What decreases VMC

[C-5 MEM]

Here is a question I could not find. What would decrease VMC speed?

 

[deceptibum]

higher altitude........less air pressure up there means less horsepower output of the working engine means less yaw tendency.

 

[C-5 MEM]

Ok Thanks for the reply. It has been a very long time since I had to get into the books for an interview.

 

[bobbysamd]

Decreasing Vmc

Couple of things. Using zero sideslip lowers Vmc; in other words, raising the wing on the dead engine. Think "raise the dead." Slipping raises Vmc.

Gear extended. Sort of like the keel on a boat. Increases stability.

Feathered prop on the "bad" (inop) engine. Windmilling prop increases drag and Vmc.

Go back to your factors of Vmc and they should suggest the answer to your question.

Hope that helped a little. Just like the above, I haven't taught multi in nine years. Miss it, too.

 

[bigD]

A hotter day, higher weight, and more forward CG will do it too.

 

[172driver]

1. Gear extended
2. Flaps down
3. Higher density altitude
4. More bank into operative engine
5. Lower power on operative engine
6. Being in ground effect
7. Higher weight
8. Forward CG
9. Lateral CG toward operative engine
10. Inop prop feathered

Remember to never associate Vmc factors with performance. Vmc is only about rudder effectiveness and thus heading control. As you can see...if the airplane is in the above configuration, you will be able to maintain heading but will be falling to the ground like a rock! When learning the factors you will want to know how the airplane is configured for certification, rather than what makes Vmc the lowest.

Oh...just read your profile...you probably already know the basics. Hope the list helps.

 

[gnx99]

I've been looking for a good take on what structural ICING does to Vmc. Anyone know?

 

[172driver]

Depends what it does to the CG, I would guess. If you have ice on the vertical/horizontal stabilizer...baaad. Nose...goood. Wings=more rearward CG...baaad. Although if you've lost an engine in icing you better keep your speed up anyway.

 

[flyboy]

If you take out bank, VMC actually increases. Try doing a VMC demo without putting your 5 degrees of bank and it will prove this true. You'll VMC 15 knots faster. Also, remember that the 5 degrees of bank is just part of the FAA regulation, you can put more than that and lower VMC even more.

 

[Hovernut]

Regs

Remember, you'll find what the certificated configuration is in 23.149

 

[Nocturnal1]

Vmc

I used to ask this to new PIC's I was instructing:
Today's not your day and you're going to lose an engine on takeoff. Which situation would you choose and why? Lightweight airplane and cold day or heavyweight airplane an hot day?

Believe it or not most "newbies" chose lightweight/cold because that would give them more thrusties for the engine-out climb. Not bad rationale, however, they didn't take into account Vmc. More importantly what more thrusties does to Vmc.

This was in a Part 25 airplane.

 

[172driver]

If it's after liftoff, at climb speed, without usable rwy, I would definitely choose light and cool. On the runway or below Vmc would be another story.

 

[Checks]

Maybe I am an idiot but I would also pick cold and light. You shouldnt be rotating before VMC anyways.

I use this acronym to remember the factors involved in determing VMC-SWAT-MUM

S-Standard day at Sea Level
W-Windmilling Prop
A-Aft CG
T-Takeoff configuration
M-Max Pwr on the operative engine
U-Up to 5 degrees bank into the operative engine
M-Max gross weight

Not included is OGE and cowl flaps in the takeoff position(usually open)

 

[bigD]

Hmmm....I'm surprised Avbug hasn't poked his head in here yet. Usually anytime anyone asks a Vmc question - his response is something like, "Vmc doesn't change! It's a number set by the manufacturer of the aircraft. Now, you may lose rudder authority at airspeeds higher or lower than Vmc, but the actual value of Vmc doesn't change at all."

Of course, don't get him started about what TBO really means!

Juuuuust kidding Avbug!

 

[Timebuilder]

Ah, you see? You have benefitted from Avbug's wisdom, already.

I vote for cool and light, too. It gives me better options, including whatever single engine climb I may need for obstacle avoidance.

Since published Vmc is something of a "worst case scenario", the actual speed where control is lost will likely be lower. I would only experiment with the region below redline with a brick s***house directly in my path, and immediately pitch back to blueline, if possible. Just as in the Vmc demo, at the moment you sense a loss of directional control, you MUST pull the power to avoid the rollover. You may survive if you land cockpit-up. You won't if you land cockpit-down.

Summary: Land if possible, continue takeoff if not possible, and know what performance may be conservatively expected before you taxi onto the runway. You might have to wait until later in the day to safely depart in a light twin.

