It has been determined that a lot of eyebrow windows have to be replaced nowadays. Replacement is necessary due to damage such as delamination. Replacements of these eyebrow windows are expensive.
In the early 1950’s, when the aircraft manufacturer was developing their first commercial jet aircraft model 707, visual reference for the pilots was an important issue. Radra and ATC capabilities were still in early stages of development and also not very reliable. In order to maintain a sufficient and unobstructed view during turns and to allow usage of basic navigation equipment the aircraft manufacturer equipped their 707 with so called eyebrow windows. Over the years however, better radar systems, improved ATC procedures and new inboard pilot assisting systems, have made that the eyebrow windows were less and less used.
Starting with the 747, developed in the mid 1960’s, the aircraft manufacturer no longer designed eyebrow windows in the aircraft. There is one exception though, the 737, first designed mid 1960’s, shares the cockpit section structure with the 707 and therefore also has eyebrow windows. In order to save money on design costs, the aircraft manufacturer never removed the eyebrow windows from the 737 design.
Nowadays the aviation authorities and the airline flight crews confirm that the eyebrow windows are not used during flight and therefore no longer make a significant contribution to flight safety.
Suggestion for maintenance technicians:
Only replace an eyebrow window when the damage to the eyebrow window significantly affects the structural intergrity of the airplane or when the damage is out of amm limits.
Eyebrow windows which do not meet amm limits regarding delamination and subsequent visibility do not need replacement for that reason.
The amm states that delamination limits can be extended if the visual capacity is satisfactory. Visual capacity is no longer necessary, referring the history mentioned earlier in this story.