What decreases VMC

[DIRT]

When I took my MEL ride a while back the DE nearly had a heart attack when we were doing a single engine go around and I retracted the gear before the flaps he said bring the flaps up first because "you may actually touchdown on the gear before you get to blue line and can begin climbing."

 

[Booker]

Booker,

You've said twice now that flaps affect Vmca in different ways for different aircraft. Can you support that argument with an explanation?

None of the aerodynamic explanations I have heard come from manufacturers or the FAA, so I opt not to offer them. However, I can cite examples of airplanes in which Vmca increases when the flaps are lowered (310, like I already mentioned, SF34, Twin Commander), as well as airplanes in which Vmca decreases as the flaps are lowered, which I will agree is likely most twins.

All this aside, I'd like to know where it is stated in official documents what effect flaps have on Vmca.

 

[172driver]

DIRT,
He was correct for not only the reason you mentioned but because the last notch of flaps is generally a much bigger drag producer than the gear. Remember that good ole drag demo? There is usually a reason for the procedures being published in the order they are in the POH which is why I think it's strange that Booker's school changes it around so much on their published checklist. Not that I think his way is any better or worse...just nonstandard. What school is it if you don't mind Booker? Just curious.

Booker,
Just got your post. You can cite unofficial sources if you like, I don't mind. Come to think of it....what are the FAA approved sources for multi stuff? Here's the only one I have, though I don't know if you consider it 'official':

ASA Multi-Engine Oral Exam Guide p. 2-4

"It is generally accepted that Vmc will be slightly lower with flaps extended. Extended flaps will increase both drag and lift. The drag produced by the extended flap on the operative engine side will tend to oppose the yawing motion of that engine..."

 

[avbug]

172,

The application of flaps is definitely aircraft specific. In airplanes that lose rudder authority with flap application, Vmca increases with application of flaps. This is often the case in aircraft with split flaps, due to the large airflow disruption with flaps extended. Some midwing or high wing aircraft will experience the same thing with flap extention.

If you study out the intent of determining Vmc, you'll find that it's predicated on a takeoff condition, or in other words, the most critical time an engine can fail. At no other time will the aircraft be exposed to as much difficulty and risk as during a takeoff. Hence, power is set for takeoff, flaps are set for takeoff, the aircraft is loaded to the most unfavorable condition. This doesn't at all mean the condition which will produce the highest Vmca...it means the condition most unfavorable for takeoff, and for losing an engien during takeoff (are there any favorable conditions for losing an engine on takeoff...other than being in a simulator??).

When considering published Vmc, many pilots and instructors erroneously assume that it signifies the highest airspeed at which directional control may no longer be maintained during critical engine loss. This isn't so. Vmca may certainly be higher than published Vmc, and it may also be lower.

What does ice do to Vmca? That's the sixty four dollar question, isn't it? The short answer is that it depends. There is no definitive. Icing which reduces rudder authority will mean less rudder is there to counteract assymetrical thrust. Vmca increases. Propeller icing decreases prop efficiency, meaning Vmca decreases. The efficacy and availability of prop deice, boots, etc, all make a difference, as does the symmetry of ice shedding/buildup, and even the type of icing.

As for when to bring up flaps or gear...don't bank on a proceedure because you were taught that way. Bad choice.

Some airplanes are best with the gear coming up first, others flaps first. Some involve flaps part way, and then gear, then the rest of the flaps. Look at the aircraft systems. An open center system with big gear means that only one object is coming up at a time, and the gear may take a long time. It may be a high drag item, while large fowler flaps may be reducing stall speed significantly. Soloution? Gear gets started up early.

In either case, never be in too big a hurry to move anything. An engine failure doesn't require lightening reflexes, and one may do well to follow standard abnormal/emergency protocal when dealing with one. Step one; sit on your hands for ten seconds and think about the situation. Then act; slowly deliberately, and one step at a time. Don't rush. Speed kills. Don't go throwing gear and flap levers; take your time. Get your airspeed, get control, then navigate, then talk, then begin adjusting and moving.

The important thing is to know your airplane. Each airplane is different. Even the same airplane may require a varied technique depending on the phase of flight. A slightly ridiculous example would be caging one on takeoff. The hapless student immediately deploys 50% flaps. The instructor warbles helplessly "What are you doing?!?" The student replies that he understands that as soon as an engine quits, one always goes to 50% flaps.

Of course, the student was meant to retract the flaps to 50%, not add them...but obeying a hard rule without understanding the airpane and the circumstances can be costly.

Consider your systems. Perhaps flaps are electrical, but gear is hydraulic. You just lost an engine, and half or possibly all of your hydraulic capability. What to do? If it's half, your gear may be operating at half speed. This being the case, perhaps you want to get it started up early; it will take longer. You won't be overburdening the system because the flaps are electrical, and these can be brought up simultaneously. Consider your options and work accordingly. It's different for each airplane, and each circumstance.

The most critical thing to know about decreasing Vmca is the most obvious, and it hasn't been mentioned yet...though I know darn well every poster understands it by heart. Pull back the good engine. That will decrease Vmca by all you need. Obviously it also decreases climb performance, and therein lies the crux of minimum control speed single engine flight. This is why gross weight is most unfavorable, even though a lighter weight will result in a higher Vmca.

That emergency landing will be a whole lot easier if it's quiet so you can concentrate, and if you're not spinning uncontrollably. Pulling back the "good" engine accomplishes this quite nicely, should the need arise.

 

[TurboS7]

Reduce power or thrust on the operating engine, reduce it all the way and VMC becomes is not a factor anymore. A curve ball curves only because the vector force of the throw decreases and lets the vector force of the low pressure on the spinning ball move the ball in the direction of the vector. Same things applies to VMC. Reduce the thrust vector to maintain control, especially with a Lear during a V1 cut if things become hairy.

 

[TurboS7]

I just read your post avbug and you beat me to the point. With the exception of yours no where in any of the post did I see reduce the thrust vector. It proves that guys need to think outside the box, procedures are great but they won't save you when things really get nasty. Every nasty thing in aviation requires and action equal and deliberate in the opposite direction to keep from producing an accident. Technology is not the answer in the case of what just happened with DHL, the TCAS told the 757 to decend, at the same time the TU-154 started its decent at the exact instant the 757 did. I am sure that just prior to impact the TCAS yelled Climb, Climb but it was too late. God Bless them all.