MCP Autopilot - LVL CH vs FL CH

[OmniAtlas 93]

Any difference between the two? 

 

LVL CH is used in the 737 MCP

FL CH in the 777. 

 

When would you use LVL CH vs ALT INV vs V/S?

 

Thanks.

 

[FLEX1978 373]

LVL CH - 737

FLCH - 777/747-400

OP CLIMB/DES - A3xx

 

All the same thing, uses pitch to maintain speed and is based on IDLE thrust or Max/derated climb thrust

 

Regarding V/S, FLCH, VNAV, I could write a whole book on when to use the correct tool for a required situation, even then it is fully open to debate since a whole variable of factors come to play.

 

Easiest thing to do is list a few basic points regarding when the modes are used in an operational sense rather then quote a manual.

 

V/S - No speed protection. In climb is generally not a good idea unless you are performing a step climb or maintaining a shallow climb to an alt restriction. In descent, very handy in the last portion of the approach when being vectored of the STAR by ATC

 

FLCH - Will not obey SID/STAR alt restriction, very good in climb if you have no ATC restrictions and want the machine to maintain best climb rate at maximum climb thrust.

 

Good for top of descent if you want the machine to descend at IDLE thrust and give you the best idle descent rate in a clean configuration.

 

VNAV - Select your target altitude and the aircraft will obey restrictions, climb/descend at the most economical rate according to what cost index & data you have entered into the performance page.

 

Saying that, with VNAV it is case of garbage in garbage out, enter the correct information and should do a damn good job, enter rubbish and it will do as its told. Something a lot of flight simmers should think about when complaining about some addons.

[g_precentralis 191]

all written in manual:

 

 

737NG:

Level Change (LVL CHG) Switch

Push –

• LVL CHG switch light illuminates

• pitch mode annunciates MCP SPD for climb or descent

• autothrottle mode annunciates N1 for climb and RETARD followed by

ARM for descent

• IAS/MACH display and airspeed cursors display target speed.

LVL CHG Mode

The LVL CHG mode coordinates pitch and thrust commands to make automatic

climbs and descents to preselectedaltitudes at selected airspeeds.

A LVL CHG climb or descent is initiated by:

• selecting a new altitude

• pushing LVL CHG switch

• setting desired airspeed.

Climb –

• autothrottle holds limit thrust

• AFDS holds selected airspeed.

Descent –

• autothrottle holds idle thrust

• AFDS holds selected airspeed.

Airspeed –

• if a speed mode is active when LVL CHG is engaged, this speed is

retained as target speed

• if a speed mode is not active when LVL CHG is engaged, existing speed

becomes target speed

• speed can be changed with MCP IAS/MACH Selector.

 

 

Vertical Speed (V/S) Switch

Push –

• arms or engages V/S command mode

• commands pitch to hold vertical speed

• engages A/T in speed mode to hold selected airspeed

• annunciates V/S pitch mode and illuminates V/S switch light.

Vertical Speed Command Mode

The V/S mode commands pitch to hold selected vertical speed and engages A/T

in SPEED mode to hold selected airspeed. V/S mode has both an armed and an

engaged state.

Engaged –

• annunciates V/S pitch mode

• vertical speed display changes fromblank to present vertical speed

• desired vertical speeds can be selected with vertical speed thumbwheel.

V/S becomes armed if:

• pitch mode is ALT HLD at selected MCP altitude and

• new MCP altitude is selected (more than 100 feet fromcurrent altitude).

With V/S armed, V/S mode is engaged by moving vertical speed thumbwheel.

V/S mode automatically engages if ALT ACQ mode is engaged and a new MCP

altitude is selected which is more than 100 feet different from previously selected

altitude.

• vertical speeds can be selected which command flight toward or away

from selected altitude.

Inhibited if:

• ALT HOLD mode is active at selected MCP altitude

• glideslope captured in APP mode.

 

 

Altitude Intervention (ALT INTV) Switch

[Option - With speed and altitude intervention]

Allows manual deletion of next FMC altitude constraint via altitude SEL and ALT

INTV switch.

Push – (during VNAV climb)

• lowest FMC altitude constraint below selected MCP altitude is deleted

• if airplane is currently at an FMC altitude constraint, deletion allows

airplane to resume climb. MCP altitude must be set above current altitude

• for each press of switch, one deletion occurs

• if MCP altitude is set above current FMC altitude, FMC cruise altitude

resets to MCP altitude. FMC cruise altitude cannot be decreased using

ALT INTV switch.

Push – (during VNAV cruise)

• if MCP altitude is set above current FMC cruisealtitude, FMC resets

cruise altitude to MCP altitude and initiates a cruise climb

• if MCP altitude is set below current FMC cruise altitude, an early descent

is initiated. Lower FMC cruise altitude cannot be entered using ALT

INTV switch.

Push – (during VNAV descent)

• the highest FMC altitude constraint above MCP altitude is deleted

• if airplane is currently at an FMC altitude constraint, deletion allows

airplane to continue descent. MCP altitude must be set below current

altitude

[Option - FMC U10.6 and above]

• if all FMC altitude constraints are deleted during VNAV path descent, an

automatic transition to a VNAV speed descent is made.

 

777

Quote

Edit: unfortunately, smartcockpit document is locked for copy/paste text, so you can open and read it yourself.

[OmniAtlas 93]

Thank you guys for the very detailed responses! 

 

Cheers. 

[Q_flyer 766]

As said, all of the mentioned modes are "speed by pitch" operations, which means they hold a given Indicated Airspeed, but not by adjusting power or thrust (as an autothrottle system would), instead by adjusting the pitch of the aircraft.      

 

As such, they apply to climbing and descending only, where variation in pitch would be acceptable.

[some1 65]

On some airplanes FLCH (or similar) mode is "smart" and for small altitude changes like 2'000 ft will apply little thrust change to provide smooth transition. On other planes it's dumb and will always move throttles to max climb or idle thrust, which is not very pleasant for passengers. For example in 737 it's "dumb" and in 777 it's "smart".

[Q_flyer 766]

Also worth mentioning that in some aircraft the "Speed by Pitch" mode is not interfaced with the throttle / power level at all. The level of power / thrust would be set manually, and the aircraft would adjust pitch to climb according to the selected airspeed.

 

Eg.. in regional jets like the CRJ (which terms this mode simply as "Speed"), or the ERJ series (which terms it as "IAS")... these aircraft have no autothrottle, so the 'speed by pitch' modes are even 'dumber' as they won't set any level of thrust, by which to then adjust pitch for.

 

The PF would need to set climb thrust (or idle).