Steep Descent with LVL CHG

[Greaser]

I was doing the LFMN VORB B approach to rwy 22. I was at 1500 feet ALT HOLD. I changed the HDG SEL to 303 and lowered the altitude to 1000 feet, and pressed LVL CHG. The plane pitched down by about 7.5 degrees. Altitude quickly dropped to around 850 feet before A/P was disconnected. It seemed too severe the descent with LVL CHG when it probably should be more gradual.  

[Morten]

Lvlch will set thrust to idle and pitch to maintain airspeed.  If you have a lot of drag out (gear, flaps) it WILL be steep.

Edited December 25, 2017 by Morten

[Litjan]

7 hours ago, johnc said:

I was doing the LFMN VORB B approach to rwy 22. I was at 1500 feet ALT HOLD. I changed the HDG SEL to 303 and lowered the altitude to 1000 feet, and pressed LVL CHG. The plane pitched down by about 7.5 degrees. Altitude quickly dropped to around 850 feet before A/P was disconnected. It seemed too severe the descent with LVL CHG when it probably should be more gradual.  

Lol, and your Captain did not hit you on the head with the weight-control folder???

[N313GM]

4 hours ago, Litjan said:

Lol, and your Captain did not hit you on the head with the weight-control folder???

I hate it when that happens. Merry Christmas.

Edited December 25, 2017 by john_s

[Greaser]

8 hours ago, Morten said:

Lvlch will set thrust to idle and pitch to maintain airspeed.  If you have a lot of drag out (gear, flaps) it WILL be steep.

I just took a another look at the replay, it pitched down between 7.5 and 10, i.e. about 8.75 degrees. The (speed with A/T On) went from 137 -> 133 -> 140 -> 133 -> 137.

So I am happy with the purchase of course, but not sure if this is accurate. It really is quite a steep (and quick) descent.

The other thing the chart shows a MDA(H) of 1500/1488, but the maximum in the plane is 999. What do you do? 

Edited December 25, 2017 by johnc

[mfor]

Well I think level change will always have idle thrust when descending (N1 when climbing) and the AP can just the adjust pitch to meet the desired speed, so the descend rate is determined by the amount of drag you currently have. An Airbus would probably yell at you but the ole 737 just does it 

The MDH is barometric in that case if I'm not mistaken, so all you do is set the little orange bug at the altimeter - no computer involved there.

 

 

[Shanwick]

Throttles will retard when you press LVL CHG to descent. The plane will take the speed that it's currently flying at and will try its best to hold that speed by pitching up and down the nose. You will have still full control over the speed by changing the speed via the MCP speed dial but doing that doesn't increase or decrease engine thrust. The plane will manage your desired the speed by pitching the nose up or down. LVL CHG works very well in the IXEG. But if you wanna meet waypoints at a certain alt V/S or VNAV is a more suitable descent mode. 

 

Edited December 25, 2017 by Shanwick

[Litjan]

In other words - the proper mode for a smooth descent in your situation would be the V/S descent mode.

You *could* use FL CHG, but you would need to advance your thrust levers again (after the RETARD-ARM cycle finishes) to add some thrust - that way the plane would descend less steep.

Jan

 

[Greaser]

I may try the same approach again with V/S down to 1000 feet. Cheers.

[N313GM]

A. I don't trust VNAV.

B. I use Level Change only when climbing.

C. That leaves SPEED control and VERTICAL SPEED control for descent. 

It's a lot of work for "busy" STAR's ie KSLC. It's the only accurate way to approach at the moment. Do you know the formula for getting semi-accurate settings?

[mfor]

Don't know if they are semi accurate - just a two rules of thumb I picked up

1.  If you set V/S in feet/min to be about 5 times your ground speed in knots (half the number and add a zero) you'll get an fairly nice 3 (2.8) degree descend and you more or less keep your speed when clean and idle.  (e.g. if you're doing 240 knots GS select ~1200ft/m )
2. for each 1000 feet you want to descend you need about 3.3 miles of space with that 3 degree descend mentioned above, i.e if you need to cross a fix at 10000 and are at 30000, you need to start your descend about 66 miles out - allow some extra miles for slowing down if you need to meet speed restrictions. (feet in 1000s multiply by 3 and add 10%)

I use these as guidelines to check what VNAV wants to do and plan longer descends, and for shorter legs I use 1) as starting point for my v/s and adjust according to the "green banana".

Edited December 26, 2017 by mfor

[Tim013]

What's this VNAV you speak of.....LOL.

Flight sims are already nothing more than button pushing simulations.  For me personally, I use LNAV, and the MCP for forward velocity, and vertical velocity. 

The funny thing is there has never, and I mean never, been an add-on aircraft that has been developed, that flys an acceptable VNAV profile.  That's why I don't get whipped up over the VNAV shortcomings of every add-on.

Tim