Top Of Descent Question

[fonawah]

A quick question about T/D. When you are approaching T/D as displayed on your FMC, do you have to obtain permission before you begin your descent? If so what happens if ATC clears you to descend to a level that will put you off your most efficient profile? Can you ask for a different level or do you simply have to adjust your vertical speed?
Hope this question makes sense. Thanks for your replies!

[Captain Stable]

When you are approaching T/D as displayed on your FMC, do you have to obtain permission before you begin your descent? Yes - every time!

If so what happens if ATC clears you to descend to a level that will put you off your most efficient profile? You ask for whatever level you need/want

Can you ask for a different level or do you simply have to adjust your vertical speed? Yes, if you don't get it you will adjust your descent profile as required.

[fonawah]

Thanks for that Captain Stable

[MasterGreen]

Hi Fonawah,
Yes you ALWAYS should get ATC approval for any change of Altitude / Flight Level. There are times when this is not an option - but this is a non-normal situation and is not relevant here.

Descent Planning :

Perhaps a little background on how this works (or is supposed to work) is better here for your query. Since with that background you will be able to see all the permutations for yourself. It ain't rocket science - although some would have it so.

The problem is : Get a large mass of aeroplane travelling at 500 kts (or so) at altitude to a complete stop at the end of a mile or so of concrete at a much lower altitude.

My usual caveat about Grandmothers applying an oral vacuum on external avian ovum growth mechanisms is implied.

As stated the problem is 250,000 kgs from 35,000ft at 500kts to 0 kts and ft. This is practically broken up into 5 phases : Initial Descent / Intermediate Descent / Initial Approach / Final Approach / Landing. I think the best way to tackle this is backwards. As it is only the first two that really interest you I will just approximately define the others. (The definitions are off the top of my head and are not complete or finite - but they will probably suffice for this discussion).

Landing - 50ft over the Runway threshold to Full stop - Say 140 Kts to O Kts

Final Approach - 2500 - 1500 Ft Gear and Flaps extended to 50 Ft. Say 170 Kts to 140 Kts

Initial Approach - 3000 Ft Clean to 2500 Ft (or Stable approach fully configured for landing). Say 250 Kts to 170 Kts

There are some planning constraints in the above, but they are pretty simple to resolve. Let me move onto the main two sections and hopefully these will become self evident.

The simple physics is enshrined in Kinetic and Potential Energy and the logic is based on economy. The most economical descent is at idle (engines) and to stay at altitude for as long as practicable (TAS / FF). So a descent point is chosen which, in theory - on an ideal (uninterrupted) descent will put the AC at the start of the Final Approach segment without opening the throttles. And this works - most of the time. Here are the variations :

In still air the average airliner likes 3 miles per 1000 ft for the Potential and an allowance for slowing down (Kinetic). The rule is 1 mile for every 10 kts of IAS at 10,000 ft (Read 310-320 kts). So you need to add 10-15 miles for the slowdown. This takes most types and most profiles into account. When you know your airplane well you can shave this a little. Wether you do this slowdown in bits (AKA 250 below 10K) or in one go or in steps depending upon company / aircraft limitations is academic. The distance is needed. However a heavy AC will need a little more distance on this allowance due to extra potential energy. On the 777-300 or a heavy IGW I use 15-20 nm. But largely this is a finesse because of winds.

Winds : Play more havoc with your descent than air traffic sometimes. This is not the place to go into the niceties of FMC programming (to predict this), but imagine this. Your descent from cruising altitude to Final Approach will take 20 minutes give or take. So a third of your average Head/Tail component in the descent will need to be factored in for a NICE descent. But enough of the detail.

As to ATC constraints. Well the energy was correct at the calculated top of descent, so if they hold you at a level - try and avoid opening the throttles and let speed take up the slack. If they descend you late, then you have a judgement call to make. I usually delay using the airbrakes for as long as possible since a late descent is sometimes indicative of a busy pattern. This will often result in longer descent paths, so save the energy until you are sure you don't need it.

I hope this gives you some background and a feel for the problem - such as it is. Generally the FMC will give you the numbers within a smigin (Smigin - Old English unit of accuracy = Half a Tad). However a quick mental check is always worth a moments brain activity since the FMC does get confused sometimes - particularly if you have a errant constraint in there. But this is, as they say, another story.

Hope all this helps.

MG