What decreases VMC

[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.'