A320 NPA question

[stm44ae]

hello Captains

please i need confirmation if this calculation is correct



3000 - 55 = 2945

2945 / 300 = 9.8

5.5 - 9.8 = 4.3

so we start desending 4.3 dme to TCH ?? is that correct ?

selected APP
thanks

[Nubboy]

That's likely to put you a bit high.

Normal technique im my company is to START down at 0.3 nm before your theoretical descent point.

If uncertain, why not use the profile dougnut, just make sure you have the crossing height and speed in as constraints on the flight plan page of the FMS. Fly it in selected, but monitor your calculated profile.

[Dan Winterland]

Or press the APP button (if you're allowed to use it). Takes all the guesswork away!

[stm44ae]

Nubboy hello in our company also same .3 so its going to be 4.6 im not sure if this calculation is correct coz tmrw going to have a test on this one .

Dan Winterland i wish if i can but its going to be Seclect.

[mcdhu]

I've probably missed something here, but what is wrong with starting down from 1900' at the KFA (ok 0.3nm before) with -3 degrees FPA. This will be 5.5nm to the threshold. So you have to lose 1900-55(elev) - 50 (over the thr) ie about 1800' in 5.5nm so you will need a little more than the 3.05 degrees suggested by the plate (327 ft/mile so try -3.3?).

mcdhu

[FlightDetent]

To follow stable 3,02 deg profile I would leave 3000 (2944 height) at 9 NM from touch down, that is 4 DME inbound "KFA". To do so, pull FPA at 4,3D.


Yours, FD (the un-real)

My preferred profile would be different though. 7 DME green dot at 3000, then FPA to obtain -500 fpm descending 1900, at 4 DME to 'KFA' deccelerate with flaps and gear to be fully configured just before 'KFA', at 2000 RA landing memo appears and cross FAF 'KFA' at ALT 1900 with final speed, checklist complete, FPA -3 deg. Continious intermediate segment without recourse to level flight.

Some would say K.I.S.S. in which case green dot to 7 DME on final track at 3000, then OP DES to 1900. At 3 DME to 'KFA' deccelerate with flaps and gear to be fully configured before 'KFA' and cross FAF 'KFA' on final speed with checklist complete, FPA -3 deg pulled 0,3 NM before FAF.

Given the location of airport, I would expect a significant positive ISA dev, which would put you somewhat above the real geometrical profile.

[stm44ae]

correct thanks alot guys

[mcdhu]

FD raises a really good point. We correct for cold temp error, but it works both ways and what sort of temps can we get in the summer in what I assume is Saudi Arabia (or is it?) - ISA +30? Correction at 1900 - minus 300'? If we do this fully managed - we're stuck with it!

[Kirks gusset]

STM44ae,

This is a stepped profile not a CDA. The 7 DME fix is 12DME from the MAP.

You should descend from 3000 to 1900 at the 7 DME arc point once within 5 degrees of the inbound 342, use level change or vs, but you must not cross the VOR less than 1900 or you will have not complied with the procedure.

Remember the speed this profile is drawn at as a standard NPA.

Once at 1900 you can set up the constant descent profile to start 0.3DME from the VOR set the path angle to 3.05 ( 3.1) degrees, fine tune as needed.

Good luck with the test.
KG

[stm44ae]

KG thanks but our company dont allow us to used stepped profile they want me to start descending from 3000 to THC which its 4.6 dme before the VOR which i must be a fully config. and FPA is -3.0

well im little confused now

[FlightDetent]

thread creep: ISA + 30

mcdhu, what would be your suggestion then? In fact, I wonder what is the behaviour of the FMGS. I would suppose the MCDU coding would display -3,0 deg, however when flown in FINAL APP then the observed fpa would show -3,5 or so as the altimeter error is bled off towards MAPt. Upon reaching MDA 530 ft the altimeter still significantly overreads so you are 100' (?) higher with 4 whites on PAPI. The brick, should probably provide corrective angular trajectory towards MAPt (last coded waypoint), where profile is at 160 ft AFE (my calculation, normally shown on LEGS page) and you would overfly with some - 40' (?) ft error compared to target geometrical profile.

If so, my guess is that many would say 'twas ze Airbouse doing strange again'; incidentally, the problem has nothing to do with FBW, sidestick or fixed TLs etc. etc. - as had never been the case.

If flown selected, one could make the correction and leave FAF on 1600 indicated ALT and then select FPA -3 deg. No, my training and fear of life prohibits me from flying below charted alt. Or, knowing that on 1900 indicated you are in fact at 2200 start FPA -3 deg 1 NM before the FAF No, my training and fear of life prohibits leaving platform 2 km before step-down fix.

My solution would be to leave FAF from 1900 [2200] but select -3,3. When visual to PAPI duck under to align and keep 3 deg profile until touchdown. Still, I admit to be undertrained.

It would be nice to learn what is the preferred technique of contributing desert specialists. Anyway, what would be the altimeter error for ISA+30 and height difference 1900' ? The deviation tablest I have are for cold weather only.

Yours,
FD (the un-real)

[FlightDetent]

Originally Posted by stm44ae

well im little confused now

Honestly I fail to see why. Your company requires constant descent from 3000 and you calculated that correctly in the very first post. 9 miles fully configured is not very fuel efficient and little slow, but perfectly legal.

Another option is to descend to platform [intermediate approach altitude] 1900 when on final and be configured at FAF. My company also prohibits stepped, dive&drive NPAs, this only applies after FAF however. So correct for me eventhough some TREs may claim otherwise.

Personally, I would adopt CDA between IF (7 DME, 3000) and FAF ('KFA', 1900) to save fuel and noise. It may seem overly complicated but in fact it is a daily routine for ILS approaches and modern ND provides all the clues to do so without unnecessary workload. To compare, the 737 FMS does calculate VNAV PTH as CDA, I believe.

If you are looking at a SIM scenario with failure(s) then of course the easiest is the best. I would fly strictly as charted.

Yours,
FD (the un-real)

[stm44ae]

thank you

[mcdhu]

That's right, no temp correction tables exist for hot weather and you don't see a hot temp limit on RNAV approach plates as you see a cold temp limit (usually around -10C) - presumably because it's on the safe (high) side.

However I believe the correction numbers are the same for ISA plus as they are for ISA minus, but with a sign change. Perhaps someone with an intimate knowledge of altimeter calibration could comment.

Thus, I would go along with FD's method of flying the approach from 1900 (or 3000 if you wish) but bearing in mind that you are probably starting out 300' high which you have 5nm to get rid of so you will need appx 400'/nm ie nearly 4 degrees FPA.

If you don't do something about it pretty quickly as you descend, you're going to have trouble with the stable approach criteria.

Lets hope we hear from some desert pilots soon and, when I get a chance, I'll have a go in the sim to see what happens.

[FlightDetent]

GAB: Even if they had published it I wouldn't dare to use them, we are in complete agreement. There is nothing in PANS-OPS either because as far as obstacle clearance is an issue there is no need to apply corrections.

Here's another point of view, help me decide if it is valid. These days it is becoming legal EU-OPS requirement to fly CDFA, so no dive and drive. Assuming the altitude diference is 300 ft at FAF, if you had selected -3 deg FPA as charted you would reach THR at 350 ft AFE, not a very good position for straight-in. If you used the DME/DIST table provided (not always the case), at 4 DME past station you have ALT 610 indicated, so maybe about 710 geometric.

That equals to 3,8 deg profile to reach 50' at THR. Airbus own FDM classifies 3,8 profile as medium severity event (amber). Airbus own FDM will trigger level 3 red risk 'Continiously high on final'. Limit for hard severity, red, event PTH HIHG is 3,95 deg.

a) you continue with 3,8 and master the flare, good chap. Not a trained or frequently practiced manoeuvre.
b) you regain PAPI 3 deg once visible at MDA. To do so, a duck under is required that will in fact destabilise the profile for a few moments and trigger level three red warnings EXCESS V/S and PTH HIGH. But the primary goal of CDFA was to have stable trajectory, wasn't it.

Throw in short runway on a cliff edge, downdrafts, wet runway (think Iraklion, Keffalonia), profile that is geometrical 3,5 deg initially, and some slices of Reason's cheese start coming together.

To further develop a good understanding of CDFA under effects of significant positive ISA dev, I think it would be nice to know how pronounced is the effect on altimetry. That's what I was after. I do not think that 100 years into powered flight we'd just discovered a killer problem, but perhaps it would be nice to know, how much margin you normally expect is taken away.

Yours,
FD (the un-real)

[potkettleblack]

If your company insists on a constant descent then for a 3 degree glide a general rule of thumb is that you will need to lose 300ft per nm.

So at 5.5 from touchdown (or zero dme since the VOR/DME is on the approach) you need to be at 1,900ft. 6.5 would be 2,200, 7.5 2,500, 8.5 2,800ft and 9.5 3,100ft and so on.

The procedure starts at 3,000ft so therefore to keep it all nice and simple you could probably expect ATC to give you a vector no tighter than 10nm from the threshold. If you are in config 2 at this stage and speed back at something reasonable like 160-180 knots then you will be in a reasonably strong position.

Since it doesn't appear that you are approved either by your company or state to do these approaches in managed mode then once cleared to intercept the 342 inbound you can select TRK/FPA. ATC will probably clear you to descend to 3,000ft as well. Using my rough maths above you can select a -3.0 "glide" on the FCU and start down around 9.5 from the threshold (set up OEMD34L on the PROG page to monitor). At 8.5 you should be at 2,800ft as a rough check. The first published check is at 5.5nm (zero DME overhead the KFA) where you should be at 1,900ft. From then on the DME will count up and you can refer to the altitude checks above the profile at the bottom of the plate.

Finally make sure that you check through the maths and discuss how you are going to fly the approach with the other guy beside you during the brief. Give them a copy of the check heights that you worked out so that they can back you up and advise if you are high/low/"on glide" as you go down.

[stm44ae]

potkettleblack your method is soooooo easy and very clear thanks alot

[Fly3]

My two pennies worth. You need to cross the threshold at 50ft. Add that to the threshold elevation (54ft) gives a height at threshold of 104ft (round it 110ft for ease of calculation and safely). To achieve that requires a ROD of 325ft/min from 1900ft at 5.5d from the threshold. If you then project this profile back along the approach you would have to commence the descent from 3000ft just inside 9nm from the threshold or 3.5d from the beacon.

[FlightDetent]

Originally Posted by Fly3

To achieve that requires a ROD of 325ft/min from 1900ft at 5.5d from the threshold.

Surely you mean 325 ft/NM starting from 0 DME to TDZE 56 ft.

[PantLoad]

In theory...

In theory, you could do that....it would work....using the VOR crossing of 1900 feet as a check. (Cannot be below 1900 feet over the VOR.)

Of course, this 'technique' is subject to your company's SOP....which may
prohibit you from doing so.....that is to say, your company's SOP may
require you to do the dive and drive technique to the VOR (3000 feet down to 1900 feet), then do the CDA down to the MDA.

We used to do a VORDME approach similar to this...the F/Os liked to do the CDA from 10 miles out from 3000 feet....I never objected....as long as
the altitude was correct over the FAF.


Fly safe,


PantLoad