Wisconsin Aeronautical Charts (WACs) are used by pilots to navigate from one airport in Wisconsin to another.

Topic 2.1 ExtendedH – Relative velocity – plotting courses

Among the many things shown on the WACs are Maximum Elevation Figures (MEFs), obstructions, beacons, radio call signs, etc.

Suppose we want to fly from Milwaukee Mitchell, to Hartford Airport.Suppose that the wind is blowing at 22 knots (22 nautical miles per hour) out of the west.Suppose further that your Cessna is capable of an airspeed of 100 knots.

In what direction should you head your Cessna, and how long will it take to reach Hartford Airport?

FYI: A nautical mile is 6076 ft (compared to the commonly used statute mile of 5280 ft).

There are two directions we have no control over:The wind, and the destination's bearing.Step 1: Draw a line through ORIGIN and DESTINATION. Step 2: With your plotter, draw a scale WIND (VAG): Use any scale you like, but END it at DESTINATION. Step 3: With SAME scale, draw VPA:It must END on the Tail if the WIND VECTOR.It must BEGIN on the ORIGINAL LINE (the red one).STEP 4: Draw in VPG:

VAG

VPA

VPG

STEP 5: Find BEARING of VPA:Line up plotter on course. Place bull's eye on a meridian. Bearing should be between 270° and 360°.

Bearing is 308°.

Find the magnitude of VPG using SAME SCALE:

Groundspeed is 83 knots.

Find ACTUAL DISTANCE between ORIGIN and DESTINATION: Be sure to use CORRECT SCALE FOR MAP:

Distance is 32.5 nm.

ORIGIN: Milwaukee MitchellDESTINATION: Hartford Airport WIND: 22 kn from west AIRSPEED: 100 kn

vPA + vAG = vPG

pla

ne

equ

atio

n

Now we can calculate the travel time: t = d/VPG...

Time is 23 minutes.

PARALLEL OF LATITUDE

ME

RID

IAN

OF

LO

NG

ITU

DE

EAST / WEST REFERENCE

NO

RT

H /

SO

UT

H R

EF

ER

EN

CE

N = 0°

E = 90°

S = 180°

W = 270°

FYI: Always measure angles WRT PARALLELS or MERIDIANS.FYI: Pilots measure angles WRT NORTH. In other words, North is 0°.