3 degree descent planning, Time vs. Distance
 

Because it's easier to multiply than to divide. You could also determine your TOD by dividing your FL by 3 i.e. FL330/3=110nm. The difference with the multiplication method is the deceleration segment.

I started a thread not so long ago asking how many pilots on the 737 multiply the altitude to lose to FL100 by 2.5 rather than 3, to account for the steeper descent gradient at high altitude and high speed. The answer to that was, nobody.

Keep. It. Simple.

altitude times 3 has always worked, even watching the airbus fmgc descend doing pretty much the same thing.

descending at a fixed vertical speed will result in varying true airspeed throughout the descent giving you inconsistent time.

fixed V/S works well on slow unpressurized planes but on jets its about the economy of idle thrust from top of descent all the way down.

What i do i also add to the calculated distance, the difference between my current IAS and and average speed of 200kts, (i.e. if before the descent i am flying at an indicated speed of 270kts, i add to the calculated distance additional 7 NM), and to conclude I also add or subtract the wind factor (10kts/1NM) depending whether i have tail or headwind.

Doing this i am usually really precise when compared with the TOD on the MCDU.

Example:

FL330, IAS:270Kts, Wind negligible

My calculated TOD will be : (33*3) + (27-20) = roughly106 NM, to be compared with the distance to treshold in the PROG page (Airbus world)

Whatever works best in your system.

B744, 757 767 777 787, VNAV does an adequate job but it is only as good as the wind profile that is entered into the box. WInd change is not linear in most instances, so any assumption of that condition will result in less than optimal path establishment. It is interesting to watch crews fly a VNAV path in an altering gradient, as the energy state of the aircraft is paradoxical to the crews actions; e.g., in path, an headwind that is greater than planned will result in a higher speed than anticipated, and heads off towards a "drag required" message, and putting out the boards, whereas the energy state of the aircraft is now low... resetting the path by going direct to the current waypoint, or by reentering descent speed etc will give the opposite result, waking up the throttles..

Airbus VNAV is disappointing in it's limited scope.

Using a multiple times your height sounds great, if you know your distance, and you are going to do the maths. Most nav systems will provide time to a WPT and as the desired outcome is what is the correct vertical path, which is dy/dx, but is flown as a rate of descent, dy/dt, then time seems a logical measure to apply. 5 minutes to a waypoint, x feet to lose... An assumption of X times distance is only valid when you have the correct value for X which changes every wind case (and different weight if a constant speed profile is intended to be flown).

In extreme cases, time works nicely; In one sporty jet, I known I can get down from FL450 to 10,000' in 2:00, anything else is a choice... (don't recommend that one at night... 30 degrees nose down at MMO/VMO is quite aggressive even when the blue is kept at the top. For a large vertical offset, such as cutting 20 miles off a distance to touchdown leaving 10nm to touch, at 300kts, FL100, well, for that particular type plonking out flaps 40, full brake and gear gives a rate of descent that can be applied to the time to threshold etc... telling the driver if it is going to work or not well before the ground rush comes up. time to target tells you something meaningful and immediate. That is also what you are seeing when you look out the window, your aiming point is the result of your dy/dt, for any given GS, dx/dt, so the basis of the analysis is time.

An efficient descent based on idle from TOD to configuration complete and established at stable condition at an appropriate altitude on final, nowdays is mainly wishful thinking. It doesn't happen in ZGGG, RJAA, UEEE, KLAX, and only the first arrival of the day may do it into EGLL..., it doesn't happen many other places, our airspace is a victim of the success of the industry. Mainly descent efficiency is compromised by STAR design constraints and traffic needs. As such, being off your original desired speed, means that the target path really is already mussed up, and what you have left to work with is time to target... giving needed sink rate.

Whatever works for you is arguably the best method.

Sounds like the HS121...

I think most pilots can do a pretty decent plan...

But some are only winging it

Oh Lord, brimstone we deserve.

My head was hurting pretty badly, but now it's even worse!! :bored:

What is wrong with winging it if it works?

In the 320 I find 3:1 with 1 mile added or subtracted for each 10 knots of tail wind or head wind or speed to reduce.

Example. Crossing restriction of 10,000 and 250 kts. Planned transition of 330 kts and starting altitude of 30,000 with a 20 knot headwind all the way down.

60 miles before for altitude lose (20x3) + 7 miles for 330 to 250 - 2 miles for the head wind. 65 miles before my restriction I'll start down. It works bang on, this is done with the trust at idle.

Some times the FMGC is not to be trusted (wrong des speed in the box or a very early Decel pt showing you at green dot over your restriction vs 250 as an example).

Because that would be indecent :}