like that.

I'll be inquiring of our friendly local regulators, who must have approved the 170 since Air Canada have them, when they decided to allow this, because it would seem to diminish VMCA as a design requirement. At least one of our designs would stand to benefit if treated the same way. Level playing field, and so on ...

If I look at the first line, for example, and make some really simple assumptions about takeoff accel, the distance from brakes release to minVR varies as follows (if you could set Vr at VMCA):

WT 47 50 55 60 65 70 75 80
VR 97 95 92 89 86 83 80 76
D 1 1.02 1.05 1.07 1.09 1.09 1.09 1.04

So the distance run to VR is a maximum at mid weights (but only 9% above the light weight case) and drops back down to only a 4% increase at max weight.

Conversely, if you were using a single fixed VMCA of 97, the distance increase at MTOW would be more like 70%. I know I'd like a paperwork change that reduced my takeoff run by that kind of magnitude! (Recognizing that there may be other limiting factors)

[FE Hoppy]

At least one of our designs would stand to benefit if treated the same way. Level playing field, and so on ...


But that would be bad news for me!!!:uhoh: :uhoh:

I think it's better to look at the min VR data rather than the vmca data for a true indication of the benifits as vmca is not always the limiting factor.

[Mad (Flt) Scientist]

I agree, but the point for a new design would be that you could size the fin to have a VMC that wasn't limiting for mid weights, knowing that any light weight cases shouldn't be a problem because the higher accel will roughly balance the higher VR. Right now we effectively oversize fins and rudders because VMC can affect all your performance.

I agree that for this case the VRs dont show any benefit at higher weights.

[novicef]

Mad (Flt) Scientist (http://www.pprune.org/forums/member.php?u=58150)

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.

Could you please give an example of aircraft types where Vmca or Vmcg are higher for future reference.

[Oktas8]

Thanks Mad Flt Scientist - again I stand corrected by your exalted knowledge. Off to change my pre-conceived ideas now... :ok:
O8

[Mad (Flt) Scientist]

Could you please give an example of aircraft types where Vmca or Vmcg are higher for future reference.
This question comes up over and over, because of that stupid interview question, and the stupid answer the interviewers seem to expect ... GIGO I'm afraid. Consequently, it's been discussed before. One previous discussion was here (http://www.pprune.org/forums/showpost.php?p=1418178&postcount=5)

To quote the specifically relevant parts:
Bombardier's Global Express has a slightly higher Vmca than Vmcg. The CRJ-700 has a lower Vmca than Vmcg. (In both cases, the differences in the speeds is small).
Here's another. The L1011-1 has a lower Vmca than Vmcg, usually by 3 or 4 kt.

edit: I'm not calling asking the question here stupid, by the way, but asking it at the interview is.

[Mad (Flt) Scientist]

Thanks Mad Flt Scientist - again I stand corrected by your exalted knowledge. Off to change my pre-conceived ideas now... :ok:
O8

Looks like my knowledge isn't so exalted after all.

A bit of asking around indicates that people ARE now being allowed to schedule VMCA with weight; you learn something every day. Sounds like it's come into play in the last few years.

Thanks to FE Hoppy for correcting my dated interpretation. :)

[FE Hoppy]

MFS-
you are most certainly welcome. I probably owe you 9999 more before we are even.

Over the last 4-5 years I've been in an environment where I've had to learn new regs and new interpretations of old regs almost every day as I have dealt with airlines from all over the world. I try not to know anything any more I just read and read again and then offer an opinion.:rolleyes: :rolleyes:
It's a complicated business this aviation lark!!

[popay]

Mad (Flt) Scientist, hi there.
Had a discussion the other day with a guy regarding the subject of VMCG in conjunction with derated thrust. I mean derated thrust allows you to reduce the VMCG consequently increasing the MTOW and so on. Here is the thing though SOP says that if using flaps 2 or 3 on A346 for derated take off don't use TOGA unless above F speed. Any idea whats that all about?
Cheers.

[formulaben]

Good grief.

This, again.

LMAO! ;)

[CaptainSandL]

I have read every Vmca/Vmcg discussion on PPRuNe since it started back in the late 90’s. I am a current, fairly technically minded, airline pilot, - and I still get confused.

As a service to us all, could somebody (MFS ?) please post up a quick pilots guide on the subject along with a few FAQs or myths. I am sure it would be much appreciated.

S&L

[Centaurus]

CaptainSandl.
While on the general discussion on VMC and I will the first to admit it is probably a stupid and far fetched question (age shall not weary us and all that sort of jazz) - but, if by chance one inadvertently enters an incipient or full spin in a 737 such as may be caused by slow reaction to a flame-out on one engine at high altitude, will full rudder be available for recovery action? - keeping in mind relatively low IAS at the time and the fact that the rudder pressure reducer would have been activated automatically at 700ft (?) after take off? :8

[CaptainSandL]

There are at least three different stages of 737 rudder modification in service. It is important that you know which you have installed as they work differently.

