Act xxx Lim CRZ - What does it mean?

[737Andi]

Hi,

I noticed this message during my flight today and I was wondering what it meant?
It prevented me from climbing, cruising and descending faster than 230 kn although the TGT speed was set higher for CLB, CRZ and DES.

I suppose its an easy answer but I could not find anything on the internet.

Thank you very much!

[737Andi]

Here is a picture showing what I mean...

[eckhard]

The ‘LIM’ part normally means that the aircraft configuration is limiting the speed. Were the flaps fully retracted?

[737Andi]

Hello Eckhard,

thanks for your reply!

just did a test flight and made sure that flaps and gear are fully up.

What else can cause this limitation?

Thank you!

[737Andi]

Ok..I did some further tests and found out that the gear is causing the problem. I do however not know if its a software bug, misconfiguration of the hardware or a normal behaviour of the 737.

Gear Up = always 230kn limitation
Gear Down = 265 kn limitation
Gear in Locked "middle" Position = Desired speed

Is it normal that the plane is limited when the gear is in the full up position?

Thanks!

[atakacs]

Is that an actual airframe or some simulation ??

[737Andi]

Its the PMDG 737 NGXu running under Prepar3d.

[eckhard]

Your software seems to freeze the gear operating limit speed at the gear lever position, irrespective of the actual position of the gear.

For gear retraction, I seem to remember that we placed the gear lever from UP to OFF (middle position) about ten seconds after the gear up indications.

For gear extension, we kept the gear lever in the DOWN position (of course).

So, with the gear down, the 265 kt limit is presumably correct.
For gear up, the 230 kt limit (Max year retraction speed?) will disappear once you’ve moved the lever from UP to OFF.

Why not try a flight with the gear up, the lever OFF and flaps at various settings? See if the LIM speed reflects the flap position placard speed.

[rudestuff]

What does your after takeoff checklist say? I don't fly a 737 but I'd hazard a guess that you're not supposed to fly around with the gear lever in the 'up' position..

[737Andi]

Hi Eckhard, thanks for your post. So the speed limitation in the up position is a software failure? Is the real 737 making a difference between OFF and UP when it comes to VNAV speeds?

[eckhard]

I don't think so.

As far as I remember, the ACT XXX LIM CRZ (or CLB or DES) takes into account the gear position and the flap position. When I say, "position", I don't know whether the FMC is referencing the actual achieved position of the gear/flaps or the maybe only the position of the lever.

With the gear and flaps up, there is no LIM speed (in this context). To be clear, I don't think that the FMC will protect the aircraft from inadvertent extension of gear/flaps at too high an airspeed, so it's still up to the pilots to respect those limits before making the selection. Once the configuration has been achieved, the FMC VNAV function will prevent overspeed.

With only the gear down (flaps up), the LIM speed should be that for flight with gear extended, or Vle.
With only flaps extended (gear up), the LIM speed should be that for the flap position achieved.
With both gear and flaps extended, the LIM speed should be the lowest of the two speeds.

Normally, the gear is only down when the flaps are extended, so the LIM speed will be that for the flaps. If your sim is showing a LIM of 230 with the flaps up and the gear up but with the gear lever in the UP position, it may be that the LIM speed is that for gear retraction (Vlo). The sim programme is interpreting the gear lever position as an intention to retract the gear and is therefore displaying that speed (230) on the FMC CDU VNAV page.

You could try a flight with the gear up and the lever in the OFF position. Then see if flap extension changes the LIM speed as I have described.

[737Andi]

Ok thanks! I will give that a try and report back.