Hi Nick, a very good question. DEBOW was instigated into the design of Concorde (so legend has it) following an engine failure in the Vulcan Olympus 593 flying test bed in the late 60's (on the ground at Filton). The engine had been undergoing repeat stop/starts over a period of time; unfortunately this finally resulted in a failure of the engine. Investigators concluded that un-even cooling of the rather long LP/HP shafts resulted in a slight 'bowing', and accelerating this engine in this condition resulted in extreme shaft vibrations, that could be 'hazardous to engine health'.
Debow was fairly simple in concept, in that when you started the engine, part of the starter interlock circuit required you to select the 'DEBOW' switch (located directly below the start switch) to ON. This switch would then input a discreet signal to the electronic
Engine Control Unit that would have the effect of limiting engine N2 to 30 %. (This you quite rightly stated is a sub-idle condition). Setting the switch also started an electronic timer, that after 1 minute caused a light set into the DEBOW switch to illuminate, this indicating that the switch could now be set to normal, as the temperatures of the shafts would have now been equalised, and the discreet was therefore removed, and the engine allowed to accelerate to normal idle.
Now if you were undergoing a 'normal' start ie. a cool engine, then all the F/E would do was as the start switch cut-out and sprung back to normal (@30% N2) then the DEBOW switch was manually returned to normal also, thus allowing the engine to rapidly accelerate to it's normal idle of about 68% N2. Now to fully answer your question, if an engine had been shut down within 15 minutes, it was deemed that bowing of the shafts had not had a chance to occur, and so a normal start could happily be performed. (I seem to remember that the limits are 15 minutes to 4 hours, for a debow start). So it is possible that a check start might have been carried out on #2 engine, so a debow start was not required. More likely, if the aircraft was a night-stopped BA003, departing JFK as the morning BA002 back to LHR, then all engines would be cool, and so a normal start could be carried out, unless a ground run had been carried out that morning on #3 engine. (ie. less than 4 hours previous).
As a matter of interest, when you started a 'warm' engine, you could feel the vibrations for several seconds, even at debow. These were not violent, but certainly noticeable. Also, after engine starting, if the DEBOW switch had been inadvertently set back to debow, nothing would happen, because as the engine passed 58% corrected N2, then the debow function was inhibited.
I hope this all helps Nick, and thanks for an interesting post
Dude
