If you look above the cockpit windows on the Boeing 737, you may see an extra set of windows that were once a useful addition.
If you look above the cockpit windows on the Boeing 737, you may see an extra set of smaller, thinner windows. These are ‘eyebrow windows’ and were once a useful addition for aircraft navigation and maneuvering. Advances in technology have seen them phased out, but you can still see them in place on some older aircraft.
Eyebrow windows – a useful addition at one time
Eyebrow windows are additional windows that were installed on many aircraft up until the early 2000s. These are usually a set of four smaller windows positioned to the side above the main windscreen. They were a common appearance on early Boeing and Douglas aircraft, including the Boeing 707 and 727.
The main use of these windows was to aid visibility outside the cockpit. They allowed extra visibility during turns, particularly tight turns when approaching airports.
They also offered an increased view of the sky for star-based navigation. This is likely to have been of less importance, as such navigation would have been using sextants which require an overhead view. Aircraft such as the DC-8 had a viewing hole in the cockpit ceiling for this. They would also carry an additional navigator crew member.
Phasing them out
With advances in technology, eyebrow windows started to become less useful. Navigation of course now is computer and satellite aided, rather than visual. And whilst clear vision is always going to be important, advances such as improved instrument landing systems and TCAS (Traffic Collision Avoidance System) made them less necessary.
They remained on many Boeing planes, including the 737, that followed designs on previous aircraft. The Boeing 757 broke the trend and did not include them in the redesigned cockpit.
They are seen too on the McDonnell Douglas MD-80 aircraft, in use still with a few airlines. American Airlines retired its last MD-80 in late 2019 and Delta is close to finishing operations.
Why then are they still seen on some 737 aircraft? Boeing started to phase them out after the start of 737 production. It was not until 2005 that the 737 started to be produced without eyebrow windows
Some early 737 still have them in place, whilst others have been covered over during maintenance or aircraft upgrades. Since 2006, Boeing has made kits available to retrofit older 737 aircraft. Many pilots comment that they would cover them over anyway. As their usefulness declined, they became annoying sources of extra sunlight, and pilots would often block them with books or maps!
Why remove them?
They could, of course, have been left in place. This is not common practice in aircraft design though. Removing them will save time and money in aircraft maintenance – an important consideration. It will also have a small positive effect on drag, and fuel economy.
In a press release covering the new 737 production in 2005, Boeing explained their reasoning as “The design change reduces airplane weight by 20 pounds and eliminates approximately 300 hours of periodic inspections per airplane.”
It also addresses pilot feedback that the additional glare was not desirable, and will have helped to reduce noise in the cockpit (another complaint with early 737 aircraft).
Originally Published at Simple Flying