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Top of Descent / VNAV PTH Descent Behavior

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Hello all,

 

I have recently started to practice cold-and-dark procedures and use the FMC more extensively. However, I have encountered a few vastly different issues:

 

Mainly, I am confused about the use of VNAV PTH for descent. I am under the assumption that the FMC will automatically calculate the appropriate top of descent point based on altitude constraints, etc. and then automatically command a descent if the autopilot is left in VNAV mode and the MCP is reset to the altitude of the next waypoint prior to the top of descent point. However, when I attempt to follow these procedures, my B747 overshoots the top of descent, before beginning a terribly steep descent of anywhere between −4000 and −7500 fpm. Is this due to a procedural inaccuracy, or should I use altitude intervention to begin the descent before the calculated top of descent? In video tutorials, it seems that proper VNAV PTH use usually produces a vertical speed of approximately −2500 fpm.

 

Secondly, is it normal that the B747 is slow to rotate during takeoff? Using the proper FMC-calculated speeds and trim setting, I am never able to reach an altitude of 35 feet at V2 (as recommended in the manual). Usually, I do not even make it off the ground when V2 is reached.

 

Finally, the B747 has two noticeable inaccuracies regarding animations that (to my knowledge) have not been fully addressed. To me, the landing gear has always been the weak point of the visual model. The PMDG B747 has the tendency to "sit" on the gear regardless of how hard the landing was. The tilt of the gear (i.e., during lowering of the gear) should be independent from the pitch of the aircraft. Also, the behavior of the spoilers (when used for additional roll control) is slightly unrealistic. Similar to the gear, the movement of the spoilers seems to be tied directly to the position of the yoke (with no delay, presumably due to hydraulics). In real life, when spoilers are raised slightly for additional roll, they take time to sink back (modeled accurately on the QW B757 and the Level-D B767). It would be nice to see fixes (or revisions in the upcoming second version).

 

Thank you for an excellent product and an extensive support forum!

 

Owen Zhang


Regards,
Owen
My YouTube

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Have you tried VNAV decent at a different airport? I usually get a similar behavior with awkward STARs at certain airports.


Dev Singh

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I try to distribute different routes to different aircraft. Both directions of the RCTP–VHHH route (the route I reserve for the B747) yield the same result. I use RouteFinder to determine the waypoints, and I use the company route feature to input the flight plan in the FMC. However, I do not use STARs—only the ILS approach function. Is this approach method incorrect?


Regards,
Owen
My YouTube

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However, I do not use STARs—only the ILS approach function. Is this approach method incorrect?

 

Yes - very incorrect.

 

The STAR is what brings you from your route to your approach (which, by the way, depending on the area of the globe, the vast majority of approaches are visual, not ILS like most in the sim community apparently think). Without the STAR, you're mostly on your own for your descent. If there are no altitudes set into the FMC, what is prompting the computer that it needs to descend? Computers are very powerful machines, but they still haven't gotten to the point of reading minds or making decisions for you. If you don't provide it with the information it needs to know when to descend, it's not going to do it for you.

 

Also, I'm willing to bet your 744 is slow to rotate because you haven't trimmed it properly.

 

If you want the plane to operate realistically, you have to start operating realistically yourself. Your posts show a very strong lack of important aspects of the aircraft's operation. You may want to look here and read through these Type Rating Courses (tutorials):

http://precisionmanuals.com/pages/downloads/docs.html

 

The explain everything (the manuals that came with the plane do as well - specifically 8-10).


Kyle Rodgers

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I did read through most parts of the manuals (procedures), and relied mainly on the type courses for more detailed operation. Please read the information provided in my posts more thoroughly, as I have already addressed some of the points you have brought up.

 

I generally prefer ILS approaches with the B747, as they provide greater accuracy. This does not mean I leave autopilot engaged until 200 or 500 feet though, as I generally fly manually from—at the latest—2000 feet. A friend of mine told me that this method effectively renders the "ILS approach" a visual approach, but I do this because of the cues that help prevent unnecessary glide slope deviation that can occur from relying solely on the PAPI indicators. Wouldn't the additional precision of the ILS system save small amounts of fuel anyway?

