Vmc

[meritflyer]

I thought this may be a good review on Vmc and the factors affecting it after reading the multi add-on thread. So, I will re-post my question to get the discussion going.

Will Vmc ever be higher than the red line on the airpseed indicator? If so, why or why not?

 

[desertdog71]

I thought this may be a good review on Vmc and the factors affecting it after reading the multi add-on thread. So, I will re-post my question to get the discussion going.

Will Vmc ever be higher than the red line on the airpseed indicator? If so, why or why not?

Just did my MEI checkride. I refuse to elaborate.

 

[tgrayson]

I thought this may be a good review on Vmc and the factors affecting it after reading the multi add-on thread. So, I will re-post my question to get the discussion going.

Will Vmc ever be higher than the red line on the airpseed indicator? If so, why or why not?

Possibly. The regs allow a 5 degree bank into the good engine. This bank is higher than what is required for zero sideslip, so you will be sideslipping into the *good* engine, increasing directional control. If you choose to maintain optimal bank (2-3 degrees), then Vmc will be higher.

Having greater than sea level power on the good engine will increase Vmc.

Poor rigging could decrease rudder travel, increasing Vmc.

You could be lighter than what was used during flight tests (difficult), increasing Vmc.

Gear down, on *some* airplanes, could be destabilizing, increasing Vmc.

If Vmc was recorded with flaps down, you might have flaps up, likely increasing Vmc.

That's all I can think of.

 

[meritflyer]

On my MEI ride, the FSDO inspector and I had a lennnggtthhyy disucssion on this subject. As I was explaining such factors that could cause Vmc to "be higher", I was stopped in my tracks.

It was made very clear to me by the inspector, that Vmc would never be higher than the red line on the ASI regardless of configuration and other factors affecting Vmc. She said CFIs dont understand what they're saying when they say "will cause Vmc to be higher".

Such factors outlined in part 23, were very difficult to obtain (ie max power at sea level on operating engine) and models were built to obtain such information. A margin of safety was also incuded within determining Vmc. Nevertheless, as long as the aircraft was flown above red line (Vmc speed) there would be no directional control or Vmc issues regardless of conditions and/or configuration.

 

[tgrayson]

It was made very clear to me by the inspector, that Vmc would never be higher than the red line on the ASI regardless of configuration and other factors affecting Vmc. She said CFIs dont understand what they're saying when they say "will cause Vmc to be higher".

I'd say your Inspector was absurdly mistaken. The intent of the certifcation standards is to give a worst case scenario, but that's only within the context of the assumptions in place when Vmc was tested.

Easy example: what happens to Vmc when you don't touch the rudder? Obviously, Vmc will occur at any airspeed you choose.

Another example, one that is documented in FAA publications, is that Vmc will be 10-15 knots higher if you don't bank at all into the good engine. Check the Airplane Flying Handbook.

 

[tgrayson]

From the Airplane Flying Handbook:

VMC is reduced significantly with increases in bank angle. Conversely, VMC increases significantly with decreases in bank angle. Tests have shown that VMC may increase more than 3 knots for each degree of bank angle less than 5°. Loss of directional control may be experienced at speeds almost 20 knots above published VMC when the wings are held level.


The 5° bank angle maximum is a regulatory limit imposed upon manufacturers in aircraft certification. The 5° bank does not inherently establish zero sideslip or best single-engine climb performance. Zero sideslip, and therefore best single-engine climb performance, occurs at bank angles significantly less than 5°. The determination of VMC in certification is solely concerned with the minimum speed for directional control under a very specific set of circumstances, and has nothing to do with climb performance, nor is it the optimum airplane attitude or configuration for climb performance.

 

[meritflyer]

I'd agree with you 100%. The point she was making was when discussing the factors assosicated with sea level, gear position, flaps, ect.

I was teaching her that Vmc was not a fixed value before I got beotch slapped.

 

[Airdale]

.

It was made very clear to me by the inspector, that Vmc would never be higher than the red line on the ASI regardless of configuration and other factors affecting Vmc. She said CFIs dont understand what they're saying when they say "will cause Vmc to be higher".

