Pmdg 737: Always "speed off" when landing. Why?

[SimFx 61]

Please have a look at the image below. Flaps 25, speed set to 150.. the aircraft logic does not seem to accept any valid speed. On the PFD, the amber minimum manoeuver speed tape and the max manoeuver tape touch each other (one continuous amber line between the red-dotted limit speed ones); no proper speed range left in between them..

On every occasion the aircraft then issues an aural "speed off" warning and automatically starts to accelerate. Happens on every single flight, and busts every landing, unless I land at flaps 15 or below. The speed selector windows shows either an "A" or a "B", so obviously ANY speed seems off.  What could I be doing wrong?

[Farlis 2,289]

Hard to tell without knowing your landing weight I just notice that you have quite a lot of fuel on board for a landing. Usually in operations a number somewhere around 2.8 Tons would be more realistic.

 

But this seems oddly similar to a similar topic of another user who had trouble with the final stages of Flaps settings on approach in the FenixA320. 

Maybe you two are using a similar conflicting add-on that somehow messes with the internal logic of the planes?

[MDFlier 2,031]

2 hours ago, Farlis said:

Hard to tell without knowing your landing weight I just notice that you have quite a lot of fuel on board for a landing. Usually in operations a number somewhere around 2.8 Tons would be more realistic.

Exactly what I was thinking. Did you fill in the Approach Ref page in the FMC? What did you input for gross weight?

The red on the speed tape at the bottom is showing that you need about 138 or139 kts to prevent stalling, while the red on the top of speed tape is showing that you can't exceed 191 kts in a flaps 25 configuration. your vRef is 133 kts (I think. I'm old and it gets a little fuzzy when I enlarge the pic) which is 5 kts below your stall speed. If you add 5 kts for wind adjustment, you are landing at exactly the stall speed of the airplane. Works fine if you fly it perfectly, but being even a little off of a number somewhere is a recipe for disaster.

Re-fly the flight with either a lot less fuel or less pax & cargo. Or both. Doing that should lower your stall speed on landing. I think dispatch messed up somewhere. 😉

 

Edited December 3, 2023 by MDFlier

[SimFx 61]

Hi Farlis, in fact I have very little add-ons running: We love vfr and a local scenery, and FSLTL traffic injector. But your observation is interesting: If it is somehow related to my system, the problem might also show up on the Fenix, so I'll try the same flight in the A320 one of these days.

As to the amount of fuel.. well I did simply let the Pmdg FMC import it from what Simbrief calculated. But now I might have stumbled on a little bug in the Pmdg Simbrief importer routine: It seems to have loaded not what Simbrief gave as required block fuel, but rather the maximum fuel value. Might be interesting to check on this some more.

I'll report back if I have any findings in either direction. Thanks!

[SimFx 61]

Hi MDFlier and thanks for your input! Indeed, following your explanations the speed values and management seem to be slightly below optimal 🙂 Still, wouldn't there always be a speed window (be it small) in between the upper/lower red and amber tapes that would allow the aircraft to fly safely in a flaps 25 config?

In the Approach ref page  I did not fill in the gross weight; it was set by default I think. I just set landing flaps to 30 with a resulting ref speed of 133.

I agree that dispatch seems not to have had its best day (somewhere the numbers might get messed up inbetween Simbrief and the PMDG FMC), but nor did the PIC who signed those papers before take-off.

As per your suggestion I'll setup the same flight with about half the fuel, and will also have all the payload taken out 🙂 Will report back how it goes!

[Farlis 2,289]

8 minutes ago, SimFx said:

Might be interesting to check on this some more.

You probably simply forgot to set the fuel by clicking on the key next to the "Set Fuel" command in the FMC.

