Newbie Vmca question
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As I understand it, the red line is the airspeed at which the rudder can no longer counter the asymmetric thrust in non accelerating flight. In that sense it would be quite accurate and determined by testing (the test pilot ran out of rudder). Note that the plane should be far from stalling, so it wont immediately spin, but directional control is need to reach a desired landing point ...
As a certificated minimum, it would have been determined in the worst possible loading condition I believe.
Cheers
As a certificated minimum, it would have been determined in the worst possible loading condition I believe.
Cheers
No, it won't necessarily be far from stalling. How does 1 kt sound? That's what one light twin has between published Vmc & Vs. And on any particular day Vmc can be anywhere on either side of Vs. Normally aspirated typically have Vmc lower than Vs once they're more than several thousand feet high or have reduced power from maximum, other factors being equal. On the other hand, each degree of bank less than 5 deg towards the live engine is ~3kts increase in Vmc.
Also the criteria for determining Vmc has two, somewhat separate tests: One is more dynamic ie the sudden failure but able to limit heading change to 20 deg or less, and the other steady state to maintain heading afterwards. They're allowed use up to 150lb force applied to the pedal, and even after trimming you can be left with 20 lb residual. It's rare in normal flight to ever apply 150lb force with your foot, let alone be called upon to do it without warning - which would be the case for an unexpected Vmc departure.
A Vmc recovery, as usually taught, is more akin to the steady state situation.
The important thing to note is that reducing power on the good engine is an important part of the recovery, not just regaining speed. Unfortunately, at low heights the time & spaces to effect a recovery is limited *and* the pilot has to go against the desire to maintain max. available power to avoid a forced landing. In a light twin, a forced landing is often inevitable, just like in a single. Doing so under control almost invariably has a far better outcome than attempting to continue flight until a Vmc departure has you turn turtle & cartwheel in.
Vmc is a place to be avoided, but just as importantly, so is Vs when asymmetric.
Also the criteria for determining Vmc has two, somewhat separate tests: One is more dynamic ie the sudden failure but able to limit heading change to 20 deg or less, and the other steady state to maintain heading afterwards. They're allowed use up to 150lb force applied to the pedal, and even after trimming you can be left with 20 lb residual. It's rare in normal flight to ever apply 150lb force with your foot, let alone be called upon to do it without warning - which would be the case for an unexpected Vmc departure.
A Vmc recovery, as usually taught, is more akin to the steady state situation.
The important thing to note is that reducing power on the good engine is an important part of the recovery, not just regaining speed. Unfortunately, at low heights the time & spaces to effect a recovery is limited *and* the pilot has to go against the desire to maintain max. available power to avoid a forced landing. In a light twin, a forced landing is often inevitable, just like in a single. Doing so under control almost invariably has a far better outcome than attempting to continue flight until a Vmc departure has you turn turtle & cartwheel in.
Vmc is a place to be avoided, but just as importantly, so is Vs when asymmetric.
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Banking into the good engine isn't done on transport category aircraft; engine-out situations are handled with the wings level (specifically line operations, as opposed to certification). Vmc numbers aren't published for all aircraft, either.
In light aircraft, Vmc can be greater or lesser than the red radial line. It should be thought of as a general reference or reminder, but nothing more. On a colder day than the day certification was performed, the engine will produce more power, and loss of directional control will occur at a higher value than the red radial line. One must also take into account the fact that discrepancies in airspeed indication, pitot placement, local airflow, lag, and other factors mean that your airplane here, today, isn't necessary displaying exactly what Joe Schimoley's airplane did 30 years ago during certification trials.
Many airplanes, unless trimmed out, can leave your leg shaking and quivering after some time doing single engine work, or engine-out work. Especially after a lengthy session. On some airplanes, it's not uncommon to need 75 lbs of force to press the rudder or hold full rudder. On some airplanes, it may require that to hold the rudder with all engines working, if using full rudder deflections. When training with lots of power changes and not much opportunity to retrim, it can be a real work-out.
Playing around near the published Vmc numbers is a bad idea. One can alleviate a lot of one's work load, including the shaking syndrome and the need to feel like banking into the other engine, by carrying more airspeed. More airspeed means less bank needed, less rudder needed, and an easier job of flying the airplane.
If you do find yourself slipping down toward the evil rad radial line, remember that it represents a solitary theoretical number from long ago, and has nothing at all to do with when you'll lose control on a given day.
In light aircraft, Vmc can be greater or lesser than the red radial line. It should be thought of as a general reference or reminder, but nothing more. On a colder day than the day certification was performed, the engine will produce more power, and loss of directional control will occur at a higher value than the red radial line. One must also take into account the fact that discrepancies in airspeed indication, pitot placement, local airflow, lag, and other factors mean that your airplane here, today, isn't necessary displaying exactly what Joe Schimoley's airplane did 30 years ago during certification trials.
Many airplanes, unless trimmed out, can leave your leg shaking and quivering after some time doing single engine work, or engine-out work. Especially after a lengthy session. On some airplanes, it's not uncommon to need 75 lbs of force to press the rudder or hold full rudder. On some airplanes, it may require that to hold the rudder with all engines working, if using full rudder deflections. When training with lots of power changes and not much opportunity to retrim, it can be a real work-out.
Playing around near the published Vmc numbers is a bad idea. One can alleviate a lot of one's work load, including the shaking syndrome and the need to feel like banking into the other engine, by carrying more airspeed. More airspeed means less bank needed, less rudder needed, and an easier job of flying the airplane.
If you do find yourself slipping down toward the evil rad radial line, remember that it represents a solitary theoretical number from long ago, and has nothing at all to do with when you'll lose control on a given day.
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Please, let's be clear on the difference between Vmc, and Vmca. The two are becoming confused.
Vmca is shown on the ASI by a red line. It is determined by a test pilot, under very specific circumstances. It represents a speed at which, if an engine suddenly fails, the test pilot was able to recover with a heading change of no more than 20 degrees.
