Help on descent...

[Guest]

I have and will continue to RTFM, but I'm outta ink until payday and without being able to refer to it while actually flying, I'm having trouble remembering things.On a descent, I choose FL CHG and it went idle thrust, maintained the airspeed by diving at about 4000fpm.This can't be right :)Pointers on realworld descents in this aircraft would be MOST appreciated.Also, where's the thingy (that's the technical term) for selecting thrust modes such as the transition from TO to climb thrust??thanks allmichael

[Guest ielchitz]

Oh.. you mean that doo-hickey next to the what-cha-ma-call-it?heh the Thrust Management Computer is controlled by a panel located next to the landing gear. I believe the panel is called the Thrust Rating Panel..As for the vertical modes - I have not experienced a -4000 ft dive using the FLCH - though that logic does not seem proper.AFIK, the FLCH mode is dependant on airspeed - and the acft changes pitch angle to keep that airspeed - and shouldn't use more thrust than whatever you specified as the limit - for descents, it sort of makes sense for it to go to idle - since that conserves fuel, but I've never experienced it before - I usually use that MODE at speeds 280 and below....Interested to find out...Ian E.

[Guest Ian_Riddell]

>On a descent, I choose FL CHG and it went idle thrust, >maintained the airspeed by diving at about 4000fpm. >>This can't be right :)Just wondering what altitude you were cruising at, Michael? I believe you will get some pretty large V/S values if you are high (where the air is thinner). If the engines are at idle and you are not exceeding any airspeed/mach limits, then I don't think there is much cause for concern. Excessive descent rate may be due to additional drag (but I'm sure you've already checked your spoilers?).Cheers.Ian.

[Guest]

Yes, I suppose it's asi that counts, not vsi, but still seems weird. This is, then, the proper way to descend? Using FL CHG?

[Guest Ian_Riddell]

If your passengers are not complaining... and ATC are not complaining, Michael, then there is nothing wrong with using VNAV for your descent. Both FLCH and VNAV (initial descent) will give you a descent with the engines in idle and therefore a similar V/S (Please note earlier messages on this forum regarding VNAV idle thrust and PIC (only) programming). However, with FLCH your speed is fixed at the cruise mach number in the MCP window (unless you intervene and change that number). This fixed number may cause problems as you descend to lower altitudes (As said in a previous message, you will have to keep an eye on your speed to make sure that you don't go too close to your max/min airspeed limits... as a fixed mach number will give you a variable airspeed with changes in altitude).It has been noted on other aviation websites that the passengers will not be troubled by V/S (It's rapid changes in airspeed, V/S, etc which will disturb them).Another consideration is rapid changes in cabin pressure as the aircraft descends/ascends at high rates. However, the cabin pressurization system on modern aircraft is designed to limit the pressure changes in the cabin (i.e. under normal circumstances) to prevent discomfort to passenger's ears. VNAV was conceived to make the life of pilots easier. With a sound knowledge of its use, it should do this.Cheers.Ian.

[Guest]

Excellent information! Thank you, Ian. I suppose it will all even out with practice. I did try a Vnav descent today, though, but she climbed at 2000fpm though I had set the MCP to the new altitude (which obviously was lower than current FL). But I think that's why after all these years of simming, I feel with PIC like I've finally found what I was looking for (no U2 allusions intentional :)). There's intricacies and nuances to this aircraft ... now if only I can be patient enough to learn then ;).michael