Vmc calculation and weight

[mrflyguy]

Under what weight condition does a manufacturer calculate Vmc speed. According to Part 23, it says most unfavorable weight condition---but what specific weight? Please cite references. Thanks in advance.

 

[Booker]

It depends on the airplane. From AC 23-8A:

Vmc should be determined at the most adverse weight. Minimum practical test weight is usually the most critical, because the beneficial effect of banking into the operating engine is minimized.

Note the use of "usually."

 

[Kaman]

Hello,
My understanding (ref: AFH pg. 14-3, para. one) of Vmc is that the aftmost legal C.G. and Max. Weight are used in the calculation of Vmc by the manufacturer. I use the nemonic: "MUFSCAM" to recall those parameters:

M= Max power on operating engine
U= Up to 5 degrees Angle of Bank into operating engine
F= Flaps T.O. position, Gear-UP
S= Std. Day
C= Critical Engine Windmilling
A= Aft most legal Center of Gravity
M= Maximum Weight

I think that the maximum weight thing is logical because even if a higher weight will decrease Vmc when you establish a zero sidesliop condition. The overall performance degradation is higher due to the heavy condition of the airplane? In other words the airplane might be too heavy for the ambient conditions even with the greater horizontal component of lift vice a lighter weight and not "spliting the ball"? Does that make sense? Just food for thought. Not trying to be argumentative.
What is interesting is that the zero sideslip condition using 5 degrees is based on CFR Title 14 regs (FAR Part 23). However, in theory at lighter weights you can still set the zero sideslip, but it requires a higher AoB.
This is one of those topics that is frequently debated and I'd like to hear the comments of MEIs and experienced ME pilots on the subject.

Regards,

ex-Navy Rotorhead

 

[bobbysamd]

Factors of Vmc

Originally posted by Kaman
M= Max power on operating engine
U= Up to 5 degrees Angle of Bank into operating engine
F= Flaps T.O. position, Gear-UP
S= Std. Day
C= Critical Engine Windmilling
A= Aft most legal Center of Gravity
M= Maximum Weight

Great mnmonic. I wish I knew that one when I was instructing.

Of course, it goes without saying that you must eat, sleep, live and breathe the Seven Factors of Vmc come multi practical time.

Good luck with your rating.

 

[172driver]

M= Maximum Weight

Vmc should be determined at the most adverse weight. Minimum practical test weight is usually the most critical, because the beneficial effect of banking into the operating engine is minimized.

OK...so which is it? Maximum or minimum? Are MEI's not teaching this stuff? I just responded to another post regarding SE climb performance with exactly the same confusion. It does seem like important information and I'm surprised it's being taught incorrectly. In defense, the new reg, for whatever reason, makes no reference to the old. Bad job by the administrator.

Any aircraft certified before Aug. 18, 1983 (all twins currently on the market) must demonstrate Vmc at MTOW. The reg changed in '83 and any new certifications must be done at the most unfavorable weight (light).

 

[pilotshan]

As an active MEI with a fair amount of multi-engine dual given, I teach that max weight is most critical. Vmc does decrease as weight increases, but the effect is very small. So, as we increase our weight, we increase our controllability by a little. But... if we're light we're going to have a much bigger advantage in the performance arena. We've discussed this at length with our examiners and this is the answer they're looking for.

 

[Booker]

OK...so which is it? Maximum or minimum? Are MEI's not teaching this stuff? I just responded to another post regarding SE climb performance with exactly the same confusion. It does seem like important information and I'm surprised it's being taught incorrectly.

Settle down, there. There's a difference between incomplete and incorrect. What everyone has offerred is accurate, just not in all cases. I can't speak for anyone else, but I simply assumed that mrflyguy was talking about airplanes that would presently be in the certification process. If he had asked instead what weight was used when certifying the Piper Seminole, I too would have dug up the regulatory history.

 

[Kaman]

Hi Guys,
I only put down what I was taught, and teach. It's also right out of the FAA Aircraft Flying Handbook. Hope I didn't ruffle anyone's feathers out there. What I know about flying is merely the tip of an enormous iceberg of knowledge to be gained from more experienced folks.

fly safe out there!

ex-Navy Rotorhead

 

[100LL... Again!]

Only 7 factors!? I got got screwed then, because my CFI made me memorize the 107 factors of Vmc, and recite them to the tune of "The Twelve Days of Christmas".

Anyhow. here goes.

Zero sideslip is seldom achieved at 5 degrees bank angle. The reason for the 5 degree limit is this: The higher the bank angle, the more directional control and the lower Vmc is. Imagine a 50 degree bank - you would need little or no rudder to hold heading, but climb performance would be measured with minus signs in front of the numbers.
Zero sideslip will be at 1.5 to 2.5 degrees for most light twins.