Vmc can, and does, vary. Therefore, it can't be published. It may be higher or lower than Vmca, and it may be higher or lower than stall speed. (Incidentally, stall speed will be lower than the published Vs, because Vs is published at idle power, and stall speed will decrease with power, even if that power only comes from one engine.) Vmc is determined by gradually slowing the aircraft down until you reach such a speed that you can't maintain control.... that is very different from Vmca in a number of respects.
Tinstaafl, where do you get your figure of 3kt per degree of bank? My school teaches recovery with wings level, but I demonstrate Vmc to students with wings level and with 5 degrees of bank, and demonstrate that it makes a difference of about 2kt to Vmca. (In fact, I don't bother doing the demo unless it's a smooth day, because the difference is so small you can't see it in bumpy weather... although it's covered in groundschool instead.)
FFF
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Vmca is shown on the ASI by a red line. It is determined by a test pilot, under very specific circumstances. It represents a speed at which, if an engine suddenly fails, the test pilot was able to recover with a heading change of no more than 20 degrees.
Vmc can, and does, vary. Therefore, it can't be published. It may be higher or lower than Vmca, and it may be higher or lower than stall speed. (Incidentally, stall speed will be lower than the published Vs, because Vs is published at idle power, and stall speed will decrease with power, even if that power only comes from one engine.) Vmc is determined by gradually slowing the aircraft down until you reach such a speed that you can't maintain control.... that is very different from Vmca in a number of respects.
Tinstaafl, where do you get your figure of 3kt per degree of bank? My school teaches recovery with wings level, but I demonstrate Vmc to students with wings level and with 5 degrees of bank, and demonstrate that it makes a difference of about 2kt to Vmca. (In fact, I don't bother doing the demo unless it's a smooth day, because the difference is so small you can't see it in bumpy weather... although it's covered in groundschool instead.)
FFF
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Yes, SNS3Guppy, they are used interchangeabley, but they shouldn't be.
You said:
For you, on any given day? It's not. It reflects a demonstrated certification number obtained by the manufacturer. Any change in weight, center of gravity, technique, configuration, air density, propeller efficiency, power setting, aerodynamics, vertical gust (turbulence), etc, will have an impact on that figure.
Vmc - the value which changes changes depending on weight, centre of gravity, etc etc etc, is generally measured and demonstrated by first failing an engine at a safe speed, then gradually slowing down.
In contrast, red line speed, Vmca, is measure by first flying at the given speed, then seeing whether control can be maintained when a sudden engine failure occurs.
By using the two interchangeably, this important difference is being overlooked (or at least might well be overlooked by someone reading this thread who doesn't know better), which might lead someone to expect an aeroplane to be capable of doing something which it can't.
(It is very important, when talking about any speed, that we are very clear which speed it is that we are talking about. We don't use Vne, Vno and Va interchangeably because they both happen to be related to the high-speed limits of the aircraft. If we did, it would cause confusion. And so we also shouldn't use Vmc and Vmca interchangeably either, because that too will cause confusion.)
FFF
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You said:
how accurate is the little red line?
In contrast, red line speed, Vmca, is measure by first flying at the given speed, then seeing whether control can be maintained when a sudden engine failure occurs.
By using the two interchangeably, this important difference is being overlooked (or at least might well be overlooked by someone reading this thread who doesn't know better), which might lead someone to expect an aeroplane to be capable of doing something which it can't.
(It is very important, when talking about any speed, that we are very clear which speed it is that we are talking about. We don't use Vne, Vno and Va interchangeably because they both happen to be related to the high-speed limits of the aircraft. If we did, it would cause confusion. And so we also shouldn't use Vmc and Vmca interchangeably either, because that too will cause confusion.)
FFF
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Yes, SNS3Guppy, they are used interchangeabley, but they shouldn't be.
(It is very important, when talking about any speed, that we are very clear which speed it is that we are talking about. We don't use Vne, Vno and Va interchangeably because they both happen to be related to the high-speed limits of the aircraft. If we did, it would cause confusion. And so we also shouldn't use Vmc and Vmca interchangeably either, because that too will cause confusion.)
Vmc and Vmca are the same. Vno, Va, and Vne are not.
Vmc means minimum control speed with the critical engine inoperative. Period.
The certification standard for most light twins, or at least those produced in the United States, is 14 CFR Part 23. The definitions for Part 23 are found in Part 1, and the definition provided therein for Vmc is quite simply "VMC means minimum control speed with the critical engine inoperative."
Provision of this information in the aircraft flight manual is a requirement of the certification regulation 14 CFR 23.1583, as noted:
§ 23.1583 Operating limitations.
The Airplane Flight Manual must contain operating limitations determined under this part 23, including the following -
(a) Airspeed limitations. The following information must be furnished:
(1) Information necessary for the marking of the airspeed limits on the indicator as required in § 23.1545, and the significance of each of those limits and of the color coding used on the indicator.
(2) The speeds VMC, VO, VLE, and VLO, if established, and their significance.
Pick up a dozen aircraft flight manuals for various light twins, and you'll find the terminology used interchangeably; some manufacturers will refer to the red line as Vmc, others to it as Vmca, and in some cases, you'll find it used interchangeably in the same aircraft flight manual or pilot operating handbook.
The FAA uses it interchangeably throughout it's literature, because Vmca is Vmc. Vmc is defined as an airborne speed by the use of Vmca, and as a ground speed by Vmcg.
The certification regulation spelling out the establishment of Vmc is 14 CFR 23.149:
§ 23.149 Minimum control speed.
(a) VMC is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and thereafter maintain straight flight at the same speed with an angle of bank of not more than 5 degrees. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure expected in service with respect to controllability.
(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 and with the airplane airborne and the ground effect negligible, for the takeoff configuration(s) with -
(1) Maximum available takeoff power initially on each engine;
(2) The airplane trimmed for takeoff;
(3) Flaps in the takeoff position(s);
(4) Landing gear retracted; and
(5) All propeller controls in the recommended takeoff position throughout.
(c) For all airplanes except reciprocating engine powered airplanes of 6,000 pounds or less maximum weight, the conditions of paragraph (a) of this section must also be met for the landing configuration with -
(1) Maximum available takeoff power initially on each engine;
(2) The airplane trimmed for an approach, with all engines operating, at VREF, at an approach gradient equal to the steepest used in the landing distance demonstration of § 23.75;
(3) Flaps in the landing position;
(4) Landing gear extended; and
(5) All propeller controls in the position recommended for approach with all engines operating.
