I am almost finished building/coding my very first RNP approach, but I need someone's technical advice to be able to complete it.

 

THE PROBLEM

I can get my aircraft to fly the designated descent path in LNAV/VNAV, UNTIL I reach 250 ft, which is about the altitude where the RNWXX waypoint kicks in.

 

The problem is, after passing 250 ft, the aircraft stops flying the required rate of descent (ROD) to the runway. I.e. it flies at a descent rate of around 800 ft/pm UNTIL it passes 250 ft/RNWXX waypoint becomes active, at which time, the descent rate changes to, say, around 500 ft/pm. As you can imagine, as I get closer to the runway, I am too high to be able to safely let the aircraft down onto the TDZ.

 

If I disconnect A/P and A/T at 250 ft, I am capable of letting down onto the TDZ without an issue.

 

If I leave A/P and A/T on, until I reach 50 ft - I'd be half way down the runway, due to the change in ROD after passing 250 ft.

 

 

With that said, why isn't the FMC calculating a continuous descent from 250 ft to at least 50 ft. Has someone encountered a similar issue or does anyone have any advice?

 

Thanks,

Michael McKenna.

Michael, there is a guy with the nick of "porthos" in the avsim forum, he actually designed the RNP approaches in Australia and was inducted in the pioneers of aviation hall of fame a couple of years ago.

Send him a PM, am sure he would give you a hand.

 

Hey mate!

I'm a captain for virgin Australia on the 737 800 so maybe I can lend a hand!

This is a hard .

 

Say your at the NONUP waypoint cruising at 40000FT and your fmc says to descent to 3000Ft In the next 85miles for your next waypoint. You start your descent and u end up way too high to land well then the aircraft is telling you that it cant make it down those 37000 FT cause it will put too much stress on the aircraft!

2 things,

1- if you have to descend quickly from a big alt to a small alt think and make sure u start your descent earlier, allowing enough time for you to get down to 3000 and being at the correct speed for flaps, gear ect!

2- if you notice your descending very slow, and your fmc is going to stuff your app up. Then do what we do and put the vertical speed on! Say maybe 1100 or even more. But with this your speed is most likely to increase, so make sure you are ready with the spoiler and pull it on say 50 percent and no more than flight detant!

I apologize if this did not help! If not reply and I shall make a YouTube clip for you

Thanks Will. I'll contact him. Thank you.

 

G'day Virgin737pilot. Thanks for the input. The RNP approach commences from 3,000 ft. So speed, I think, in this case, isn't so much of an issue as I'm fully configured for landing at the designated decision point (which isn't the DA(h)) which sits at around 2,000 ft.

 

The issue is between 250 ft and 50 ft, where ROD changes from 800-ish ft/pm to <500-ish ft/pm. Coincidentally, the ROD changes from -800 to -500 exactly as the active waypoint changes. I.e. from XXXXX waypoint to the runway waypoint RNWXX.

 

Explaining it in another way - at 1,000 ft, and also 500 ft, I meet stable approach criteria. As soon as I pass 250 ft in LNAV/VNAV with A/P and A/T engaged, the aircraft decreases it's ROD from -800 ft to -500 ft.

 

Michael.

QF 737

 

In the real world I do lots and lots of RNP Approaches, considering this normally if the weather is good and I have a clear visual on the runway we disengage the autopilot at 1000 FT

 

I'm actually quite confused on your question! Are u asking why the aircrafts ROD decreases past 250FT?

Mate I think I may have the answer

I used to be a FO on the Embraer 170/190 before I came a captain on the 737!

At 250FT Is the Default decision height! So if you don't set your minimums and your Decision height correctly before landing rthe fmc tells the aircraft that the default decision height is 250FT. If you have the autopilot engages at 250 ft (the default decision height) the autopilot tells the aircraft to start prospering for the flare, the nose up action

So basically your descending at a VS (VERTICAL SPEED) of 800 and you haven't set your decision height up, well the at 250FT your aircraft pitch is going to increase, readying it for the flare.

 

So know if you set your baro to say 100FT OR 50FT you will notice passing through 250FT you will still be at a VS of 800 because of your new decision height! So as soon as u hit your set baro ( decision height ) you will notice the pitch will now be at 500VS

 

SO again getting back to the embraer. The ejet required a massive nose up action, so the ejet required a decision height of 300 or more because I requires such a bigger nose up action!

 

If you don't understand I will explain Abit clearer next time

 

Jack Dutton

Are u asking why the aircrafts ROD decreases past 250FT?

Yes. That is my question.

 

Mate I think I may have the answer

I used to be a FO on the Embraer 170/190 before I came a captain on the 737!

At 250FT Is the Default decision height! So if you don't set your minimums and your DH right the fmc tells the aircraft that the default decision height is 250FT. If you have the autopilot engages at 250 ft (the default decision height) the autopilot tells the aircraft to start prospering for the flare, the nose up action

So basically your descending at a VS (VERTICAL SPEED) of 800 and you haven't set your decision height up, well the at 250FT your aircraft pitch is going to increase, readying it for the flare.

