Sign in to follow this  
captain420

PMDG 777, LOC/APP button necessary anymore?

Recommended Posts

It's been awhile since I've flown the 777, but before taking off, I would normally load up PFPX to plan my flight and save the route and load it up in my FMC on the 777, and then select DEP/ARR and then activate. When browsing through the legs page to see all the different points, it seems very complete from start to finish.

 

With the flight plan bringing us all the way down to the runway and I see that it includes the ILS as well. Is pressing the LOC/APP button on the MCP to capture the localizer and glideslope necessary anymore? And if so, at what point do you press the LOC button? Is there a rule to this?

Share this post


Link to post
Help AVSIM continue to serve you!
Please donate today!

LNAV/VNAV will get you only A to B and altitude management, APP/LOC on other hand will land you :) is short answer.

 

I am simplifying this heavily:  

LNAV/VNAV will only get you down to predefined decent point of the STAR at which point you can ether do manual landing or engage APP for CAT  approach, in some cases like real life you will have to be vectored into the runway, at which stage lnav/vnav mostly does not play any role and u rely on LOC and adjustments manually to speed and altitude based on what ATC tells you.  I would recommend googling LNAV/VNAV navigation and then ILS approaches CAT etc to see how they play and what specific part of the flight plan u use them.

 

I am sure alot will reply here with even more detail but i would recommend doing reading on these to understand what they do.  They are both necessary lets put it that way :).

Share this post


Link to post

If you're doing an ILS approach then you absolutely need to use the LOC/APP button if you want to fly it through the autopilot. In the real world, if a procedure has a navaid in the title (ILS, VOR, NDB etc) you are legally required to have the appropriate navaid tuned and identified. If the procedure requires that you track an ILS beam, the only autoflight mode that will do that is the LOC/APP mode.

 

LNAV/VNAV is only as good as the FMC position. You might know you're on the magenta line, but how do you know the magenta line is in the right place?

 

There's no reason why you can't leave the autopilot in LNAV/VNAV all the way up to establishing on the ILS (though that would be highly unusual), but at that point you should be changing to LOC or APP mode. The vertical and lateral paths from the FAF when you select an ILS approach from the FMC database are not guaranteed to be accurate, and whilst they may get you somewhere near the airfield, there's no guarantee it won't fly you in to an obstacle.

Share this post


Link to post

Thanks for the explanation, but when should you arm the LOC and APP buttons? Is there a rule of thumb when doing this?

 

I am having a lot of problems when coming close to the arriving airport. Seems like sometimes my flight plan is trying to make a very sharp turn when coming into the approach, and misses the next waypoint all together. 

 

Not sure what I'm doing wrong. I'm using the flight plan thats produced in PFPX and then loading that into my FMC. So either PFPX is messing up my waypoints at the end of the flight plan or something else is happening. 

Share this post


Link to post

In my aircraft (real) I never select the approach mode on the autopilot until I have been cleared for the approach by ATC.

Share this post


Link to post

Once you have the ILS tuned and identified, and you are on an intercept heading (generally within 30 degrees of the final approach course) you should arm LOC.

 

Once you have captured the localiser, you can then arm APP (which will then capture and follow the glideslope). It's generally considered good practice not to engage APP until established on the localiser, as the glideslope is only guaranteed to give you obstacle clearance along the localiser path itself (plus a small margin each side).

 

You may be better off using HDG SEL to position yourself on an intercept heading for the localiser: depending on the STAR and approach procedure selected, there may not be a direct connection from the end point of the STAR and the FAF. Bear in mind that in the real world ninety nine times out of a hundred you'll be radar vectored for sequencing on to the ILS, so this is how the procedures are set up. The other reason it is generally a good idea (though not necessarily always mandatory) to intercept the ILS in HDG SEL is because there is a small risk that if you FMC position is slightly out (or the approach in the database is slightly out) you may parallel the ILS in LNAV without actually properly intercepting the localiser and LOC/GS becoming active.

 

Also consider your speed during tight turns: the higher your speed the wider your turn radius will be. If you have to make a sharp turn you may need to slow down and configure early: if you're leaving LNAV and VNAV engaged there's a good chance that it will continue to command 240kts up to the FAF, which may not be conducive to a tight turn.

Share this post


Link to post

But how would you slow down if the speed is already configured in your FMC? I'm trying to make the flight as automated as I can. I'm more of a casual simmer so I don't really need for it to be realistic. I just want to be able to depart from one airport and land at another without any problems.

Share this post


Link to post

You could probably programme it in the FMC, but honestly the easiest thing to do is to use the Speed Intervene function on the MCP if you're in VNAV: push the speed knob and wind it down to the speed you want, extending the flaps as you approach the various manoeuvring speeds on the speedtape. If you're in VS or ALT mode the speed window will be open anyway.

