What decreases VMC

[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...