As far as weight goes, Vmc is marginally lower at max gross weight. But the problem here is that not enough focus is placed on the performance robbing aspect of higher weight. Who cares that Vmc drops a tiny bit at MaxGW? Irrelevant.

It is fine to memorize these factors, but several of them contribute nothing to the safety of flight. Let's have a look:


Crit Eng Inop/Windmilling ----------------------
Important, but obvious. The difference between crit & non crit engin is minimal. If you are counting on counter-rotating engines to save your butt, do not fly twin engine airplanes.

Full power on Op engine -----------------------
Again, important, but mostly obvious. At the point of engine failure, is it important to philosophize about the Vmc factors?

Max 5 degrees bank ----------------------------
This is a CERTIFICATION maximum, not a FLYING TECHNIQUE. Fly 1-3 degrees, you'll be all set. Go ahead and forget that 1/2 ball stuff also, if you prefer.

Full rudder up to 150 lbs -----------------------
This varies according to certification rules. Again, does it matter?

Gear up --------------------------------------------
This can confuse: some pilots think that extending gear will help stabilize the airplane. The opposite is true: If you are rolling over, the problem is insufficient rudder and/or airspeed. Dropping the gear will contribute nothing, yet some CFIs come away from the '7 factors of Vmc' discussion with just this impression.

Flaps set for takeoff -----------------------------
Again, the most important point is that you get the flaps up.
Do not get the impression that putting them down will lower Vmc enough to stop a rollover.

Aftmost CG ----------------------------------------
Again, nice to know, but irrelevant. Fly the airplane.
If it is aft, you may need to exercise more awareness, but an honest-to-goodness engine out will probably give us all the alertness we can handle.

Max Gross Weight -------------------------------
Irrelevant as far as Vmc is concerned. Far more important to be considering your single-engine climb rate and accerate-stop and -go distances rather than the effect on Vmc.

Sea Level, Std. Day -----------------------------
Reread the section on weight. Same deal.


Continued below.

 

[100LL... Again!]

NOW BEFORE ANYONE FLAMES ME:

I am not saying that we should not learn these factor, but consider this: we do not memorize the certification criteria for any other situation that I can think of. We simply fly the airplane.

The gist of it then is that it is more important to know th OPERATIONAL FACTORS rather tha the certification factors, which are way more abstract. Quick, what is the compression ratio in your engine? What is the proper fuel/air mixture ratio (by weight)? Do you care? I hope not. Because you cannot change them.

I would like to challenge the more creative of you to come up with a scenario for any or all of the 9 factors I have listed where knowing the certification criteria would change how you fly the airplane. In other word, is any of it 'save the day' info.

You may create such a scenario, and I would enjoy reading it.


In the meantime, then here is all you need to know to stay alive in twins as far as the Vmc factos go:

Maintain blueline (or higher, if able) like your life depends on it. It might.

Use a gigantic amount of rudder and lots of aileron. Do not fly banked into the dead engine UNLESS YOU ARE TURNING, and are at blueline or better. Do not be a wussy limp-handed pilot when it comes to aircraft control.

If you are heavy or it is hot out, you won't probably make it on one engine unless you are D-MN GOOD. Even then, I wouldn't count on it.

Bank 1-3 degrees into the good engine for best climb.

Know your engine-out drill. If it takes you more than 5 seconds to think the drill through sitting in a chair, do not fly twins until you have refreshed yourself on the procedures.

Use turbine engines when at all possible.


Comment eagerly awaited.

 

[bobbysamd]

Vmc

The above two posts bring home the point that you must be able to explain factors of Vmc to the same level of detail for your practicals.

As an aside, I do not remember having to explain Vmc to such depth when I took my multi and MEI practicals fifteen years ago. At that time practical test standards were just being introduced. But my examiner was a former manager of the OKC FSDO and had a reputation for being tough. I went to him three times because I got along well with him and he was extremely fair. Then, I remember Riddle multi stage checks to be extremely brutal regarding Vmc, much more so than the FAA. Just be prepared.

 

[100LL... Again!]

Bobbysamd-

I think it it is slightly helpful to know the nine factors and also know how changing them affects Vmc, i.e. gear down lowers vmc, cg fwd lowers cg, etc.

However, to some degree I think that it is a lo like the old days of airline orals where you had to memorize tons of irrelevant info - such as turboprop gear reduction ratios, etc.

The big question is then why we do not go into all the factors behind other aircraft limitations, such as Vne, Vno, Vy, Vx, etc.
We know the number and when it is used, but no concern is given to how it is arrived at (thank goodness).

