Vmc question

dwe213

Well-Known Member
Does the FAA determine Vmc at maximum gross or minimum gross weight?

Careful: as the hcl increases, so does rudder affectiveness. Therefor, Vmc decreases with weight.
 

Ralgha

Well-Known Member
The FAA determines it using the most unfavorable weight, which should generally be the lightest.
 

Radu38J

Well-Known Member
What does "generally the lightest weight" mean? One hour of gas & a 95 lb. pilot?
According to my MEI, it means average size pilot and minimum fuel - don't know the exact standards. Also, they rig it somehow for the most aft CG, so the 2 go together.
 

splash

your social justice comic center
You can't see but your safty pilot is not doing a good job you know. Nice glasses though.;)
 

CoffeeIcePapers

Well-Hung Member
Careful: as the hcl increases, so does rudder affectiveness. Therefor, Vmc decreases with weight.
True, but not all that much. You have to consider P-Factor as well. As the weight increases, you need more lift, so you increase the angle of attack. P-factor also increases with angle of attack. The added P-Factor raises Vmc. The Airplane Flying Handbook says what you said, so I assume that the benefit from the HCL overrides the p-factor somewhat.
 

tgrayson

New Member
That article is very old and the information is out of date. The regulation used to read "maximum takeoff weight, unless a lesser weight is need to demonstrate Vmc" (or something like that). Even then, flight testing usually used lightest weight. At the highest weights, the aircraft would stall before Vmc.

The current regulation is under 23.149.
 

Fly_Unity

Well-Known Member
After a certain year, FAR 23.149 was changed to read that VMC was to be tested under most unfavorable weight. However there are some planes like the Piper Seminole that was tested before the 23.149 came out. On my MEI checkride (June Bonesteel, probably the most knowledgeable woman on earth about aviation haha), she stated that the Piper Seminole was tested at gross weight, and she showed me what the regulation was during the time of testing. (I forget what the old regulations were called before FAR 23.149 came out) And she also showed me in the airplane flying handbook that all V speeds are tested at gross weight unless otherwise specified. The POH dont dont say what weight it was tested at. So its to be assumed that it was tested at Gross weight. I emailed Piper Aircraft, and they confirmed it was "probably" tested at gross weight, however they also stated this was so many years ago, so that know one really knows, nor cares.
 

Ralgha

Well-Known Member
After a certain year, FAR 23.149 was changed to read that VMC was to be tested under most unfavorable weight. However there are some planes like the Piper Seminole that was tested before the 23.149 came out. On my MEI checkride (June Bonesteel, probably the most knowledgeable woman on earth about aviation haha), she stated that the Piper Seminole was tested at gross weight, and she showed me what the regulation was during the time of testing. (I forget what the old regulations were called before FAR 23.149 came out) And she also showed me in the airplane flying handbook that all V speeds are tested at gross weight unless otherwise specified. The POH dont dont say what weight it was tested at. So its to be assumed that it was tested at Gross weight. I emailed Piper Aircraft, and they confirmed it was "probably" tested at gross weight, however they also stated this was so many years ago, so that know one really knows, nor cares.
Of course nobody cares, it's irrelevant. Know what affects it, know how to recognize it, and know how to recover from it. Better yet, know how to never get yourself into it in the first place.
 

tgrayson

New Member
However there are some planes like the Piper Seminole that was tested before the 23.149 came out.
The Seminole is a Part 23 airplane, and, according to the TCDS, was certified under the 1978 Part 23 regulations, which contained an earlier version of 23.149. I've attached a timeline of the changes to 23.149 below. Weight wasn't mentioned until 1978 and it's not clear whether the Seminole was operating under this definition or not, since it occurred the same year the Seminole was certified. The addition read:
The maximum sea level takeoff weight (or any lesser weight necessary to show <st1:stockticker>VMC</st1:stockticker>)
According to a conversation I had with a certification guy at the FAA's Small Airplane Directorate, the "maximum seal level takeoff weight" was essentially irrelevant, since "the lesser weight necessary to show Vmc" was normally the determining factor in establishing the weight at which the tests were done.

Plus, every credible article and book I've read on the subject said that testing was done at light weight, even before the change in regulations. In fact, the change in regulations came about, according to the guy at the Small Airplane Directorate, to reflect how the testing was actually done.


