Not at flap 20 with the gear down they weren't!
How much extra vertical speed you get from the SPBK depends on what your
IAS is. The drag varies as the square of the speed (form drag =
1/2V
2SCd). The resultant increase in VS is too difficult to quantify unless you were flying the aircraft under pure flight-test conditions, flying a constant IAS and you can eliminate variables such as descending through shear layers.
Generally the effectiveness of the SPBK falls off dramatically as you slow down below 250kts Once below 200 kts, Flap 2 (add gear if you really want to descend) is the best configuration.
I'm trying to get some idea of how many track miles would be too few if you're suddenly given a short cut for an approach for example.
Try this:
Altitude x 3 = miles required.
Add 6 miles for deceleration to GRN DOT.
Add or subtract 1 mile per 10 kts tail/headwind.
Add or subtract 1 mile per 10 kts IAS above/below 290kts IAS.
Subtract an allowance for field altitude if it is significant.
EG: Descending into MAD, 290 kts IAS, 50kts tailwind, passing 13000ft QNH.
13 x 3 = 39 miles
50kts tailwind = 5 miles. Total 44 miles.
Deceleration + 6 Miles. Total 50 miles.
Field elevation 2000 ft amsl - 7 miles. Total 43 miles.
So, 43 miles is the required distance to touchdown. You can do a quick mental recalculation at any point.
You're now at 210kts/GRN DOT and given close-in down-wind vector.
sequence the Flt Pln to give you CF as the "TO WPT" ( extend the centreline)so your minimum required track miles is shown bottom right on the MCDU.
Let's say it shows 18 and you are passing 9000ft (still at MAD, 2000ft amsl).
FMGC distance makes no allowance for the turn so add 3 miles for the base turn/intercept onto the LOC. You have 21 miles to go.
You're at 9000ft (field 2000ft) so you're 7000ft above TDZ. At 21 miles. Perfect.
Suppose you were at 10000ft. You'd need 24 miles. Let's say for the sake of argument that your G/S downwind is 240 kts (fairly typical) so your datum VS to give a 3 degree descent path is 1200fpm (G/S x 5 or half first two digits).
Make sure you have Flap 1 and extend the SPDBK (if you are in a 320, you could disconnect the A/P to give full SPDBK). Check resultant V/S. Is it 1500fpm or more? (increase of 300fpm will lose you 1000ft in approximately 12 miles) so you should be on the glideslope at 9 miles albeit at 210kts so you'll need to drop the gear early (I suggest 2500ft above TDZ) to reduce the speed.
If you see that it is not losing your excess height quickly enough, gear down, flap 2 and 180kts will allow you to descend at up to 2000fpm without the speed increasing.