VMC question......

[LoadMasterC141]

Page 12-2 of the AFH states that "Unless otherwise noted, when V-Speeds are given in the AFM/POH, they apply to sea level, standard day conditions at maximum takeoff weight".

However,

FAR 23.149 sates VMC airspeed must be calculated at the most unfavorable weight. Unless there is a situation I don't know of, the lightest weight gives the least horizontal component of lift to aid the rudder.

So the FAA has contradicted itself? Had a student bust an initial MEI because he said that the most unfavorable weight should be used to determine the VMC airspeed, and that is the lightest. The examiner cited 12-2 and failed him.

 

[braunpilot]

If he would have stopped at most unfavorable weight he probably would have passed. I agree that it is a pretty cheap shot though.

 

[JK7177]

Max T/O weight is how its calculated i believe, at least in the Seminole.

 

[braunpilot]

Max T/O weight is how its calculated i believe, at least in the Seminole.

Again, most unfavorable weight....for the aircraft.

 

[LoadMasterC141]

Max T/O weight is how its calculated i believe, at least in the Seminole.

Piper certified the Seminole VMC airspeed at 2730 lbs, about as light as they could get it. This would be most unfavorable because it would give the least amount of load factor to the horizontal component of lift. Therefore, less of that gives less aid to the rudder. More rudder deflection...less rudder authority and more drag.

The only thing I can come up with is that the FAR states "UP to 5 degrees of bank into the operating engine". It does not say that any bank is required. Therefore, if a manufacturer chooses no bank, then weight would not be a factor in determining VMC. Higher weight would affect stall though of course.

If he would have stopped at most unfavorable weight he probably would have passed. I agree that it is a pretty cheap shot though.

Yeah I agree with that. It was a very cheap shot. The guy was doing very well and is a good student too, so it wasn't like he was just pulling the trigger on soemthing as a result of an overall bad performance.

 

[braunpilot]

Heard of a DPE in STL area that would ask CFI applicants about if they could remove the rear seat in a Tampico (TB9). The problem was that the French portion of the manual said no, then the next page was the DOT verision that said Yes. My friend was the first CFI applicant to get that question and said that the would call the FSDO for clairification so he failed him....15 mins from the end of the CFI Oral. Guy got a bad rep and now is a ASI with the FSDO. GO FIGURE.

 

[Hubbs]

I've heard a couple of different arguments about weight affecting Vmc. Firstly, less weight = less horrizontal component of lift, thus lightest is least favourable. Secondly, more weight = more inertia and so when the critically engine is 'suddenly' inop the yaw will have already began and there is more inertia to overcome with the rudder to bring the nose back. Whereas, during ME training the critical engine failing is never that 'sudden' and you can get on top of it before the inertia builds up.

 

[Cav]

If he would have stopped at most unfavorable weight he probably would have passed. I agree that it is a pretty cheap shot though.

Agree 100%. A wise old CFI told me when I was a student pilot that during an oral you should always answer as though you are on a witness stand in court...yes or no answers until something more is asked for. Even then be short and to the point. Never offer more information than is required!

I also remember during my upgrade training that the people who "knew" the most about the systems seemed to have the most trouble with the systems oral!

 

[Cessnaflyer]

Page 12-2 of the AFH states that "Unless otherwise noted, when V-Speeds are given in the AFM/POH, they apply to sea level, standard day conditions at maximum takeoff weight".

Why didn't he fight and say that it is noted that the weight is somewhere else, likke the POH that they used?

 

[braunpilot]

Why didn't he fight and say that it is noted that the weight is somewhere else, likke the POH that they used?

The student probably should have. There is nothing wrong with asking for clearity...as to why that was a failure in fact.

 

[amorris311]

i agree with braun on this one. shoudl have stopped at most unfavorable weight. if the examiner then pressed the question i would have opened the poh to give him the exact numbers. there is a solid argument on both sides of the table. lighter v. heavier. i have always been a lighter most unfavorable kind of guy.

 

[LoadMasterC141]

He did ask for clarity and cited FAR 23.149 as to the certification.

I don't think there is any argument as to whether light weight is most unfavorable IF bank is used in certification.

Certainly though, the student should have kept his mouth shut in the first place. I always try to tell them that, but he seemed to think he realy understood VMC and wanted to keep it on that subject. That thought process got him busted.

