Flighter1 From , joined Dec 1969, posts, RR: Posted (12 years 3 months 2 weeks 1 day 15 hours ago) and read 3003 times:
On Boeing 707 jets with turbofan engines, why is the #4 engine pylon shaped differently than the pylons on engines #1-3? What do engines #1-3 have that is not needed on engine #4? You can see the difference between a #3 pylon and a #4 pylon very clearly in Airliners.net photo id#82418, a Boeing 707-323C.
Spitfire From France, joined Feb 2001, 800 posts, RR: 7 Reply 6, posted (12 years 3 months 2 weeks 1 day 2 hours ago) and read 2923 times:
Ceilidh, for someone who said he flew 707, it's a quite poor explanation.
In fact due to FAA certification in the 50's they do not allow air conditionning and presurization air to be picked directly from the compressor of the engine. They fear to have a mixed of engine oil in this air. So the imposed to have an high pressure bleed air turbocompressor wich take air directly outside the engine.
Here is exactly what is written in the PAN AM 707 Operation Manual (Pneumatic System 707.161 p1 -- Sep13, 1974) :
After the engine are started, the primary pneumatic source is from engine air-driven turbocompressors. Ambient air enters a turbocompressor ram inlet on top of the engine nacelle, is compressed, and is then routed to the manifold.
Why is there a difference between the engines mount?
Simply because there is only THREE turbocompressors, mounted only on engines 2,3 and 4.
Hope it helps.Rgds.
Sabena ... Never to be forgotten (11 years already , what a shame !! )