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framer
14th Nov 2009, 05:28
A new call has crept into our operation. We operate B737 300's and 400's and quite a few guys have started asking for "level change" subsequent to the "set climb thrust bug up" call and the flaps being retracted.
My understanding after looking in the books is that this is a pointless exercise as the auto-throttle is already commanding climb thrust after N1 button was pushed. The proponents of the practice suggest that the thrust is "at TOGA" and there is a time limit. ......others say that the a/c is in "TOGA pitch mode" and will accelerate faster in level change. My opinion is that it is a pointless exercise but I thought I'd ask here to in case I am wrong and am interpreting the books incorrectly.
Your thoughts?

lederhosen
14th Nov 2009, 09:07
You should obviously follow your company procedure. For us on reaching flaps up the pliot flying orders 'VNAV or LVL CHG/speed....'

I am not sure I understand your question? If you always blast off into the middle distance with LVL CHG 250kts then it might be OK. But most departures I fly require some strategy and a decision as to mode and speed which has to be called.

RAT 5
14th Nov 2009, 21:28
"set climb thrust bug up"

Not sure what this means. Never heard it and can not desypher it. After N1% the thrust will be in CLBnot TOGA. TOGA is a pitch mode for speed control. If you are accelerating to climb speed to go en-route it would be normal to use VNAV after flaps up. I did hear an opinion that TOGA pitch mode, when used to accelerate, was about 40% accel & 60% climb, whereas LVL CHG was 60% accel & 40% climb. Like I siad this was an unsubstantiated opinion. You'd have to ask a Boeing Test P. However, once these 'opinions' get around they become fact. I never heard any opinion about the ratios in VNAV. However, you are talking of seconds over a normal flight, so anyone thinking they are making a big difference is deluding themselves. If they want to accelerate fast use V/S.

Wizofoz
15th Nov 2009, 03:34
What's wrong with VNAV?

ClimbSequence
15th Nov 2009, 03:53
Hi Framer,

Boeing's normal procedures dictates to set climb thrust at Thrust rerduction height (by pushing N1 button on the MCP) and verifying the thrust mode display have changed to CLB or R-CLB (whichever be the case) and set Flaps up speed at Accceleration height to retract the flaps on schedule. Once flaps and slats have been fully retracted, set VNAV by pushing it on the MCP.
Level Change is a Supplementary procedure and it is not contemplate on the Boeing's amplified normal procedures, however if your company's SOP dictates to use LVL CHG then you have to do it.
I remember some pilots to push the VNAV button at 1000' AGL, the airplane reduced to Climb thrust and set speed 250 KIAS all at once and start to command flap retraction immediately thereafter. But again, it was not a normal procedure nor adhered at any SOP.

lederhosen
15th Nov 2009, 10:43
Indeed I cannot see what is wrong with VNAV. If You push one button you get the same as pressing LVL CHG and twiddling the speed selector, surely more efficient? If you want best rate of climb that is another story.

Denti
15th Nov 2009, 14:14
Depends on your company procedures again. We set 3 times 1500 for normal climbout procedures and SOP says VNAV on at 1500. We had our SOPs allready changed for 10.8 (VNAV/LNAV selected at the gate), however boeing really did a blunder with that version. Had to change all aircraft back to 10.7 pretty soon after they introduced the bugged version.

If you select VNAV before the selected climb thrust reduction altitude nothing will happen until that altitude, and then you will get a normal climb thrust reduction and speed cursor moving up.

Checkboard
15th Nov 2009, 17:41
RAT 5: "set climb thrust bug up"

"Set Climb thrust" = push the N1 switch on the glareshield Mode Control Panel to engage N1 mode (from THR HLD or ARM), initiating FMC commanded climb thrust. Engaging an autopilot above 400 feet or selecting another pitch mode terminates the takeoff mode, and does this automatically. Some airlines hit the switch as an SOP to cover all takeoffs (manual control etc).

"Bug 'UP' " = Adjust the IAS/Mach selector until the magenta speed bug covers the green 'up' manoeuvring speed reference on the speed tape. Some airlines call "Set clean speed" to cover this.

framer
16th Nov 2009, 08:53
Sorry I wasn't clear enough with my original question.

"set climb thrust bug up" is as checkboard describes.
But most departures I fly require some strategy
Yes us too. What I meant is that if the situation warrants something other than VNAV,( for the many reasons I'm sure we're all aware of), then the flying pilot requests level change and then the speed they desire rather than just requesting the speed and leaving the pitch mode as is.So what they are doing really is changing the pitch mode from TOGA to MCP speed, the thrust doesn't change. So my question should maybe be "what is the advantage of doing this?"
What's wrong with VNAV?
I'm just talking about the situations when VNAV isn't the best option.
Level Change is a Supplementary procedure and it is not contemplate on the Boeing's amplified normal procedures, however if your company's SOP dictates to use LVL CHG then you have to do it.
Man o man there must be some restrictive SOP's out there, I don't think of Level Change as a supplementary proceedure so much as 'flying the plane'.
So anyway, I'll rephrase it; If , after flap retraction, you want a speed of say....240 kts, and the VNAV is not programmed for that, is there a good reason to say "Level Change 240kts" as opposed to saying " set 240kts".
Cheers,

Checkboard
16th Nov 2009, 12:58
Selecting another pitch mode (VNAV or LVL CHG) or engaging an autopilot terminates the TO/GA pitch mode, so your discussion is only valid for manual take-offs.

AFAIK, the TO/GA pitch mode commands:

10 degrees down to 60 knots, then
15 degrees up, until "sufficient climb rate", then
V2 + 20 knots, in MCP speed mode, although TO/GA is still displayed, because
If an engine failure occurs, the engine failure speeds of V2, existing speed, or V2 +20 will be commanded.

I would say that for all normal (i.e. non-engine failure cases) the TO/GA and LVL CHG pitch modes are identical MCP speed modes.

framer
16th Nov 2009, 13:50
Yeah I tend to agree Checkboard. it still adjusts the pitch for the commanded speed so I can't see any difference or any reason to do it.

Capt Pit Bull
16th Nov 2009, 14:07
OK, I might be getting my types muddled up here... been flying a desk too long now..

IIRC though, comparing TO pitch mode and Level Change, do the mid vintage 737s not behave identically in pitch as far as acceleration schedule etc, but there is one difference:

TO will not auto change from IAS to Mach number.

So, if you had a continuous climb and a combination of circumstances that precluded VNAV, then with sufficient lack of attention you could exceed MMO.

In my last outfit, IIRC, there was a positive check that we were no longer in TO mode on passing FL100.

I might be getting mixed up with the prophylactic jet though.

BOAC
16th Nov 2009, 14:23
framer - never checked this on the NG but I seem to recall the Classic had a 2.5 minute inhibit after t/off on changing engine limit from TOGA to Climb? I think LVLCHG was therefore likewise inhibited. That was the reason the call WAS 'N1 Speed XXX' (especially in the days before VNAV:eek: - yes, boys and girls , we DID manage). With the advent of VNAV I think that mode change overrode the time limit, but I cannot recall clearly now.