I had a general question, if you use the 3,3 rule for descending Ex: So your altitude is 33,000 ft if you take 33 X 3 = 99 round to 100 miles out what would you set your feet per minute to? TIA
Fpm is approximately 5x ground speed in knots for a 3 degree descent. But in the bus you can switch to FPA mode and set the descent angle directly!
We don’t set a rate of descent, rather we conduct our descent in either OPEN DESCENT (flight idle) or DESCENT (notionally flight idle also)
When we do this we are targeting a standard descent profile of Mach number into 280 knots till 10,000 then 250 down to 3000, all at flight idle if all goes to plan.
Indeed we expect it to fluctuate as the wind changes but to gross error check the profile we multiply our altitude by 3 and add 5 miles to the result for the slow down from, in our case 280knots to 250 knots then another 5 track miles is anticipated for the slowdown from 3000 feet at 250 knots to green dot speed.
Of course all this assumes an uninterrupted descent at about 60 tonnes in nil wind. We would typically add 1 nm mile per 1 tonne more or subtract 1 nm for 1 tonne under 60 tonnes.
Similarly, we tend to add 1 track mile to the expected distance for descent for each 10 knots of tailwind or conversely, subtract 1 nm per 10 knots of headwind component.
All this is of course simply a gross error check to the FMGC provided TOD point which is always correct unless you’ve programmed it incorrectly. It’s easy to get lazy and not check it because it is rarely a problem.
What you are referring to (3:1) means for every 1000 ft you’ll travel 3 nm. So for descent planning, when cruising at FL330 you should start descent 99 nm out. It works quite well on most jets. You should correct for wind though, roughly 1 nm per 10 kts. For example, 10 kts tailwind, add 1 nm. Then you should allow some distance to reduce speed for approach (deceleration segment) as follows:
So for calculating the TOD (3 degree path):
- TOD = FL / 10 x 3 + wind effect + deceleration (e.g. FL100 = 30 nm + wind + deceleration).
Take into account that the above should be height, if the airport is at 5000 ft it becomes 15 nm.
- Wind effect is 1 nm per 10 kts tailwind.
A jet aircraft usually needs to reduce vertical rate or level off in order to decelerate and configure for approach while a (turbo) prop can easily decelerate and configure while maintaining a 3 degree path.
- deceleration = 2 nm per 10 kts at 1000 ft/min
- deceleration = 1 nm per 10 kts in level flight
You can cut the above distances in half using speedbrakes.
For a (turbo) prop, height to start deceleration:
- Height = IAS * 10 (e.g. 200 kts = 2000 ft)
This will ensure you are stabilized at 500 ft AAL, in IMC add 500 ft to be stabilized at 1000 ft AAL.
Thats all there is to it. Its not an exact science anyway, height in FL / 10 x 3 + 10 nm or so will probably work in most situations. You will eventually know where and when to start descent and deceleration for particular aircraft depending on weight and wind. Those are nice rules of thumb when you start flying a new type. I used those in real life for years, they work quite well.
Just use this.
I usually add 20nm to slow down.
You could do altitude x 3 + 10 or 20 nm before even opening that app .
I multiply my difference x4 @ 250 knots with a lil buffer to slow down. If you go a little faster you need to have a bit more room though. I do usually fly in live weather so yeah if a tail wind you might need a little extra wiggle room too I think.
That is way too much, you don’t need more distance when flying faster, you just need a higher rate of descent… For tailwind you do need to correct indeed, 1 nm per 10 kts. I’m in commercial aviation for over a decade, the rules of thumb as described in my previous post work fine for most turboprops and jets.
Or you can do it quicker and fill out what I posted.
If your are interested in realism then no. As I said before, you can perform the calculation yourself before even opening an app or webpage, these are not orbital mechanics we are talking about . Its not like you can open a webpage during flight and punch in some numbers… Besides, its such a simple calculation and easy crosscheck. You don’t need a app or calculator for that? In real life you simply crosscheck height versus distance during descent, since everybody is flying CDOs nowadays there is no need to add a deceleration segment, so during descent:
- Altitude x 3 = TOD, e.g. FL100 (10 x 3) = 30 nm, FL200 = 60 nm, etc.
The other way around during descent for crosschecking height versus distance to go:
- Distance / 3 = height, e.g. 60 nm (60 / 3) = 20.000 ft, 45 nm = 15.000 ft, 30 nm = 10.000 ft, etc.
Vertical rate in the meanwhile:
- Groundspeed x 5 (or x 10 / 2 if you find that easier), e.g. 200 kts (200 x 5) = 1000 fpm, etc.
Forgot to add my 1500 feet per min descent sorry Keeping it smooooth. Of course if I’m not paying attention or overshoot I will slow right down to compensate. Took me a while to get used to the numbers though. I used to always be early on descent now I am always cutting it pretty close. DC6 math is my favorite though it’s just 5 miles per 1000 feet =)
Not very fuel efficient but ok .
Thank you. Bookmarked this
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