Descent to a fixed point

[Groundbased]

Hi,

I have a question about descending to a fixed point.

When you get a clearance to be at a certain level, by say 25 miles (is that NM by the way?),
before a specific waypoint or navaid, what is the best way to meet the restriction?

Would you create a new waypoint in the FMC route and let VNAV do the work, or do
some speed/ROD calculations and then use v/s or lvl ch modes?

Also is there any tolerance in this type of clearance?, +/-

Thanks in advance for any answers.

GB

[411A]

Usually do a quick calculation and descend manually (v/s mode), especially for close-in restrictions. Have watched guys try to do these close-in descents with the FMS, only to have it miss by rather a lot.
In this case, and they are the PF, have THEM explain to the ATC controller why things didn't work out....
Works for me anyway.

Also have watched a rather experienced First Officer (who thought he was 'ace of the base') begin descent using the FMS only to have an intermediate level off from ATC. He programmed the box OK, but then sat there watching the aeroplane NOT level off as programed...and when I pointed this out, his only excuse was..."well, it was SUPPOSED to level off".

Back to basics sometimes (usually) works better.

[fatboy slim]

These clearances tend (in Europe) to be

'descend when ready FL XXX make level 35 before YYY'

so i do a quick mental calculation of 1000s of feet needed to lose divided by 2, which equals the number of minutes i need at a ROD of 2000'/min. A quick Groundspeed/Distance calculation will tell me how many minutes to go until the point is reached.

Most FMSs will do some sort of a descent profile for you , the one i use isn't a huge help so its mostly sums in the head!!

[Dragon Knight]

The way i do it:

"yyy" in scratch pad and add a "/-35" and it should look like this yyy/-35

I insert the point where yyy is, and a new waypoint is generated on the track prior to yyy, with a distance of 35 miles to yyy. I restrict the new waypoint with XXXB (means Below FL XXX), and let the box do the work. It works every time, and it only takes 10 sec. to program, so why burden yourself.

[OzExpat]

I've been a bit more cautious with this over the years. In some places, ATC have been known to impose some pretty unrealistic requirements that would necessitate a massive rate of descent and, in one well recalled situation, nearly VERTICAL!

So, needless to say, I always work it out in my head first, before doing anything. I don't trust anyone, or any thing, until I've done that.

[Moneyshot]

Flying a machine without Vnav ,FMS etc I tend to do a quick calculation based on the fact that I will go 3 'nautical' miles per 1000 feet of altitude lost in nil wind conditions. With a headwind I won't go as far and with a tail wind I'll go a bit further. This assumes that I will maintain a speed close to .7 Mach or 280 knots depending on altitude. Once established in the descent ,I can monitor whether or not I am going to make the assigned level and make adjustments accordingly.
In the climb it's more difficult as any calculations you make are more sensitive to the changing wind with altitude (your groundspeed) and your weight (your rate of climb). Helps if you've got a computer to help you but you have to do a calculation based on time=Distance/groundspeed and then look at your rate of climb to see if you're going to make it!

[eyeinthesky]

QUOTE

Also is there any tolerance in this type of clearance?, +/-

UNQUOTE

In the UK, many of these 'level by' restrictions are to keep you separated from crossing traffic in the neighbouring sectors so there is no tolerance in them. Better to be level a few miles early than late.