'Bleed system problems. On our -100 course we were taught four different bleed system configurations - and they were the ones that worked (more or less)! The compressor had either had more air than it could handle or not enough; so it was prone to compressor stall: Getting the balance right seemed to have been a matter of trial and error by P&W in the early days of the JT9.'
Ahhh. the old JT9......
When I did my ground engineers course when the instructor came to describing the bleed system he said that when P&W were first designing the bleed system they threw a hand grenade in the front and where the shrapnel came out through the casing, that is where they put a beed valve.
I had a tailpipe fire on one engine run I did early one morning. Fuel on at max motoring (22%) and there was instantaneous light up. I shut the fuel off, kept motoring and watched the EGT literally go off the clock. Just as the fire brigade turned up the EGT started to go down. Afterwards the headset man said he did see a mist coming out of the back of the engine when I started motoring but he had assumed it was the rain he was standing in.
It turned out that what had happened was that the condition actuator (HP Cock) had stuck open after being functioned in the hangar (with the guy in the cockpit just watching the LP Valve postion light as a check for correct operation). So when I started motoring the fuel pump pumped all the fuel that was in the supply pipe from the wing spar valve into the combustion chamber just waiting for the ignitors to burst in to life. Then when the start lever was placed to run the ignitors came on and the spar valve opened, replenishing the the fuel supply in the pylon pipe. So the EGT would only decrease when that fuel was exhausted.
On stripdown the faulty Condition Actuator was found contaminated with engine oil. The JT9 oil leaks being another story.