2 to 1 descent mental calculation

[darshanpatelh]

hello everyone, looking for some pointers calculations to do in the head for 2 to 1 descent. the 3 degree rule is widely used but I am not sure how to computer 2 to 1 in my head.

any help is greatly appreciated.

thank you

[stilton]

Don't know why you'd want to do 2 for 1 but just like 3 to 1 its pretty simple.


If you have 30,000 feet to lose times 2 you would start down 60 miles out, you're going to need speed brakes though !

[BleedingAir]

Assuming you're talking about turboprops, 2:1 profiles can have numerous advantages (can, not will). I used to fly them quite regularly.

For a straight-in approach, my rule of thumb was Height x 2, plus 4nm. This will bring you down a 2:1 slope to a point 10nm from the field at 3000', which on every turboprop I've flown was a comfortable place to be to decelerate, configure and be nice and stable as required, regardless of speed on the descent. Also means minimal adjustment to the profile is required if joining an instrument approach. So if you're at FL250 arriving at a sea level airfield, your top of descent will be 25 x 2 + 4 = 54nm.

If you're talking about jets - best of luck.

[darshanpatelh]

thank you for the info. I am indeed asking for turboprop related.

can anyone chime in on calculating feet per minute. 3 degree is gs / 2 + 00

what would it be for2 to 1?

thank you again for the help.

[BleedingAir]

This doesn't really answer your last question exactly, but I never used to bother working out precise rates of descent, as it never unfolds exactly as you expect.

A good baseline rate for a 2:1 profile was 2500fpm (based on a descent speed of 230-240 KIAS). I'd tweak it up a little with a tailwind, down for a headwind, and just monitor the profile all the way down.

It's worth mentioning that the profile I mentioned before is rather generic, and I'd use it in anticipation of a visual approach - if I was joining an ILS or other instrument approach, I'd adjust the profile to ensure a smooth and stable transition, as you don't want to be coming out of a 2:1 at glideslope intercept.

[darshanpatelh]

thank you for the info. I was just curious as we normally do freighters were we are empty back and the captain would like to utilize the steep descent option. as I recently started flying turboprop I am just curious to known there were general rule of thumbs for the 2 to 1 descent.

everyone's response so far has helped very much. thank you again

[mustangsally]

Decent rate for 3 to 1 is three time your mach number.

[oggers]

can anyone chime in on calculating feet per minute. 3 degree is gs / 2 + 00

what would it be for2 to 1?


There is another form of the rule of thumb you quoted and it can be used for any descent angle. It is:
speed in miles per minute x descent angle x 100
eg: for 3º approach @ 120 gnd spd you would have:
3 x 2 (nautical miles per minute) x 100 = 600fpm
...as can be seen it is the same rule at the end of the day.

The rule of thumb for descent angle is number of FLs (ie increments of 100ft) ÷ miles.
eg for a '2:1' descent you would have:
10FLs ÷ 2 miles = 5º angle

Putting it all together for a '2:1' @ 180 gnd spd:
10FLs ÷ 2 = 5º
5 x 3 = 15. Rate of descent = 1500fpm.

Rules of Thumb: Descent VVI (http://code7700.com/rot_descent_vvi.html)

[27/09]

For ROD for 2X profile try 8 X G/S. Quite easy to do in your head

All speeds in the 200 knot range will be 1600 fpm plus the extra for the amount above 200 knots.

All speeds on the 300 knot range will be 2400 fpm plus extra for speed above 300 knots.

e.g. 250 knots = 1600 + 8 X 50 = 1600 + 400 = 2000 fpm

It's not an exact calculation as 2 X descent rate for 300 knots is 2500 fpm. However close enough.

[darshanpatelh]

thank you all for the info. much appreciated

[604driver]

If you use (height loss requited x 2) + a "factor" = miles to touch down

you can work a TL closed descent profile, even in most exec jets.

NOTE: The "factor" varies according to weight and wind component and if you have to be fully configured by 5 miles or at 20 miles to touchdown.

EG

Top of Descent FL450
Bottom of Descent 0 feet

45x2=90
Plus a factor of say 30 = 120 miles This would be your TOD if you don't have excessive weight or a high tail wind component in the descent.

So how do you actually fly this?

Well at TOD you close the TL's and steadily increase the VS as you descend to maintain barbers pole -20kts or around 300kts IAS. NOW, here's the magic. You never want to be closer in track miles to touchdown in NM's than (Alt x 2) +15. If you are, you will likely need to use the speed brake at some point. If you are more than (Alt x 2) + 25 you will need power in the descent at some point... or wait till you level off at 5000 feet 25 miles out and drag your ass in using power :( :( :(

So once established in the descent, you adjust the VS to keep you around (Alt x 2) +20 = track miles to touchdown. If you are closer increase ROD. If you are further out reduce ROD.

I'm giving you a simplified version here. But this method keeps the T/L's closed and allows you to vary VS to maintain an efficient profile. So at 15,000 feet you could end up with a calculation of 15x2+15= 45 miles, so that's the point you now plan to slowly switch to the 3 degree profile. However you will have excess energy, so assuming obstacles aren't an issue, you can descend below your 3 degree profile, build in a slow down period at your level off alt, and recapture the 3 degree profile, without excess energy.

Always keep an accurate track miles to touchdown and you will always have a good indication of your energy state. It takes time getting used to, but it works.

Most Auto Throttle systems will fly a path and vary the speed, using thrust so you get throttle pumping all the way down the descent, which personally i hate.

Anyway, as i said, this is the short version and I'm happy to explore it more if you like.

All the best