Mirrodie - thought you might like some actual examples. I drive Avro RJ 100s. As in most jets, the most fuel efficient way to descend is to spend as long as poss. at cruise level, then reduce power to flight idle.
We have a basic rule of thumb for calculating where to chop the power - Multiply the number of thousands of feet to lose by 3, then correct the resulting number of nautical miles for :-
(1)weight (lighter aircraft, greater descent rate - I took a while getting my head round that too! Its opposite to what you would expect),
(2) head or tailwind during the decsent, and
(3) whether or not your using the airframe de/anti-icing kit. This saps power so the engines are automatically spooled up a bit to compensate, and it won't descend so rapidly.
Typically one has to slow to 250 knots by 10,000 ft so we add 5 miles for that.
So to run an example - a 38 tonne aircraft with a 60 knot tailwind, icing kit required on for 5000 ft of the descent, cruising at 33,000 ft, to get down to pattern altitude of 3,000 ft.
30,000 ft to lose = a starting figure of 90 miles.
2 tonnes lighter than the reference figure of 40, so subtract 2 miles per tonne = -4 miles (its lighter so requires less distance to descend due to the higher rate)
Take 2/3 of the tailwind component, 40, add a mile per 10 kts = +4 miles.
The deceleration = +5 miles.
1 mile per each thousand feet the icing kit is on = +5 miles.
So 90-4+4+5+5= 100 miles out, about 20 minutes, which is a remarkably consistent figure for all types of jet transport.
As we get closer to the field ATC restrictions start messing with our planning, but the descent progress is monitored all the way down and if we're too high, add some airbrake, too low, add some thrust. In either case the planning wasn't quite right, but there are so many variables affecting the descent, its more a black art than a science.
Sure, in some types the vertical nav mode of the Flight Management Computer works out the TOD, top of descent point, but they work on a 3 degree slope, and don't use idle thrust. They therefore compensate for all the abovementioned variables by changing thrust automatically, and the bottom of descent point is more predictable. Avro RJs and to my knowledge 146s don't have VNAV in the sense that modern types do.
Ground idle in our RJ 100s is approx 25% N1 (Fan Speed, representative of thrust). Flight idle could be up to about 50% N1 at high levels and this figure also varies for a number of reasons as mentioned in earlier posts. When de/anti-ice is selected the min. speed of the engine's core is increased to ensure it copes with the air bleed demand, but there is no automatic compensation on this type for flaps/gear. Small engines need less time to spool up in case its necessary.
On final approach with flap 33 (max) and gear down, the thrust will be around 60%, i.e. typically from about three miles out or wherever we get into the landing configuration. This is reduced to idle at about 50 feet.
So the objective, and apologies if this sounds familiar, I mentioned it in a reply to someone else's topic recently, is to go to flight idle at cruise level and not have to add any power until final approach. It rarely happens, but is very satisfying if it does. Its called a CDA, continuous descent approach, and is beneficial in terms of noise for residents near airports. The less time spent flying level in the terminal area, i.e. with thrust on, the better.
So there you go. Been to Australia yet?
Regards - Musang.