 

Regarding the altitude restrictions, I believe I do follow the recommendations in the type courses correctly. I confirm the FMC altitude constraints so they are consistent with those specified in the ILS approach diagrams (i.e., make the small font size go to the big font size to specify a hard constraint as noted on the approach diagram). Specifying these altitudes and speeds caused the top of descent marker to move closer to my current position, but excessive descent still followed. Is it correct that the computer cannot calculate a descent path, even if I specify the altitude and speed constraints of first (lower) waypoint after cruise?

 

Regarding my rotation issues, please note the content in my original post. I set the stabilizer trim to the FMC specific trim on the takeoff page, yet the aircraft still never makes it off the ground before V2.

 

I have tried to put in some decent effort into reading the manuals, so please be patient with me, as the B747 is my first advanced aircraft. Thank you for your support so far.


Regards,
Owen
My YouTube

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Are the ALT restrictions sensible i.e:

 

HON ---- FL150

then 7nm later

BNN FL130

 

rather than:

 

HON---FL150

7nm

BNN FL20.

 

The a/c would have to decent very rapidly to drop from 150 - 020 in 7 miles!


Dev Singh

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No, I didn't mean restrictions that were that extreme. It was more specifying the restrictions (e.g., must pass GUAVA at 240 nm/h and at 8000 feet instead of just leaving the speeds and altitudes as FMC predictions). Even if my procedures were incorrect, how come I still see the descent path indicator on the navigational display?


Regards,
Owen
My YouTube

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Owen,

 

I apologize for the gruff posts. Part of it just comes from being on here so much over the years that I'm a little jaded. My job for the past two years was to learn computer programs at an expert level in 1-3 days and then go teach them to people. All I'd head in those classes was "I'm doing it exactly like it says to do it, and it's not working," and sure enough, when I sat and watched them do it, they were doing it incorrectly. That's why my approach is so...doubting.

 

That in mind, I see what you're writing, but I can only go so far. When you're telling me that you're not flying STARs to the airport, you're highlighting that there's a lack of understanding in operating the aircraft (yes, I know the vast majority of airports in the world don't have SID/STAR procedures, but where the 744 flies, this is less of the case).

 

If a very simple navigational aspect of flying is being left out (STARs), it shows me that there's somewhat of a lack of learning the details of the way things work. With that, when you're saying you're reading the manual and so on, I can only put so much weight in that. The reason is that there's no lack of information - even at a simmer's level - all over these fora, on the interwebs, and it's all over YouTube (I'm presuming because the current generation hates reading). Of course, there's always the manuals (that even I find difficult to read). So, while I believe you're reading the manual and the tutorials, it seems as if there needs to be a more in-depth look.

 

This may be your first complex aircraft, but the aircraft is no less forgiving to you than it is me. It's a steep learning curve, to be sure, but that's a testament to the quality of the developer. Furthermore, if this is your first aircraft, and you've admitted you're still learning how it works, I'd hold off on attempting to point out errors of the aircraft.

 

 

 

Regarding the ILS approach, you'll find that many pilots will let the aircraft capture the LOC and sometimes the G/S because there's little to no effort after that point. Many pilots also set the plane up for the ILS anyway, just in case ATC asks the pilot to intercept the LOC and shoot the visual. It's not wrong, really. I was just pointing out that ILS Autolands aren't the standard as many simmers seem to think.

 

 

 

Regarding the takeoff issue, how heavy is the aircraft? Are you loading it up with full fuel and passengers? Are you able to even rotate prior to V2?


Kyle Rodgers

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Okay. I will use STARs. For some reason, it just did not seem obvious that they were necessary to the standard operating procedures of the aircraft (I thought that STARs were only optional ATC aids to help coordinate traffic efficiently). Thank you for the clarification.

 

I did not mean to be arrogant when I pointed out some of the minor animation issues. I just wanted to point them out, in hopes that they would be corrected in the next release. Overall, however, it is remarkable that desktop simulations these days can be so complex, and I praise PMDG for its accomplishments.

