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.