Top of Drop ???

[Flap42]

The question is: Given a choice of profile without ATC obstruction, would you descend at idle thrust all the way down to approach platform altitude or use a 3degree profile from the CRZ?

For example 35000 ft *3 = 115 nm before destination and at a ground speed of 420 kts a V/S of 2100 fpm, adjusting the V/S as you go to take in to account of the change in ground speed. This would give you a 3-degree profile, good for the press system and pax comfort.

Or

Use a Path/Vnav/Profile used by the FMS that calculates idle thrust all the way down. This gives high rates of descent 3000-5000 fpm on the aircraft I fly and always seems like a bit of a rush!!

I have been a F/O for about 8 months now and most of the skippers I fly with have different ideas! Some constructive help from the more experienced of you out there would be great.

[Notso Fantastic]

The most efficient way to fly is to maintain cruise as long as possible to give you an idle power descent. Any power used during the descent is inefficient as that fuel would have been better used in the cruise. A 3 degree descent from cruise is not good to aim for- it does not allow for strong wind components and in a strong headwind will result in a premature descent and power all the way down to maintain a 3 degree profile. Similarly, a strong tailwind would get you high on a 3 degree profile causing extra drag needed, showing you cruised too long and wasted fuel. Remember, any power application, use of speedbrakes or large speed adjustments during descent shows you have misjudged and cost yourself fuel (barring external causes like ATC, STARS etc. You should not think in terms of 3 degree descents until on final approach.
Pressurisation systems are designed to cope with an idle power descent from maximum pressurisation altitude quite happily.
Any wild RODs of 3-5000fpm are due to very coarse FMS control and should not be accepted. FLCH on the 747 gives idle power/maintain speed sometimes better than VNAV control. You should never get more than 2500fpm. Until you are quite experienced, 250 kts below 10,000' is advised.

[Flap42]

Notso Fantastic, Thank you.

What does your FMS know about the wind? Is it only as good as the wind that you have put in your self or does it know the real time wind and forcast for different levels on the way down?

If you were not going to use the FMS for a TOD poit, how would you work it out for idle thrust all the way down?

[Notso Fantastic]

FMSs work in different ways. On ours, you can enter descent winds for 4 levels that come off the flight plan, but these can be forecasts nearly 24 hours old! I believe some wash out the last cruise wind down to zero during descent. It's not really critical, you're only in the descent about 20-25 minutes, and other factors come into play, like track miles to touchdown become more important.
It's not a bad idea to get to know your performance. Calculate a desired end descent position and altitude. See how your aeroplane descends- work out 3x alt in thousands, probably +10, allow a bit for wind, and see how that does it. It was fine on basic 737s.

[FullWings]

Flap42,

This was discussed last year in:

http://www.pprune.com/forums/showthread.php?threadid=147304

Hope it's useful.

[411A]

Rocket science this ain't.

Used to have a First Officer who, as soon as he reached top of climb, would calculate for hours just where he wanted to descend, and would spin the bezel on his Breitling watch until it nearly fell off...and in the end, when TOD came 'round, he would
still have it bollexed up.

Descent at 100nm worked just fine in the 'ole tri-motored Lockheed, with ROD adjusted for wind.

[catchup]

ETA minus 30.

keep it simple;)

[18-Wheeler]

747's are easy.
However, you have to take into account the altitude, weight, head/tail wind, other restrictions ....

Then go down at 110 miles.
:)

[Capt Fathom]

Damn! I thought it was at 100nm. No wonder I was always high!

[Capt Claret]

The venerable 146 doesn't like idle descents from top of drop. Lots of dings as air valves don't stay open and aircon packs drop off line, added to internal seals requiring some airflow to work properly.

BAe advise a descent schedule of 70% N2 above FL250, 65% N2 above FL250 and Flight Idle up to FL200.

My employer has a SOP of 70% N2 to FL150. BUT then one must factor in icing conditions if present, in which case a power scheduel of 72%N2 plus 2% N2/5000' AMSL must be maintained to keep the ant-ice/de-ice going.

Then you couple this with an autopilot that in IAS hold can give a ROD of between 1000 FPM and > 4000 FPM. :uhoh:

A colleague has perhaps the best system, FL >250, TOD 70nm, FL< 250, TOD 60 nm. :}

[alexban]

The Eurocontrol is discussing about the implementention of CDA ,continuous descent app,which will mean FMC descent from t/d to G/S intercept.
It's very nice,from T/D you'll receive cleareance to descent at 2500' (for ex.) and intercept the ILS,or call on base leg.
You will descent at idle thrust ,in FMC descent,which has in the descent profile a series of 'gates' through which all the planes,no matter type,must pass. This way the controller will be able to monitor the descent,and plan his flights,and not intervene in your descent.
The planes arriving or departing will fly through virtual tubes,with no further need for atc intervention.
We did some tests in the sim for the eurocontrol,it was interesting,and now they are also instructing the atc controllers from our area,in order to fully implement,for testing,these procedures.
So flap42,you should better use the vnav profile,and if you want to avoid high rates of descent you should change the descent speeds ( i.e.77/300kt for 737 will give you a nice descent ).The vnav profile will take you through the mandatory altitudes ,so comply also with the STAR's.
Brgds Alex

[Carpathia]

Then you couple this with an autopilot that in IAS hold can give a ROD of between 1000 FPM and > 4000 FPM

Yes, it really is that bad. Use of IAS above FL100 is not very sensible in my humble opinion, particularly in the descent. Not difficult at all to descend in VS and accurately keep the desired speed using the throttles. I personally favour VS for any descent except if you intend to descend at idle, which is quite rare. Accurate control of the vertical profile using IAS and power is nigh on impossible.

[DFC]

The planes arriving or departing will fly through virtual tubes

Sounds like tunnels in the sky.

Now where have I heard that one before? :rolleyes:

Regards,

DFC