The so-called “eyebrow windows” were eliminated from Boeing’s cockpit designs in 2004 after their function was made more or less obsolete due to the progress of modern avionics. But why exactly were they installed in the first place?
On older Boeing 737s, manufactured up until the early 2000s, the cockpit is equipped with a couple of small extra windows, located diagonally above the pilots’ heads. These were given the nickname “eyebrows” due to their anatomically corresponding placement above the main front windows.
These four little windows were present on all of the earlier Boeing models, such as the 707 and the 727, and remained part of the 737 up until 2004. While they are no longer a standard feature, they are still available upon customer request.
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Visual circuits and spotting traffic
These windows allowed for better visibility in tight turns, for example, during a visual approach to a runway. They could also help pilots in better spotting other aircraft that could be on a potential collision course.
There is also the idea circulating that they were to offer a better view of the sky if navigation by stars was required. However, this latter raison d’être does not really make much sense. Astral navigation would require pilots to see a much bigger part of the sky than what is possible through the small “eyebrows.”
However, there are aircraft that have such windows, like older versions of the 747. They are located on top of the plane, as you need to be able to stand up and use what is known as a sextant. This is an instrument that, simplified, measures the angle between a celestial body and a horizontal line of reference.
Plugs reduced maintenance hours
The eyebrow windows were mostly a concession to the FAA, which expressed concerns that the extra visibility was essential during the process of certifying early models of Boeing’s aircraft.
After more research, feedback, and of course, the progression of modern avionics, Boeing decided that the windows did not really serve much of a purpose anymore. So, it altered the production of new planes to leave them out and made plugs for the older models to be fitted in the spaces where the windows used to be.
Airplane windows have heavily regulated maintenance intervals. Removing just these four small windows reportedly saves around 300 hours of maintenance over the lifetime of the aircraft. It also reduces, albeit not to any great extent, drag, thus contributing to a slightly better fuel economy.
The windows were no longer needed for visibility, as navigational systems became more sophisticated. The certification of the Traffic Collision Avoidance System (TCAS) in the late 1980s also rendered the spotting-oncoming-traffic function of the windows obsolete.
Pilots would cover them with check-lists
They also entailed a bit of a nuisance for the occupants of the cockpit. When the sun was high in the sky for longer periods, for example, during the summer months, it would shine right in on the foreheads of the flight crew. Often, the pilots would fill the windows with check-lists or other documents to keep the light out, as the windows served no purpose during regular flight.
They weren’t of use anymore, they were more of a bother than anything else, and removing them saved maintenance hours. As such, they were rendered obsolete.
So you will hardly ever see the eyebrow windows on commercial aircraft anymore. However, you can still glimpse them on the military variants of Boeing jets, as they require that extra fraction of visibility in potentially more extreme maneuvers.