PMDG 737 RNP Approach

[ATRguy]

So just for sake's sake, here's the iFly 737 MAX doing an RF turn into Palm Springs; it's admittedly a very nice day there, no wind, but it just rolls into the turn, and holds it the whole way.  I'm sure the flap schedule isn't accurate, I was trying to mess it up with config changes and see how it reacted, but it holds it basically rock steady. 

It does fly quite nicely, IMO.  The ATR I fly IRL flies RF legs extremely well, so it's nice to see.

 

 

[ianb2469]

That does look nice and smooth. I think I’ll end up pushing the button on the Max at some point 🤦‍♂️

[LRBS]

40 minutes ago, ATRguy said:

 

It does fly quite nicely

 

Yes, this is a beta version. I'm not sure if you're familiar with the VSD, but it works like a charm on Ifly. Unfortunately, there are many issues with the PMDG. I hope they can resolve this "crisis" before it’s too late.

[Stearmandriver]

6 hours ago, somiller said:

I thought the flight director is what "directs" the autopilot...no flight director, no autopilot.

Naw, the flight control computers generate guidance for both the flight directors and autopilots - one doesn't follow the other.  You can turn the flight directors off in a Boeing (or they can fail) and the autopilot keeps on trucking.

[Stearmandriver]

6 hours ago, jon b said:

 

Maybe it’s type specific as I remember on the 744 it was a requirement to use the autopilot, the reason being there wasn’t a high enough resolution on the flight director.

Did you have nav performance scales on the 74?  That's one difference I'm aware of for operators that must use the autopilot below a certain RNP - they often don't have NPS.  Back when we did these things in the -400s, the PM was required to monitor progress 4/4 to continuously monitor nav perf as those aircraft didn't have NPS.

The resolution of the flight director itself though is plenty good.  At a manually set RNP value below .3, the LNAV guidance gets nice and sensitive.  The ND is also part of your primary scan at that point; laying the white turn predictor "noodle" over an RF leg makes it pretty simple.

Actually, thinking about it, a blanket autopilot requirement wouldn't work for us anyway as we also do RNP departures with RNP below .3.  Can't turn the autopilot on until 1,000ft on departure and you already need to be in fairly aggressive turns by then on some of these.

Edited October 23, 2024 by Stearmandriver

[jon b]

No, we didn’t the NPS on the 747-400 though the 747-8 does I believe as it’s based on the 787 suite including IAN.

The FD resolution was the reason given why we had to use the AP, however this may have been something concluded by our training and fleet office, in which case a large bag of salt maybe required when considering its accuracy, if you catch my drift.

The FD was good enough for a cat 1 ILS , so 🤷 

Edited October 23, 2024 by jon b

[Stearmandriver]

7 hours ago, jrw4 said:

It knows the current wind, but can it predict the future presence of variations in that wind field as it flies into it? I have never flown anything more than a 172 irl, but I recall making constant changes in flight in response to the wind. A Boeing has much greater mass and momentum than a 172, of course, but do we know how well MSFS simulates the real atmosphere's wind variability?

Many thanks to the real world pilots who contribute to this and other discussions.

Kind of, as it has three different altitudes' worth of descent forecast winds loaded in.  These are more for VNAV path calculation, but they'll play a part in the box's predicted groundspeed in various segments. 

Even when bank angle adjustments are necessary for wind though, they're very smooth.  There is definitely never a feeling that the aircraft is wandering, or overshooting a correction and backing off etc.  Corrections are smooth, minor, and exactly accurate, because wind is a minor player at this point; if you think about it, wind speed and effect is a very small percentage of overall motion vector of an aircraft flying an average of, say, 180kts (starting the procedure around 220kts and transitioning through the procedure to a final speed of 140 -150kts.). Your normal 10-20kts of wind is not having an enormous effect.

[LRBS]

1 hour ago, Stearmandriver said:

Did you have nav performance scales on the 74? 

I guess it is about SOPs and FMC versions, as an example, on our 744, we don't have any restrictions regarding AP. They only get the UNABLE RNP, and that's the end of that type of approach; on the 748, all the messages are the same, plus on the ND, you can see the lateral path deviation distance, RNP, and ANP.

To make it interesting, there are new FMCs versions on the 400 that also show the same info on the ND as the -8 has. As you mentioned, some operators stay on PROG 4/4 to monitor even if there are automatic messages. Such is life. 

[somiller]

On 10/23/2024 at 12:02 PM, Bobsk8 said:

He said the resolution on the flight director wasn't enough, not that it was missing altogether. 

 

delete

[somiller]

On 10/23/2024 at 6:03 PM, Stearmandriver said:

Naw, the flight control computers generate guidance for both the flight directors and autopilots - one doesn't follow the other.  You can turn the flight directors off in a Boeing (or they can fail) and the autopilot keeps on trucking.

Thanks for the clarification - I wasn't aware of that.