DETERMINING VMC SPEED

Determining Vmc Speed Lesson by wifiCFI

New Airspeeds

Vxse: Best Angle of Climb with a Single Engine

Vyse: Best Rate of Climb with a Single Engine/Least Rate of Descent

Vsse: Safe Single Engine speed

Vmc: Minimum controllable airspeed (more information later)

Definitions

Vmc:

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.

Critical Engine:

The engine, that when failed, most adversely affects the handling and performance characteristics of the airplane.

Determining Vmc

In determining Vmc a manufacturer most follow the standards set forth by the FAA in FAR 23.149.

By following this process the aircraft manufacturer can come up with a published Vmc speed.

However, an aircraft’s actual Vmc speed will vary with external conditions.

The acronym used to determine Vmc speed is: SMACFUM.

Standard Day at Sea Level

Max Power on the Operating Engine

Aft Legal CG

Critical Engine Inoperative and Propeller Windmilling

Flaps and Trim set to takeoff position, Gear up

Up to 5 degrees of Bank into the Operating Engine

Most Unfavorable Weight

Most all of these set up the airplane for worst case scenario. Variations to these standards will result in a lower Vmc speed.

Standard Day at Sea Level

An aircraft equipped with naturally aspirated engines will have more power at sea level.

This makes it more difficult to maintain directional control.

Max Power on the Operating Engine

This makes it more difficult to maintain directional control. 

Aft Legal CG

The further aft the CG, the shorter the arm from the CG to the Rudder.

This makes the Rudder less effective in maintaining directional control.

Critical Engine Inoperative and Propeller Windmilling

A windmilling propeller creates more drag than a feathered propeller.

This creates a stronger yaw effect into the inoperative engine.

Flaps and Trim set to Takeoff, Gear Up

When flaps and gear are extended they act like a keel on a boat.

The keel helps the boat maintain it’s heading (directional control).

With the flaps and gear retracted, the aircraft has a more difficult time maintaining directional control.

Up to 5 Degrees of Bank into the Operating Engine

Banking 5 degrees into the Operating Engine transfers some vertical lift into horizontal lift to aid in directional control.

This standard lowers Vmc speed.

Most Unfavorable Weight

The most unfavorable weight for directional control is light.

Heavier aircraft must generate more lift in order to fly.

Because the heavier aircraft generates more total lift, it has more to transfer to horizontal lift when the aircraft is banked.

This increase in horizontal lift factor makes the heavier aircraft more capable of directional control.

FAA Sources Used for This Lesson

Airplane Flying Handbook (AFH) Chapter 12

Federal Aviation Regulations (FAR) 23.149