(d) A minimum speed to intentionally render the critical engine inoperative must be established and designated as the safe, intentional, one engine inoperative speed, VSSE.
(e) At VMC, the rudder pedal force required to maintain control must not exceed 150 pounds and it must not be necessary to reduce power of the operative engine(s). During the maneuver, the airplane must not assume any dangerous attitude and it must be possible to prevent a heading change of more than 20 degrees.
(f) At the option of the applicant, to comply with the requirements of § 23.51(c)(1), VMCG may be determined. VMCG is the minimum control speed on the ground, and is the calibrated airspeed during the takeoff run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane using the rudder control alone (without the use of nosewheel steering), as limited by 150 pounds of force, and using the lateral control to the extent of keeping the wings level to enable the takeoff to be safely continued. In the determination of VMCG, assuming that the path of the airplane accelerating with all engines operating is along the centerline of the runway, its path from the point at which the critical engine is made inoperative to the point at which recovery to a direction parallel to the centerline is completed may not deviate more than 30 feet laterally from the centerline at any point. VMCG must be established with -
(1) The airplane in each takeoff configuration or, at the option of the applicant, in the most critical takeoff configuration;
(2) Maximum available takeoff power on the operating engines;
(3) The most unfavorable center of gravity;
4) The airplane trimmed for takeoff; and
(5) The most unfavorable weight in the range of takeoff weights.
Let's not forget Vmcl, which is the minimum control speed in landing configuration. Then there's Vmcl-2, which is applicable to aircraft with three or more engines (before anyone gets uppity about this being a light airplane forum, let's nor forget that there are indeed light airplanes with three or more engines; the Trilander springs to mind). Clearly it's not quite as simple as you might think.
Vmc is the basic definition, and simply means the speed at which directional control can no longer be maintained. It's specified, or clarified to the phase of operation by the addition of letters. In the case of the topic under discussion Vmca, to signify the minimum control speed while airborne.
Lest we simply read Part 23 and let it drop, let's keep in mind that not only do various manufacturers interchangeably use Vmc and Vmca, but so does the FAA throughout it's various publications. In fact, in some handbooks, such as the Pilots Handbook of Aeronautical Knowledge, Vmca is used, rather than Vmc. In Appendix A of Part 60, dealing with certification of simulators, for example, the FAA only refers to Vmca. In Appendix F, providing definitions, we find the following:
Vmc Minimum Control Speed.
Vmca Minimum Control Speed in the air.
Vmcg Minimum Control Speed on the ground.
Vmcl Minimum Control Speed--Landing.
Vmca Minimum Control Speed in the air.
Vmcg Minimum Control Speed on the ground.
Vmcl Minimum Control Speed--Landing.
• VR — Speed at which the rotation of the airplane is initiated to takeoff attitude. This speed cannot be less than V1 or less than 1.05 x VMCA (minimum control speed in the air). On a single-engine takeoff, it must also allow for the acceleration to V2 at the 35-foot height at the end of the runway.
FAA H-8083-25a, the Pilots Handbook of Aeronautical Knowledge (under Chapter 10, Aircraft Performance), we find the terms defined as:
• VMCG—minimum control speed on the ground, with one engine inoperative, (critical engine on two-engine airplanes) takeoff power on other engine(s), using aerodynamic controls only for directional control (must be less than V1).
• VMCA—minimum control speed in the air, with one engine inoperative, (critical engine on two-engine aircraft) operating engine(s) at takeoff power, maximum of 5° bank into the good engine(s).
Advisory Circular AC25-7A, a flight test guide for certification, states:
(1) General.
(i) Prior to beginning the minimum control speed tests, an evaluation should be conducted to determine which engine's failure will result in the largest asymmetric yawing moment (i.e., the "critical" engine). This is typically done by setting one outboard engine to maximum thrust, setting the corresponding opposite engine at idle, and decelerating with wings level until full rudder is required. By alternating power on/power off from left to right, the critical engine can be defined as the idle engine that requires the highest minimum speed to maintain a constant heading with full rudder deflection.
(ii) For propeller-driven airplanes. VMCA, VMCG, and VMCL (and VMCL-2, as applicable) should be determined by rendering the critical engine(s) inoperative and allowing the propeller to attain the position it automatically assumes. However, for some engine/propeller installations, a more critical drag condition could be produced as the result of a failure mode that results in a partial power condition that does not actuate the automatic propeller drag reduction system (e.g., autofeather system). One example is a turbopropeller installation that can have a fuel control failure, which causes the engine to go to flight idle, resulting in a higher asymmetric yawing moment than would result from an inoperative engine. In such cases, the minimum control speed tests must be conducted using the most critical failure mode. For propeller-driven airplanes where VMCA is based on operation of a propeller drag reduction system, VMCA should also be defined with the critical engine at idle to address the training situation where engine failure is simulated by retarding the critical engine to idle. If VMCA at idle is more than one knot greater than for the engine failure with an operating drag reduction system, the idle engine VMCA should be included in the Normal Procedures section of the Airplane Flight Manual (AFM) as advisory information to maintain the level of safety in the aforementioned training situation.
(iii) Airplane Flight Manual values of VMCA, VMCG, and VMCL (and VMCL-2, as applicable) should be based on the maximum net thrust reasonably expected for a production engine. These speeds should not be based on specification thrust, since this value represents the minimum thrust guaranteed by the engine manufacturer, and the resulting minimum control speeds will not be representative of what could be achieved in operation. The maximum thrust used for scheduled AFM minimum control speeds should represent the high side of the tolerance band, but may be determined by analysis instead of tests.
(iv) When determining VMCA, VMCL, and VMCL-2, consideration should be given to the adverse effect of maximum approved lateral fuel imbalance on lateral control availability. This is especially of concern if tests or analysis show that the lateral control available is the determining factor of a particular VMC.
