Continuous Descent Approach
 

As I understand it in the latter stages of the descent ATC provide track miles to touch down I had some questions about interpreting this information.

1. Where vertically in the descent profile does this information get given.

2. Do you If flying Autopilot VNAV/FLCH/VS alter FMC/MCP to alter the descent profile given the new information, if so what do you do ? (eg alter alt. restrictions)

3. What do you do if hand flying ?

4. Do you use the predicted altitude intercept semi circle ? (Not on Airbus ?)

5. Are you using P-RNAV procedures for the CDA ?

Thanks in advance

2. VNAV is made for the CDA. It will attempt to cross each point at its restriction height, and plan a constant descent profile to that point.

3. Use all the magic in the FMS and FD and Nav Display just as if Otto was flying!

4. Sometimes. It's a great help if you want to use Vertical Speed instead of VNAV, and a good backup in any case.

You have your programmed track and vertical profile in your FMC. When you're on vectors what's in the box won't necessarily be correct, and that's why ATC tells you how many track miles you can expect. A good way to check if you're high or low is the 3 x alt thumbrule. If you're high you go LVL change + speed brakes, if you're low you go VS and reduce your ROD. Simple as that.

CDA's
 

Hi Guys, just read the post and thought i'd throw my pennys worth inot the mix.

I have been on the 757 for about 18 months now and can say that having done some serious line training before being let loose on the real world so to speak, I must have been told about 40 different ways of doing a CDA.

However the way that works for me is to use the FMC and VNAV down to about 15000' and then go for FL CH and V/S to fine tune everything, especially as like you say above ATC start vectoring you about the place, VNAV becomes more of a hinderance.

That said though, if you do get the chance one late evening and get direct to the centre fix of wherever from about 300nm try and let VNAV do its bit, it is pretty good when it works in a straight line and you get a descent clearance right the way down without levelling off etc...

One more note though...how good is it when ATC ask you to maintain 320kts minimum, its FL CH all the way down, and just following the barbers pole up the ASI


Fly Safe..

2. No VNAV on me bird and no FMS either. VS mode only :}

3. Couldn't care less, If I'm hand flying and ATC suddenly gives up vectoring and offers short visual approach, props forward, gear down and half flaps will see us happily following 10° glideslope.

When I'm told track miles to touchdown I just multiply my current height AAL with 3 to check position relative to 3° glide and adjust VS accordingly.

Do you know if you've flown the CDA perfectly, or bust it?

Only useful if the range is given at a reasonable altitude, say 5-7000ft.. at LHR it would normally be given leaving FL70.

This is my method...
1. Multiply track miles by 3 to get Hundreds of feet for a CDA, eg 25 nm x 3 = 75, so 7500 ft is ideal. You know straight away if high/low/on profile.

2. Relate V/S to Ground Speed; for 300 ft/nm halve the GS, add a zero.
eg 240 kt halved = 120 so 1200 ft.min
180 kt gives 900 ft/min etc.
By using G/S as reference you cater for wind effect as you turn base/final etc. (Think ahead!)

3. Adjust V/S as necessary to get on and stay on the profile - speed control usually from ATC.
Works on every a/c!
TP

Flightman - I guess if he did a continuous descent from 6000', then he flew the CDA perfectly....

CDA's
 

Well by defenition a constant descent approch is just that, one without any level flight.

Although I do believe it only applies below a certain level, and is defined as a minimum vertical speed throughout the manoeuvre. however my challenge to myself is to not touch the thrust levers, apart from guarding them! until we are fully established and need to spool up for the landing, usually about 8nm or so as we get asked to do 160kts to 4dme, which needs flaps 20 and some power to maintain it....

that said its a great feeling when it does work out and you dont need to touch the thrust levers until you need to spool up.... big smug grin on face at that point.....

When were given track miles can vary greatly between airports flown into, we went into LBA the other day and at FL90 were told 20 miles to run by a new controller i think, bearing i mind we were still doing about 250kts... but then thats what the speedbrakes for isnt it....

It all adds to the excitement, however flying a 757 on FL CH against the speedbrake can produce some pretty impressive rates of descent.

There was a document from sept 2001,(Noise From Arriving Aircraft- An Industry Code Of Practice)which stated:

For practical purposes a working definition of CDA is Below 6000ft,

*No level flight or

*One phase of level flight not longer than 2.5nm

1)Due to constraints of the GEMS system and the different elevations of airports ,Cda achievement will be monitored from a height of 5,500ft AAL

2)'Level flight' is interpreted as any segment of flight having a height change of not more than 50ft over a track distance of 2nm or more, as recorded in the airport NIK system

Ahh, but as a CDA is defined as "no height change of more than 50ft, over a track distance of 2nm, as measured by the airport NTK system" ( which i manage ) how would he know? :)

The definition from whatdoesthisbuttondo taken from the Arrivals Code of Practice is spot on BTW,a s it should be!

We recieve (at LGW) a list of non-CDA approaches: date, Flt No. etc.

I guess it comes from this NTK boffinery.

I wonder if similar list but operator specific can be obtained, let's say on the operator's request via official channels?

Yes, I meant that the list is for my airline in LGW so you can tell if you or your matey achieved a CDA.

Best definition of a CDA by RYR-738-JOCKEY. You may need to add a couple of miles for deceleration. But Post Number Three sums it all up. It doesn't matter if you are in a glass or non glass cockpit... the 3 for 1 rule plus one mile for each 10 knots of speed to lose works like a charm every time whether you are in a 737 or 747. if necessary, just pull out your pocket cray computer and do the math.

Happy aviating folks.;)

Depending on the wind of course!

Just out of interest. Does a CDA require you to intercept the glideslope from above?

Not necessarily, but sadly all too often!

Castle Donnington have the perfect solution to CDA. They leave you high enough for long enough, turn you in and clear you for the approach. Power isn't needed after that point until reverse is applied !

You guys need Fam flights :eek:

My CDA method (1 of thousands)..assuming no ATC speed :E

1. Put into the Box something meaningful....i.e. 8 Mile centre fix at 180kts / 2400ft

2. Pre plan energy gates..i.e 10000ft 35 miles (to go) 300kts
5000ft 20 miles (to go) 270kts
3. V/S -500fpm at 4000ft..ish slowing to clean speed...and configured with initial approach flap at 180kts..i.e. where I want to be, just like Ive put in the box.

CDA method number 2

Track miles times 3 plus 10 miles to slow down from 300kts to 180kt in V/S-500fpm (add only 5 if at 250..and 3 if at 220)

works a treat on the mighty 75.

Try this with flap 15, it also works as long as your at 180kts with 15 flap at G/S intercept :ok: