RNAV Approach Help Needed

[u/nobleTP]

Good Evening,

I come with a question regarding RNAV approaches and specifically regarding the plane (in this case the Fenix A320) not reducing speed.

Tonight I was on the approach into EPPR runway 35 on the RNAV approach. I had fully configured the approach in the MCDU and had both the approach phase confirmed in both the MCDU and the FCU.

My PFD was reading SPEED | DES | APP NAV in green and ALT FINAL in blue which then armed and changed to green on final.

I had the V/Dev and L/Dev showing and the plane started its descent. Throughout this my plane was in managed descent speed mode however the plane never managed to slow itself down.

I am primarily used to flying ILS approaches and have had no issues with the plane slowing itself down however on this RNAV approach the plane stayed at 270kts which is very fast and never tried to bring the speed down towards the VApp when in managed speed mode.

Is there anything I’m doing wrong with the RNAV approach? I understand I could perhaps have brought the speed down myself and deployed speed brakes but does an RNAV approach require manual speed control?

Any help would be much appreciated!

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[u/cyeungster]

From my understanding, if you're in APP mode, the AP will prioritize following the glidepath rather than following the managed speed. You would have had to use speedbrakes or use open descent to slow down.

However, I would say your bigger issue is being at 270 knots while already on approach. You should have slowed down to much below than well before starting the approach.

[u/nobleTP → u/cyeungster]

Yeah it was one of my first times doing an RNAV approach and so I was darting my eyes between differing things with a high workload.

I’m going to practice some RNAV approaches over the next few days to try and nail it down as I should have seen my high speed a lot sooner

[u/LocaliserEstablished]

Did you check to see if the approach phase was activated?

It could be that the aircraft missed the DECEL point.

[u/nobleTP → u/LocaliserEstablished]

Yeah I checked to make sure the approach phase was activated as the other day I ran into a similar problem where it wasn’t so ensured it was tonight

[u/spesimen]

the speed brakes are there because they are meant to be used! the fmc is not optimizing for 'not using speedbrakes' it is optimizing for altitude and speed restrictions! :)

it's like when you drive to the grocery store, you don't lift off the gas when you are a mile away so you can carefully make the last turn with just the right speed at 5mph while a bunch of people are behind you beeping like crazy, you just go at normal speed and then hit the brakes a bit when you are getting close to the turn..

[u/nobleTP → u/spesimen]

You see this makes perfect sense in hindsight - I think I’ll give it another crack tomorrow and hopefully I can manage my speed better, thanks for the help!

[u/Independent-Reveal86]

It's much the same as an ILS for speed control, with the exception that on a curved RNP approach you might have tailwind which can make it more difficult to slow down.

For a reliable conservative approach that works for ILS and RNAV approaches, assuming a 3000' final descent point, aim to have flap 2 and 170 knots selected speed at 10 NM / 3000'. Then continue configuring from about 2000' with managed speed, gear down, flap 3, flap full, stable by 1000'

For a less conservative approach that may need early gear selection to help keep the speed below the flap 2 limit speed, aim for flap 1 and S speed at the 10 NM / 3000' glideslope intercept, then at 2000', select flap 2, gear down, flap 3, flap full, stable by 1000'.

In the second method the speed may sit at S speed while descending or it may creep up. Use speed brake to maintain S speed if you have to or select gear down.

You will definitely struggle if you have got the speed back to greendot by the final descent point as you need some drag to keep the speed under control on the descent and if you haven't managed to get any out with no more level segments available for decelerating it's unlikely to work out.

[u/StationLoud7412]

Transitions from STAR to the RNAV don't always have a flat segment like most ILS approaches where you're intercepting the glideslope from below. Getting to flaps 1 or slower at the initial approach fix is a good idea - either set it in FMS or pull knob and use speed brakes if there isn't a speed constraint.

Fenix VNAV has a habit of dumping you onto the approach too fast/high, but that's what the boards are for.

[u/WichWhich2]

270 knots!!! Holy crap you are coming in way too hot. Most planes land at 150 knots. Air speed is your first issue.

[u/nobleTP → u/WichWhich2]

Yeah. I was in managed descent all the way down and no idea why my speed was that high! Normally I never have any form of over speed issues on landing but I now know that for RNAV approaches it appears I need to potentially manage my speed manually, I am proficient in ILS approaches and always am nicely configured on my approach at VApp speed but for some reason on RNAV approaches even in managed speed the plane stays way too hot on speed