What decreases VMC

[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...

 

[Booker]

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, Id, Feather, Gear Up, Flaps Up, Secure

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

Where I work, the SEMI checklist for engine failure above Vr - Takeoff Continued is as follows:

Directional Control - Maintain
Throttles - Full Forward
Props - Full Forward
Mixtures - Full Rich
Fuel Selectors - On
Flaps - Retract
Gear - Retract
Fuel Pumps - On
Inop Engine - Identify (verify)
Inop Throttle - Idle
Inop Prop - Fx
Inop Mix - ICO
Bank - 2-3°
Airspeed - 88 KIAS.

Why this sequence? It's gets all possible power in, then removes all possible drag (incidentally, we NEVER take off with flaps in the SEMI: Flaps 25 is almost the same as lowering the gear), then secures the bad engine. Another nice thing is it makes for a nice flow pattern. For those unfamiliar with the PA44, we call it the softball pitch or bowling ball flow.

For engine failure in flight - speed above Vmca, the checklist is as follows:

Inop engine - Identify (verify)
Operative engine - Power as req.
Airspeed - >= 88 KIAS

Why this? You've set up the best-case scenario for climb/descent performance, short of shutting stuff down. The non-memory checklist then specifies all troublshooting items:

Fuel selector - On
Carb Q - On
Mix - Rich
Fuel Pump - On
Fuel qty - Check
Oil P/T: Check
Mags: Check
Carb Q: As req.

Again, this sequence makes for a nice flow pattern. If the engine is truly gone, then you proceed to securing (throttle/prop/mix, in that order).

 

[172driver]

I don't think you should be wasting time with the fuel selectors and fuel pumps if you have an engine failure at low altitude. I also think putting the throttles fwd first doesn't help if the mixture is too lean for the engine to run. Also, putting a/s last on the checklist is a bad move, IMO. We all know that a/s is a very high priority with an engine failure in any twin. You may have already hit the ground by the time you feather.

Heading
Pitch attitude(airspeed)
Mix
Props
Throttles
Flaps(most drag)
Gear
Identify
Verify
Decide(feather or restart)

This is the way I have been trained and, much more importantly, the way the POH says to do it for the PA44. If you have altitude, performance, time, oil pressure on the dead engine, you may opt for the restart. Then you can start with the fuel selectors and run through your flow. No point in wasting time with these things if you're dropping like a rock at low alt or your engine has lost oil pressure...it's not going to restart no matter how much fuel you pump into it.

 

[Booker]

All your points are valid. However, there are a few things we teach that aren't on the checklist. First, we all know that airspeed and directional control are what keep you alive in a twin. As such, "pitch for 88" is always the mantra for an engine failure close to the ground. It's last on the list only as a reminder, in case it slipped while completing the other items. We also set the FD before each takeoff to the approximate SE angle to help maintian blue line. The reasons the selctors and pumps are on there is that the only times we've *ever* had real engine failures here is when the selectors were in the wrong position or the EDPs failed. Besides, it takes an extra half-second to slide your fingers across them in between the other stuff you're moving. As for the mixture, even if the bad engine sputters more when the throttle goes up, again, it takes a fraction of second to move from the throttle to the mixture, even with the props in between. You're almost doing everything at once whether you go left-to-right or right-to-left. Heck, you could just pretend it's an Antonov and move everything up at once. The end's the same.

Finally, you made mention of the importance of using the POH checklist. I won't disagree. However, our checklists are not something we cooked up overnight and just started using. Each one has been seen and approved by the FAA. As such, if you don't use either our list or the POH list, you get busted for not completing the appropriate checklist. But if you use ours, it holds the same weight as the POH version in the local inspector's eyes.

 

[bigsky]

I wont argue that warmer then standard temps/ high density altitudes and a heavy airplane lower vmc(at least on the drawing board)
1) a heavier airplane is will resist vmc longer/ is harder to displace
2) the 5 degree bank into the good engine is taking advantage of horizontal component of lift to counteract the vmc moment- hence the heavier the airplane the more lift or more horizontal lift being produced.

Catch 22
In real life you lose an engine/ obstacles are getting much bigger in the windshield.... I know for sure I would prefer a light airplane because as long as I fly it correctly vmc will not be a factor and we will climb.

ON THE OTHER hand the heavey plane will have a lower vmc but... correct me if Im wrong but at the same airspeed a heavier airplane will have a higher angle of attack.. which in turn causes more P- factor and more asymetrical thrust which seems to cancelout any benefit of being heavey.

 

[bigD]

It is kind of a catch 22, but I figure that with my policy of not flying the Duchess below blue line under any normal operation, even at light weights I won't be anywhere near Vmc. Therefore I'd rather take the lower weight. I'd imagine that I'd have to think about it some more if I were flying a plane with a higher Vmc.

 

[SigTauPilot]

Just was reading through the post and a ways back 172driver mentioned a list of thangs that lowered Vmc, and flaps down was one of them. Correct me if i am wrong but i always thought that extended flaps raise Vmc----extending the flaps increase the lift on the operating engine due to the accelerated slipstream, which increases the yawing and rolling tendency that must be compensated by the downward deflection of the aileron on the dead engine side. further adding more yaw and drag that must be compensated with more rudder.

Also on engine out procedures i was always taught:
Control: pitch and bank
Power: throttles, props, mixtures
Drag: Flaps and Gear (no particular order)
Identify: dead foot dead engine
Verifiy: verifiy dead engine with the trottle

For that light and cool or hot and heavy? Nice question, but what is my field elevation.

 

[172driver]

SigTau,

Flaps increase keel effect, stabilizing the yawing tendency, thus lowering Vmc. Vmc is not about roll which is what your theory seems to imply. I guess you could just as easily argue that the accelerated slipstream on the operating engine hits the flap. producing more drag, yawing the aircraft in that direction.

 

[Booker]

Just was reading through the post and a ways back 172driver mentioned a list of thangs that lowered Vmc, and flaps down was one of them. Correct me if i am wrong but i always thought that extended flaps raise Vmc----extending the flaps increase the lift on the operating engine due to the accelerated slipstream, which increases the yawing and rolling tendency that must be compensated by the downward deflection of the aileron on the dead engine side. further adding more yaw and drag that must be compensated with more rudder.

This one doesn't seem to die. Truth is it depends on the airplane. In some (e.g., C310), flaps increase Vmca, in some, they lower it.

(Insert comment about the difference between Vmca and Vmc here.)

 

[L.A.pilot]

If you lose an engine, light and cold is better:

Better climb performance
-You have a better chance of not being forced to land

Less distance is required for takeoff AND landing
-You have a better chance of surviving a forced landing

True airspeeds will be lower due to higher air density
-If you do hit something, you will hit at a lower speed

K.E.=1/2 m v^2

The kinetic energy you need to dissipate when crashing varies with mass and the square of velocity.

If you are heavy in less dense air, you are more likely to inadvertantly pitch up and slow below Vmc because the ground will be rushing up toward you!

 

[172driver]

Booker,

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