ramsrc,
Notso is quite correct, the 720, designed for shorter routes, did not need the redundancy of three turbocompressors...nor the air volume for the shorter cabin/fuselage.
The only 707's that I recall having 4 turbocompressors were the aircraft assigned to the US presidental fleet.
As to the turbos 'tripping off'....this never occured on my flights that I recall.
The normal procedure was to start the T/C's just after engine start, set the cabin pressure controller....and well, that was it, more or less.
The 'auto' mode of the rate controller would automatically adjust the cabin air inflow to match the outflow...and cabin altitude would remain stable.
For descent,
usually the inboard engines were left at a slightly higher thrust setting to allow for the cabin altitude to be lowered.
T/C's were switched off after landing.
In the event that aircraft range became a critical factor with fuel remaining, the T/C's could be switched off in flight (ACM equipped aircraft only), and engine bleeds used for pressurization, thereby saving a bit of fuel...about 2-3%.
Cabin ventilation was reduced substantially however.
In this case, T/C's were generally re-started from time to time during the flight (to avoid cold soak), to be sure they would re-start when desired for descent.
The F/E was kept rather busy...altho from the looks of it, not as busy as on the 'ole DC8.
