Descent Planning

[tingtang]

In the descent from say FL350 to FL250 it is easy to use the 'LVL CHG' function in say a 737 for example and plan that for every 1,000ft of descent the a/c will approximately travel 3nm. Therefore to arrive at FL250 at a specific fix will require a TOD 30nm before that fix.

If 'VNAV' is used, the TOD is worked out automatically.

If you didn't use VNAV or LVL CHG, and used 'VS' mode, you can approximate the ROD required by using 5 x ground speed (or halve the ground speed and add a zero on the end), and approximate the distance covered in the descent in the same way as above.

The approximations used above are all based on 3degree descent angles which is obviously useful since most glidepaths on ILS's are 3degrees and the maths is simple to approximate in flight. You could easily choose any angle of descent, but doing the trigonometry everytime wouldn't be fun.

My question is - do most descents (not just final approaches) in a/c that fly high and fast work on the principle of using a 3degree angle of descent (or 5%)? Do FMS's use 3degrees by default as the descent path angle when working out VNAV profiles? I would like to know what other pilots do for descent planning, other than using the FMS to work everything out.

Thanks!

[NW3]

Hi Tingtang,

In reality, and as an aside, it's not normally that much of a science. Many places we go to (in Europe and the USA at least) might ask you to descent before 'your' TOD point (as calculated by the FMC) for airspace reasons, so even if you're trying to be clever and economical and descend in VS-500 to get back on profile, they will spot it and ask you to descent faster.

However, given a free choice, and without an FMC to help you, 3 times the flight level, plus 1 mile per 10 knots back to 180, is a pretty good guide, and basically what I use on every descent. It works in the 757 and 767 - other aircraft might have different factors.

It's also useful to use standard energy 'gates': If you are at FL100, 250 kts, with 40 track miles to go, you'll probably be fine. If you're still at 300 kts, and at FL120, when you have 40 miles to go, you have too much energy and need to get rid of some.

The VNAV descent will be calculated based on an idle descent at whatever speed you have put into the computer for the descent.

This can be:

- ECON (based on the cost index... likely to be about 260-280 knots)
- A selected speed (e.g. 0.80/300 which means descending at .80 but then holding 300 knots when the IAS increases to that value)
- a couple of other ones too

If the speed is very slow, then the descent will start much earlier, as the nose will need to be a lot higher, and the ROD therefore much lower. The overall angle will then be quite shallow.

If you descend at a very high speed, the TOD point will be much further along, but when you do get there, you will stuff the nose down and descend at a much steeper angle.

Hope that makes some sort of sense - sorry about rambling a bit.

NW3