Originally Posted by
TheBusFlyer
Thanks for your answer! I get your point, but having speed on the elevator and thrust controlling rate of climb/descent does not make sense to me. Let’s take climb as an example, in flight school we are all taught to just apply full power and pitch for target speed. Vertical speed is a parameter that is never controlled, it’s always incurred.
Talking about a 777 that is trimmed for speed, starting from a level flight position at a given speed…simply adding thrust will put the aircraft in climb at the same speed. No (or very few - to damp oscillations) elevator input will be required. Having thrust controlling rate of climb and elevator controlling speed…won’t both interfere with each other ? Pitching for speed will change rate of climb/descent thus changing the commanded rate of climb by our new thrust setting.
Thanks !
In the scenario you just mentioned, a 777 trimmed for speed the elevator IS controlling speed. Add thrust and you climb - that's FLCH.
When you're flying straight and level the autopilot is keeping altitude with pitch and the autothrottles as controlling speed. If you want to climb at 500fpm, imagine a 1° line climbing away. Use the elevator to fly along that line and you'll need to add a bit of power to keep your airspeed - that's V/S. Anything that involves flying a PATH (vnav path, straight and level or a defined VS) - then elevator is the primary control. You'll notice in a geometric descent in VNAV path the autothrottles control the speed - because the priority is maintaining the path. If you open the speed window the priority changes to VMAV speed and you will diverge from the path. Going backcto your flight school plane you always have the option to reduce the thrust and reduce the rate of climb, its just that you normally wouldn't. All phases of flight are a combination of pitch and power. You say the interfere with each other, but you could also say they compliment each other. Boeing logic is all about
priority. FLCH proritises speed above all else and generally in a long climb/descent it does behave exactly like your flight school plane. Also, it tends to get you started with thrust management then it leaves you to it. Once it goes THR HOLD the the thrust levers are yours to control in the descent.
If ATC ask me to maintain 1000fpm descent rate I
could use FLCH and fiddle with the thrust levers for the next 30 seconds trying to fine tune the correct ROD - but its a ****load easier to just dial in -1000fpm and let the autothrottles fine tune the thrust for me.
Most of the time you'll be in VNAV anyway. FLCH is useful for small altitude changes which don't need full power as well as flying when you're on vectors and don't have a path, as well as being a general get-out-of-jail-free button. V/S is useful to reduce the rate of climb to avoid TA/RA, for bringing the nose up to slow down quickly and for getting a nice CDA. Ultimately you can use VNAV, FLCH or VS whenever you like - you could pick any one and do the whole flight from start to finish if you wanted to it's personal preference.