It is my understanding that if you have the RPR and/or RPL installed rudder authority is reduced by 1/3 when above 1000ra on take-off or 700ra on approach. So full rudder authority is not available at high altitude.

If you are in an NG you have load limiters instead of the RPR/RPL, these reduce rudder authority by 25% at blowdown speed. The LL’s become active above approx 135kts with no altitude constraints. So full rudder authority is available at any altitude if below 135kts. This may catch your spinning scenario.

Bringing this back to Vmcg/Vmca, does this mean that the 737 has two different Vmca’s depending upon whether or not the rudder limiter is active or not? I suppose it must.

[SR71]

Centaurus,

My understanding would be that in the situation you describe, if you fly an a/c without the RSEP modifications but with a RPR you would retain full rudder authority in this case because there is another condition which allows full system pressure back to the actuators, namely, when the difference between left and right N1 exceeds 45%.

The situation with a RSEP modified aircraft, wouldn't appear to be any different?

Am I missing something?

:confused:

[Centaurus]

Thanks for most informative info, Captain Sandl and SR71. I'll ponder that lot for a while.

[CaptainSandL]

SR71,

Non-RSEP a/c you are correct; full rudder pressure will return with a 45% N1 difference at any altitude.

Quote from Vol 2:

“A rudder pressure reducer is connected to the A system hydraulic line upstream of the main rudder PCU. Hydraulic pressure to the rudder is reduced when the airplane climbs above 1000 feet AGL. Hydraulic pressure returns to normal when the airplane descends through 700 feet AGL, or if B hydraulic system depressurizes, or whenever the N1 difference between the left and right engines exceeds 45%.”

However for RSEP a/c the only factor is airspeed:

“At speeds above approximately 135 kts, both hydraulic system A and B pressure are each reduced within the main PCU by approximately 25% each. This function limits full rudder authority in flight after takeoff and before landing.”

Pulling this back to the thread again does Vmca change depending upon your a/c mod status???

It is all very confusing. :confused:

[alexban]

The limitation of the rudder displacement comes from the hydraulical force available to fight the aerodinamic force.With low pressure the rudder displacement will be limited by the amount required to fight the blowdown on the rudder at that actual IAS.
In case of speed near stall the blowdown will be reduced,so my guess ,the rudder will go all the way, even with the RSEP.
It's not a mechanical limitation,it's a pressure force one.

[Pugilistic Animus]

Is your interview as a CFI -ME?

in that case the real signifigance of the lowered VMC [many training twins far23] have only one Vmc and many times it's very near-or below- the stall speed so as the engine power is reduced with the temperature ratio at a certain point Vmc will be lower than the stall speed-on the ground this is fine- but for the required Vmc(a) demonstration it cannot be done and as it is required for the test standard--you have to use your feet to block the student's pedals and create a fake vmc.

in this application only does the question makes sense to me,
otherwise a very silly interview question IMHO

[victoriano]

Hello.
Does anyone know how to get the book “Jet Airplane Performance” of Lufthansa Consulting; it has 140 pages. It was printed 1988.
Thanks.

[john_tullamarine]

MFS has a very sound background in this stuff so I would counsel heeding his commentary. For my thinking, Vmc/Vmcg are potentially hazardous certification animals and, as line pilots, we are better off giving them a moderate berth where feasible. Better to leave the risky stuff to the QTPs of the world (who are trained in managing those risks).

Does anyone know how to get the book “Jet Airplane Performance” of Lufthansa Consulting

I don't think I have read that particular book but there are others of a similar vein on the net, eg Performance of the Jet Transport Airplane etc by Prof Trevor Young (Wiley), and others. Also the Boeing Performance Engineers Manual and the Airbus Getting to Grips manuals are a good read. These can be found on the net.

you have to use your feet to block the student's pedals and create a fake vmc.

In a similar vein, a colleague had need to pick up a GA twin endorsement prior to renewing an instrument rating years ago. His experience didn't warrant much of the stuff commonly done on light aircraft. So, when the mentally programmed instructor set out to have him demonstrate Vmc (which you can't really do, anyway) he held some rudder in reserve. This perplexed the instructor who then proceeded to demonstrate the exercise. My colleague held the rudder blocked to keep things sensible and the instructor went home quite confused by what he had experienced ......

[FlightDetent]

Diamond of a thread. Many thanks, just observing the conduct is well worth the read.

[vilas]

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 !
Download airbuswin app. It's free. There are ten Airbus flight test videos. They show and explain how it's done. In addition you can read articles in Airbus safetyfirst.