Hi gang,During the last "few" flights I've noticed the odd behaviour of FL CH. I first thought it was some anomaly that would go away by itself, but it hasn't, and now I'm getting ######...The problem is this:When I'm using the FLCH for descents, it will NOT pitch down enough to capture the desired airspeed. Instead, it will kind of try to hold a much lower airspeed than actually selected on the MCP, thus completely ruining the descent.An example. I'm cleared down to 5000ft, and controller says to speed back to 270. So I select FLCH (instead of VNAV I was on previously) and dial in 270 for speed. The speed starts to bleed off (was 300) slowly, and then it goes PAST 270 and the plane starts to slowly pitch up and the descent rate decreases to ~ 500fpm. I'll let it try a little while longer, but at 210 KIAS and ~ 300fpm descent rate I select V/S and SPD and use those instead, and it works fine. Only, it's NOT the way it is done, nor should be done.For the record, it worked fine before, and I'm not sure exactly when it has started to malfunction, but it's been this way at least 1-2 months, and I'm getting fed up! Also, every other part of the autoflight system works 100%.Any suggestions ?rgdsTero

Tero, Make sure your joystick throttle is in idle and then press F2 to retard the throttles - it works fine.I have seen a real B734 captain making sure his throttles are idle or move them to idle after selectng FLCH for descent.Also keep in mind that you will have some +10 to +15 kts during descend more that what's shown on MCP. Samuel KalachorasAthensGreece

Hi Samuel,Well, as I have selected the "A/T inhibits manual throttle" option, the idle selection wouldn't have any effect. Also, the throttles don't have to be at idle when using FL CH. In FL CH descent the A/T allows for manual use of thrust until ALT CAP. (ie. even if one has selected the "a/t inhibits manual throttle", it's possible to adjust the thrust manually) This way the pilot can adjust the rate of descent in FL CH mode. Add thrust, and the plane pitches up to maintain the selected speed, and descent rate decreases. Decrease thrust, and the plane will pitch down to compensate for the thrust loss (again, to maintain the selected speed)and descent rate increases.I don't agree with your comment about the speed. Whatever is selected on the MCP SPD is maintained exactly. If I select 250, the aircraft better keep the 250, and that's how it works in real life, and that's how it has worked in PIC up until now. As I said before, this problem is entirely new to me, and the FL CH mode worked perfectly before... :(However, thanks for the comment!Tero

I have never seen this problem with FLCH, usually very solid in PIC, especially at the lower altitudes...Side note: I was not aware that FLCH could be altrered manually with thrust as you say it can be...I was always told FLCH will (with A/P on) -climb- at the TRP limit and -descend- at idle, the throttles being commanded by the A/T and not able to be overidden...I will have to try the 'adjust' feature when I get home tonight.Only a few things I can think of...and you probably have thought of them already, but here goes...1) Are you using the PIC767 AIR file?2) Have you applied the taxi thrust modification / adjusted your aircraft.cfg file?3) Joystick/throttle noise can still be a factor even with the A/T inhibit active?4) Are your speed brakes up / do you have a control input mapped to the speed brake that could cause interference?Hope something helps here, best I can do for first thing in the AM.Rob.

Is the F/D showing correct commands that the A/P is not following, or is it the F/D commands that are wrong?(No idea about a solution, just trying to diagnose the problem.)Martin767 fetishistIt's a lot like life and that's what's appealing

No time now to expand on this but Tero has an issue I can confirm. I have noticed this behavior for well over 6 months, long before the taxi thrust mod came along. It does not happen all the time, I would say perhaps 30% of the time. Engines are at idle but rate of decent holds at about 800 to 1200 FPM and airspeed bleeds off 10 to 30 knots below commanded, I even think I would go into a stall if not shutting off the AP or selecting VS and setting a faster rate of decent.No idea what is going on but I had that issue on our approach to PHX just last night. I anticipate it and work around it but this would be a good one to figure out.Mike

