737 Fuel Flow switch and AIR COND questions
Hi!
Undergoing the type rating, I've had a couple questions that the SFI's havn't been able to answer fully. They may not be entirely necessary to know, but it would be nice to get a definitive answer (preferably with reference to official documents).
1. One of the procedures we use before transfering fuel is to check for possible leakage by switching the fuel flow switch to USED and crosscheck if it is correct in relation to what was planned for that stage (Xcheck w/ PLOG). In the lighter aircraft I used to fly, we had the same kind of switch, but it would show us a figure that was derived from the fuel flow transducers, and not the difference in actual tank quantity, which would mean that using this feature wouldn't indicate a leakage even if there was one. So does anyone know where the fuel flow switch gets its information from when moved to the USED position in the 737?
2. When checking takeoff N1 settings, we must turn APU bleed air on in order to not get BLEEDS OFF takeoff figures. This does not make sence entirely to me, as you could very well take off with the APU supplying bleed air with engine bleed air off, and therefore thrust not being affected. What actually prompts the FMC to indicate BLEEDS ON N1 settings? Someone told me that it was when the manifold was pressurized, but the person telling me this wasn't entirely sure. Anyone have an idea?