Note this last sentence, "of a particular Vmc." There is more than one Vmc; various terms are used to cllarify Vmc. Vmc means only the speed at which directional control is lost. It doesn't define which speed is to be used, hence the application of Vmca, Vmcg, and Vmcl (and of course, Vmcl-2).
Vmca is Vmc. Vmcg is Vmc. Vmcl is Vmc. Vmcl-2 is Vmc.
The terms may be used interchangeably, except that when the terminology "Vmc" is given, it doesn't specifically state whether in flight or on the ground, or reference the configuration or circumstances under which control is expected to be lost. It is,however, used frequently by the governing bodies, certification authorities, manufacturers, and test facilities and personnel, in an interchangeable basis with Vmca.
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Wow - someone's very bored to have found that many quotes!
I wasn't aware that the FAA use the term Vmc so loosely, and I stand corrected. I don't have anything from the CAA to quote back at you (I'm not bored enough!), but certainly from the point of view of the training which is given as part of the MEP rating, and the associated written exam, Vmc has a more definite meaning on this side of the pond - which is why it's always bothered me when people use the term "incorrectly".
However, I don't think my Vne, Vno, Va comparison was apples and oranges at all. Let's say we invent a phrase, "Vmax", which we use interchangeably to mean either Vne, Vno, Va, or any other maximum speed under a given set of circumstances. In other words, there would be more than one Vmax. It would be very confusing, wouldn't it? Well, isn't the FAA use of Vmc, just as confusing? Because it can apply to many different speeds - you say yourself that there is more than one Vmc - and it needs additional clarification as to exactly which speed it refers to each time it's used.
Anyway, thanks for the informative quotes. I will bear in mind the FAA's loose definition of Vmc in future!
FFF
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I wasn't aware that the FAA use the term Vmc so loosely, and I stand corrected. I don't have anything from the CAA to quote back at you (I'm not bored enough!), but certainly from the point of view of the training which is given as part of the MEP rating, and the associated written exam, Vmc has a more definite meaning on this side of the pond - which is why it's always bothered me when people use the term "incorrectly".
However, I don't think my Vne, Vno, Va comparison was apples and oranges at all. Let's say we invent a phrase, "Vmax", which we use interchangeably to mean either Vne, Vno, Va, or any other maximum speed under a given set of circumstances. In other words, there would be more than one Vmax. It would be very confusing, wouldn't it? Well, isn't the FAA use of Vmc, just as confusing? Because it can apply to many different speeds - you say yourself that there is more than one Vmc - and it needs additional clarification as to exactly which speed it refers to each time it's used.
Anyway, thanks for the informative quotes. I will bear in mind the FAA's loose definition of Vmc in future!
FFF
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Why do it if it's not fun?
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(Quick aside... I've found my debate with SNS3Guppy really interesting, but I do feel we owe an apology to FlyingKiwi for hijacking his thread. The discussion has gone way beyond the scope of the original question!
And it's so refreshing to have a good debate on PPRuNe without it turning personal in any way!)
FFF
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And it's so refreshing to have a good debate on PPRuNe without it turning personal in any way!)
FFF
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Dammit, FFF, you.....you......you bloody apologist, you! So take that!
Oh, the approx. 3 knots / degree I saw used in various USA based reference texts & online but can't remember where. It's only approximate so your 2 kts /degree could well be more accurate in the type you fly. It could also be related to situation ie static vs dynamic heading control.
Vmc vs Vmca et. al.: The FAA in its 'Airplane Flying Handbook' makes it clear that Vmc can vary and also that the published Vmc is under a particular set of conditions. Generally I think context provides enough distinction as to whether one is referring to the published or 'at the moment' Vmc. Published Vmc is only a guide because unless every certification criteria is met then it is unlikely to be correct for the moment. I tend to use Vmca/Vmcg to distinguish between themselves if there is likely to be confusion about which one I mean.
Oh, the approx. 3 knots / degree I saw used in various USA based reference texts & online but can't remember where. It's only approximate so your 2 kts /degree could well be more accurate in the type you fly. It could also be related to situation ie static vs dynamic heading control.
Vmc vs Vmca et. al.: The FAA in its 'Airplane Flying Handbook' makes it clear that Vmc can vary and also that the published Vmc is under a particular set of conditions. Generally I think context provides enough distinction as to whether one is referring to the published or 'at the moment' Vmc. Published Vmc is only a guide because unless every certification criteria is met then it is unlikely to be correct for the moment. I tend to use Vmca/Vmcg to distinguish between themselves if there is likely to be confusion about which one I mean.
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Tinstaafl,
I know when I've been beaten! (Or, to put it another way, we're never experienced enough to not be able to learn something new.)
As for my 2kt, though, I meant 2kt altogether, not 2kt per degree. Typically, I might find that Vmc (the static one, decreasing speed gradually, with the particular conditions of the day) would be 70kt with no bank, and 68kt with 5 degrees of bank. That's considerably different to whatever you read! Yes, dynamic vs static could explain the difference... although your misunderstanding of my earlier post means that the difference is considerably more than I think you thought I was saying.
FFF
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I know when I've been beaten! (Or, to put it another way, we're never experienced enough to not be able to learn something new.)
As for my 2kt, though, I meant 2kt altogether, not 2kt per degree. Typically, I might find that Vmc (the static one, decreasing speed gradually, with the particular conditions of the day) would be 70kt with no bank, and 68kt with 5 degrees of bank. That's considerably different to whatever you read! Yes, dynamic vs static could explain the difference... although your misunderstanding of my earlier post means that the difference is considerably more than I think you thought I was saying.
FFF
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I will bear in mind the FAA's loose definition of Vmc in future!
Vmc means the speed at which directional control can no longer be maintained, period.
What Vmc doesn't tell you is when and where.
What Vmc does not do is give you specifics. Vmc in the air is not the same as Vmc on the ground. Vmc in the air under the conditions predicated for Vmca is not the same as Vmc while configured for landing, Vmcl (or Vmcl-2, as applicable).
