TOD and wind

[GSLOC]

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?

[Tarq57]

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...

[MarkerInbound]

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.

[Basil]

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).

[hawker750]

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.

[screwballburling]

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.

[hawker750]

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!!

[hawker750]

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

[fireflybob]

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!

[Bagot_Community_Locator]

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.

[Capn Bloggs]

speed brake = "Pilot Error Correction Lever"

Profile Correction Lever or Fixit Stick. :ok:

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

[fireflybob]

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!

[Basil]

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.

[ImbracableCrunk]

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/src:www.pprune.org/get/images/smilies/badteeth.gif

[sodapop]

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.

[hawker750]

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".

[hawker750]

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.

[Kirks gusset]

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!

[Basil]

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".

'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.

[Intruder]

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.

[bubbers44]

Make it so simple a caveman can do it. 3 X altitude +/- 10% for winds 10% of TAS or multiples of and add 7 for the slowdown at 10,000. Sometimes you are taken off an arrival routing and cleared direct so if you expect it will happen like going into MEX or BOG most of the time you are ready and don't look like an amateur speedbraking all the way down.

33,000 X 3= 99 + 10 for 50 K tailwind + 7 to slow for 116 miles. Easy and on descent as the speed diminishes you can update progress to fudge the speed a bit to always be in the slot with no power or speedbrakes. Our airline considered it poor planning if you used speedbrakes or power before 1,000 ft unless you were given vectors or speed restrictions on the descent. It made it fun. At normal descent speeds of course you had to add a few miles if you were heavy and take a few off if you were light.

VNAV works most of the time but takes you out of the loop especially if you didn't notice that holding pattern on the arrival that you won't really be doing.

[cosmo kramer]

Whats been said about speed brakes in this thread... what a load of BS.
The speed brakes are a normal part of the aircrafts flight controls - use them as appropriate.

Perfect example is when ATC tells you to keep the rate and slow down. You can be an ass and say that you are unable (hopefully they will sequence you behind the other more reasonable and professional pilots). Or you can help to make the traffic flow work by complying as best as possible within the limitations of the aircraft (witch includes the use of speed brakes).

Another good example of airmanship use of speed brakes is if you don't know how long your distance to go would be (for the Microsoft pilots that appear to have been commenting here, that would be in the case of where ATC is giving you radar vectors). For the sake of economy it would be advisable to keep a descend path somewhat between what you think would be the shortest distance and the longest that ATC would vector you). Thereby keeping the path just high enough that you can take the shorter distance with the speed brakes and the longer distance with a minimum use of thrust (=fuel).

Posters commenting in this thread, who are retired dinosaurs used to fly in an airspace as the only aircraft, are excused...

[bubbers44]

I guess you would consider me as one of your dino over the hill guys because I retired at 60 a few years ago but my currently flying friend is having a cocktail as we speak across the table from me and says everything is the same today as a few years ago so I disregard your last post as BS. A good pilot will use energy management to efficiently fly his aircraft as he is paid to do. It takes a little planning but that is what he is paid for. If you want to make every descent with speedbrakes, knock yourself out but you are wasting fuel and your fellow pilot will wonder why you don't plan better no matter which seat he is in. It is uncomfortable because the passengers feel the buffeting and guys like me know you weren't on your game for that descent. We usually know when the speedbrakes were an operational necessity or to fix a screw up. Have a nice day. Think I will top off my wine glass.

[FlightPathOBN]

VNAV takes into account many variables you dont want to think about calculating...current weight, temperatures, GPA, and speed control.

Speed brakes are over used, most of the time due to lack of experience, and not in anyones best interest....de-stabilize flight, give the passengers a rough ride, and waste fuel, not to mention the FOQUA bust.

[John Citizen]

Whats been said about speed brakes in this thread... what a load of BS.
The speed brakes are a normal part of the aircrafts flight controls - use them as appropriate.


So its ok to waste fuel then ?

fair enough, I suppose you are not paying for it

I suppose Microsoft pilots never have to worry about wasting fuel and making someone else pay for it.

[cosmo kramer]

John Citizen
So its ok to waste fuel then ?

fair enough, I suppose you are not paying for it

I suppose Microsoft pilots never have to worry about wasting fuel and making someone else pay for it.

I just gave an example above where the calculated use of speed brakes actually saves fuel. But I will be happy to explain it again and with more details that even someone who is not a pilot can understand. ;)

To be able to avoid using the speed brakes you have to calculate your descend according to the shortest possible route. But when ATC then gives you another 20-30 nm vectoring you are down low and will use more fuel.
I say, it's more efficient to stay a little higher and if indeed you get the shortcut, pull the brakes. Of course you shouldn't plan for the longest imaginable route either, as the speed brakes will then not make it up if you get the shortest of the short cuts. So it's a compromise, and knowing you destination and how the traffic usually flows.

Now looking over a number of flights, there are 3 outcomes:
1) Sometimes you get the longer way and don't have to use the speed brakes, hence you saved some fuel by not having to fly level at a lower altitude.
2) Sometimes you were bang on.
3) Sometimes you got a shorter route and pulled the speed brakes.

Statistically, it's more likely that you didn't get the shortcut every time and hence in the other circumstances you saved fuel. And hence (Bubbers44), I will not use the speed brakes in every descend. However sometimes I will use the speed brakes as part of a planned, deliberate and calculated action.

If you always plan for the shortest route and doesn't get the shortcut you are wasting fuel.

Clear now?

Of course, if the situation allows I also prefer to increase speed before pulling the speed brakes. However, in the scenario outlined above with high traffic density it's rarely an option. I don't know about the traffic levels in Florida or where ever you flew, but in the major European airports there is a reason if ATC tells you to keep 1500 or more and reduce to 250 or less. You don't want to bump into the guy crossing your path 20 miles in front of you and you don't want to catch up on the guy ahead of you. ATC didn't screw up, these are the circumstances - deal with it!

If I fly to low traffic density airports I will of course plan for the shortest route, and if kept high due to crossing traffic use speed to negate that.

What irritated me enough to write the previous post was Bagot's and Hawker's extremely simplified and ignorant comments about the use of speed brakes.

I believe I argued my point why it's ok to use the speed brakes, and will agree to that much that they are not there to make up for lack of planning. However, they can be part of the plan too.

[Bagot_Community_Locator]

I edit my original post

Speed Brake = "pilot error correction lever"

OR

"ATC f#%k you around lever" (ATC unexpectantly slow you down /reduce your track miles to run)

[fireflybob]

If you always plan for the shortest route and doesn't get the shortcut you are wasting fuel.

Unless you reduce speed (assuming atc allow).

[Basil]

Like a few on here I spent most of my working life in civil aviation and, it should go without saying, am familiar with the principles of descent energy management (or should that be 'decent'?).
Yes, of course, under appropriate circumstances, one can, e.g. having failed to obtain further descent clearance, reduce ROD and speed and then, when cleared, accellerate with a transient increase in ROD.
What if you have been allocated a speed by ATC or are approaching min speed? Well, then you have to apply power and put unwanted energy into the system which will, at some stage, require removal.
Suppose you have the choice of speedbrake or extended routing to lose altitude; remember this: during the increased time of extended routing your engines are still using idle thrust fuel.

I now fly only as a passenger and it worries me that my PiC may think that speed brake = loss of face. I'd rather it was used when NECESSARY - you'll know when that is. Don't let misplaced pride get you into a rushed approach.

Finally, when landing easterly at LHR could you all please arrange to be at idle thrust and on or a little above the path when turning base from the north? :ok:

Bas, Marlow.