Fuel Jettison and balance questions

[torrence]

I've been working on a situation similar to a flight I took in Dec.   Lihue HI to KLAX.  Aircraft was a 757.  On climb out the pilot announced that they had a leading edge flap that had not retracted.  After some time - I assume trying to cycle flaps - he told us we would divert to PHNL, but would have to hold for a while to burn off fuel to get an allowable landing weight for the longest runway, 8L (used by the USAF also) and would land faster than usual we probably wouldn't notice.  All went well but we had to overnight in Honolulu and get afternoon flight out the next day.  

I've been doing this flight in PSX - although 744's don't normally use PHLI.  Instead of burning off fuel I decided to use the Fuel Jettison system to speed up getting down to 630 k lb and noticed a number of things:

1. Jettison worked as advertised.  In this case I started at ~147 k lb of fuel and had to get down to ~90 k lb.  When fuel was reduced below 120 k lb I suddenly notice that jettison had stopped - my friendly virtual PNF was checked for 'silent tasks' and
switched to Tank-to-Engine configuration, turning off the the Ovr/Jettison pumps in the process.  Very persistent pilot - I tried to over-ride a couple of times and he kept switching them off again.  Found two ways of continuing jettison: first, obviously, unchecking the 'silent tasks' box on the Instructor station and second, I found that I could leave the pumps on as long as I left the crossfeeds in tank-to-engine and the PNF wouldn't complain and jettison proceeded.  In both cases I opened the usually closed Fuel Trans 1 and 4 valve since it looked like I would end up with fuel imbalance otherwise.

2. A couple of questions - could the PNF fuel management 'silent task' be disabled if Jettison is on?  And second, is using the gravity transfer from main tanks 1 and 4 the correct solution?

Ended up only using about half the available runway with max autobrakes.

Cheers,
Torrence

[Hardy Heinlin]

Good point. In the next update I will inhibit the PNF's fuel management when jettison is on. It is already inhibited when malfunctions occur, but I forgot the jettison stuff.


Cheers,

|-|ardy