Flch
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Joined: Sep 2002
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From: Earth
Flch
I'd like to talk about FLCH please.
For starters, what are its vertical speed limits? Is it a mode that works only with constant rate of descent? When do you choose between V/S and FLCH?
Also what thrust setting does it use? How does it choose a thrust setting?
Why do pilots transition from VNAV to FLCH at the lower levels? What exactly are the benefits?
For starters, what are its vertical speed limits? Is it a mode that works only with constant rate of descent? When do you choose between V/S and FLCH?
Also what thrust setting does it use? How does it choose a thrust setting?
Why do pilots transition from VNAV to FLCH at the lower levels? What exactly are the benefits?
Joined: May 2000
Posts: 3,204
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From: Seattle
It depends on the airplane. My experience is with the 744...
The airplane attempts to reach the selected altitude in 2 minutes. It will use thrust from idle to climb thrust to meet that goal at the selected airspeed. If idle or max climb won't make the time goal, it will use the max climb/descent availalbe while holding the airspeed constant.
FLCH controls airspeed more closely than VNAV. FLCH tries to hold it exactly; VNAV has a +/- 15 KIAS tolerance to follow the projected VNAV Path. When expecting vectors off the programmed route, FLCH is more predictable.
The airplane attempts to reach the selected altitude in 2 minutes. It will use thrust from idle to climb thrust to meet that goal at the selected airspeed. If idle or max climb won't make the time goal, it will use the max climb/descent availalbe while holding the airspeed constant.
FLCH controls airspeed more closely than VNAV. FLCH tries to hold it exactly; VNAV has a +/- 15 KIAS tolerance to follow the projected VNAV Path. When expecting vectors off the programmed route, FLCH is more predictable.
Buttonpusher
Joined: Oct 2004
Posts: 449
Likes: 4
From: Bloody Hell
I think one of the major benefits of going from VNAV to FLCH is speed control, on the 7576's speed control in VNAV is not as accurate as FLCH.FLCH will give you idle throttle in descent and Climb power in the climb, VNAV will vary during descent in order to make any restriction you have entered in the FMC. I generally switch from VNAV to FLCH at 10000' as the default setting on the FMC's (for our birds anyway) is 240 kts. So I'll switch to FLCH to maintain 250kts and not screw up the traffic flow into the airport. Thats my 2 cents worth....
Joined: Jun 2004
Posts: 1,843
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From: Australia
As Intruder says, it does depend on the airplane, my own experience is with the B777, very similar to the B747-400, but in some others I've flown it's not much more advanced than the basic Speed / Mach hold on earlier aircraft.
I agree with the others here that FLCH does offer tighter speed control than VNAV, but the biggest advantage (to me) is that when the pre-planned expected descent path is not achieved, then FLCH is the only practical alternative. With VNAV the STAR, with all of it's speed schedules is planned and executed exceptionally well by VNAV, but the inevitable Radar vectors giving Track shortening / Lengthening and differing speeds can make VNAV more of a liability than an aid. If the STAR and arrival is flown as planned (rare), then VNAV is wonderful. Properly done, and with appropriate SOPs, VNAV is wonderfully accurate for the non-precision approach.
Regards,
Old Smokey
I agree with the others here that FLCH does offer tighter speed control than VNAV, but the biggest advantage (to me) is that when the pre-planned expected descent path is not achieved, then FLCH is the only practical alternative. With VNAV the STAR, with all of it's speed schedules is planned and executed exceptionally well by VNAV, but the inevitable Radar vectors giving Track shortening / Lengthening and differing speeds can make VNAV more of a liability than an aid. If the STAR and arrival is flown as planned (rare), then VNAV is wonderful. Properly done, and with appropriate SOPs, VNAV is wonderfully accurate for the non-precision approach.
Regards,
Old Smokey
