Hi Mark,

I believe that the bottle (containing high pressure air) is used to start the #3 engine when a ground power/air cart was not available or not working. Then, once #3 was running, you would use the blead air from #3 to start the remaining engines. The light would come on and remain on because you discharged the bottle and ground crew would have to recharge or replace the bottle again.
Normally, you would use low pressure (ground cart) to start #3. The bottle would not come into play then.

Maybe Lou are some one else can correct me if I"m wrong, as I'm not a real pilot.

Rick

Rick is correct. The high pressure bottle was used only as a last resort at some out station where start carts were not available. With the high pressure start you could start on only the battery for ignition. In all the years I flew the 707, the only time we used the bottle was in the simulator. If anything went wrong with the start ( hot, hung, no ignition etc.) you had no way to clear the engine. With the start cart air available, you could motor the engine to clear out the hot gases or raw fuel.

Now for the start sequence. Engine start was sometimes controlled by Jetway placement, bag loading, etc. If the front (L1) door was still opened we would not start 1 or 2 because of the noise. It did not matter which engine you start first, however only engine 1,2 & 3 had turbo compressors so you would rarely start 4 first in case the cart would die. Some (very few) 707 had 4 turbo compressors. Also, you would delay start on an engine that did not have a working generator for the same reason - in case the ground power would quit - which they did often.