 

[cvsfly]

When you you have fininished ME training and ME instructing you probably will never do VMC demos again. (Of course we are always training, right?) Hopefully you will never be at a speed even close to VMC if you ever loose an engine unless you are already on the ground.

 

[Hovernut]

Weight

Checks,

The weight isn't necissarilly max gross. 23.149 says "weight most unfavorable."

Typically this will be lightest! Here's why. If you remember your H.S. physics, you'll be familiar with the equation F=M*A. Force equals mass times acceleration. Rearranging, F/M=A, so for the same force (takeoff thrust), a reduction in mass yields greater acceleration....VMC yaw/roll tendancies! You can displace a light a/c easier and faster than a heavy a/c. Hold a brick in one hand and a tissue in the other, blow on both and see which one displaces easier!

Regards

 

[172driver]

Aircraft certified before 8/8/93 are allowed to use max gross weight for certification.

The new reg begs the question...what is the most unfavorable weight? Enough gas to get around the pattern and a 90 pounder flying? Doesn't seem very standardized.

 

[Timebuilder]

Acording to what I have seen, the interpretation is max takeoff weight, "or less if required for the demonstration", before 8/8/93.

 

[172driver]

That is the old reg. The newer one definitely specifies 'most unfavorable weight.'

 

[Booker]

Aircraft certified before 8/8/93 are allowed to use max gross weight for certification.

The new reg begs the question...what is the most unfavorable weight? Enough gas to get around the pattern and a 90 pounder flying? Doesn't seem very standardized.

Usually it is minimum practical test weight. Reference AC 23-8 (get ready - it's 228 pages).

The FAA rationale is found in the Airplane Flying Handbook. Up to five degrees of bank may be used in testing. Reduce the weight, reduce the lift required to sustain flight. Reduce the lift required, reduce the horizontal lift that opposes the yaw at five degrees of bank. Reduce the horizontal lift produced, increase the rudder force required for a given airspeed.

Furthermore, flaps' affect on Vmca is aircraft specific.

 

[flyboy]

I think the reg actually says most unfavorable cg, not most unfavorable weight.

 

[Timebuilder]

>>That is the old reg. The newer one definitely specifies 'most unfavorable weight.'

I'd say that's appropriate, since most of the twins we fly were indeed so certificated. Both the Seneca and Navajo I few were built in 1978, so the "old reg" applies to them, and perhaps 90 percent or more of what we train in nationwide.

If you are forunate enough to be training in a twin manufactured since August of 1993, you are one of the very few who would be concerned with the "new reg".

So, since discussions of Vmc happen in a training scenario, I'd be able to describe the method under which the particular aircraft received its certification for purposes of an oral for multi or MEI.

 

[172driver]

Agreed, and just to clarify, although you probably intended this already. It doesn't matter when the actual aircraft was built, only when its type was certificated. Example: We fly brand new Seminoles but still follow the 'old' reg because the PA44 was type certificated long before 8/93.

 

[Timebuilder]

True.

My point of mentioning an aircraft built before Aug 1993 included the inherent idea that the type certificate would have preceeded the building of the plane.

Who has type certified a twin since 1993? I can't name one off the top of my head.

 

[172driver]

I was curious about that myself. I bet someone on this board can come up with one.

 

[Checks]

I stand corrected about the max gross weight, thank you.

I would still take off light and cold.

Here is another question for you guys,

When doing engine failures I have always done:

Mix, Prop, Throttle, Gear Up, Flaps Up, Id, Feather, Secure

I heard an instructor tell a guy Flaps Up then Gear Up. Was I taught wrong?

 

[Timebuilder]

I was taught, and I taught my students:

Mixture rich

Props forward

Throttles open

Flaps up

Gear up

Identify

Verify

Feather (if enroute, attempt the restart first)

In a takeoff scenario, where immediately puting the airplane back on the ground is not possible, you must get rid of the drag, ASAP, then identify and verify the failed engine, put you hand on the prop lever in question and make sure you haven't nervously grabbed the wrong one, then feather. Pitch to blue line, and avoid obstacles. If a safe return to the field is possible, which is most often unlikely in a piston twin, then do so. Otherwise, use the good engine to help you pick WHERE you will make your emergency landing.

 

[bigD]

Checks - I've seen it done both ways. I was taught to retract the flaps first, and then the gear, mainly because the flaps are a more draggy item. However between the all of one second that it takes to move from the flap handle to the gear handle, or vice versa, I'm not sure if it'd really make much difference. Plus, for me, the gear comes up when there's no available runway left (unless I'm departing from a long runway), so if the gear is down when I lose one - in theory I should be closing the throttles and landing anyway.

 

[Booker]

I think the reg actually says most unfavorable cg, not most unfavorable weight.

Not anymore. 23.149(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...