I think the FSDO inspector needs to do a retest on her inspector certificate if she thinks an airplane won't Vmc above redline. I think thats the dumbest thing I've heard so far come out of the mouth of an examiner.

Fact is, Vmc is never the same. It changes everyday with changes in temperature and air density. It changes with altitude, it changes with aircraft loading, and it changes based upon who is at the controls.

Vmc will decrease approx. 3 knots per degree of bank angle. The only safe single engine airspeeds you have are Vxse and Vyse (blueline). The airplane can safely maintain directional control at those airspeeds when flown with up to 5 degrees of bank into the op. engine (in *most* cases).

Every multi engine airplane handles differently during an engine failure, and every airplane has different Vmc characteristics. The Seminole has docile Vmc characteristics and is designed as a training airplane. In *most* cases, Vmc will be below a stall in the Seminole under *most* conditions with a good pilot at the controls. Whereas a Piper Commanche has much more violent Vmc characteristics due to its laminer flow wing.

To teach students that Vmc is a set number, or that Vmc will never be above redline, is extremely dangerous and can someday get somebody killed with that mentality. Try doing a single engine go-around in any twin and see how quickly that airplane Vmc's above redline when you shove that throttle forward. Single engine go arounds and Vmc never occurs above redline all have the making for an NTSB report.

It doesn't matter what airspeed loss of directional control occurs at, its irrelevant when you're in an emergency and there is no way to calculate it anyway. The only thing that matters is recognizing loss of directional control - full rudder into the operating engine and the aircraft continues to yaw and roll toward the failed engine. Does not matter what airspeed that happens, the immediate action is the same - reduce power, reduce angle of attack.

Manufacturer's publish Vmc speed as per the FAA so that pilots are aware of the Vmc speed with the FAA's set worst case scenario.

Critical Engine Prop Windmilling - Increases Drag, which must be over come with power, which increases Vmc speed.
Aft Cg reduces rudder effectiveness by decrease its arm, this increases Vmc speed
Standard day conditions yield high density air, which increase engine performance and increases Vmc speed
Take-Off configuration (flaps/gear up) Flaps/gear have a stabilizing keel effect which reduce Vmc speed. Put them up, increase Vmc speed
Bank up to 5 degrees into Op. engine - creates zero s/s - decreases Vmc
Operating engine Max power - increases Vmc speed (which is actually the simple cause of loss of directional control, which is why a reduction in power immediately reduces Vmc speed)
Weight most unfavorable - Light. Light airplane is easily displaced by power, increases Vmc speed.

Those are the conditions they use to determine Vmc speed, and those are the conditions that affect Vmc. Simply put, Power = Vmc. Remove power = Remove Vmc.

Piper determined the Seminole's Vmc speed with the above conditions to be 56kts. 99% of the time the aircraft is not flown with a windmilling prop (if you follow proper procedures), it should not be flown with an Aft Cg - period. Standard day conditions - well a cold day in the winter will maximize engine performance and thus increase Vmc speed, but typically conditions are above standard. T/O config, part of the eng. failure checklist is Flaps/gear up. You should definitely bank into the operating engine, rarely will the airplane be extremely light. Even full of fuel with no persons on board, the aircraft is around 3200lbs, which is much heavier then what they used for Vmc determination. So in *most* conditions, Vmc will be less then 56kts. Vs is 57kts, which will put Vmc below a stall in *most* conditions.

Notice I say *most*. Vmc is a variable, its not a constant indicated airspeed.

 

[tgrayson]

The point she was making was when discussing the factors assosicated with sea level, gear position, flaps, ect.

Since we agree on the incorrectness of her statement in regards to the bank, why grant her any credibility on the rest? Inspectors don't receive any training on this stuff. And most people without a scientific background place too much faith in a number. Empirically derived numbers are only valid under the conditions under which they were derived.