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S<o></o>ec. 23.149
<o></o>Minimum control speed.
<o></o>(a) <st1:stockticker>VMC</st1:stockticker> is the minimum calibrated airspeed at which, when any engine is suddenly made inoperative, it is possible to recover control of the airplane with that engine still inoperative and maintain straight flight, either with zero yaw, or, at the option of the applicant, with an angle of bank of not more than five degrees. <st1:stockticker>VMC</st1:stockticker> may not exceed 1.2 with--
(1) Takeoff or maximum available power on each engine;
(2) The rearmost allowable center of gravity;
(3) The flaps in the takeoff position; and
(4) The landing gear retracted.
(b) At <st1:stockticker>VMC</st1:stockticker>, the rudder forces required to maintain control may not exceed the limitations set forth in Sec. 23.143, and it may not be necessary to throttle the remaining engines. During recovery, the airplane may not assume any dangerous attitude or require exceptional piloting skill, alertness, or strength, to prevent a heading change of more than 20 degrees.
<o></o>
1968
<o></o>Sec. 23.149 Minimum control speed.
(a) <st1:stockticker>VMC</st1:stockticker> is the minimum calibrated airspeed at which, when any engine is suddenly made inoperative, it is possible to recover control of the airplane with that engine still inoperative and maintain straight flight, either with zero yaw, or, at the option of the applicant, with an angle of bank of not more than 5°. <st1:stockticker>VMC</st1:stockticker> may not exceed 1.2 with--
(1) Takeoff or maximum available power on each engine;
(2) The rearmost allowable center of gravity;
(3) The flaps in the takeoff position;
(4) The landing gear retracted; and
[(5) The propeller of the inoperative engine--
(i) Windmilling, with the propeller speed or pitch control in the takeoff position; or
(ii) Feathered, if the airplane has an automatic feathering device.]
(b) At <st1:stockticker>VMC</st1:stockticker>, the rudder forces required to maintain control may not exceed the limitations set forth in Sec. 23.143, and it may not be necessary to throttle the remaining engines. During recovery, the airplane may not assume any dangerous attitude or require exceptional piloting skill, alertness, or strength, to prevent a heading change of more than 20°.
<o></o>
1978<o></o>
Sec. 23.149
Minimum control speed.
(a) <st1:stockticker>VMC</st1:stockticker> is the calibrated airspeed, at which, when the critical engine is suddenly made inoperative, it is possible to recover control of the airplane with that engine still inoperative, and maintain straight flight either with zero yaw or, at the option of the applicant, with an angle of bank of not more than five degrees. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure with respect to controllability expected in service.
(b) For reciprocating engine-powered airplanes, <st1:stockticker>VMC</st1:stockticker> may not exceed 1.2 (where is determined at the maximum takeoff weight) with--
(1) Takeoff or maximum available power on the engines;
(2) The most unfavorable center of gravity;
(3) The airplane trimmed for takeoff;
(4) The maximum sea level takeoff weight (or any lesser weight necessary to show <st1:stockticker>VMC</st1:stockticker>);
(5) Flaps in the takeoff position;
(6) Landing gear retracted;
(7) Cowl flaps in the normal takeoff position;
(8) The propeller of the inoperative engine--
(i) Windmilling;
(ii) In the most probable position for the specific design of the propeller control; or
(iii) Feathered, if the airplane has an automatic feathering device: and
(9) The airplane airborne and the ground effect negligible.
<o></o>
1993
Minimum control speed.
<st1:stockticker>VMC</st1:stockticker> 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 then maintain straight flight at the same speed with an angle of bank of not more than 5 degrees. The ability to maintain straight and level flight at <st1:stockticker>VMC</st1:stockticker> in a static condition with a bank angle of not more than 5 degrees must also be demonstrated. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure, with respect to controllability expected in service.
(b) <st1:stockticker>VMC</st1:stockticker> may not exceed 1.2 , where is determined at the maximum takeoff weight, with--
(1) Maximum available takeoff power or thrust on the engines;
(2) The most unfavorable center of gravity;
(3) The airplane trimmed for takeoff;
(4) The maximum sea level takeoff weight, or any lesser weight necessary to show <st1:stockticker>VMC</st1:stockticker>;
(5) The airplane in the most critical takeoff configuration, with the propeller controls in the recommended takeoff position and the landing gear retracted; and
(6) The airplane airborne and the ground effect negligible.
(c) A minimum speed to intentionally render the critical engine inoperative must be established and designated as the safe, intentional, one-engine-inoperative speed, VSSE.]
(d) At <st1:stockticker>VMC</st1:stockticker>, the rudder pedal force required to maintain control may not exceed 150 pounds, and it may not be necessary to reduce power or thrust of the operative engines. During [the
maneuver,] the airplane may not assume any dangerous attitude and it must be possible to prevent a heading change of more than 20°.
<o></o>
1996
<o></o>Sec. 23.149
<o></o>Minimum control speed.
(a) <st1:stockticker>VMC</st1:stockticker> 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) <st1:stockticker>VMC</st1:stockticker> for takeoff must not exceed 1.2 , where is determined at the maximum takeoff weight. <st1:stockticker>VMC</st1:stockticker> 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.
<o></o><o></o> (e) At <st1:stockticker>VMC</st1:stockticker>, 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°.
 

deadstick

Well-Known Member
Could the "lesser weight to show Vmc" be to increase the spread between Vmca and Vs? If you are flying a plane with Vmca VERY close to Vs, then you'd want to do a Vmc demo at a lighter weight so that you don't accidentally go into a stall while doing the demo. I think the PA30 had a problem with this, but it's been awhile. However, I don't know why the manufacturer wouldn't publish the max gross Vmca, too.
 

Brian Z

Well-Known Member
Could the "lesser weight to show Vmc" be to increase the spread between Vmca and Vs? If you are flying a plane with Vmca VERY close to Vs, then you'd want to do a Vmc demo at a lighter weight so that you don't accidentally go into a stall while doing the demo. I think the PA30 had a problem with this, but it's been awhile. However, I don't know why the manufacturer wouldn't publish the max gross Vmca, too.
I do not want to take this too much off of the op's question. Where I train one would stall the plane before reaching Vmc due to the altitude. We either recover at the horn or limit rudder travel to simulate loss of effective rudder.
 

Patrick

Well-Known Member
Could the "lesser weight to show Vmc" be to increase the spread between Vmca and Vs? If you are flying a plane with Vmca VERY close to Vs, then you'd want to do a Vmc demo at a lighter weight so that you don't accidentally go into a stall while doing the demo. I think the PA30 had a problem with this, but it's been awhile. However, I don't know why the manufacturer wouldn't publish the max gross Vmca, too.
I'm not sure if this is so much an individual problem with the PA-30, but rather a function of Vmc decreasing with density altitude. Many light twins will have actual Vmc at or lower than Vs at the altitudes where the demo is often performed.
 

tgrayson

New Member
Could the "lesser weight to show Vmc" be to increase the spread between Vmca and Vs?
I think the idea is more to create a worst-case speed. If the speed were determined at max gross weight, it would steadily increase as you burned off fuel or disembarked passengers.

I recall the Comanche's Vmc was increased after the plague of Vmc training accidents in the airplane.
 
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