What are the small rectangular fairings with the square shaped holes in them, that are located on the bottom aft end of the engine cowlings on the 747-4 in this photo used for?
A wild guess on my part is that they are drain ports for oil that's leaked down onto the bottom of the engine cowling from the gearbox and bearing seals, etc.
Photo © David Morrell - Avid Creations
After doing a good photo search of engine shots, I believe I've determined that the Pratt & Whitney PW 4056, General Electric GE CF6-80C, and Zeus Olympic engines all have this small fairing on their engine cowling. However, the Rolls Royce RB-211 does not. I guess the reason for this is simply based on a different engine design. What's the difference?
Here's some photos of engines that have this fairing .........
Photo © Mark Donoghue
Zeus Olympic Engine (you can just see it).
Photo © Marian Musil
With the fairing (I don't know the engine types, but I'm sure they're either GE or PW engines).
Photo © Anatol Kolendo
Photo © Toby Willis
Photo © Oliver Brunke
RB211 Engine (I believe) - without the fairing.
Photo © Will Lanting
Without the fairing (I don't know the engine types for sure, but I believe 3 of the jets have RR RB211 engines).
Photo © Glenn Stewart
Photo © Greg Weir
Photo © Darren Howie
Photo © Jonathan Simmons
Does the JT9D-3A engine have this fairing? (I can't tell from the photo).
Photo © Jack Hannen