 

Now you know why checklists are so vital to flying. Never skip a preflight checklist. You would have noticed that you had the wrong amount of fuel loaded if you had gone through the checks properly. 😉

[SimFx 61]

21 minutes ago, Farlis said:

Now you know why checklists are so vital to flying.  😉

Right you are. Amateurs flying complex airliners for an hour or two to relax after their office work.. desaster must be lurking right around the corner ;)

[Farlis 2,289]

1 minute ago, SimFx said:

Right you are. Amateurs flying complex airliners for an hour or two to relax after their office work.. desaster must be lurking right around the corner 😉

Excuses, excuses. 😜

[Stearmandriver 2,312]

The plane is definitely heavier than the box thinks it is... But that doesn't explain all of this.  In reality, a 737 can be landed at MGTOW, you just write up the overweight landing for an inspection.  This is why the airplane doesn't have the ability to dump fuel for an emergency return - it doesn't need to.  You can land flaps 30 or 40 in those cases, and you may be within the lower amber airspeed band (below full maneuver speed for that flap setting) but you'll not have an A in the airspeed window, nor would you have a complete lack of maneuver margin present on the speed tape.

I've never tried this in the PMDG, so I can't say if it's a problem with your setup specifically, but some of the NG models do have strange indications for low airspeed at high flap settings.  On the -700 for instance, the red low speed band (shaker activation) is not even visible at approach speed.  One of those low priority items I expect they'll get to eventually.

[MDFlier 2,031]

49 minutes ago, SimFx said:

In the Approach ref page  I did not fill in the gross weight; it was set by default I think. I just set landing flaps to 30 with a resulting ref speed of 133.

If you went into the approach page and selected flaps 30 well before your landing, then the gross weight may have been very high because you hadn't burned off a lot of fuel yet.

Before selecting the approach flaps, go to the PROG page. At the bottom, you will see your current fuel quantity. Just above that, you will see your destination airport with the projected fuel quantity when you get there. Subtract the fuel quantity at the destination from the current fuel quantity. Then, go to the approach ref page and subtract that number (difference between current and arrival fuel) from the gross weight that you see when you open the approach reference page and input that weight in the gross weight before pressing the flaps 30 LSK twice. This will calculate the approach speed based off of what the plan will weight when you get there instead of what it weighs now.

See if that gives you a little more headroom at the bottom of the tape.

[Gazzareth 811]

1 hour ago, SimFx said:

Hi Farlis, in fact I have very little add-ons running: We love vfr and a local scenery, and FSLTL traffic injector. But your observation is interesting: If it is somehow related to my system, the problem might also show up on the Fenix, so I'll try the same flight in the A320 one of these days.

As to the amount of fuel.. well I did simply let the Pmdg FMC import it from what Simbrief calculated. But now I might have stumbled on a little bug in the Pmdg Simbrief importer routine: It seems to have loaded not what Simbrief gave as required block fuel, but rather the maximum fuel value. Might be interesting to check on this some more.

I'll report back if I have any findings in either direction. Thanks!

I've never seen the Simbrief importer load the block fuel into the aircraft - either in the FMC itself or in the fuel loading page..

 

That and payload (zfw) I always set manually...

 

G

[Stearmandriver 2,312]

11 minutes ago, Gazzareth said:

I've never seen the Simbrief importer load the block fuel into the aircraft - either in the FMC itself or in the fuel loading page..

 

That and payload (zfw) I always set manually...

 

G

It's a new tablet feature.

[Bobsk8 12,792]

47 minutes ago, SimFx said:

Right you are. Amateurs flying complex airliners for an hour or two to relax after their office work.. desaster must be lurking right around the corner 😉

That's why they call it "study level" 

[Bobsk8 12,792]

13 minutes ago, Gazzareth said:

I've never seen the Simbrief importer load the block fuel into the aircraft - either in the FMC itself or in the fuel loading page..

 

That and payload (zfw) I always set manually...

 

G

It has been automatic since the UFT was introduced

[Gazzareth 811]

9 minutes ago, Bobsk8 said:

It has been automatic since the UFT was introduced

Must be me flying bleary eyed, sure I set it manually on the last couple of flights. Although, thinking about it I would probably have set it before I even touched the tablet .. ..

 

G