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Originally Posted by amead
Thank you for this information - very interesting. I'm basing my questions off experience with MS Flight Simulator/VATSIM, so it might just be lolworthy non-real world stuff. Consider this hypothetical:
You are cruising at FL350, and it's getting toward the time to start descent. You are cruising at some speed obviously greater than 250KIAS. I'd assume you'd generally get cleared for lower, so let's say that you're cleared down to FL180. Do you just go idle thrust and set your AP for FL180 at a standard rate of descent? Would you base your rate of descent at a certain rate that wouldn't accelerate the plane beyond your current cruise speed?
As you might expect, there are several ways to do this and it varies with the situation. All modern airline planes are capable of LNAV (lateral navigation, the one that keeps it on course) and VNAV (vertical navigation, which plans fuel optimized descents).
If using VNAV, the flight director will command a path for the optimum descent and will adjust autothrottles as necessary to maintain the VNAV speed which is normally derived from a Performance Setting determined by our dispatchers, but we can override that speed if we want to. The descent path has priority, so it may exceed the predetermined speed if the throttles are at idle and it has to increase the descent rate.
If we're not using VNAV, we can set a descent rate on the Mode Control Panel and the autothrottles will try to maintain selected speed, but the descent rate has priority if we've selected it. Or we can descend in Speed mode, in which case the descent rate will be a function of where we set the power.
All of this sounds confusing I know. It's one of those things easier to visualize if you can actually see it in practice. One of the big determining factors in what mode we select is whether or not we have an altitude restriction, i.e. the clearance "Descend and maintain Flight Level One Eight Zero" has no urgency implied, so we will normally just set the new altitude and descend with a comfortable descent rate (perhaps 1000' per minute). Sometimes we get a clearance like "Descend and maintain Flight Level One Eight Zero, be level within 4 minutes" or "Descend to cross 40 miles east of Providence at Flight Level One Eight Zero." Both of these have a restriction and we will hustle down if necessary, letting the airspeed go right up to the barber pole (Vmo) if we haven't also been speed restricted.
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Now you're at FL180 still at cruise speed (would you have slowed down already? Still stay as fast as you can?), and get cleared down to 8,000.
We usually stay fast as long as they'll let us. At FL180 we'll be out of the mach regime and be going by IAS...probably about 310 kts or so.
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You're going to need to hit 10k at 250KIAS, so I assume that means you need to descend while slowing down. Does this just mean that you'd set your rate of descent to such a rate that would allow for you to slow however much you need to in the 8,000 feet of descent that you have before hitting 10,000?
Again, this depends on what restrictions we are given by the controller. If there were no additional restrictions, we could just set 8000 in the FMC (Flight Management Computer) and on the MCP (mode control panel) and let VNAV do its thing. It will automatically adjust descent to achieve 250 kts by 10,000 before continuing down.
If flying without VNAV, we can come screaming down at 310 kts and then dial in 250 kts at around 13,000 and the plane will be close to 250 by 10,000. If we see it's not going to be completely slowed, we can step in and adjust the descent rate to, say, 500' per minute down and it will slow quickly at idle power. Failing all that, we can hit Altitude Hold at 10,000 and it will level off and then slow quickly and then we can continue down.
Lots of ways to achieve the goal. The easiest is just to let VNAV do its job.
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I'd always end up being too fast in the sim world, so the timing of when to slow/how to slow while descending intrigues me.
Since you're not in VNAV, try adjusting the speed as you pass about 13,000 and see how that works, or try adjusting Vert Speed to 500' per minute and you pass around 11,000.
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Side question: How long did it take in your training to get used to steering with your feet while on the ground? I always found my brain compelled me to use my hands in the few landings I did in the C172, and I'd touch down and start drifting to the side and generally sucked at it, lol. I assume that you need a deft touch to not oversteer especially at higher speeds right after touchdown.
Using the hands is a classic rookie tendency and a transference from your driving experience. You'll get used to steering with your feet. If the winds are light, try taking your hands completely off the wheel when at taxi speed.
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Did I mention this thread is a classic?
Thanks.
Last edited by W0X0F; 06-19-2014 at 06:27 PM.