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Guest cliffie1931

Which to choose..... FLCH or the Altitude button?

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Guest cliffie1931

We're cruising at FL30 and are instructed by ATC to decend to FL25.In what circumstances do we reset the altitude from 30,000 to 25,000 and then hit the ALT button? And in what circumstances do we reset the altitude, choose the vertical speed then press the FLCH button?Cliff

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>We're cruising at FL30 and are instructed by ATC to decend to>FL25.>>In what circumstances do we reset the altitude from 30,000 to>25,000 and then hit the ALT button? >>And in what circumstances do we reset the altitude, choose the>vertical speed then press the FLCH button?>>CliffHi Cliff,If you are trying to maintain a climb or descent at a particular speed, use FLCH (dependent on the speed dialed in, and your VS will adjust for the speed). If you are tring to maintain a certain vertical speed, use VS (speed will fluctuate to maintain your dialed in VS). Your question is very general, so this is a general answer.Hope it helps.- John


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- John Drago

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ALT button is only used when you want to hold current altitude. You can think of it as a "panic" button if ATC directs pilots to stop their climb/descend say because of some arising conflict. In normal situations you would not use this button.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg


Michael J.

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Guest cliffie1931

Michael and John........I thank you. I'd previously tried to find a simple undertanding but now your two explanations give me all I need. Enjoy your weekends.Cliff

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Cliff,glad you found what you were looking for, however the autoflight system isn't that simple. One proof to that is a study which showed that many B744 pilots were unfamiliar with some of the system features. Here is a manual quote relevant to your question, which I believe greatly expands on the answers offered so far. Hope this helps:"When pushed during cruise, the MCP ALTknob will transfer the MCP ALT value to theFMC, and the new altitude becomes thecruise altitude. If in VNAV ALT or VNAVPTH modes, VNAV will automatically initiatethe required climb or descent.If at cruise, and within 50nm of the Top OfDescent point, selecting a lower altitude inthe MCP altitude window, then pressing theMCP ALT knob causes the DES NOWfeature to become active, and the AFDS willinitiate a 1,250 ft/min descent rate untilintercepting the VNAV calculated descentpath, at which point it will enter the VNAVdescent path.">Michael and John........I thank you. >>I'd previously tried to find a simple undertanding but now>your two explanations give me all I need. >>Enjoy your weekends.>>Cliff


Regards, Opher Ben Peretz

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Guest cliffie1931

Opher, thank you too. Your expanding on the previous tips is one of the reasons I find our hobby so satisfying.The 747-400 is a complicated bird (for me at least, having first learned to fly on a Tiger moth) but the helpfulness of Forum Members continues to surprise me. Time is precious yet so many offer their time and knowledge to complete strangers.Would that we came across this generosity more frequently in the "real world".Enjoy your weekend.Regards,Cliff

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Thanks for the information on the MCP altitude button. I had discoverd this operation all except the 50 miles from T/D part. I might add that when using the altitude button to change altitude it also recalculates the T/D. I'm wondering about the FL CH button. It doesn't change the cruise altitude on the FMC and does not switch back to VNAV, whereas when using the altitude button autopilot stays in VNAV and everything is changed to match the new cruise altitude. I'm somewhat at a loss about the FL CH button since FL CH stands for "Flight level change." I guess what I'm asking is why would you use FLCH rather the altitude button method of changing altitude?Owen

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Hi Owen,FLCH Switch: Pressing FLCH (Flight LevelChange) disengages other active pitchmodes and integrates pitch and autothrottlecontrol to effect altitude changes.If the IAS/Mach indicator is blank: Indicatorwill un-blank and display the FMC targetspeed. If the FMC target speed is invalid,then FLCH will use the existing airspeed forthe climb or descent. If the IAS/Machindicator is not blank: Command speed forthe climb will remain as displayed.The Autothrottle will adjust power asrequired to maintain the desired speedduring climb or descent. AFDS will use pitchcontrol to moderate speed in associationwith thrust.When MCP altitude is reached, the pitchmode changes to altitude hold and ALT isdisplayed on the PFD. The autothrottleholds the commanded speed and SPDmode is engaged.It's all in the autoflight manual.>Thanks for the information on the MCP altitude button. I had>discoverd this operation all except the 50 miles from T/D>part. I might add that when using the altitude button to>change altitude it also recalculates the T/D. >>I'm wondering about the FL CH button. It doesn't change the>cruise altitude on the FMC and does not switch back to VNAV,>whereas when using the altitude button autopilot stays in VNAV>and everything is changed to match the new cruise altitude. >I'm somewhat at a loss about the FL CH button since FL CH>stands for "Flight level change." >I guess what I'm asking is why would you use FLCH rather the>altitude button method of changing altitude?>>Owen