The factors I listed will affect the Vmc number because physics says they must. Engineers when testing Vmc cannot take these things into account, because the factors either have a low probability or are highly variable. The resulting Vmc's could be all over the map and there would be no useful number to provide to pilots. Instead, they choose a set of unfavorable, but well-defined, conditions and base the Vmc number off that. It's conservative, but not the absolutely worst number that you could see.

Just like your stall speed. It's given to you at a forward CG, max gross weight, 1-G. It's intended to be conservative, but you know full well that if you violate the assumptions in the measurement of the speed, you'll stall when going faster. Bank angle, load factor, wing contamination, coordination can all make the speed higher.

So I still reject the whole direction of where your inspector was going. IMO, you demonstrated understanding, but she demonstrated rote memorization.

 

[meritflyer]

The notion that Vmc increases 3 knots per every degree not banked into the engine is another interesting concept. On my commercial ride with a United FO here, we discussed this concept as well in depth. He asked me what would I do if I was making left hand traffic in the patten with a inop left engine. Would I make that 20-30 degree turn into that dead engine? I used the 3 knots per degree explanation why I would be cautious.

So wouldnt turning 20 degrees in the patten increase Vmc from 56 KIAS in a Seminole to over 100 KIAS? I told him infact it would.

He told me to go ahead and make such a turn anyways and that Vmc wouldnt be a factor. He never really offered an explanation why but we did it out in the air and sure enough, no real challenges except overcoming my nerves doing it and dealing with the thrust.

 

[tgrayson]

Would I make that 20-30 degree turn into that dead engine? I used the 3 knots per degree explanation why I would be cautious.

Yes, you would, although I understand your reason for caution. Here's the important concept:

The reason why you have the bank in in the first place is to avoid sideslip towards the dead engine, which increases Vmc. As long as the turn towards the dead engine is coordinated (not ball-centered), then you will not encounter any sideslip and will experience no increase in Vmc.

When I say coordinated in this context, I mean appropriate use of rudder, or lack thereof, in order to keep the ball 1/2 out of center.

Let's say that the left engine is dead, as per tradition. You're using right rudder and 2-3 degrees of bank to the right. The ball will be 1/2 out. To make a left turn, simultaneously bank towards the left and remove some of the right rudder. The yawing tendency of the asymmetric thrust will counteract the adverse yaw, and the turn will be coordinated (not ball-centered).

How do I know? Theory says it's true and a yaw string proves it. If you keep the yaw string straight, there is no sideslip. IMO, you can't teach someone well about multi-engine aerodynamics without using a yaw string.

Even in the absence of good coordination, you're flying blue line anyway, well above Vmc, so any increase in Vmc just isn't that important.

So the "don't turn into the dead engine" is pretty much a myth, and I've never seen it advocated in print. It just seems to be an old wive's tale (OWT) handed down via instructors to their students. I've heard some examiners have had candidates make steep, 360 degree turns towards the dead engine in order to prove the point.

 

[ananoman]

So the "don't turn into the dead engine" is pretty much a myth, and I've never seen it advocated in print. It just seems to be an old wive's tale (OWT) handed down via instructors to their students. I've heard some examiners have had candidates make steep, 360 degree turns towards the dead engine in order to prove the point.

I always had my students do this after they secured an engine for the first time. We had one examiner who would always bitch about doing turns into the dead engine, but you just had to humor him. He was borderline senile anyway.

 

[Airdale]

Turns toward the dead engine are perfectly fine as long as two conditions exist: Its a coordinated turn and its a constant speed turn. If you're fighting to hold altitude in a turn toward the failed engine, you'll pull it right up into a Vmc condition. I have students do steep turns into the dead engine, but I stress to them, make it coordinated and maintain your blueline airspeed. Sure you will loose altitude, but you won't Vmc. Pull a tight turn into the dead engine, yank bank on the yoke to hold altitude and watch the airplane roll over - quickly. With an engine failure in the pattern, you have to keep the turns shallow and avoid overbanking-overyanking. Shouldn't be doing steep turns in the patter period IMO.

 

[meritflyer]

This is good input and constructive dialogue.

Last but not least, is Vmc primarily a roll condition or yaw (and dont say both )?