To assign the red radial line on the airplane as Vmc is correct, because that's the published speed at which directional control may be anticipated to be no longer available under the prescribed certification conditions for air operations. Vmcg is not shown on airspeed indicators, nor is Vmcl. therefore, the red line is Vmc, one of several Vmc's but it is specifically Vmca. To refer to it as either one is correct, because this is a correct definition. Not a "loose" one. Vmc on the airspeed indicator is the published certification number at which directional control can no longer be maintained within 20 degrees when the critical engine is suddenly retarded, the good engine maintained at takeoff power, an aft center of gravity with the least favorable weight, in the takeoff configuration. This is certification Vmc, and as it's an airborne number in the takeoff configuration, it is also Vmca; the terms are synonymous for this application.
The same cannot be said of the red radial line and Vmcg, Vmcl, or Vmcl-2. Those numbers are not the same as Vmca (although they are Vmc numbers). Accordingly, whereas we know that the red radial line speaks to specific conditions of establishment, we know that this particular Vmc value is only an airborne, takeoff-configuration number. It is Vmca, but it a Vmc value none the less. To refer to any of the Vmc values as Vmc is correct, but to refer to them in context is more correct, or more precisely, more precise. That is, while Vmca, Vmcg, Vmcl, and Vmcl-2 are all Vmc, a more exact rendering of "Vmc" when referring to these speeds appends the letter or letters which speak to the circumstances under which the speed is derived.
It is not correct to say that Vmc is the red radial line, but that Vmca is the actual speed at which control will be lost. Neither is it correct to say that Vmca is the red radial line, and Vmc is the actual speed at which control will be lost. Vmca is Vmc. Vmc is Vmca. Vmca is Vmc in the air. Vmc in the air is Vmca. This isn't a loose definition; it's a regulatory one, a design and standards one, and one spelled out throughout technical manuals, handbooks, manufacturer publications, certification standards, and so forth. It's not the definition which is loose in this case, but perhaps your understanding of the subject. I realize that you may have been taught differently in class, but that doesn't change the fact.
As Tinstaafl noted, whether one chooses to refer to the red radial line (or published numbers) as Vmc or Vmca makes no matter. It's a general guideline. The actual airspeed under which control might be no longer possible to maintain will vary with aircraft loading, configuration, ambient conditions, center of gravity, and so forth. The actual Vmc (or Vmca, if you will) varies, and can be lesser or greater than published.
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SNS3Guppy,
Given the points you made by your earlier quotes, I'd agree with almost everything you said in your last post.
I think the only thing we're now disagreeing on is whether Vmc is a "loose definition". This might be subjective, and I think we'll have to agree to disagree, because despite agreeing with what you're saying, I still think that it's fair to call any definition which can have a number of different meanings "loose". I don't expect you to agree with me on that, it's just my opinion.
FFF
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Given the points you made by your earlier quotes, I'd agree with almost everything you said in your last post.
I think the only thing we're now disagreeing on is whether Vmc is a "loose definition". This might be subjective, and I think we'll have to agree to disagree, because despite agreeing with what you're saying, I still think that it's fair to call any definition which can have a number of different meanings "loose". I don't expect you to agree with me on that, it's just my opinion.
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You're doubtless familiar with the story of the man who bought two horses named Arnie and Sam, and took them back to his farm. One was a rider, the other a breeder. He tried to work them for several days, but in the end he took them back to the seller.
"What seems to be the problem?" asked the seller.
"It's quite simple," said the farmer. "I can't tell them apart. I never know which to call, and it's got me in a jam."
The seller looked the horses over and then said "Come back tomorrow. Let me think on this for a while. You can pick them up in the morning."
In the morning the farmer returned, and the seller handed him the lead ropes to the horses. "I figured it out." He said.
"Really?" asked the farmer. "That's great. How do I tell them apart?"
"Well," said the farmer, pointing to the crown at the top of the mane of the horse on the left, "if you look real close, you'll see that the black one is about an inch taller than the white one."
I understand where you're coming from, but I think it's more that you either disbelieve the concept here, or don't understand it.
Both horses are just that. Horses. Vmc and Vmca are both minimum control speeds (or the speeds at which directional control can no longer be maintained). One horse is a breeder, one horse a rider. Both horses are capable of breeding, but one is specifically for riding. Vmc and Vmca are both minimum control speeds, but only one specifically points to minimum control speed in the air. That's Vmca.
We could have many horses in a herd. We could have an Arabian, a Quarterhorse, an Appaloosa, and a Lipizzan. All are horses. The Arab is good for mountain and endurance riding. The Quarterhorse is good for cutting cattle and roping. The Appaloosa is excellent for western riding disciplines, and the Lipizzan for show. Each one a horse, one would not be remiss in calling any of them horses, for horses they are. To define each of them as a horse would be quite correct.
If one were to select the Arab for a trail ride, one would refer to the horse as a "horse" or an "Arab" interchangeably, because the horse is both. One could not refer to the Arab as a Lippizan, or the Quarterhorse as an Appaloosa, because they are not the same thing, even though they are each horses. To refer to any of them as a horse is not a loose definition; it's exactly what they are. To refer to any of them by their proper breed name is also not inexact or incorrect, because they are exactly that breed. To refer to any of the breeds by the proper breed name, or to refer to them as a horse, interchangeably, does not a loose definition make, because both are fully correct, and both describe the animal well.
Likewise, Vmca, Vmcg, Vmcl, and Vmcl-2 are all versions of Vmc. One can refer to any of them as either Vmc, or by their specific name. Therefore, Vmca is very much Vmc. Likewise, Vmcg is very much Vmc. Vmcl is very much Vmc. Vmcl-2 is very much Vmc.
Horse is a generic name for the type of animal. Arab, Quarterhorse, Appaloosa, or Lippizan are all horses, but are unique in their applications and strengths. An arab is a horse. A horse that's an Arab may be referred to as a horse, or as an Arab.
Vmc is a generic name for a situation in which directional control can no longer be maintained. Vmca, Vmcg, Vmcl, and Vmcl-2 are all Vmc scenarios, but are unique in their aplications and criteria. Vmca is Vmc. Vmc that's in the air may be referred to as Vmc, or as Vmca.
