When To Initiate Descent In Relation To The Top Of Descent Icon... Options

[heyjoojoo]

In sim flying the Airbus 320, I often have trouble determining when to begin the descent phase of the flight. I generally wait for the ATC or wait for the Top of Descent marker to show up. At what distance (in miles) from the TOD icon on the ND should initiate the descent phase?

- Kareem
::FSXing::

[Piltdown Man]

If it's like the real thing, at the TOD marker. But I think most of use 3 (4 in A330) miles per thousand feet. Boeing boys then add 10 to slow up.

PM

[Aspiring Boeing ...]

When I am flying along in my boeing 737 classic at FL300 I usually initiate my decent 80 miles from the threshold. There is a calculation you can use (which I use) to calculate my decent rate. Every so often I will re-calculate to make sure I am decending at the right rate.

I am sure loads of us finish up approaching too high in flight simulator. Sometimes I have to face this prob.

I usually do as the manual says, the tip in the lower page says "our Target airspeed/altitude will be 30 miles from the threshold at 10,000 feet AGL at 250kts... any approach can be completed if you pass through this window"

This is the rule of thumb to figure out a good decent:

1) take your present altitude (in my case 30,000ft)
2) Subtract the altitude you want to be (the threshold elevation or your next altitude over a certain VOR etc)
3) Multiply answer to above by 3
4) this is the number of miles needed to decend.

So If I am at FL300 and landing at Denver with an elevation of 5,000feet for example then we do 30 - 5 = 25
then 25x3= 75. So for this decent you will need 75 miles to make a good decent. And a little amout of space for error. Lets say 5 miles to round it to 80 miles.

Kep re doing this calculation in order to make sure you are decending.

It says in the book that this does not really matter in FS because the differences in pressure and air density, temp etc are not simulated, but to make sure you are ok at least do it a couple more times throughout the decent.

If you are to go to the FIX page in the CDU and select the landing airport, if you type /30 and enter it, it will show a green arc around 30 miles all around the threshold. This will help to guide you on the decent so you know when you should be at 10,000ft. Keep calculating so you don't end up too high or too low in proportion to the distance from threshold.

This is a little confusing at first but with help of the manual in fornt of me I get more farmilliar with this method each time. Keep reading over until you understand.


Regards,
Chris

[heyjoojoo]

Thank you Chris and PM. I'm practicing on an Airbus 320, short hop from Sacramento to SFO airport, IFR, using Autoland.

I will take another stab at it tonight. This is fun, but a little challenging. Another thing is that it's hard to follow the ATC and get the same results. Are there other really good Flight Tutorials for the A320 or other airliner type aircraft?

[Ranger]

3X the altitude that you want to lose plus 1 mile for each 10 knots of airspeed that you might want to lose. Example:

Level at FL 310 and cleared to cross a fix at 9,000 feet. 31,000' minus 9,000'= 22. 22 X 3= 66 miles. If we assume that we're in the U.S., we need to slow to 250K before we descend below 10,000'. If we are descending at 300K then we need to lose 50K before we descend below 10,000'. That's another 5 miles. Sooooo, 66 + 5 miles= 71 miles. Even with the FMS this mental exercise sould ALWAYS be done. Why? Because occasionally the automation reaches back and screws you.

[Aspiring Boeing ...]

3X the altitude that you want to lose plus 1 mile for each 10 knots of airspeed that you might want to lose. Example:

Level at FL 310 and cleared to cross a fix at 9,000 feet. 31,000' minus 9,000'= 22. 22 X 3= 66 miles. If we assume that we're in the U.S., we need to slow to 250K before we descend below 10,000'. If we are descending at 300K then we need to lose 50K before we descend below 10,000'. That's another 5 miles. Sooooo, 66 + 5 miles= 71 miles. Even with the FMS this mental exercise sould ALWAYS be done. Why? Because occasionally the automation reaches back and screws you.

Did not know of the latter part. Interesting, thanks for that. I suppose that is the way in which not to get too high on approach or too close to the airfield before TOD summed up. I will also take that into account. You learn something new everyday, even when off school at the weekend.

Regards,
Chris

[chris_pilot]

Once you've done that, you'll want to get a rate of descent that will have you at the altitude you want to be at and when you want to be there. To do that, pick a navaid near your destination or where you're descending to; and work out the ETA in hours (distance in knots / descent Vspeed in (knts/hr)). To get the number of minutes take the ETA in hours and multiply by 60. Once you have the ETA in mins, work out the amount of altitude you need to loose (current altitude - desired lower altitude), change to feet (multiply by 6076.115) and divide this figure by the number of minutes. This should give you a rather rough descent rate to adhere too. Obviously there are lots of variables, like you'd need to maintain the same speed throughout the descent; and over long descents this might be a bit harder to follow through with thanks to fuel burning and enviromental differences such as temp and pressure; which could through you off course vertically a little. Still, it provides a decent indication as to what you're aiming for.
If needed, divide the descent up into smaller parts and work out each 'leg' seperately to get different descent rates for each part of the it. This will be far more accurate than just lumping everything together and hoping for the best as it can account for changes in speed etc.

Of course, your FMC will be far more accurate; but this was the little plan I devised myself back in the days of default FS2000 flying...
I'm sure there are formulae out there that are much easier, quicker and accurate to use than that, but hey, I'm stuck in my old ways... or maybe I just don't know 'em..

Cheers,
Chris

[The Airbuser]

3X the altitude that you want to lose plus 1 mile for each 10 knots of airspeed that you might want to lose. Example:

Level at FL 310 and cleared to cross a fix at 9,000 feet. 31,000' minus 9,000'= 22. 22 X 3= 66 miles. If we assume that we're in the U.S., we need to slow to 250K before we descend below 10,000'. If we are descending at 300K then we need to lose 50K before we descend below 10,000'. That's another 5 miles. Sooooo, 66 + 5 miles= 71 miles.

By the way, I know this works for every aircraft, but what is the decent rate that this thumbnail guide will give us? In the Cherokee is about 500fpm, but how tedious a descent would be descending from FL410 to Miami at 500fpm...

[bartdepiloot]

By the way, I know this works for every aircraft, but what is the decent rate that this thumbnail guide will give us? In the Cherokee is about 500fpm, but how tedious a descent would be descending from FL410 to Miami at 500fpm...

i usually maintain 1800 ft/min (the default descent rate in FSX)
if i'm flying on the PMDG 747-400, the VNAV system will automatically calculate the descent rate