Is the descent performance of the PMDG 737-800 accurate?

DJZorrow
  • Is the descent performance of the PMDG 737-800 accurate? DJZorrow

    I often fly the PMDG 737-800 (with winglets) in my home simulator and most of the times, I find it hard to slow the aircraft down for landing when I descend. Most of the time, I need to deploy the speedbrakes to keep descending without increasing speed. I usually start my descent earlier than the marked top of descent point to make the flight a little smoother.

    So I was wondering if this is a bug or if the real aircraft actually behaves like this. I have wondered about this for some time. I can only dream about becoming a pilot and find out myself cause of my bad eyesight.

    Here is a video of me landing the 737 where i had to deploy speedbreaks: http://www.youtube.com/watch?v=8BGL5lH-VXY

    Since this video is getting a little outdated: I can no longer tell exactly the payload or weight of this aircraft. But i normally fly with heavy payload.

  • I have not flown the 737, but I can tell you that jets in general have a relatively low amount of drag (they are very "clean"). This makes them hard to slow down and they build up speed easily and quickly. The 737-800 is a Next Generation 737, and has the updated wing that includes (among other enhancements) winglets, so this problem would be even more prevalent than on the classic 737's.

    Slowing down and going down at the same time is very hard to do in airplanes like this, and use of spoilers is almost always required when the airplane is in a clean configuration if you need to do both. Descent planning is an important part of flight in such aircraft.

  • Until someone with more authority steps in...

    I have a friend who flies 737's and he has mentioned several times how well they glide and how difficult they are to slow down. He tries really hard to not to use the speedbrakes because "they just shake the plane" making an uncomfortable experience for the passengers. They simply manage this "problem" (actually a really good thing in terms of fuel burn) by planning the descent properly, and always starting down early enough to not require spoilers.

    I can't speak as to the accuracy of PMDG, but I would guess the performance you're seeing is at least roughly similar to real world, and it's your descent planning which needs adjusting. What surprises me is that the FMS wouldn't automatically select a reasonable top of descent, but perhaps there is a difference between how you are flying the descent and how the FMS expected it to be flown.

  • You haven't provided everything possible to really ascertain the issue noted, but based on what you have I've noticed the following things:

    • You'll find most 737s at 240-250 KIAS passing through 10000 feet, rather than the 220 knots that you were going at (while 220 KIAS is above minimum clean speed at normal landing weights, it's still slower than most)

    • Being heavy actually makes minimum drag and max L/D speeds higher, which means that for the same speed, you'll actually descend at less of a gradient than if you were lighter

    • The Boeing 737-800W has a idle descent ratio of about 1:20, which means that at 250 knots you're only going to be descending about 1200fpm -- the 737-800 (especially with winglets) is a very slick bird.

    For your reference, you can see the distances required for an idle descent (.78/280/250), plus allowances for a straight-in approach:

    |=====================================================|
    |          |       |      |       DISTANCE (NM)       |
    | PRESSURE |       |      |---------------------------|
    | ALTITUDE | TIME  | FUEL |  LANDING WEIGHT (1000 KG) |
    |   (FT)   | (MIN) | (KG) |  40  |  50  |  60  |  70  |
    |----------|-------|------|------|------|------|------|
    |   41000  |   27  |  340 |  102 |  119 | 133  |  142 |
    |   39000  |   26  |  340 |   97 |  114 | 127  |  136 |
    |   37000  |   25  |  330 |   92 |  108 | 121  |  130 |
    |   35000  |   24  |  330 |   88 |  103 | 116  |  125 |
    |   33000  |   24  |  320 |   84 |   99 | 111  |  120 |
    |   31000  |   23  |  320 |   80 |   94 | 105  |  113 |
    |   29000  |   22  |  310 |   75 |   88 |  98  |  106 |
    |   27000  |   21  |  300 |   70 |   82 |  92  |   99 |
    |   25000  |   20  |  300 |   66 |   77 |  86  |   92 |
    |   23000  |   19  |  290 |   61 |   71 |  79  |   85 |
    |   21000  |   18  |  280 |   57 |   66 |  73  |   78 |
    |   19000  |   17  |  270 |   52 |   61 |  67  |   72 |
    |   17000  |   15  |  250 |   48 |   55 |  61  |   65 |
    |   15000  |   14  |  240 |   44 |   50 |  55  |   58 |
    |   10000  |   11  |  200 |   30 |   34 |  37  |   39 |
    |    5000  |    7  |  150 |   18 |   19 |  20  |   21 |
    |    1500  |    4  |  110 |    9 |    9 |   9  |    9 |
    |=====================================================|
    

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