"PTOBO", question from a friend

[ChristiaanJ]

Does anybody know what "PTOBO" would refer to?

This is in the Concorde Vol IIa flight manual.
The aircraft must not be taxied with a PTOBO in excess of :-
3300 kgs for a 53.5% take-off
1800 kgs for a 54% take-off
Could stand for something like "Pre-Take-Off Balance-Offset", but solving this riddle would please both me, and the friend who asked me the question :ok:

CJ

[EXWOK]

Pre Take Off Burn Off

The fuel required to be burnt during taxy to allow txfr from tank 11 to tanks 5 & 7 to achieve TO CG (53.5% or 54%).

Used when ramp CG aft of TO CG (ie high fuel load)

Alternative was Pre Take off Transfer when ramp CG ahead of TO CG - fuel txfrd to tank 11. Much quicker and less hassle.

[ChristiaanJ]

Many thanks, EXWOK !

That fully answers the question.

Cheers,

Christian

[EXWOK]

And it doesn't take a genius to work out that the limits you refer to (3300kg and 1800kg) are just another way of defining a max aft CG for taxying.

[ChristiaanJ]

EXWOK,
Never had to do Concorde "fuel-and-CG math" myself, so I take your word for it.
(I helped to build 'em, not to fly 'em.)

I would assume that when you got up to the limits, a close watch was kept on what went into the rear baggage hold, as well.
Also, to start worrying about PTOBO, you'd probably already would have "over-filled", no?
(For those unfamiliar with the aircraft, it's a Concorde trick, where you "top off" each tank with a carefully metered dose of fuel after the "high level" indication comes on. Don't know if it's done on other aircraft.)

CJ

[EXWOK]

No need to overfill....you'll recall that tank 11 sits higher than all the others. So CG remains roughly constant until tanks 1-10 are full, then it starts to run aft as tank 11 continues to fill. So PTOBO vs PTOTR is a function of ZFCG and total fuel load.

We would sometimes encourage the loading of the aft hold in order to achieve a 54% take-off if heavy for its (small) fuel burn and performance benefits. Careful ZFCG management would minimise the Burn/txfr..

There was always more to the operation of this aircraft than first appearance suggested!