TOD and wind
 

Say normal descent distance 100 nm, tailwind component is 50 kts (assuming at all flight levels while descending). Would this require to start descent 50 nm before normal TOD, or start descent from normal TOD, but with increased V/S & speed adjustment?

In other words: offset TOD or adjust speed?

Without knowing the authoritative answer (which probably depends somewhat on whether you want speed or economy) there is an obvious flaw in the 'rithmetic, unless you're intending to spend an hour in steady descent...

FMCs will adjust the top of descent to account for winds. The tradeoff between time and fuel also comes into play. For the shortest time you would cruise to a point and then push the nose over to barber pole all the way down. To save fuel you would cruise to a point where you can just get to airport gliding down at best L/D. Many times with a low cost index in the FMC you may slow from your cruise speed for the descent. Both of these points will move with your head or tailwind. Closer with a headwind and farther from the airport with a tailwind. But not a mile a knot.

D Alt: 33,000
x3=:.........................99
+10:.......................+10
+/-3/10kn wind:........+15

ToD Point:................124 nm

That doesn't include adjustment for flying at non standard speeds etc.
I flew for an outfit which was paranoid about calculating extremely accurate ToD point. Load of blks! Always gets messed around so you increase speed (dive it off) or use speedbrake (that's what they're for).

Quote:
use speedbrake (that's what they're for).

Wrong, speedbrake is for when you cock it up or are too idle to do a proper job.

Or, if you are trying to do a proper job and atc demand a speed reduction, to say 220 kts or even minimum clean speed. If you are not quick enough for them, they may put you in the hold. I don't like using speed brakes if I can help it, as it can upset some passengers. Cant win sometimes.

Screwball
I remember going into LHR decades ago and ATC were really messing up a United aircraft asking for all sorts of speed changes ROD changes etc etc. Finally the ATC girl asked them to slow up and increase rate of descent. In that lovely deep American accent that you guys seem to develope (is it real or practiced?) United said " We can do one of those which one would you like? The girl told them to use their speed brakes. United replied "Lady I use speed brakes when I screw it up not when you do" End of conversation.
In those days if the handling pilot used speedbrake he had to buy the first 2 rounds of beer that night. 4 or 5 of us up front but could be really expensive if the cabin crew noticed!!

Quote
he had to buy the first 2 rounds of beer that night

Of course those were the days when it was deemed acceptable to have a couple of beers on a night stop with out fear of being tested and found to have .0000005 mg of alcohol in one's blood

Descent takes about 20 mins = one third of an hour - use one third of descent wind component to correct.

So in this case, tailwind 50 kt - go down 17 miles early - this is what we used to do/teach before we had all this fmc stuff and did it manually (btw very well) and it worked fine!

ps we didn't use the speedbrake either - that was for atc use!

speed brake = "Pilot Error Correction Lever"

Basically you f$&*ed up your top of descent calculation/profile

if you need more than idle thrust on the way down before 1000' HAA, you f%$#ed up again.

Profile Correction Lever or Fixit Stick. :ok:

I hope the OP doesn't fly big jets for a living...

Capn Bloggs, agree with you there - speedbrake lever is a flight control and one of the tools in the box to help ensure safe and efficient operation - if you need it, use it!

hawker and bagot,
Did I once refuse to fly with you two? Naah, couldn't have, you're too young.

Now, you youngsters, listen to me!
Flying a REAL big jet is not a computer game.
If you need speedbrake, use it!
If you need power, use it!

Finally, may heaven protect you from captains who make a huge song and dance about your use thereof.

Basil,

At my airline the speedbrake is also known as the Lever of Shame.

I'm not sure that that's poor corporate culture or poor CRM or neither. Use it if you need to, but if it's your own fault, you didn't think ahead.

Then again I think the term Cowboy Authority is in our Op-Specs, as well. http://images.ibsrv.net/ibsrv/res/sr...s/badteeth.gif

It's all to do with the TIME you will be exposed to the wind. (CARP calculations for any ex-tac airdrop boys or firefighters or anyone else who drops stuff from airplanes)

60 kts wind = 1nm/min
30 kts = .5nm/min

divide alt to lose by normal descent rate (usually 2500 fpm for 3 degrees/idle thrust) and multiply by wind factor related to the above.

i.e. 15000ft to lose = 6 min; with 60kts tailwind = 6x1 = additional 6 nm required

30000ft to lose = 12 min = additional 12 nm required

Normally this is a tad too much as the wind usually decreases as you descend.

Basil
Ha Ha You remind of an old sod I flew with on DC10's. Going into LAX, ATC did the normal trick of 350 knots one minute and 250 the next 4000 fpm one minute and 1000 the next. In the end we were asked to speed up and increase rate of descent at the same time. We had full speed brake out at the time so he increased power. I pointed out this fact and was told "sonney, speed brakes are for going down and throttles are for going faster". Well I guess he was corect in a kind of way.
The majority of occasions speed brakes are used can be avoided by decent planning and energy control. Also anticipating what ATC will want of you helps. One bit of energy control is to close some throttle at your normal point of descent even if you are waiting for clearance to descend. Get rid of energy in a fuel efficient maner not inefficiently by using speed brake. But I guess that is not a normal SOP and would be frowned upon by the "never out of the box brigade".

Westcott Snatch
No, not some pretty little hosties living NW of LHR. Any Trident guys here who can enlighten us on the efficient (or not) use of energy?? I thnk many of the younsters here will not believe it.

Call me old fashioned but we just use VNAV path and speed intervention if it has a problem, although with geometric path software it usually keeps up. Of course, the forecast descent winds and cruise winds need to be input, but all this calculation stuff.. nope.. even if we get " direct" to its no problem to review the remaining time and convert that to a ROD... strangely, the latest Co-mail advice is not to " try and do a better job than VNAV" applies to climb and descent. Speed brake is hardly every needed,, and it's not a " handle of shame" otherwise it would not be fitted in the first place!

'struth! There's always one! No, actually there's a lot more than that :rolleyes:

Had aged skip who, when minor error pointed out by FO or FE, would rotate erect thumb and say "Just testing, old boy." He WAS being intentionally amusing.

Start with a WAG if you don't have an FMS:

Normal TOD is about 100 NM; normal time of flight after TOD is about 30 minutes, for about 5 NM/minute on average.

60 Kt = 1 NM/minute, or 30 NM over the course of the descent, so a good wind correction is half the wind. Assume average wind is half the wind at cruise. Therefore:

TOD correction = 1/4 head/tail wind component at cruise.