Do you ccept the FMC descent prediction or over-ride it?

[Continental737]

 

 

Or do you trust in the FMC to bring you in safely as planned?

I would never trust the FMC 100%. I always back it up with the 3:1 rule to make sure that FMC calculated T/D is in the ballpark of where it should be.

[cliffardus]

Continental1737 the 3:1 makes sense to me and I thank you for this information. However I’m not sure how to use it correctly.

 

I understand that with the throttle in idle the aircraft is estimated to lose 1,000 ft for every three horizontal miles.

 

My understanding of how to use this system is as follows:

 

1.       Select the FIX INFO page by pressing the FIX key.

 

2.      Type the relevant Waypoint into the scratch pad.

 

3.      Hit LS1L to add the Waypoint and open a queue of dashed lines under the RAD/DIS.

 

4.      We get the FIX page to create a dashed green circle around the Waypoint by adding an entry like, for example, “/66”. This will create a circle with a 66 mile radius.

 

At this point I become hazy about how to usefully proceed and I’d be grateful if you would confirm the above as being correct and explain how you use this facility in your flights.

 

Cliff

[Flamin_Squirrel]

If you're flying a SID that has a transition that takes you straight onto the approach, there is (generally) no reason why you can't leave the MCP in LNAV/VNAV and let the aircraft fly using the automatics all the way, providing you deploy the flaps on schedule. If you try and it doesn't maintain profile then you probably did something wrong.

 

While posters above are right in that it's unlikely you'd ever do this in reality, it's worth doing so you can learn how the automatics perform.

[CTBlankenship]

Cliff:

 

Here are my criteria for using the FIX page.

 

First, enter descent profile as per the STAR as indicated above.  Here is an example ... on the DEAKK3 arrival into KTPA ... at DEAKK the chart says Expect altitude of exactly 11,000 ft. and 250K if landing North.  I make sure I enter this into the CDU. 

 

Enter Airport in LSK L1 (KTPA)

 

Enter /10 as the first distance from  KTPA for example (I want to be at Minimum Flaps Up Maneuvering Speed for the aircraft ... roughly 206KIAS for the 737NGX ... slowing down to 180ish and Flaps 5).  My experience at KATL (which is where most all of my flying is either into or from) the ATC likes to enter me into the downwind leg at this speed.  When maneuvering into the base leg I want to be at 180K Flaps 5 and slowing.  When entering into the final approach (LAGO INT ILS RWY 1 at KTPA ... which is 10nm from the threshold) ... I want to start slowing and performing the step down into a stable approach at the FAF.  I said all of this to point out that to arrive at the final distance circle I usually calculate the lengths of all the INTs on the approach (.1 + 6 + 1.8 + 2.1 for ISL RWY 1)

 

Enter /30 as the second distance from KTPA (I want to be at or below 250KIAS and at or below 10,000 ft. AGL ... depending upon crossing restrictions).

 

Enter 3-2-1 TOD (usually around 100nm) as the third distance from KTPA ... if the FMC and the calculated TOD are relatively close allow the plane to naturally enter into the descent.  If the TOD for the 3-2-1 calculation are not close at all ... I use whichever is the farthest away from the airport.  To do this I hit DESC NOW on the DESC page of the CDU.

 

Here are the general guidelines for Delta Virtual Airlines:

 

https://www.dropbox.com/s/iffj4aynww0ewtw/ApproachSpeeds.jpg

 

 

Hope this helps.

 

CT

[Continental737]

Continental1737 the 3:1 makes sense to me and I thank you for this information. However I’m not sure how to use it correctly.

 

The 3:1 rule is pretty simple. If you are at FL350 and need to cross ABC VOR at 15000, all you need to do is 35-15=20   20 X 3 = 60 nm is required to descend on a 3 degree slope. So the T/D indication should be around 60 nm from ABC VOR. Keep in mind that a normally the VNAV Path calculates the descent at a steeper angle so it will not be exactly 60nm. The FMC will also a just the T/D for wind if you enter it on the DES page.

 

It is more of an approximation in order to validate the FMC calculation.

[kjjj11223344]

If you're flying a SID that has a transition that takes you straight onto the approach, there is (generally) no reason why you can't leave the MCP in LNAV/VNAV and let the aircraft fly using the automatics all the way, providing you deploy the flaps on schedule. If you try and it doesn't maintain profile then you probably did something wrong.

 

While posters above are right in that it's unlikely you'd ever do this in reality, it's worth doing so you can learn how the automatics perform.

I agree with this. I have flown, offline of course, with cost index of 500 and have just let LNAV/VNAV handle the flight until final approach, and the FMC handles everything perfectly. The only time I've had problems is when my weather engine inputs a sudden change of wind, which sometimes requires a little use of speed brakes. On Vatsim with fully staffed London Control, however, I almost never use VNAV but instead LVL Change and V/S in order to comply with ATC's instructions, as others have mentioned.

[Ashatsea]

Cliff:

 

Here are my criteria for using the FIX page.

 

First, enter descent profile as per the STAR as indicated above.  Here is an example ... on the DEAKK3 arrival into KTPA ... at DEAKK the chart says Expect altitude of exactly 11,000 ft. and 250K if landing North.  I make sure I enter this into the CDU. 

