I had to switch out from vnav to level change as i got early descend instructions it started out fine but i noticed about fl120 for 5000 it was only descending at 600 fpm? Does anyone know why such a low rate ?

I had to switch out from vnav to level change as i got early descend instructions it started out fine but i noticed about fl120 for 5000 it was only descending at 600 fpm? Does anyone know why such a low rate ?

 

FL CH is all about climbing/descending using pitch to maintain speed.

 

So, if you command 250 in the speed window and you:

...give it enough thrust that it would accelerate if you were in level flight, it will pitch up to maintain the commanded speed.

...give it less thrust than it needs to maintain level flight, it will pitch down to maintain the commanded speed.

 

So, it's all about what you're allowing it to do.  Are your thrust levels above the idle position?  What were you doing when you selected FL CH?

 

I'd recommend looking into your various AP modes.  I think the FCTM goes into some of it (I know the FCOM will likely include more detail).  If you'd prefer to avoid those, I have a YouTube video going over them:

My descend speed was still 273 before fl100 so not sure what caused it i re-engaged lvl change and seemed to descend a bit steeper!

 

All i can do next time is just keep an eye out for it don't think its the first time this has happened though.

My descend speed was still 273 before fl100 so not sure what caused it i re-engaged lvl change and seemed to descend a bit steeper!

 

All i can do next time is just keep an eye out for it don't think its the first time this has happened though.

 

Let us know.  Also, you don't need to change modes if you get an early descent instruction.  Just go to the VNAV - DESCENT page and pres DES NOW>.  It'll start the descent earlier than T/D, but at a shallower rate, and then descend more steeply when it re-captures the original profile.

I will do and let you know the details!

 

About staying in vnav that wont work because you'll be descending at 1000 fpm and ATC will be naging for you to get down quicker?

SPD INT to enter VNAV SPD works, too.

Pressing this just before an early descend ?

A video of what you are experiencing would be helpful if possible...

I don't know how to make a video! Basically during descend at some point in level change it wouldn't go past 700 fpm or something close to that. I'll keep an eye out next time!

I've seen similar behavior myself...I go to LVL CHG and see, maybe, 1000 fpm descent, then it will change to 1200 or 900 or 1000. I'm not sure why.

 

I thought I read somewhere that LVL CHG would (should?) descend you at 1000 fpm and I have seen that behavior, but I've also seen other rates of descent. I just looked but I can't find a reference now to that 1000 fpm down rate.

 

Maybe Kyle or Matt can comment on that?

Yep kinda similar to me ralph! I'm looking everywhere on the net and manuals, still nothing!

 

Have you removed your ILS tutorials from youtube...

>Have you removed your ILS tutorials from youtube.

 

No, they are all still there!

 

https://www.youtube.com/user/rfresh1011

LVL CHG when descending is conducted with a/t commanding idle thrust and a/p commanding pitch to maintain the pilot selected airspeed. The higher the selected speed, the steeper the descent (fpm) and vice versa.

 

Headwinds and tailwinds will also affect the angle of descent for any given airspeed and give a different result in terms of fpm.

 

 

I thought I read somewhere that LVL CHG would (should?) descend you at 1000 fpm 

 

No. LVLCHG does not descend at 1000ft/min. Only V/SPD selected to -1000ft/min would do that.

 

LVL CHG attempts to slow/speed up to the Indicated Airspeed to the Indicated Airspeed selected in the MCP speed window (Knots indicated airspeed) 

 

It will then adjust pitch to maintain that speed.

 

Normally it will do that at either (flight) idle thrust or climb thrust, unless you manually change the throttle yourself.

 

It uses climb thrust or idle thrust adjusted for GRAVITY and drag to maintain the INDICATED KNOTS AIRSPEED to determine how many feet per minute it will descend at.

 

Decent/climb rate in feet per minute will change second by second in reaction to:

• Weight of the aircraft
• winds
• altitude
• drag
• thrust

when in FLCH mode.

 

It will also always remain in a descent when the MCP altitude is below the current altitude, and remain in a climb when MCP altitude is above the current altitude.

 

Sometimes this will be as little as +/- 100ft/min, (one hundred) and sometimes as much as 7000ft/min (seven thousand) depending how much thrust/speed/drag/weight/head or tail wind there is, and what the commanded Indicated Knots Airspeed or Mach is on the MCP.

Yes, now I recall, that's right...you're description of lvl chg is right on.

 

I wonder where im thinking about that 1000 fpm down rate? Am I getting that from vnav descend?