Range To Altitude Arc B744

[QAVION]

Yet another question relating to overweight gliders.... :D

Assuming dry fuel tanks at 35,000'... engine flameout and a gliding 747-400... Would the "Range To Altitude Arc" on the Nav Display still be displayed? I have this feeling that the APFD Mode Control Panel is not powered by the Standby Busses... and that the Sel Altitude readout would remain locked at the last selected altitude. If you lost power to the MCP before you had dialled in the altitude of the airport you were intending to divert to, would the Range To Altitude Arc no longer be of any assistance?

Unfortunately, I can't remember what generates the arc.... and I have no access to a sim. I'd guess that the IRS's provide IVS (descent rate) and that the ADCs and MCP provide the altitude differential, but what does the calculation... the EIU's, the FCC's, the FMC's? Does the aircraft remember the last Sel Alt setting if power is lost to the MCP?

Thanks.
Q.

[Fright Level]

The range arc would be a useless tool in this case as it bases it's assumption only on instantaneous descent rates which will vary as you descend into thicker air. It also doesn't know the effect of drag etc until it is presented with it (speedbrake/gear etc).

A glide approach and landing will be better assured if old fashioned techniques were used in this case (ie keep plenty of spare energy until you can guarantee a landing, then use drag to waste it.

Rod

[Cornish Jack]

Hi QAVION
Just surfaced from lunchtime post-prandial torpor so the thinking may be even more woolly than normal.
Anyway - while the 'green banana' may not be much use in the big glider scenario, perhaps the FIX page might be usable. If, for example, a spot height of, say, 2000 feet was entered on the ETA/ALT line at 6R it could be used as a 'LOW KEY' point or equivalent visual reference on the ND??? Not absolutely sure that it would work in those circumstances but if I manage to get any Sim 'playtime' in the near future I'll try to remember to have a look at it.

[QAVION]

Thanks for the response, gents :)

I did confirm during tests today that the MCP values are frozen during Stby Power Ops, so even if you wanted to use the Range To Altitude Arc, you couldn't change your mind about a target altitude.

Looking forward to the results of the sim session.

Cheers.
Q.