How do you decide how far out to decend

[magpienja]

Hi all, just had a week away in cyprus flew with cyprus airways A320 manchester/pathos, and being a sado asked for a seat just behind the wing to watch the operation of flaps alerons ect I find it fascinating, all my flying is a passenger with a mate in a PA28, and what are those panels that pop up top side of wing when coming into land and on touchdown, I suspect lift dumpers or air brakes is this correct, anyway the main point of my post the aircraft had one of those moving map displays to show out alt/speed/position, and on the way back on crossing the channel I was thinking will will start our decent anytime now, but no it was not till just south of birmingham, which did supprise me, I thought it would take much more distance than that to get down from 36000ft to get into manchester, we used 06, how do you guys decide when to start your decent or is there a computer to give you that info, just one more point are the engines at idle on the way down.

Thanks for taking the time to read from Nick in cheshire.

[BigHitDH]

The "Flight Management Computer" (FMGS on the Airbus) will calculate a vertical profile, with a "T/D waypoint" or "Top of Decent". This is annunciated on the navigation display.

When you plan an approach into the FMC, usually it will include altitude/speed restrictions, for example, below 5000ft, the aircraft will be programmed to hold 190 knots.

The reason you decended so late is because of the cost. It's much more cost effective to stay at cruise for as long as possible, and then "glide" the aircraft is such a way so as to use as little fuel as possible during the decent - all it means is the pilots made a good job of planning their vertical profile.

On the subject of the "panels" on the outer wing: They are "Spoilerons". In flight thay are used to augment the ailerons to effect roll control, plus help to create drag to slow the aircraft down. Upon touchdown the "Spolierons" deploy fully to help kill the lift from the wing end create extra drag. This helps stick the aircraft to the ground so the brakes can do their thing.

Hope that helps,

[Dualbleed]

Another crude, but ok way to determine this is the divide by 3. It does not take into account the wind. i.e 30 000 feet = start decent 100 NM out. 18 000 feet = 60NM out.

[Joles]

Dear Nick,

Yes you are right, those are brakes used to cut speeds. Deployed immediately on landing - rather touchdown- they ( along with the pedal brakes in the F/C ) help to bring the plane to a fast stop.

The descent is a factor of various elements, in addition to the distance. I remember once we were in a small plane ( Dornier 228) and the ATC requested us to maintain 3000 ft till we were circling the destination airport - due to heavy traffic down there. So in case the airport has traffic and your flt has been asked to circle a few times you may start the descent later than normal or closer to the airport
Incidentally for a straight approach the descent normally starts about 30 mins ( in a Jet a/c) prior to descent.
Trust this helps
Other PPruners, please feel free to correct !

YA
Joles

[ROB-x38]

My spamcan method:-

- you know your ETA
- you know how much you have to lose
- 500 ft/min ie: 2 mins per 1000' plus a couple of minutes for mum

eg: - A055
- ETA time 47
- overfly height 2000'
- need to lose 3500' = 7 minutes (500' / min)
- add a couple -> 9 mins
- TOPD time 38

:ok:

[BOAC]

magpie - this (http://www.flightinfo.com/rulesofthumb.asp) will give you an insight into how we cerebral pilots actually cheat on these calculations:D

[ravenx]

Mike Jenvey - I've sent you a PM

[Xeque]

I always thought 3 miles a thousand was the rule of thumb, at least when doing it in your head.

However, with all the Flight Management stuff coupled to the autopilot and autothrottle, I suppose this can now be calculated to the nearest nano-whatsit with no flight crew input whatever.

[magpienja]

Thanks guys for taking the time to reply, I get the picture now very interesting reading, must be very satisfying to get the flight spot on close the throttles and glide the rest of the way saving all that fuel.

Regards Nick.

[Pilot Pete]

XequeI suppose this can now be calculated to the nearest nano-whatsit with no flight crew input whatever. Computers are only as good as the info put into them. Sitting back and letting it just get on with it is not something that pilots do. They are constantly monitoring their descent profile using a 'rule of thumb' to ensure the machine is 'in the ball park' with what it is doing. For instance, what if ATC give you a shortcut and you lose 10nm? That's 10 miles of descending at the current rate that you have just lost so you will have to do something about that (usually increasing your rate of descent), but if you are already at idle power the only way you can do that is to increase speed (dive it off) of put out extra drag (like speedbrakes) to make you descend back onto your (new) ideal profile.

In the 75/767 we use 3 x the height to lose (in thousands of feet) plus 1nm for every 10kts of airspeed you need to reduce back to minimum clean speed. For instance, at 20,000ft and at 300kts we are looking for 20x3=60nm, plus say 9nm to reduce from 300kts to 210kts.

The descent is all about energy management. If you are high and you have enough track miles to run you can increase speed (ATC and birdstrike speeds etc permitting) to dive down increasing your rate of descent. Using the above rule of thumb though, you will have to dive through your ideal profile as you now have more speed to have to lose, so to get yourself back on the profile you have to dive through it, then lower your rate of descent whilst reducing that excess speed to end up back on the ideal profile at the required speed! Sounds complicated, but it becomes reasonably easy with a bit of practise and good spatial awareness.

If ATC won't give you further descent when you level off at an intermediate level during your descent then you can end up becoming high on profile (as you fly level and the descent profile drops away from you) All is not lost though at this stage as you may be able to trade off speed whilst waiting for the descent clearance. For instance as soon as you level off you wind the speed back (say from 300kts to 250kts) which will take about 5nm to do. During these 5 miles the thrust levers will stay shut (engines at idle just as in the descent) and you are burning off no more fuel than on a constant descent. You are then pushing and 'twitching' for ATC to give you further descent, at which point you increase the speed again towards the 300kts, the aircraft nose drops to dive to regain the speed and you end up with a higher rate of descent diving back towards your ideal profile. That's good energy management, but obviously can only be taken so far until such point that you have to add thrust whilst flying level!

If you end up low on profile (perhaps ATC extend your routing) then there is not a lot you can do other than to increase thrust to reduce your rate of descent (even to nothing and fly level), which of course is what we are trying to avoid due to the large amount of fuel that these engines burn when low down. So it may be better to increase the thrust earlier to get back on the profile higher up rather than keep descending with the levers shut until you reach your cleared level and then have the thrust levers come all the way up to fly level for a long time with a high(er) fuel burn.

Hope that gives a bit more of an insight and shows you that even with all the automatics in the world we don't just sit back and let it get on with it! There are too many variables screwing up the automatics; short cuts, extended radar vectors, winds not as predicted, unforseen intermediate level off due traffic etc. Remember, computers work on the principle of sh*t in, sh*t out!!!!;)

PP