I am not sure if this is the right forum to post but since I cannot see anything appropiate I try anyway

 

On many airspeed indicators I notice one or several white arcs ( ex Cessna,Moonet Bravo etc ) .

I know it has something to do wirh flap extending speeds,however why are there 2 white arcs.One from 40 to 80 knots,

the other one from 90 to 130 knots ( as an example )

 

Can anybody explain ?

 

Hubert Werni

You should see the white arc or flaps operating range. The green arc is the normal operating range. The yellow arc is the caution range, and the red line is the never exceed speed.

 

For more info you can check the operating instructions for the plane's AI.

 

Jose

Hubert,

 

The colored arcs are pretty straight forward, and there is only one white arc. The Top of which is equal to the aircraft's maximum speed with flaps extended, and the bottom equates to the aircraft stall speed with full flaps and landing gear down.

 

The 2nd white arc you may be seeing is likely in the lower left of the instrument and is the True Airspeed indicator. You would set the current temperature deviation, and pressure altitude in the top window with the knob and read the resultant "True Airspeed" in the window. Without the correction the normal speed read on the indicator is "Indicated Airspeed".

 

Airspeed can be a somewhat confusing discussion. There are many types to consider:

 

Indicated Airspeed - Speed read off the indicator uncorrected for atmospheric density, installation error, or instrument error.

 

Calibrated Airspeed - Indicated airspeed corrected  for installation or instrument error.

 

True Airspeed - Calibrated airspeed corrected for temperature deviation and altitude.

 

Groundspeed - The actual speed of the aircraft over the ground.

 

I hope this helps!