The terms, each exactly correct, may be used interchangeably, and neither are "loose." Vmc means directional control, and if we're talking about the red radial marking on the airplane, it's a given that it's Vmca, because that's the only Vmc marking required on the airspeed indicator. Vmcg, Vmcl, and Vmcl-2 are not required on the airspeed indicator. When establishing the minimum control certification number and instrument marking,then, we can correctly say we're establishing Vmc, because it's the only one that's going to be put on the indicator. If we're establishing something else, we'll note as much by being specific by saying it's the ground minimum control speed, Vmcg, for example. We know that Vmca is the red radial line because only the airborne takeoff configuration Vmc applies as marked on the airspeed indicator. We can say it or simply know it, but it's still Vmc, and it's still Vmca.
Vmc, of course, defining the speed at which we lose directional control, or specifically, at which directional control can no longer be maintained, varies as previously described. The published number is a reference only, and one should never assume that it reflects real-world conditions, except in approximation. You could correctly say, then, that Vmc as published, and Vmc as experienced in flight, are different.
Put another way, we're all familiar with Vx and Vy. While these numbers are known and published, they change with altitude, and they also change with ambient conditions. Vx and Vy are predicated on power, and where more power is available, the numbers are different than published. Many people disregard the difference or don't know how to figure it, but that usually won't hurt someone. Disregarding Vmc will. Therefore, while it's okay to fly the published numbers for Vx and Vy and be "close enough," it's not okay to assume the published numbers for Vmc are close enough; directional control may be lost before or after that published number. Vmc published and Vmc actual are different animals. For a worse-case scenario, it's Vmca that interests us: low, slow, and high power setting is what will bite us, as on a takeoff or go-around. It's Vmc, very much Vmc, but it's the Vmca (of all the Vmc's) that generally interests us the most.
The published number may correctly be called Vmca because it is Vmca. It may be correctly called Vmc because it is Vmca. The published number Vmc is always the Vmca number, whether we call it that or not, but it's very appropriate to use the terms interchangeably, because they are. Nothing loose about it. I harp on this point because as you said previously, we shoudn't be ambiguous about our terminology when discussing airspeeds. It was you who said we shouldn't confuse them, and you're right. Hence the lengthy clarification.
"What seems to be the problem?" asked the seller.
"It's quite simple," said the farmer. "I can't tell them apart. I never know which to call, and it's got me in a jam."
The seller looked the horses over and then said "Come back tomorrow. Let me think on this for a while. You can pick them up in the morning."
In the morning the farmer returned, and the seller handed him the lead ropes to the horses. "I figured it out." He said.
"Really?" asked the farmer. "That's great. How do I tell them apart?"
"Well," said the farmer, pointing to the crown at the top of the mane of the horse on the left, "if you look real close, you'll see that the black one is about an inch taller than the white one."
I understand where you're coming from, but I think it's more that you either disbelieve the concept here, or don't understand it.
Both horses are just that. Horses. Vmc and Vmca are both minimum control speeds (or the speeds at which directional control can no longer be maintained). One horse is a breeder, one horse a rider. Both horses are capable of breeding, but one is specifically for riding. Vmc and Vmca are both minimum control speeds, but only one specifically points to minimum control speed in the air. That's Vmca.
We could have many horses in a herd. We could have an Arabian, a Quarterhorse, an Appaloosa, and a Lipizzan. All are horses. The Arab is good for mountain and endurance riding. The Quarterhorse is good for cutting cattle and roping. The Appaloosa is excellent for western riding disciplines, and the Lipizzan for show. Each one a horse, one would not be remiss in calling any of them horses, for horses they are. To define each of them as a horse would be quite correct.
If one were to select the Arab for a trail ride, one would refer to the horse as a "horse" or an "Arab" interchangeably, because the horse is both. One could not refer to the Arab as a Lippizan, or the Quarterhorse as an Appaloosa, because they are not the same thing, even though they are each horses. To refer to any of them as a horse is not a loose definition; it's exactly what they are. To refer to any of them by their proper breed name is also not inexact or incorrect, because they are exactly that breed. To refer to any of the breeds by the proper breed name, or to refer to them as a horse, interchangeably, does not a loose definition make, because both are fully correct, and both describe the animal well.
Likewise, Vmca, Vmcg, Vmcl, and Vmcl-2 are all versions of Vmc. One can refer to any of them as either Vmc, or by their specific name. Therefore, Vmca is very much Vmc. Likewise, Vmcg is very much Vmc. Vmcl is very much Vmc. Vmcl-2 is very much Vmc.
Horse is a generic name for the type of animal. Arab, Quarterhorse, Appaloosa, or Lippizan are all horses, but are unique in their applications and strengths. An arab is a horse. A horse that's an Arab may be referred to as a horse, or as an Arab.
Vmc is a generic name for a situation in which directional control can no longer be maintained. Vmca, Vmcg, Vmcl, and Vmcl-2 are all Vmc scenarios, but are unique in their aplications and criteria. Vmca is Vmc. Vmc that's in the air may be referred to as Vmc, or as Vmca.
The terms, each exactly correct, may be used interchangeably, and neither are "loose." Vmc means directional control, and if we're talking about the red radial marking on the airplane, it's a given that it's Vmca, because that's the only Vmc marking required on the airspeed indicator. Vmcg, Vmcl, and Vmcl-2 are not required on the airspeed indicator. When establishing the minimum control certification number and instrument marking,then, we can correctly say we're establishing Vmc, because it's the only one that's going to be put on the indicator. If we're establishing something else, we'll note as much by being specific by saying it's the ground minimum control speed, Vmcg, for example. We know that Vmca is the red radial line because only the airborne takeoff configuration Vmc applies as marked on the airspeed indicator. We can say it or simply know it, but it's still Vmc, and it's still Vmca.
Vmc, of course, defining the speed at which we lose directional control, or specifically, at which directional control can no longer be maintained, varies as previously described. The published number is a reference only, and one should never assume that it reflects real-world conditions, except in approximation. You could correctly say, then, that Vmc as published, and Vmc as experienced in flight, are different.