 

Thank you again for the clarification regarding your original statement about ILS approaches. When I mentioned ILS, I did not mean to imply that I always perform autolands. I rarely perform them—only to observe the behavior of the autopilot.

 

I generally set the weight of the aircraft so it is consistent with a passenger load factor of approximately 85%, a cargo load factor of approximately 70%, and fuel calculated using the flow charts in the B747 manual. I am able to rotate four or five degrees, but I am unable to leave the ground. Could this be a calculation error with the V speeds?


Regards,
Owen
My YouTube

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Secondly, is it normal that the B747 is slow to rotate during takeoff? Using the proper FMC-calculated speeds and trim setting, I am never able to reach an altitude of 35 feet at V2 (as recommended in the manual). Usually, I do not even make it off the ground when V2 is reached.

 

Do not feel the need to have the aircraft fully of the ground by V2, if you have a heavy load you are looking for a slow rotate from V1 to gently pull the aircraft up, taking about 4 seconds to get from "rotate" to 15 degrees nose up .The last thing you want to be doing is grabbing the yoke and violently rotating to get the aircraft into the air, this causes tail strikes. Get a program such as TOPCAT to help you with calculating V Speeds


Regards,

 

Richard Nobes

 

Yes, I sometimes exceed 250kts below 10,000ft! Imagine that....

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I always use the V speed calculation features of the FMC, which I believe should be highly accurate and realistic (as with anything that involves PMDG).

 

The strange thing is, my aircraft weights for the RCTP–VHHH are quite light, with my gross takeoff weight around 600,000 pounds. Of course I use thrust derated by 15% for regular takeoffs on this route. FMC calculated V speeds are relatively close together. I have observed that many people on YouTube are unable to lift off before V2 (contrary to what the documentation recommends and real-world aviation videos), but am wondering what some of the highly experienced PMDG B747 pilots here are experiencing with V speeds and rotation behavior.


Regards,
Owen
My YouTube

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The speeds of V1, VR, and V2 in the real bird are a mathmatical calculation which performs calculations based on data entered into the CDU. The data it looks at is GW, , Flaps setting, CG, Air Temp, winds, runway length, and thrust setting. While each has its own calculating factors, remember that V1 is all about a safe Rejected Takeoff. This means it is more about runway length and braking distance than actual flight characteristics, however, lift does reduce braking action, therefore, no matter what, V1 is reached before the aircraft has enogh lift to rotate. Therefore, the exact same aircraft with exact same weight and weather should have different V1 speeds for different runway lengths.


Branton Turner

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The speeds of V1, VR, and V2 in the real bird are a mathmatical calculation which performs calculations based on data entered into the CDU. The data it looks at is GW, , Flaps setting, CG, Air Temp, winds, runway length, and thrust setting. While each has its own calculating factors, remember that V1 is all about a safe Rejected Takeoff. This means it is more about runway length and braking distance than actual flight characteristics, however, lift does reduce braking action, therefore, no matter what, V1 is reached before the aircraft has enogh lift to rotate. Therefore, the exact same aircraft with exact same weight and weather should have different V1 speeds for different runway lengths.

 

That's not strictly true, on long runways it's very common to find V1 = VR on a great deal of aircraft and flights. Even a 744 with a TOGW of 330 tonnes has enough runway to RTO at VR at EGLL (27R) for example, and that's the kind of weight a 744 would be at MZFW and going across the pond. It isn't always the case that V1 is achieved before the aircraft has enough lift to rotate, since as soon as you deploy the spoiler (as standard in an RTO) the negative effect the lift has on the braking is pretty much taken out of the equation. However, I do believe some airlines stipulate in their SOPs not to have V1 = VR for numerous reasons, but from my own experience of being in the flight deck, V1 = VR on most flights (this is with 737/A320s departing from 3000-4000m long runways).


Luke Harvest

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V1 is often Vr on a lot of the smaller birds, too (J41 as an example).


Kyle Rodgers

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