 

So know if you set your baro to say 100FT OR 50FT you will notice passing through 250FT you will still be at a VS of 800 because of your new decision height! So as soon as u hit your set baro ( decision height ) you will notice the pitch will now be at 500VS

 

SO again getting back to the embraer. The ejet required a massive nose up action, so the ejet required a decision height of 300 or more because I requires such a bigger nose up action!

I understand. I've been setting the DA(h) to 250 ft baro. I'll change my DA to 50 ft baro on my next test. Fingers crossed. I'll be back in 15 minutes and report the findings.

 

Cheers Jack, much appreciated.

 

Mike McKenna.

Good on ya mate let us know how it goes?

 

Jack Dutton

I don't think BARO setting has any effect on this. That's just a reminder. However, that's typically the lowest altitude you'd have the A/P engaged. RNP is not autoland, of course.

 

What does your FMA say? Are you still in VNAV PATH? Do you get an ALT HOLD message?

 

Have fun!

That's also very true!

 

Again, RNP isn't ILS so if wont be Autoland !

 

On FSX pmdg may be telling you to disengage the autopilot and manually fly the plane, or it's saying your pitch is too low to land and you will fail your flare massively ! Again this is different in the real 737

So... the 50 ft DA baro had nil effect as predicted by Matt.

 

What does your FMA say?

MCP SPD | LNAV | VNAV PTH... from 3000 ft all the way down, until I disconnect A/P A/T (at 50 ft).

 

Are you still in VNAV PATH?

Yes, until I disconnect A/P and A/T at 50 ft.

 

Do you get an ALT HOLD message?

No, I don't get ALT HOLD annunciated at any stage in the RNP approach.

 

I'm aware that an approach in LNAV/VNAV is not the same as Autoland.

 

All i'm basically trying to achieve is a reliable descent path to 50 ft (which is the lowest altitude you can have A/P A/T engaged to). I have created a reliable descent path from 3,000 ft to 250 ft. However, from 250 ft to 50 ft, the descent decreases (from -700/800 ft/pm to 400/500 ft/pm).

 

Cheers. Michael McKenna.

May I ask

What country is this in?

 

And does this happen to all airports u land at or only a few?

What country is this in?

BNE. Rwy 01.

 

does this happen to all airports u land at or only a few?

Only this one that i've created. I have ZQN RNP which was created by someone else. Those procedures get me down to 50 ft no problem.

 

So clearly i'm missing something. I just don't know what.

 

Mike.

Ahh runway 01 landed on that runway many of times

Seeming Brisbane is my based airport for the 737!

I actually landed on runway 01 at 1.30 today !

 

Mate I don't think your doing anything wrong!

 

Simply a RNP Approach is definitely not close to a Autoland! Basically at 250FT is like I said the decision height for a RNP approach, so at 250FT Above the runway if you do not have a visual on the runway you must go around! It's on our books it's a MUST!!!

 

So a RNP is not a approach u can fly all the way down to the centre line of a runway!

A RNP approach is specialized for manual flying! So you can have it on Auto pilot until 250FT ! As soon as u see that pitch rising its telling u to go around cause the aircraft thinks u haven't got a visual on the runway yet!

 

Do u understand?

 

 

Okay let me try and pit this simple!

 

Your In bad foggy weather flying in to ybbn rwy 01 arrival!

You have your star set up for arrival your 500 ft above the runway with the VS at 800

You don't have a visual on the runway yet!

Finally you hit 250FT the decision height and u still have the autopilot on plus still no visual on rwy 01

U notice how the pitch goes from a VS of 800 to 500 it's because the autopilot is still on

The aircraft is telling you it's pitching up because you haven't disengaged the auto pilot !

It thinks you haven't got a visual on the runway yet ! It's telling u it's a the decision height and u still don't have the autopilot disengages so where pitching the Nose up for a go around!

 

And also landing into Brisbane or Sydney Melbourne wherever, we always always use ILS even if it's clear skies and u have a visual on the runway from 30 miles away!

 

U only use RNP on approaches like the good coast or a runway with no ILS!

 

Again virgin Australia states that if we are flying a RNP into the gold coast and we have a visual on the runway to disengage the auto pilot and hand fly the jet!

 

Try flying the ILS Blacka approach into Brisbane? Save the RNP approaches to the gold coast and other places with no ILS!

Make sure that if your in heavy fog and your still in the autopilot that you have a visual before 250FT Or to do a go around this is why your plane pitches up at 250FT with the autopilot still engaged!

 

 

Wow I hope this helps

Yeah. I usually disconnect above 1,000 ft and throw it on the TDZ without a problem. Usually I tune 109.5, follow LNAV/VNAV to 250 ft, then I use the G/Slope for guidance below 250 ft. (In VMC of course).

 

Why do Virgin not use RNP approaches as a default? ILS approaches are long and dragged out. Add to that the incessant holding into BNE

 

Cheers.

 

Mike McKenna.