Fuel Jettison

[Will]

Apparently, the fuel jettison time is estimated for the first 90 seconds, and then updated based on real-time data about the actual jettison rate.  What is the usual difference?  (And Hardy... are you modeling the update in the jettison time after 90 seconds? grin...)

Will

[Jeroen D]

Well,
Here's some stuff from some AOM's:
Jettison time is initially estimated using preprogrammed rates. The system begins updating the estimate based on actual fuel quantity rate of change ninety seconds after jettison begins.
So I read that after ninety seconds it will constantly monitor the rate of change on the total fuel quantity and calculate the remaining jettison time.

During jettison the system also controls fuel balancing between main tanks 2 and 3. It does so by de-activating the override/jettison pump in the low tank, untill tanks are balanced. This will of course, influence the remaining jettison time.

Jeroen

[Jeroen Hoppenbrouwers]

Does anybody know whether there are fuel flow sensors in the jettison circuit (exact measure of fuel being dumped), or the system uses the FQIS to guesstimate the outflow? If the former, I would expect a waiting time much less than 90 seconds for the first update. If the latter, 90 seconds per quantity probe seems quite logical.


Jeroen

[Jeroen D]

Everything i read in my AOM's seems to indicate the system does not have any flow sensors but relies on the measuring the change in the total volume/mass of fuel remaining.

Makes sense, as all the required sensors are there already. Whereas flow sensors in the jettison circuit will be extra, adding weight/complexity.  

Jeroen

[Hardy Heinlin]

I think PS1 works in the same way. The rate is computed by comparing the current total fuel quantity with that from 90 seconds ago (or whatever time interval). That's why it takes one cycle until the first computation can be done.


|-|

[delcom]

No flow sensors...calculations only.

cheers,
delcom

[OKD]

Hardy & Jeroen...

Got the feedbacks below from a QF pilot....hope that helps?


The aircraft tells you an estimate of the time when you first select the amount to remain.  But it updates it when it sees the actual rate of jettison. ie the amount the total fuel decreases with time.

This changes as the individual tanks empty.and the number of fuel pumps operating decreases, and the number of fuel pumps operating decreases

Some tanks have stronger pumps which over-ride and those tanks empty first.  So the calculation is dynamic.  It changes as the fuel decreases

The initial estimate is to give the pilots some idea of how long they have to deal with the emergency.... fire/failure etc

[delcom]

The time until jettison complete is determined by subtracting the total fuel
quantity value from that calculated 30 seconds ago, multiplied by 2 to get quantity per minute, and then dividing by the total fuel quantity minus the fuel to remain value. During the first 30 seconds of jettison operation, the time is calculated based on the predicted jettison rates depending on the number of jettison pumps operating.

cheers,
delcom

[delcom]

Jettison time calculation and FTR indication is done by the FJCC.

sincerely,
delcom

[John H Watson]

FJCC = Fuel Jettison Control Card/s  

[Hardy Heinlin]

Thank god it's not done by the FIFA.

[Will]

To alert the crew that jettison is taking place, a loud vuvuzela noise plays throughout the entire procedure.

[Jeroen Hoppenbrouwers]

... in the cabin.

[Hardy Heinlin]

You are joking, but the jettison nozzles themselves are indeed vuvuzelas, they are also depicted on the fuel panel.