 

Enter Airport in LSK L1 (KTPA)

 

Enter /10 as the first distance from  KTPA for example (I want to be at Minimum Flaps Up Maneuvering Speed for the aircraft ... roughly 206KIAS for the 737NGX ... slowing down to 180ish and Flaps 5).  My experience at KATL (which is where most all of my flying is either into or from) the ATC likes to enter me into the downwind leg at this speed.  When maneuvering into the base leg I want to be at 180K Flaps 5 and slowing.  When entering into the final approach (LAGO INT ILS RWY 1 at KTPA ... which is 10nm from the threshold) ... I want to start slowing and performing the step down into a stable approach at the FAF.  I said all of this to point out that to arrive at the final distance circle I usually calculate the lengths of all the INTs on the approach (.1 + 6 + 1.8 + 2.1 for ISL RWY 1)

 

Enter /30 as the second distance from KTPA (I want to be at or below 250KIAS and at or below 10,000 ft. AGL ... depending upon crossing restrictions).

 

Enter 3-2-1 TOD (usually around 100nm) as the third distance from KTPA ... if the FMC and the calculated TOD are relatively close allow the plane to naturally enter into the descent.  If the TOD for the 3-2-1 calculation are not close at all ... I use whichever is the farthest away from the airport.  To do this I hit DESC NOW on the DESC page of the CDU.

 

Here are the general guidelines for Delta Virtual Airlines:

 

https://www.dropbox.com/s/iffj4aynww0ewtw/ApproachSpeeds.jpg

 

 

Hope this helps.

 

CT

CT Thanks for that simple easy TD method I just used it today and it works well.

[downscc]

Carefull, the 3:1 tool is not a rule but a quick estimate. You might be dumping a lot of altitude from up high but don't forget your groundspeed is up there too. I have found that 250ft/nm descent angle is just about right at any speed, the faster you go means the higher the descent rate. A 3-deg descent is close to 318ft/nm and that turns out to be about the steepest descent angle for the entire letdown.

 

As to the original post, I have found that the descent profiles for both NGX and MD-11 are spot on for even the most difficult arrivals IF and ONLY IF you have nailed the forecase winds. I usually fudge by adding a little more tailwind component to the forecast just to hedge against any FSXisms.

[cliffardus]

I can't say I'm amazed at the degree of guidance you've offered but I most certainly am grateful as I've learned an awful lot from you all.

 

It's such a pleasure to see how Members take the time to help others. I hope that one day I can do the same.

 

Warmly..........Cliff

[vr-pilot]

Although a bit off topic, I wanted to leave a word on the "children of the magenta" studies video, linked earlier in this thread. The lesson from this course is of great importance for everyone dealing with automation. The instructor is the old-school hands-on guy which I prefer very much to the "doctor doctor" guys. He makes a very good performance with just one logical weak point: he names the first of the three levels of automation "manual level", which is a contradiction. I think he ment level 1 = FD (guided mode), 2 = AP, 3 = AP+FMC.

This guy reminded me somehow of "Patroni" (George Kennedy) from the movie "Airport 1970", just without the cigar.

"Thank you, Mr. Boeing..."

 

Greetings,

Claus

[Porthos]

Use of VNAV in the terminal environment (aside from RNAV approaches and stuff like that) is actually relatively rare in the real world. It's way easier to reach up and hit LVL CHG and HDG SEL to comply with an unexpected ATC instruction than it is to sit there attempting to figure out how to program what they just gave you into the FMC. There's a reason those modes exist in the AP system and this is it...

 

 

Tabs thats a pretty sweeping statement. Relatively rare???? Disagree.

 

The FMS gives the most efficient VNAV path from TOD to touchdown using a STAR connected to an IAL to the runway.

 

If the original author is talking about PMDG NGX I can understand as the modelling probably doesn't capture the logic of the FMS to a tee. BUT IRL its the FMS that gives the best efficiency to the runway. My statement is made with the usual disclaimers..not totally accurate wind etc but to say its rarely used is quite simplistic.

 

Granted in the TMA when you cop an unexpected vector and speed go the HDG SEL and LVL CHG or vert speed. Otherwise the real airplane flies itself nicely to mother earth (and efficiently) using the FMS.

 

My 2.2 cents worth (GST inc)

 

 

 

[scandinavian13]

 

 

The FMS gives the most efficient VNAV path from TOD to touchdown using a STAR connected to an IAL to the runway.

 

It's relatively rare that this type of procedure is used in the United States, which is what he was answering to.

[Porthos]

It's relatively rare that this type of procedure is used in the United States, which is what he was answering to.

 

It might be in the USA..but there was no reference to USA in any of the preamble to Tabs response. 

[scandinavian13]

It might be in the USA..but there was no reference to USA in any of the preamble to Tabs response. 

 

No, but taking the fact that he lives there, and his statement that it's pretty rare - a truth in the United States - it doesn't take much to figure it out.

 

Just like I can look at your location and understand why you have such an objection to it.  Outside of the United States it's common because most places don't have the traffic volume or the fleet mix to go against magenta lining it from departure to destination.

 

Either way, what he said is quite true, just not all across the globe.  Sweeping, sure.  Inaccurate?  No.

[Porthos]

Never mentioned anything about Tabs being inaccurate Kyle.

 

and my comment was in no way an objection. It was....well..... a ....comment....