Regards, Opher Ben Peretz

KLMMD-11.jpg

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Thanks for the reply, I appreciate it. I had read the manual and when experimenting with the FL CH it does exactly as stated. What I guess I'm trying to say that when using the FL CH switch, in my opinion, it doesn't complete the job like the altitude knob does. When using FL CH and you want to continue on using VNAV you must change FMC cruise altitude switch back to VNAV. Altitude knob does everything, resets MACH to IAS (if below 31,00ft or so,) resets the FMC cruise altitude and recalculates Top of Descent. The method of maintaining the path is different too, FL CH uses VNAV PATH and changes vertical speed to maintain forward speed and the altitude knob uses a fixed descent rate of 1250 ft/min and uses SPD to maintain forward speed.There must be a purpose for both, just don't know why you'd use one over the other. To me it seems like the altitude knob method would be best for most all situations. IMHO.Owen

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Guest Darren Howie

Unless i am wrong FLCH on descent commands idle thrust and controls speed through pitch using the selected or commanded speed.FLCH isa way of descending the aircraft without using the pre computed VNAV path.Hence idle thrust and pitch controlling speed.Great for around the terminal and for whatever reason you do not want to follow the VNAV path.A phrase to remember "FiLCH is your friend below 10" implying its worth around the circuit where you can command an idle descent at whatever speed you want.Its a great way to energy manage the aircraft on a descent.VNAV hasits place but around the terminal is normally not one of them.Going heads down getting distracted by the box causes major issues which is where FLCH really comes in.Its a quick and easy way to descend at a selected speed at idle thrust.CheersDarren

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Thanks Darren. Now that makes sense. So when you use FLCH you normally would not want to go back to VNAV mode. My use of the altitude knob method of descending is shat I should use. Descending from FL35 or FL40 down to 30,000 ft and continueing on in VNAV. When at 30,000ft the FMC will cause speed to drop back to descent speed about 8 - 9 miles before T/D but above 35,000 it won't. At those altitudes I've found that it will not attempt to drop to descent speed till you've already started down the hill.Thanks again,Owen

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Guest glenlee

>Descending from FL35 or FL40 down to 30,000 ft and.......Hi, I think there is something in airliners operating proceedures about when and where you can use each descent mode, I'm sure most airlines don't allow descents to be made in VNAV, you have to use FLCH. Can anyone expand on that?Also if ATC instructs you to descend (say 5000ft) and you dial the new altitude into the ALT window and push the button and you are near the TOD point calculated by the FMC then the descent rate would not be sufficiently steep enough for ATC. The only time it wouldn't matter is if ATC give a 'Descend when ready' instruction.Glen LeeP.S. Owen / Cliff, just a polite pointer - FL35 = 3,500 feet FL350 = 35,000 feet. The 0 was missing from a couple of posts so I thought I'd help out!!

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Thanks Glenn, that makes sense too. FLCH would produce a much quicker assent of descent. Sorry about the FL35, yes I meant FL350. Thanks again,Owen

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Guest Darren Howie

HI OwenFLCH is more of a short term measure rather than a long term thing if that makes sense.It is used to quickly descend or climb when the pre computed VNAV path does not suite the situation.It is used quite a bit around the terminal when ATC take you off your STAR for instance and VNAV really has nothing to work with for track miles to run etc.So in general its an easy way to descend at a commanded speed without getting distracted programming the box when you really should be heads up.CheersDarren

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