 

The FMC is great for 'strategic' long term control of the flight path: but the MCP (HDG SEL/VS/SPD etc) is much more convenient for short-term 'tactical' interventions. Even the most automated modern flight decks require you to do some of that pilot stuff at some point!

Share this post


Link to post

 

 


But how would you slow down if the speed is already configured in your FMC? I'm trying to make the flight as automated as I can. I'm more of a casual simmer so I don't really need for it to be realistic. I just want to be able to depart from one airport and land at another without any problems.

 

As many have stated before, an ILS is a precision approach that have to be flown VIA the nav radio. The ILS/LOC is the only approach that the FMC/FMS can't do even though its listed in the data base. Relying on FMC calculated speeds can get you in trouble. Some of the calculated speeds may not be what you want at the time. The FMC is efficient and will try to keep speed up till the last minute. This can screw your approach. In my jet, the speed is automatically controlled by the FMS based on configuration, altitude and distance from the airport. On arrival, the plane will also fly a certain speed based on flap handle setting. For flaps 20, it automatically selects 150kts. On approach, I don't want to drone at 150, id rather do 200 or 180. So i have to select manual speed and dial it in. Going into holding, the speed automatically gives max endurance speed, this can be very low, so again i use manual speed. As skelsy stated, you have to do some of that pilot stuff even if you don't want to be realistic. The FMS/FMC is based on flying on a perfect day with no ATC restriction, vectors or traffic. In the real world you have to be on top of speed to arrive on speed, course and glide path.

Share this post


Link to post

It is sometimes possible to land using LNAV/VNAV, and you can manually set the final approach speed on the legs page for the FAF (Final Approach Fix) and runway way point, but as several others have said, LNAV/VNAV is not designed to be as accurate as an ILS, localizer or manual approach.   I control the final approach speed by manually intervening on the speed, and I believe that is standard procedure.

 

There is such a thing as an RNAV approach, using LNAV/VNAV (for example if there is no localizer or GS) but the pilot must monitor the approach carefully and take over manually by the FAF. 

 

Finally, as has already been pointed out, not all STARs lead directly to the first fix of the final approach.  If you look at the charts for KIAD (available on flightaware.com you will see that on several STARs coming from the north, the STARs lead directly to the south-going runways (19R, 19C, 19L) but not to the north-going runways (1L, 1C, 1R).   So often you will need to intervene using heading select to choose a reasonable approach course. 

 

Probably PFPX is producing a flight plan where there is a provision for vectors from the last waypoint in the STAR to the first waypoint on the final approach.  That is why you are getting very sharp turns.  After the last waypoint of the STAR, RW you would be vectored by ATC to line up w/ the runway.  With no ATC, you need to vector yourself.

 

Mike

Share this post


Link to post

An RNAV (GPS)  approach does not have to be hand flown after the Final Approach Fix. It is perfectly legal to allow the autopilot fly it to the Missed Approach Point, or Decision Altitude (Or height depending on the aircraft). 

 

It is sometimes possible to land using LNAV/VNAV, and you can manually set the final approach speed on the legs page for the FAF (Final Approach Fix) and runway way point, but as several others have said, LNAV/VNAV is not designed to be as accurate as an ILS, localizer or manual approach.   I control the final approach speed by manually intervening on the speed, and I believe that is standard procedure.

 

There is such a thing as an RNAV approach, using LNAV/VNAV (for example if there is no localizer or GS) but the pilot must monitor the approach carefully and take over manually by the FAF. 

 

Finally, as has already been pointed out, not all STARs lead directly to the first fix of the final approach.  If you look at the charts for KIAD (available on flightaware.com you will see that on several STARs coming from the north, the STARs lead directly to the south-going runways (19R, 19C, 19L) but not to the north-going runways (1L, 1C, 1R).   So often you will need to intervene using heading select to choose a reasonable approach course. 

 

Probably PFPX is producing a flight plan where there is a provision for vectors from the last waypoint in the STAR to the first waypoint on the final approach.  That is why you are getting very sharp turns.  After the last waypoint of the STAR, RW you would be vectored by ATC to line up w/ the runway.  With no ATC, you need to vector yourself.

 

Mike

 

 LNAV/VNAV approaches are not as accurate, but an LPV approach can be. LPV approaches can get down to 100', but usually 200 which is the same as a CAT 1 ILS. These approaches can be flown exactly the same as an ILS, as in the autopilot can take you down to minimums. The in the ProLine 21 with FMS 3000 (and other versions of the FMS I am sure) you still select APPR to conduct the approach while conducting these types of approach. Even an LNAV with Vertical Guidance you can still allow the autopilot bring you down. It's all about knowing the capabilities of your aircraft as well as knowing the legalities behind what you are doing.

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this