Calculating Top of Descend

[proxus]

Hello there !

I am having trouble with finding the correct way to calculate TOD
and was wondering if anyone out there have a good teaching skill so he or she could teach me to compute when to start TOD.

This is for flight planning for ATPL and I have an understanding on the subject but I would like to nail it down perfectly.

Best regards.

Proxus

[High Wing Drifter]

For MRTJ open CAP 697. Goto fig 4.5.4 page 89.

1) Two tables. Choose one for your desired speed (0.7 or 0.74M).

2) With your pressure alt and estimated landing weight look up the time required for the descent.

3) Work out your ground speed from the provided wind component or find your TAS and use the whizz wheel to extract the ground speed (the quick way of determining TAS is to cross index your gross weight, mach and pressure alt in the appropriate integrated fuel planning for cruise tables.

4) Extract the correct NAM from the table and use the following formula to workout your ground distance (NGM). NGM = NAM x GS/TAS.

5) Subtract the NGM from the BOD to get the TOD.

[RichardH]

proxus

Please check private messages.

HWD is correct in his example. However, some of the TOD calculations don't require you to use CAP697.

[High Wing Drifter]

Richard,

Are you talking about the type of questions such as where you are provided with two altitudes, GS (or whizz wheel data) and a preferred descent rate?

If not I am interested too.

[df1]

One method that seems to be used often is as follows (giving your TOD as a distance from destination/specified point):

Altitude to loose (in tens) multiplied by 3, + 10 for deceleration.

e.g. 33,000 ft cruise, apt elev 3000 ft.

= 30 x 3 + 10

= 100 nm.

i.e. start your descent 100 nm from desination/point.

Use caution though as this may not be accurate enough for your ATPL studies.

[RichardH]

HWD - Yes

Given two altitudes and ROD work out TOD from beacon type question.

df1 - gives a real world rule of thumb and has he says is NOT accurate enough for exam world with answers such as
20.6 nm

[Send Clowns]

df1, you are right this is not accurate enough, but it is also too accurate ( :confused: ). The given ROD and speed need to be used, but the deceleration distance is ignored.

There are two typical, similar questions: (i) giving altitude at top and bottom, rate of descent and some measure of mean groundspeed, asking for distance taken to descend and (ii) giving altitude at top and bottom, some measure of mean groundspeed and distance available, asking for rate of descent.

In both cases the key is time in the descent.

(i) find the time by dividing the height to be lost (make sure the pressure reference is the same) and divide by ROD.

Multiply mean groundspeed by time to find distance.

Note: if the question asks you to be level some distance before the beacon, don't forget to add that distance

(ii) find the time by dividing the distance available by mean groundspeed.

Divide height to be lost by time to give rate of descent.

You will notice that each case is simply two speed - time - distance calculations (rate of descent is just a speed, downward, altitude change is just a distance). Time = distance ÷ speed followed by either distance = speed × time or speed = distance ÷ time. Time is the key: the movement across the ground takes place in the same time as the descent.

Good Luck

Send Clowns
Navigation Instructor, BCFT