Is there any wood trim around the aluminum frame, or does the fin of the aluminum frame go behind the brick veneer???<br /><br />If you have wood trim the job should be very easy, just take accurate measurements of the opening and the manufacture would deduct for the replacement window to fit the opening!!!
The outside picture looks like the fin trim goes behind the brick veneer, if it has a fin!!!<br /><br />You may have to do a little exploring from the inside!!! Remove the slipper casing,the finish board that goes aganist the interior aluminum, there should be a space between the window frame and wood,take a thin saws-all blade or pick etc: and push it towards the outside and you have to determine if there is a fin trim and if there is then you a job cut out for ya!!! Most aluminum windows do have a fin!!!<br /><br />One other thing is when interior window sash is in the open position look and see if any screws go into the wood framing!!! Could be 2-3 screws along the jamb and if that is the case you may be able to pull the window out after releasing the screws on each side!!!! 
but.........I spent two days watching exactly how it was done and learning the little tricks <br />Next time we did it ourselves to replace aluminium framed windows and it wasn't difficult at all and with a nice bead of silicone you can "hide" some mistakes <br /><br />Of course maybe my translation is wrong in which case ignore this post 