Put another way, we're all familiar with Vx and Vy. While these numbers are known and published, they change with altitude, and they also change with ambient conditions. Vx and Vy are predicated on power, and where more power is available, the numbers are different than published. Many people disregard the difference or don't know how to figure it, but that usually won't hurt someone. Disregarding Vmc will. Therefore, while it's okay to fly the published numbers for Vx and Vy and be "close enough," it's not okay to assume the published numbers for Vmc are close enough; directional control may be lost before or after that published number. Vmc published and Vmc actual are different animals. For a worse-case scenario, it's Vmca that interests us: low, slow, and high power setting is what will bite us, as on a takeoff or go-around. It's Vmc, very much Vmc, but it's the Vmca (of all the Vmc's) that generally interests us the most.
The published number may correctly be called Vmca because it is Vmca. It may be correctly called Vmc because it is Vmca. The published number Vmc is always the Vmca number, whether we call it that or not, but it's very appropriate to use the terms interchangeably, because they are. Nothing loose about it. I harp on this point because as you said previously, we shoudn't be ambiguous about our terminology when discussing airspeeds. It was you who said we shouldn't confuse them, and you're right. Hence the lengthy clarification.
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I thought this might have come to an end when I agreed to disagree, but ok, let's continue!
I wasn't, but I am now. It's a good analagy - and it makes my point very well.
If I wanted to buy a show-horse, I would go to the horse dealer and ask if I could buy a Lipizzan. I'd then get what I wanted. If I simply asked him for a horse, I'd quite likely come away with the wrong type of horse - unless he sought further clarification. The word "horse" works perfectly well in many circumstances, but is too ambiguous to be of any use when I'm at the horse dealer telling him what I want to buy.
"Vmc" is exactly the same as "horse". It is often perfectly acceptable to use it, either because it doesn't matter exactly what Vmc we are talking about (rare), or because the context provides clarification (more common). But it can also be ambiguous.
Let me ask you some questions:
a) What happens to the control of the aircraft at Vmc?
b) Can you explain why Vmc is lower when 5 degrees of bank towards the live engine is used compared to when the wings are level
c) Does Vmc change with height?
In question a, it doesn't matter which Vmc we are talking about, because the answer is the same regardless: the answer is that control is lost.
In question b, the context tells us that we can't be refering to Vmca, we must be talking about the actual speed at which control is lost. If we were talking about Vmca, the question would be nonsense because Vmca is defined as not being wings level, but the question asks about Vmc with wings level and implies that Vmc exists, and is different, if wings are level.
In both of these cases, the phrase "Vmc" is absolutely adequate.
But in question c, it is not. The answer to the question depends on what Vmc we are talking about. If we were being asked about Vmca, we might answer "No, Vmca is always quoted for sea level". If we were being asked about the actual speed where control is lost, we might answer "Yes, Vmc decreases with height in an aircraft with normally-aspirated engines because the good engine produces less power the higher it is."
So, in the case of question c, the phrase "Vmc" is ambiguous.
I believe the concept thanks to your quotes, and I understand it perfectly. Just because what you are saying is factually correct, that doesn't mean I have to think it's a good idea!
FFF
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You're doubtless familiar with the story of the man who bought two horses named Arnie and Sam, and took them back to his farm
If I wanted to buy a show-horse, I would go to the horse dealer and ask if I could buy a Lipizzan. I'd then get what I wanted. If I simply asked him for a horse, I'd quite likely come away with the wrong type of horse - unless he sought further clarification. The word "horse" works perfectly well in many circumstances, but is too ambiguous to be of any use when I'm at the horse dealer telling him what I want to buy.
"Vmc" is exactly the same as "horse". It is often perfectly acceptable to use it, either because it doesn't matter exactly what Vmc we are talking about (rare), or because the context provides clarification (more common). But it can also be ambiguous.
Let me ask you some questions:
a) What happens to the control of the aircraft at Vmc?
b) Can you explain why Vmc is lower when 5 degrees of bank towards the live engine is used compared to when the wings are level
c) Does Vmc change with height?
In question a, it doesn't matter which Vmc we are talking about, because the answer is the same regardless: the answer is that control is lost.
In question b, the context tells us that we can't be refering to Vmca, we must be talking about the actual speed at which control is lost. If we were talking about Vmca, the question would be nonsense because Vmca is defined as not being wings level, but the question asks about Vmc with wings level and implies that Vmc exists, and is different, if wings are level.
In both of these cases, the phrase "Vmc" is absolutely adequate.
But in question c, it is not. The answer to the question depends on what Vmc we are talking about. If we were being asked about Vmca, we might answer "No, Vmca is always quoted for sea level". If we were being asked about the actual speed where control is lost, we might answer "Yes, Vmc decreases with height in an aircraft with normally-aspirated engines because the good engine produces less power the higher it is."
So, in the case of question c, the phrase "Vmc" is ambiguous.
I think it's more that you either disbelieve the concept here, or don't understand it.
FFF
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I thought this might have come to an end when I agreed to disagree, but ok, let's continue!
Let me ask you some questions:
a) What happens to the control of the aircraft at Vmc?
In question a, it doesn't matter which Vmc we are talking about, because the answer is the same regardless: the answer is that control is lost.
a) What happens to the control of the aircraft at Vmc?
In question a, it doesn't matter which Vmc we are talking about, because the answer is the same regardless: the answer is that control is lost.
b) Can you explain why Vmc is lower when 5 degrees of bank towards the live engine is used compared to when the wings are level
In question b, the context tells us that we can't be refering to Vmca, we must be talking about the actual speed at which control is lost. If we were talking about Vmca, the question would be nonsense because Vmca is defined as not being wings level, but the question asks about Vmc with wings level and implies that Vmc exists, and is different, if wings are level.
In question b, the context tells us that we can't be refering to Vmca, we must be talking about the actual speed at which control is lost. If we were talking about Vmca, the question would be nonsense because Vmca is defined as not being wings level, but the question asks about Vmc with wings level and implies that Vmc exists, and is different, if wings are level.
You still seem to think it represents something other than what it does. Rather than explain it again (I think it's been very thoroughly covered thus far, insofar as the terminology goes), read again.
