The windows in the cab must be double-glazed. The specifications call for float glass. Float is glass made in flat sheets by pouring molten glass onto the surface of a bath of molten tin. Most conventional windows are made from float glass, but the float glass used in cab windows conforms to higher specifications than ordinary windows. They must both be at least a quarter of an inch (six millimeters) thick, but less than half an inch thick. The total thickness of the whole window unit can't be more than one and a half inches. Where high sun might be an issue, low-E (low emissivity) glass is recommend. Low-E glass has a microscopic layer of metal bonded to the surface, cutting down the amount of heat that gets through. Tempered glass isn't authorized for use in cab windows, thought it can be used in overhead windows provided that it doesn't get in the way of a crew's view of aircraft flying overhead.
Both panes need to be a certain type, class and quality of glass. The inner pane must be Type I, Class I, Quality q3.
The outer pane must be Type I, Class II, Quality q3. If the outer pane is not more than a quarter of an inch thick, the glass needs a slight blue/green tint to cut down on glare.
The two panes of float glass making up the double-glazed window units will need to be half an inch (13 millimeters) apart, separated by an air space. The air space needs to be dried out with a drying agent certified for at least five years. If there is moisture present, it could condense and obscure the windows.
The windows need to be certified for the wind pressure they will have to tolerate. They must also be certified for the tower's elevation above sea level, as this may affect the window unit's performance.
