747-400 Airspeed Bug Behaviour

[QAVION]

A question for pilots who forget to enter the winds for descent into their FMC's :D :

On a 747-400, during VNAV descent (Idle) with unexpected tailwinds...

When the aircraft attempts to maintain VNAV path by speeding up, does the Command Airspeed Bug (CAB) on the PFD follow the aircraft airspeed as it rises... or does the CAB remain fixed to the original descent target airspeed?

Thanks.
Q.

[static]

It stays fixed on the original target speed.
You get a scratchpad message:"drag required".
And the nice thing is: If you do nothing, you will get an overspeed.

[Denti]

Just like it is in my small 737, does that imply i can fly the big thing?

[scanscanscan]

Qavion... You will find the information you require for the 757/767/ 747-400 des. and much more info.in the book called "FMC Users Guide" ($45.00 check payable to Leading Edge Libraries)
Mail to Bill Buffer
2031 River Falls Drive
Kingswood Tx 77339
Usa
Reading it should keep you happy for a week!

[QAVION]

"And the nice thing is: If you do nothing, you will get an overspeed."

This appears to be contrary to all my manuals, "Static" (both for the 767 and 747-400). According to these, the aircraft should leave the path if the aircraft is unable to maintain the path by increasing its speed (i.e. by up to 15 kts(in the VNAV idle phase)... but only if this extra 15kts does not violate any VNAV speed restrictions or Vmo/Mmo).

"You will find the information you require for the 757/767/ 747-400 des....in the book called "FMC Users Guide" "

Thanks, "Scanscanscan", but I already have this manual. No mention of bug behavior seems to be made in the chapter concerning VNAV descent. I may have to put question to the Bluecoat forum.

Rgds.
Q.

[whoop-whoop]

Although the manual states that at bug +15kts VNAV path will default to VNAV speed, its depends on each individual A/C. Some I have flown will as advertised default to VNAV speed at spot on bug +15, whilst others will cling on to the path for dear life.

With the former case, if you sellect FLCH and increase the descent speed to 'dive' below the path (approx 300'below/10kts increase in inspeed B757/B767) then resellect VNAV, as the speed reduces back to the target airspeed you will recapture the path from below.

There are many ways to skin a cat and this is only one suggestion.

Accurate descent planning is a fine art not an exact science.

[Intruder]

Qavion:

In the descent with tailwind scenario you posted, VNAV will allow up to 15 KIAS above the bug speed, but the bug will stay in the same spot (controlled by the setting in the VNAV Descent page). Overspeed protection is still active. If the winds diminish, the airplane will return to the original commanded speed after the VNAV path is regained.

If VNAV Path cannot be followed at +15 KIAS, options are: speed intervene, reset VNAV Descent speed, go to FLCH at a higher speed, deploy speedbrakes, extend landing gear and/or flaps...

[QAVION]

Many thanks to all.

Cheers!
Q.

[hopharrigan]

BTW, you don't need to set the winds in the descent page. Check out the book for the correct procedure, and see the default. If the descent winds are decreasing at the pro rata rate, then inserting the descent winds is a waste of time.

[Nice Wing]

What I found many pilots did not know was that leaving the cruise winds in for lower altitudes affected the descent profile calculations. For example if you had in winds for 310/350 at a waypoint near departure, and were now at 390, it would still factor in the 310/350 winds for the descent. If they were not valid, as soon as the aircraft started to descend, the profile would change. Deleting cruise winds after climb to the next level and all but the final level before descent solved the problem for me.

[scanscanscan]

Nice Wing...I agree with that, and I also found the 757 required more attention on descent profile,and felt a bit more slippery, than the fat 767.
The 757 I felt was also less inclined to slow down than the 767.
I programmed in the descent winds, from the cfp provided, and used the m.78/290kts and 250kts below 10000 ft above the airport elevation,speed decent profile option.
This avoided being caught out at high speeds above turb speed if turbulence or cloud was encountered at night and gave more time to correct if being high on profile due ATC. There is the added advantage of a more gentle deck angle in descent for the cabin crew,generally no use of speed brakes, and a slight saveing in fuel, and stress on the nh pilot, as he observes you stand a good chance of makeing the approach first time.
Maximum descent speeds,high speeds below ten, and supper clean fuel saving approaches,we also did some of these, and you enjoy the buzz, however my friends that always did them, sometimes screwed them up and either died or lost their job for unstable approaches, incorrectly configured landings,unstable go-arounds, etc.At the end of a 12hour night duty, I simply did not have the energy for a dead duck or red brick type descent/approach, it was an age thing.