 

[husky16]

When I was doing my MEI oral I was teAching VMC and it's certification standards for the Seminole and said The most unfavorable condition was light due to a light weight being easily displaced.The DPE said they certify at max gross weight and we moved on. When teaching students I taught both, heavy vs light depending on the DPE they got for their checkride. Depends what the DPE wants to hear.

Tough bust

 

[deadstick]

Well, how about this?

Is Vmc REALLY critical (ok it is always critical, but relatively speaking....) during cruise or a cruise climb? Only if you sleep through the engine failure. How about descent/landing? Nope, you're at a reduced power setting, and the remaining power and altitude are potential energy sources that can be traded for airspeed and altitude. So, the remaining phase is takeoff and initial climb. Single engine, fighting gravity close to the ground at a high angle of attack (climb), full power(takeoff), AND AT MGTW will result in a more rapid deceleration from the "low" airspeed (blue line?) to Vmc. The the minimum amount of excess power is available when the aircraft is at MGTW.

Then again, they had to set the bar somewhere? The empty weight of two BE58s can be very different -- radar, boots, AC, etc. So, assigning Vmc to some arbetrary perentage or weight over that would not work. It's setting a standard like the max 5 degress of bank.

 

[meritflyer]

From a recent AOPA article -

By regulation, manufacturers must determine V_MC by duplicating a certain configuration during flight tests. This configuration includes:

• A windmilling — or unfeathered — propeller on the dead engine. In airplanes with clockwise-rotating propellers, this engine is the left, or critical, engine.
• Maximum continuous power on the operating engine.
• Landing gear extended.
• Flaps at the takeoff setting.
• Maximum gross weight.
• Most aft center of gravity.
• Establishment of zero sideslip toward the operating engine. (For minimal drag and best single-engine performance the airplane should be slipped enough to keep the rudder ball between one-half and three-quarters of a ball-width out of the center position, toward the operating engine. In addition, you should set up a 5-degree bank into the operating engine — "raise the dead," as some instructors say.)

 

[LoadMasterC141]

From a recent AOPA article -

[/list]

Thanks Merit. Again though, FAR 23.149 says manufacturers must certify VMC based on the "Most Unfavorable Weight". That would be lightest if Zero Sideslip is set. It just confuses me. The FAA says this in their regulations, yet manufacturers certify at max weight? Are they not violating an FAR? Further, given that the FAR says this, and the AFH says "Max T/O weight", is the FAA not contradicting itself?

It all seems pretty stupid to me. The FAA has, as usual, made something simple incredibly complex, and now are busting people for it because their own employees differ on the understanding of it.

 

[casey]

Thanks Merit. Again though, FAR 23.149 says manufacturers must certify VMC based on the "Most Unfavorable Weight".

What did that regs say during the year the Seminole recieved it's certification? Regs change and the wording today may not be the wording the airplane was certified under.

 

[amorris311]

if a multi checkride is getting that precise im sure the student was making mistakes on other topics. checkrides dont usually end on one mistake. now if the student could explain why he made his decision and sounded like he had a good knowledge of the topic i would as an instructor rip into the dpe. i had to sit in on an oral of one of my students and the dpe was confusing two different topics. the student as a result became confused and the dpe apologized and then said he too was prone to making mistakes.

 

[nosehair]

checkrides dont usually end on one mistake.

:yeahthat:

 

[casey]

What did that regs say during the year the Seminole recieved it's certification? Regs change and the wording today may not be the wording the airplane was certified under.

Certification Basis Type Certificate No. A19SO issued March 10, 1978.
Date of application for Type Certificate, January 17, 1976.

PA-44-180: Federal Aviation Regulations (FAR) Part 23 effective February 1, 1965,
through Amendment 23-16 effective February 14, 1975; FAR 23.1557(c)(1) as amended
by Amendment 23-18 effective May 2, 1977; and FAR 36 effective December 1, 1969,
through Amendment 36-4.

Sec. 23.149

Minimum control speed.

(a) VMC 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. VMC 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 VMC, 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.

Sec. 23.149

Minimum control speed.

(a) VMC 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°. VMC 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 VMC, 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°.

The next revision to change that reg was 23-21 (Mar 1 1978) and the seminole was only certified under part 23 up to revision 23-16, so the version just above is the version that was in force when the seminole was certified.