B737 classic level change

[framer]

Framer, I am curious as to why you would ask opinions and then discount them

Hi Kirk. Sorry if my simple "Nope" seemed a little dismissive. Earlier in the thread endofeng said that it may be something to do with avoiding levelling in VNAV, I responded to that with

Nothing to do with levelling in VNAV
( I can see why you thought that though.)

so when the idea was put up a second time my response was simply "Nope" because I felt like I had been clear about that. Again, I apologise if it seemed rude or dismissive.

Here's a scenario, you depart. select VNAV and then are told to "Climb Now" fl XXX or ALT XX, how are you going to do this.?. ATC expect the climb to start pronto, and getting the PNF to dig in the FMC and remove the altitude constraints and then remain in VNAV is not a good idea. I dont want to get into a debate about thrust modes etc or FMAs.. just an opinion on how you would achieve this without using LVL Change or VS,

If I was in that exact scenario I would say something along the lines of "Level Change .......kts " and continue to follow the flight directors. I'm not sure why you think I wouldn't use Level change.... I wouldn't use V/s going uphill but thats just my preference (unless of course there was likelyhood of a TA)....anyway, this thread is not about that..........
Jolly,

Also on the Classic selecting LVL CHG automatically puts you in the N1 mode

See this is where I start to come unstuck....it is already in N1 mode from the moment you pushed N1 at acceleration altitude. The A/T is commanding climb thrust in N1 mode . If you then go and push Level change it will still say N1 and it will still command climb thrust. The only thing that will change is that the pitch mode will go from TOGA to MCP speed.....now my problem with that is that they behave in an identical manner as far as I can see, if you bug up to 250 or 280 while pitch mode is in TOGA it pitches for that speed, it will stay in it and pitch for whatever speed you want until you select another mode (ie VNAV or MCP speed by putting the a/p in or selecting Level Change. So maybe my question should be "what is the difference between TOGA and mcp speed pitch modes when clean and above acceleration height?.
Anyway, thanks agaiin for your replies, still not sure if there is a benefit to doing it .....more thinking required.

[InSoMnIaC]

Farmer - I think I get the jist of what you are asking. You want to know why someone would call for LVL change after having selected N1 during the climb out because according to your understanding of the AFDS, You can simply just wind up the speed and it will follow the speed anyway (without pressing LVL change).

according the this FCOM VOL 2 - in auto flt section, it says

that after the aircraft is airborne the FD commands 15deg pitch until a predetermined ROC is reached then it commands MCP plus 20kts (ie V2 plus 20)


So by just winding the speed selector up at your acceleration altittude and not selecting LVL CHG the aircraft will accelerate to the new MCP SPD PLUS 20kts (ie the pitch mode of the FD will still remain in TO MODE).

This is overcome by selecting a new pitch mode. eg. LVL CHG which will force it to follow the actual MCP speed. VNAV is not selected until flap retraction due to possibility of flap overspeed issues.

btw by selecting LVL CHG at 1000 and not N1 the aircraft will fly V2 speed and if there is less than 2.5 mins since takeoff then engine will remain at TO thrust.

[Sciolistes]

Pressing n1 ends t/o mode so toga does not command bug+20.

[InSoMnIaC]

Sciolistes

Pressing n1 ends t/o mode so toga does not command bug+20.

pressing N1 just reduces engine thrust. It does nothing to the FD TO mode

[Jolly1]

Quote:
Also on the Classic selecting LVL CHG automatically puts you in the N1 mode
Do you have a reference. My reading of FCOM2 says that LVL CHG does not automatically select N1 mode. VNAV does though. Do you have reference.

Yes I do. Chapter 4 Section 20 with the exception that there is a 2 1/2 minute inhibit during initial takeoff.

Quote:
Also on the Classic selecting LVL CHG...and is the recommended mode during engine out acceleration and climb up to software U10.7 even on the NG (U10.8 and later it is LVL CHG or VNAV).
My FCTM says that LVL CHG is not not used for single engine acceleration, which makes sense to me. The normal non-normal procedure we use is to bug up at acceleration height (i.e. TOGA FMA pitch mode) and then set A/T off, LVL CHG and MCT when flaps up and clean speed.

I was speaking of what happens at flaps up. See Flaps Up - One Engine Inoperative in Chapter 3 of the FCTM. We are referring to "after departure" which I believe is what the initial question in this thread was. Also the Boeing FCTM only says "Ensure autothrottle is disconnected prior to reaching level off altitude so what you describe is a Company procedure.

Quote:
U10.8 and later it is LVL CHG or VNAV
In my FCTM the diff for U10.8 and later is to do with VNAV. LVL CHG has nothing to do with the FMC.

LVL CHG or VNAV are MCP Selections I never said they were FMC choices. However if you select VNAV one of the differences in U10.7 & U10.8 is in "Engine Out Logic" so you will have different results from selecting VNAV at that point U10.3 only changed the label of the ENG OUT CLB page.

Quote:
By using LVL CHG as a standard you are using a procedure that stays the same during both 2 engine and engine out climbs.
Harminisation? Oh you mean as in single engine go around harminisation

There are many different reasons companies adopt certain SOPs grasshopper.

[framer]

Farmer - I think I get the jist of what you are asking. You want to know why someone would call for LVL change after having selected N1 during the climb out because according to your understanding of the AFDS, You can simply just wind up the speed and it will follow the speed anyway (without pressing LVL change).

Exactly.....why didn't I just say that?

But I am still wondering because

So by just winding the speed selector up at your acceleration altittude and not selecting LVL CHG the aircraft will accelerate to the new MCP SPD PLUS 20kts (ie the pitch mode of the FD will still remain in TO MODE).

is not correct, at least not in the a/c I fly, they happily pitch to the speed selected in the window.....all the way up into flight levels if you don't select anything else, and the whole time the FMA displays N1 in the a/t box and TOGA in the pitch box.
So.....why do they do it? There will be a reason I'm sure. I am not doubting it's the normal or maybe even best thing to do.......I just can't see the point in it and I wouldn't mind knowing.
thanks again for the answers.

[RAT 5]

This question was asked to our HOT years ago. The reply followed the lines of: you are asking the a/c to climb and accelerate. There is a ratio between the 2 parameters. TOGA gives 60/40 to climb/accel & LVLCH give 60/40 to accel/climb. True or not I have no idea. It could have been B.S. or true Boeing stuff. Either way, in his opinion, TOGA should remain until the takeoff is completed (it is a takeoff FMA) and then an en-route FMA should be displayed. Neater. You could stay TOGA & N1 to CRZ if you wanted, but it is untidy.

[Sciolistes]

Insomniac,
Yes, that was a fairly rash statement. But the fact is that that TOGA does not command bug+20 if you increase the speed on the MCP.

Jolly,
I don't think it says that LVL CHG automatically selects N1, it just says that it inhibits automatic selection for a period of time. That doesn't mean to say it doesn't, just that it doesn't say it does (at least not unambiguously).

Rat 5,
I don't think so, the acceleration pitch seems roughly the same for MCP SPD and TOGA FMAs accordingly. There is a mention of acceleration bounds in the FCTM for OEI with the advice to leave it in TOGA until flaps up clean speed.