I still hope someone proves me wrong and comes up with a scenario where knowing one of the factors changes how you fly the airplane, or creates a no-go decision.

The only no-go I can come up with is too hot or too heavy to climb on one engine, or too little runway for accel-stop and -go.
BUT these are not Vmc issues. Hah!

Looking forward to more discussion.

 

[Flymach2]

Hi...

Here are a few thoughts regarding the different opinions on the subject of weight vs. Vmc.

It should be obvious that within this discussion that there are more than one point of view.

As mentioned, FAR § 23.149 indicates the most unfavorable weight in the range of takeoff weights in addition to the other factors.

When an airplane departs at maximum gross weight as opposed to a lower weight, the angle of attack required for flight will be at its highest in comparison to any other weight. Subsequently, asymmetrical thrust or P-factor will also be at its highest in comparison. The increased P-factor will have the effect of increasing Vmc.

Another point of view is this.

Since the airplane is at its heaviest, (max. gross weight), the total lift generated by the wings is also greatest. A 3-5 degree bank into the operative engine will result in the horizontal component of that lift being greater, subsequently having a greater impact in the reduction of Vmc.

Food for thought.

Regards

 

[172driver]

The big question is then why we do not go into all the factors behind other aircraft limitations, such as Vne, Vno, Vy, Vx, etc.

The reason we don't go into those factors is because those numbers are relatively constant. Sure, Vx and Vy change by a few knots with altitude but that's it. Vmc can change from the published value by 30+ knots. That's why we need to know about it.

Here's your scenario...

Take a high performance twin up, preferably turbocharged. Load it all the way aft and away from the operative engine, take minimum fuel and pax, windmill the prop, flaps and gear up, bank ten degrees into the inop engine, apply full power to the operative engine. Think you'll be safe if you're flying above the red line? At the blue line? Let us know.

 

[bobbysamd]

Immaterial head nonsense for practicals

Bobbysamd-

I think it it is slightly helpful to know the nine factors and also know how changing them affects Vmc, i.e. gear down lowers vmc, cg fwd lowers cg, etc.

Agreed. Absolutely. I wasn't discounting your points. It's basic aeronautical knowlege that every multi pilot should know, however . . . .

However, to some degree I think that it is a lo like the old days of airline orals where you had to memorize tons of irrelevant info - such as turboprop gear reduction ratios, etc.

Which is the point I'm trying to make.

All your points are well taken.

Shall we turn this into an anti-Riddle (or FSI - which wasn't quite as bad - or anti-Comair or anti-wherever) stage check discussion?

 

[172driver]

Shall we turn this into an anti-Riddle (or FSI - which wasn't quite as bad - or anti-Comair or anti-wherever) stage check discussion?

We can if you like but we're talking about the importance or nonimportance of being able to explain the factors of Vmc. The FAA has deemed it to be important so, we as pilots, stage check pilots, CFI's, must ensure that Vmc is being taught and learned in detail. This obviously includes not only the factors but the principles behind them.

The reason we should know how the airplane is configured when certified is that we've got to know how they determined where to put that red line. We need to know that Vmc can occur at speeds much higher than the red line. In my opinion, it's as important as knowing that we can stall an airplane at speeds much higher than the bottom of the green arc.

PTS:

b) Effects of density altitude on Vmc
c) Effects of airplane weight and CG on control
d) Reasons for variations in Vmc
e) Relationship of Vmc to stall speed

 

[100LL... Again!]

172-

You've posted a scenario, but I don't see how you have made your point that knowledge of Vmc factors changes the way the pilot approaches the flight. Also, how do you get such an aft CG with such a low weight? Could happen, but hard to do in most aircraft.

Also, Unless you are carrying bowling balls inthe nacelle locker, I don't think you would likely be able to shift the CG enough laterally to make an appreciable difference.

I think the point you are trying to make is that rudder effectiveness would be reduced, but power would remain the same, plus the laterally diplaced CG, etc could cause Vmc to rise above redline.

Even if the aircraft can produce the same power at altitude (or more, in the case of a Seneca II), the prop efficiency is still reduced. Rate of climb diminishes with altitude even if manifold pressure and rpm remain constant. Your scenario is remotely plausible, but highly unlikely.

Last point: What in the world would anybody be doing flying at redline? (Other than training).

If a pilot does not possess the skill or sense to maintain blueline or better, all bets are off.

 

[Deftone45075]

However, to some degree I think that it is a lo like the old days of airline orals where you had to memorize tons of irrelevant info - such as turboprop gear reduction ratios, etc.

26:1 on the Garret TPE-331's.

 

[mrflyguy]

So if the manufacturer must use minimum weight, what would they use for minimum weight? It seems you could always take a little more fuel out or find a little lighter test pilot.