Drafts around the sides of the Pella casement window are usually an indication that the weatherstripping requires replacement. The weatherstripping is inside the jamb on each side of the opening. It is a vinyl plastic bead, much like what you see holding glass in place on Pella storm doors. Replacing weatherstripping on Pella casement windows is a matter of unsnapping the old and pressing in a new piece. Replacement weatherstrip for the Pella line is available at Lowe's.
Open the Pella casement window with the hand crank. Once the casement window is fully open, you'll have access to the weatherstripping on the inside of each window frame jamb.
Insert a putty knife blade between the weatherstripping and the window frame. Pry the weatherstripping away from the jamb. Continue prying the weatherstripping along the length of the jamb until it is completely away from the window frame. Repeat for the opposite side.
Measure from the bottom of the window frame to the top with a tape measure. Cut a piece of vinyl weatherstrip to the measurement with a hacksaw. The teeth on the hacksaw blade make a clean cut on the vinyl. Repeat for the opposite window frame jamb.
Insert the angled side of the weatherstripping into the window frame jamb first. Press the opposite side into the window frame jamb with your thumb. It takes some pressure before the weatherstripping snaps into place. Repeat for the opposite side.