hellocould anyone describe what lvl ch is for? I don't understand it. already know that lvl ch cooperates with vnav and also fmc. please help me, if I am wrong, correct methanks in advanceAndrew Kasperek

LVL CH changes the altitude of the aircraft to the altitude set on the MCP, keeping the speed constant to the value set on the MCP as well, keeping the engines on IDLE if it is a descent. So if the plane needs to descent at a rate of 1800 ft/min to keep the set speed, it simply does that without checking with the actual VNAV path settings.On VNAV mode, let's say the FMC tells the autopilot to descent 1500 ft/min keeping IAS at 260 knots according to the VNAV path, but the environmental factors (winds etc) forces the aircraft to slow down at this rate of descent, it will add thrust to the engines to compansate the difference, trying to keep the 1500 ft/min - 260 knots. Likewise if the plane tends to fly faster on this rate, the fmc warns the pilot to add some drag (speed brakes). It would to neither of them if it was on LVL CH mode. It would just increase or decrease the rate of descent to keep the 260 knots.Shortly; of the three modes VNAV tries to keep both airspeed and rate of descent as they were set on the FMC, LVL CH tries to keep airspeed constant and V/S mode keeps the rate of descent (or climb ofcourse) constant.This may not be 100% accurate ofcourse since I am just a simulation enthusiast like most of us. Maybe it would be better if we wait for a professional response.Cheers.Doruk

Hey Doruk and Andrew. Doruk, I read your explanation of LVL CHG and it came across to me differently than what you were probably trying to explain. (It could be the way I read it though!).So what Doruk says about speed is correct. LVL CHG will hold the set airspeed with reference to what is set on the speed in the MCP. Now if you are decending the power will be right back to idle and if you are climbing it will be at full power. LVL CHG doesn't care about vertical speed. Only IAS. It will hold this IAS (by changing the pitch of the aircraft AND thats all it uses to hold airspeed) until it reaches the altitude set in the MCP. And like I said, if you are climbing to the altitude set in the mcp, it will use full power. If you are descending it will use idle power. Its that simple! LVL CHG doesnt care about anything in the FMC. Hope this help!JackColwill

LVL CH changes the altitude of the aircraft to the altitude set on the MCP, keeping the speed constant to the value set on the MCP as well, keeping the engines on IDLE if it is a descent. So if the plane needs to descent at a rate of 1800 ft/min to keep the set speed, it simply does that without checking with the actual VNAV path settings.On VNAV mode, let's say the FMC tells the autopilot to descent 1500 ft/min keeping IAS at 260 knots according to the VNAV path, but the environmental factors (winds etc) forces the aircraft to slow down at this rate of descent, it will add thrust to the engines to compansate the difference, trying to keep the 1500 ft/min - 260 knots. Likewise if the plane tends to fly faster on this rate, the fmc warns the pilot to add some drag (speed brakes). It would to neither of them if it was on LVL CH mode. It would just increase or decrease the rate of descent to keep the 260 knots.Shortly; of the three modes VNAV tries to keep both airspeed and rate of descent as they were set on the FMC, LVL CH tries to keep airspeed constant and V/S mode keeps the rate of descent (or climb ofcourse) constant.This may not be 100% accurate ofcourse since I am just a simulation enthusiast like most of us. Maybe it would be better if we wait for a professional response.Cheers.Doruk

Exactly right!(But I am also only Flightsimmer )David

One more thing, in descents, once the thrust mode goes to ARM, you can adjust it to whatever N1 you would like.

Hey Doruk and Andrew. Doruk, I read your explanation of LVL CHG and it came across to me differently than what you were probably trying to explain. (It could be the way I read it though!).So what Doruk says about speed is correct. LVL CHG will hold the set airspeed with reference to what is set on the speed in the MCP. Now if you are decending the power will be right back to idle and if you are climbing it will be at full power. LVL CHG doesn't care about vertical speed. Only IAS. It will hold this IAS (by changing the pitch of the aircraft AND thats all it uses to hold airspeed) until it reaches the altitude set in the MCP. And like I said, if you are climbing to the altitude set in the mcp, it will use full power. If you are descending it will use idle power. Its that simple! LVL CHG doesnt care about anything in the FMC. Hope this help!JackColwill

Yup, I tried to explain the exact same thing, I forgot to mention climb though, just tried to explain descent. :) Just one thing that I now realised I am not sure about: When it is a climb that "full power" you mentioned would be the N1 setting for CLB, right?

Yes CLB or CLB 1 or CLB 2. JackColwill

so one thing that I don't understand

LVL CHG doesnt care about anything in the FMC. Hope this help!JackColwill

but you are talking about CLB, CLB 1 and 2 and as I know these are values limited by fmc so it looks like LVL CH takes these values (limits) from fmcAGAIN - correct me if I am wrong :)

Or MCT or CRZ maybe or basically any limit displayed on upper DU.

FMC computes the N1 limits for the engine. LVL CHG doesn't use VNAV info.

Try not to confuse something so simple. The fmc doesnt really matter. If it was crapped out you can still do derated takeoffs, etc, just like in the old 200's. Thats what the N1 Dial is for above the engine DUJackColwill

Yes N1 comes from the FMC.CLB 1 is 10% less.CLB 2 is 20% less

To clarify, 10 and 20 % thrust, not N1.Bert Van Bulck

And to further clarify, even if you were limited by CLB 1 or 2, the acft automatically adjusts to full climb power after a certain altitude. Wanna say 14500' MSL but am not sure.