Where do you find Vmca defined as wings not level? Vmc is Vmca. Vmca is Vmc. Vmca is Vmc in the air. Remember?
As to your question, the five degree bank isn't used with all aircraft. When you move up to transport category aircraft, for example, you'll find that wings-level is the order of the day when addressing an engine-out, in most cases.
In both of these cases, the phrase "Vmc" is absolutely adequate.
Vmc, of course, can also refer to the same thing on the ground, which is Vmcg, or in approach configuration, which is Vmcl, or Vmcl-2.
c) Does Vmc change with height?
But in question c, it is not. The answer to the question depends on what Vmc we are talking about. If we were being asked about Vmca, we might answer "No, Vmca is always quoted for sea level". If we were being asked about the actual speed where control is lost, we might answer "Yes, Vmc decreases with height in an aircraft with normally-aspirated engines because the good engine produces less power the higher it is."
So, in the case of question c, the phrase "Vmc" is ambiguous.
But in question c, it is not. The answer to the question depends on what Vmc we are talking about. If we were being asked about Vmca, we might answer "No, Vmca is always quoted for sea level". If we were being asked about the actual speed where control is lost, we might answer "Yes, Vmc decreases with height in an aircraft with normally-aspirated engines because the good engine produces less power the higher it is."
So, in the case of question c, the phrase "Vmc" is ambiguous.
For certification purposes, the number tends to remain fixed, but the relationship to actual directional control varies with the aircraft and powerplant system in use. It's fair to say that EAS and TAS for Vmc changes with altitude, though you may or may not see an actual change in the indicated airspeed at which directional control can no longer be maintained. You haven't provided enough information.
Where Vmc, meaning the ability to no longer maintain directional control, changes with altitude, the term is not at all ambiguous, because it means just what it says: the ability to no longer maintain directional control.
The numbers may be different, but the term is still valid and precise.
Re-read again. You're missing something. Can you figure out what it is?
You may have to have the conversation alone for a few days. Time to go fly.
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I get the feeling you are arguing for the sake of arguing now.
- We agree that Vmc can have many different values.
- It's clear that you don't have a problem with this.
- For my part, I accept that this is true - I have stated that I don't like it, but there's nothing I can do about it.
- In all other respects, we seem to agree.
As far as I'm concerned, there's nothing else to discuss. I've found this conversation useful and enlightening - I will no longer assume, when I hear people use the term Vmc, that it's being used to describe minimum control speed in any particular circumstances as I used to, but will seek clarification if required. And I'll give clarification where required when I use the term Vmc myself.
You also said I don't understand what Vmca means. It's a pity you can't concisely correct my misunderstanding, rather than referring me back to the same posts that I had misunderstood earlier. Generally, when someone misunderstands something I say, I try to re-word it to help them understand.
However, I re-read much of what you've written, and I think you may also be suggesting that Vmca can have many values, that it doesn't only refer to the certification criteria used to work out the red-line value but that it can also be used to refer to actual in-flight values. Is that what you mean? If so, then I will take everything I've just said about Vmc and apply it to Vmca too.
If you feel I've still misunderstood something, then I look forward to being further enlightened when you return from flying. If you want to disagree with my opinion then that's fine, but I can't see any point in debating further something on which I know we disagree.
FFF
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- We agree that Vmc can have many different values.
- It's clear that you don't have a problem with this.
- For my part, I accept that this is true - I have stated that I don't like it, but there's nothing I can do about it.
- In all other respects, we seem to agree.
As far as I'm concerned, there's nothing else to discuss. I've found this conversation useful and enlightening - I will no longer assume, when I hear people use the term Vmc, that it's being used to describe minimum control speed in any particular circumstances as I used to, but will seek clarification if required. And I'll give clarification where required when I use the term Vmc myself.
You also said I don't understand what Vmca means. It's a pity you can't concisely correct my misunderstanding, rather than referring me back to the same posts that I had misunderstood earlier. Generally, when someone misunderstands something I say, I try to re-word it to help them understand.
However, I re-read much of what you've written, and I think you may also be suggesting that Vmca can have many values, that it doesn't only refer to the certification criteria used to work out the red-line value but that it can also be used to refer to actual in-flight values. Is that what you mean? If so, then I will take everything I've just said about Vmc and apply it to Vmca too.
If you feel I've still misunderstood something, then I look forward to being further enlightened when you return from flying. If you want to disagree with my opinion then that's fine, but I can't see any point in debating further something on which I know we disagree.
FFF
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(Quick aside... I've found my debate with SNS3Guppy really interesting, but I do feel we owe an apology to FlyingKiwi for hijacking his thread. The discussion has gone way beyond the scope of the original question!
And it's so refreshing to have a good debate on PPRuNe without it turning personal in any way!)
And it's so refreshing to have a good debate on PPRuNe without it turning personal in any way!)
Thanks all
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Without wanting to spark a whole new debate, I will offer the following photo for an example. I took this in stable flight, during flight testing in a Navajo last week. I was applying full rudder to maintain control.
Though hard to see, the indicated airspeed is 10 MPH less than the red line, and the aircraft is banked to the right, slightly climbing in a left turn. The stall warning was sounding while I did this.
To be fair to Vmc(whichever - I'm not trying to incite an argument here), the prop is feathered, and not windmilling, and it was at altitude. Maximum power was applied on the right engine. This is becasue I was checking the general handling, and stalls, with light aft C of G. Vmc(whichever). I did stall the aircraft in this configuation, with no unusual outcome. I will be determining if the actual speed of Vmc was affected by the modification next week, with the prop windmilling.
Though hard to see, the indicated airspeed is 10 MPH less than the red line, and the aircraft is banked to the right, slightly climbing in a left turn. The stall warning was sounding while I did this.
To be fair to Vmc(whichever - I'm not trying to incite an argument here), the prop is feathered, and not windmilling, and it was at altitude. Maximum power was applied on the right engine. This is becasue I was checking the general handling, and stalls, with light aft C of G. Vmc(whichever). I did stall the aircraft in this configuation, with no unusual outcome. I will be determining if the actual speed of Vmc was affected by the modification next week, with the prop windmilling.