Why or why not?

 

[meritflyer]

Pull a tight turn into the dead engine, yank bank on the yoke to hold altitude and watch the airplane roll over - quickly.

If I understand what you're saying correctly, you're saying a Vmc roll would occur. Not to say that it wouldnt but Vmc rolls are another OWT in the instructor community. There was a recent article about them I will see if I can dig up and post.

This idea of the airplane suddenly barrel rolling about the longitudinal axis once the plane hits Vmc is very misunderstood.

 

[tgrayson]

This is good input and constructive dialogue.

Last but not least, is Vmc primarily a roll condition or yaw (and don't say both )?

Hmmm....both? irate:

Actually, it depends on the airplane. Some airplanes are rudder limited and some are aileron limited. The moment you run out of either control, you've reached Vmc. Low powered propeller airplanes (and jets) tend to be rudder limited. Some larger propeller airplanes, such as a King Air, might have such a high induced airflow over the wing that you lose roll authority before you lose directional authority. The Flight Test Guide to Part 23 Airplanes (AC 23-8B) hints at this distinction, but you have to be looking for it to see it.

However, roll and yaw are coupled....when you get one, you get the other. As you lose directional control, the aircraft yaws. Both the dihedral effect and the accelerating wingtip induce a roll. And as the aircraft starts to roll, it sideslips, inducing a yaw, etc.

If it weren't for the roll, an airplane losing directional control would just start making boat turns in the sky. If the airspeed continued to decay, the vertical stabilizer would eventually stall. That wouldn't be pretty.

 

[meritflyer]

Again, I would agree. Here is how it was explained to me from a guy flying the Airbus A320 (a DE).

Vmc is primarily a yaw issue or movement about the vertical axis. As the very definition of Vmc is the speed at which the aircraft will be unable to maintain directional control, it was viewed that "directional control" being a heading. I could see that.

Okay, so using the Airbus as an example, the DE said when an engine is lost they do nothing more than put a little rudder trim in. No worries about roll (sideslip I guess).

Using the Seminole as an example he pointed out a Vmc demo and the elements related to such. When would we recover? Answer being loss of heading, stall buffet/warning horn, or actual stall. So, do we ever recover at a rolling moment? I answer, I guess not. The question is re-stated. Is it a roll or yaw issue. I say yaw (as I knew it'd get me past that subject).

Concluding Vmc would primarily be a yaw issue as it relates to rudder authority and loss of directional control (heading).

:insane:

 

[tgrayson]

<<Vmc is primarily a yaw issue or movement about the vertical axis.>>

If the airplane is rudder limited, yes. If it's aileron limited, then there would be uncontrollable rotation around the longitudinal axis (initially).

<<As the very definition of Vmc is the speed at which the aircraft will be unable to maintain directional control, it was viewed that "directional control" being a heading. I could see that.>>

Well, 23.149 actually says "straight flight" and "maintains control". But uncontrollable roll would cause problems in these areas as well.

<<Okay, so using the Airbus as an example, the DE said when an engine is lost they do nothing more than put a little rudder trim in. No worries about roll (sideslip I guess).>>

No induced airflow over the wing, so not much roll, except what might be caused by the sideslip into the dead engine.

<<Using the Seminole as an example he pointed out a Vmc demo and the elements related to such. When would we recover? Answer being loss of heading, stall buffet/warning horn, or actual stall. So, do we ever recover at a rolling moment? >>

This being a light twin, the induced airflow over the wing is relatively small. Rudder should be limiting and I agree that you would see yaw first. Now, if you aren't aggressive enough on the rudder, the slight yaw will produce a roll, but the yaw came first.

<<Concluding Vmc would primarily be a yaw issue as it relates to rudder authority and loss of directional control (heading).>>

For most light twins and jets, sure. But he led you to that conclusion through a limited sampling of the data. It's the exceptions which test the universality of our rules, and some airplanes apparently are unable to maintain wings level while they still have rudder available. This means that Vmc for them is a roll issue.

My observation is that the more information one collects about a subject, the less definitively one can give answers.