Rob!Thanks for the suggestions.The only thing that I can think of is the speed brake thing, I have it mapped to a yoke lever, and often use it "partially", ie. not fully out during descents. Also the joystick noise is one thing I'll need to check out!Regarding FL CH functionality, check this (from PIC manual, didn't have time to dig the info from actual AOM, so this will have to do :))7- Flight Level Change Mode (FL CH):- It engages the plane in an airspeed-dependant climb or descent.- It engages the autothrottle in a FL CH mode that controls powerautomatically to make the aircraft climb or descend as required.- It automatically selects Speed (SPD) as the vertical mode since pitch is now dependent onthe selected airspeed.- Resets the airspeed window to the current indicated airspeed.- If a climb is required the TMC is automatically set to a climb mode (CLB, CLB1 or CLB2 depending on TRP settings).- This mode will fly the airplane to the altitude set in the ALT window. If a climb is required, it gives you climb power and pitch to maintain the selected airspeed. If a descent is required it gives you IDLE thrust and pitch to maintain selected airspeed.- In a FL CH descent the autothrottle changes to throttle hold mode (THR HLD) which allows the pilot to alter the vertical speed of the descent by adding or removing power. The autothrottle re-engages in the SPD mode during altitude capture.- FL CH and SPD are annunciated on the EADI.rgdsTero

Cool - still learning new things about the PIC...never ceases to amaze me. I will try using FLCH at higher FL's now that I know I can trim up the descent with some power.Martin's question about the problem being indicated with the FD cues or other is interesting...Bye for now.Rob.

FLCH -> large alt changesSPD/VS-> small adjustmentsI for one always use FLCH if i'm above FL100 and if the altitude change is more that 2k ft.Otherwise i find SPD/VS to be more appropriateespecially in the very busy sectors(e.g. the arrival area of EBBR) where i find that SPD/VS has more 'control' over the plane and FLCH does 'slip up' sometimes.

You didn't mention the use of VNAV??V/S + SPD I would only use if I was chasing (descending) to a Glideslope....otherwise VNAV and/or FLCH seem to get me where I need to be quite nicely. For example, when a certain controller in PHX tells me to turn to intercept the LOC at 7 miles out and 8000 feet, V/S comes in very handy ;-) Fortunately that does not happen too often and I have extra strength vomet bags for the pax.Rob.

Guys,I just did a 20 minutes FL CH training session to try to locate the problem source.Alas, I didn't find it... However, it was interesting that during the 20 minutes of descending in different speeds, configurations and power settings, the FL CH mode only ONCE made the "dreaded mistake" of letting the speed bleed off and not pitching down. It happened when I was decelerating through 210 KIAS and had FULL spoilers down.Interestingly, the flight director commanded the aircraft to pitch up at that moment, so the error does not lie in the autopilot's capability to follow the f/d, but in the FCC itself that drives the f/d!This is so weird, and doesn't seem to follow any logic. Perhaps there are some variables, that need to be met to set the FL CH to "crash course". Whatever it is, I don't have a clue.Also, when doing large changes to speed, the FL CH causes the aircraft to oscillate quite a bit. It's something like a badly configured PID-controller trying to adjust an electrical process; too much.. too little.. too much... a tad too little... a bit too much... about right.. and so on :). Does anyone know if the real aircraft has this tendency to oscillate when chasing the right angle for the desired speed ?rgdsTero

This is real odd Tero... I can't say I notice a whole lot of oscillaiton trying to capture the speed in FLCH. If I am slowing, FLCH usually gives me 500fpm down until the speed bleeds off. In climb, I do notice -some- oscillation if I change speed settings, but not much. This I contrast with PSS 747/777 FLCH which has the effect of a yo-yo. I dusted off the 777 the other day for fun...doing things like that always gives me a better appreciation of the PIC.Rob.

I can't seem to overide the idle thrust in FLCH descent as it was suggested here I should be able to...always returns to idle...if I turn off the a/throttle I can use the cues and manually control power in FLCH mode...any else confirm this?

"I can't seem to overide the idle thrust in FLCH descent as it was suggested here I should be able to...always returns to idle...if I turn off the a/throttle I can use the cues and manually control power in FLCH mode...any else confirm this? "No problems here, Rob.When you say you cannot override idle thrust, what A/T mode is displayed on your EADI, "IDLE" or "THR HOLD"? Perhaps you are trying to push the throttles forward too soon, i.e. before the mode change to Thrust Hold?Cheers.Ian.

Rob,Ian is right.You have to wait until it says 'THR HOLD' on the EADI. It doesn't activate immediately after starting FL CH descent, you have to wait like 1 minute or so. I'm sure it will work for you too :).rgdsTero