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

I often fly the PMDG 737-800 (with winglets) in my home simulator and most of the time, 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 on YouTube of me landing the 737 where I had to deploy speedbrakes.

Since this video is getting a little outdated: I can no longer recall exactly the payload or weight of this aircraft.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

• The "willingness" of the aircraft to descend depends on a few factors, including weather conditions, aircraft weight, your speed and the resulting rate of descent. You might want to add those specifics so we can see whether that would compare with a real-world aircraft or not. – Qantas 94 Heavy Jan 23 '14 at 13:39
• I actually have a video of me landing the PMDG in Heathrow, London. It can be found here: youtube.com/watch?v=8BGL5lH-VXY About the weight, wind and weather factors: I am not sure as it differs a lot. I usually let FSX choose a random payload before each flight. And the weather and wind is not always the same. The video might show some of it. I know i did mistakes during that landing (like using the anti-ice or doing a "clean-up" on the runway). So please don't point at those if it doesn't play a factor about this subject. – Alexander Johansen Jan 23 '14 at 13:50
• Try the 737-800 with dual winglets, the plane literally floats and loosing altitude can be tricky. I have had drop rear gear in one instance. – user7759 Mar 25 '15 at 0:23
• According to "Pilot_H" on Twitch, who is a Ryanair 737-8 pilot and regularly streams the PMDG 737 in P3D/FSX, the PMDG performs very similarly to the actual aircraft in almost every phase of the flight. – Jon Story Oct 18 '16 at 15:25

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 |
|=====================================================|

• I like the table. Could you cite the source for it? – Bret Copeland Jan 26 '14 at 5:29
• @BretCopeland: some oldish internet FCOM, not exactly legitimately sourced. – Qantas 94 Heavy Jan 26 '14 at 5:42
• Are these for a fixed IAS or a fixed angle-of-attack? What is the meaning of "(.78/280/250)"? This answer would be made much more useful to some of us non-airline-pilots if the meaning of ".78/280/250" were clearly explained. – quiet flyer Mar 10 at 12:51

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.

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.

I fly now with Prosim and the Prosim Flight Model. Before I jused PMDG and I also had a difficult time with bleeding the speed off during decsent. Almost everytime I got to high and too fast on visual approach to my home airport using real VFR-routes and descent-paths.

Free speed descents" which the airliners get from ATC every now and then was never sucsessful.

But with the prosim flightmodel I experience alot more sucsess on descent. It's much more realistic drag simulation in my opinion. Fuel usage is spot on and climb performance are spot on. So I dont think there is too much drag either..