Which do you trust, dispatch or weight & balance computer?

[Will]

I recall our previous discussion about what to do when you're on the ground ready for takeoff, and the OFP from the dispatch office predicts you'll land with proper fuel, but the FMC says you'll land below some regulatory or operational limit. (The answer was that the OPF is controlling, and to disregard the FMC's prediction at this stage of the flight.)

Q1. What about a case where the OFP differs from the W&B computer by several thousand pounds? It looks to me like it's easy to override the W&B computer, which implies that the OFP would be controlling. Is this the case?

Q2. If there were a discrepancy, would you flag it in the log somewhere so that the maintenance guys could check the calibration?

Q3. How often is there a discrepancy?

Q4. If it's off by a small amount, say 300 lbs, do you still override the W&B computer with the OFP number?

Thanks as always.

[localiser]

Hi Will,

I don't think you should be going anywhere with that sort of a difference between the ofp and the fmc.

I can't imagine a situation where you'd leave the gate without having picked it up in one of the cross-checks. Big in change in ZFW for example, where they send you figures that are way out from the ofp.

You might get a minor difference, perhaps there was a delay and the apu was running a bit longer than planned, or some of the winds in the route data page are out, or wrong flight level in the cruise page. Not on the 747 but on the 737 years ago even an incorrect top of climb wind/temperature could create a weird figure, let's say you were departing from a hot country transiting to a colder area for arrival, but normally you knew where to go to correct it. The ofp should also have printed on it somewhere the fuel or drag penalty for that airframe, and you can check that on the FMC ident page. Any difference there would involve a call to ops and/or engineers.

But you shouldn't be adjusting the FMC to match the figures you were expecting. Anything suspicious should be treated as suspicious...

As I say, anything really significant should get picked up fairly early on.

[GodAtum]

There's always a difference between Simbrief and PSX, I just ignore it