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Guest JTH

Question about when to descent with regard to ATC

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Guest JTH

Forgive how basic this question is but say when I am flying the 744 and the FMC calculates the Top of Descent and I can see it on my route. Anyways, say ATC has not cleared me to descend from cruise yet. What do I do? Do I just not reset the MCP alt knob? Do I just wait till ATC says "descend to x thousand feet" and then spin the knob down to that altitude?The other thing I am wondering is how to act when ATC clears you to land and says something like "maintain 3,500 until established on the localiser". Does that mean that I press the APP button there and then, and then let the plane descend when it finally gets on the ILS track? Or do I wait till I am closer to press the APP button?Thanks in advance for any help clearing up any of this! Regards, Paul.

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Guest guyjr

I can't really help you with the operation of the MCP, but can help you with the ATC instructions. Obviously if you're not cleared to descend, you can't descend. As PIC, it's up to you to ask ATC if you think they missed a call to you, but normally there's a good reason for them delaying a descent instruction.As for the "maintain 3,500 until established" call, that literally means stay at 3,500 feet until you start tracking the localizer inbound. Just seeing the needle move isn't enough, you need to actually be somewhat lined up with the center and not deviating all over the place before you can start descending. Of course in practice if you're getting closer than you'd like to the runway you can start your descent as soon as you see the needle move, but make sure you don't overshoot! :)

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If you're at cruise altitude and you're at your TOD without a descent clearance, by all means ask for it, "Center, SWA333 requests descent." They will either give you a descent clearance or tell you when to expect it. This is not an unusual request.


Dan Downs KCRP

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Guest JTH

Okay thanks for the replies, that's cleared some stuff up for me. One thing though, say if they give you delayed descent clearance, i.e. your at top of descent and haven't got descent clearance, you ask for it, and they say expect it in 2 mins. So you pass the top of descent, knowing that you haven't got clearance yet but you will any minute. Then you finally get it, two minutes later. So what exactly do you do now? Do you reset the MCP alt, as you would have before reaching T/D? Although I have a strange feeling that if you do the plane won't descend as it should because you've already passed the T/D. If this is the case, how do you tell VNAV to make a new descent path from where you are now to where you need to be? Hopefully you can understand this, I am basically wondering how you get the 744 in this instance to make and execute a new, steeper descent path to make up for the delayed descent clearance.Thanks a lot once again! Paul.

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Don't rely on VNAV... there's as many techniques as there are pilots but what I do is set MCP Speed and VertSpeed to a value in the ballpark of what is indicated on the DES page (greater descent if you're above the path). I have found that this gives me the most control over my letdown. There is usually not a problem from ATC giving you a descent as soon as you ask, they are just as interested as you are in getting you down to the next altitude constraint so they can work a handoff to the next sector.What I have read in this forum from jet experienced jocks is that they normally use VNAV for climbout but not for letdown. I've pretty much adopted this in my simming, plus I enjoy having a little more of an active role in flying.


Dan Downs KCRP

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This is a very common occurrence in the real world and depending upon how long you are left there is not too much of a problem. Dan is right. There are lots of ways to regain your profile and that really depends on just how high above it you are. Leave the MCP altitude set at your current cruising altitude. You will notice that the FMA on your ND will be indicating VNAV ALT. When cleared by ATC to descend set the altitude on the MCP and press to release the aircraft from VNAV ALT. Because you are above the path the FMA will change to VNAV SPD. No problem. You now need to help VNAV regain the path (VNAV PTH). First of all if there are no ATC speed restrictions then just open up the speed window on the MCP and crank the speed up. The higher above profile you are the more speed you need ( up to VMO if needed). It is an effective way of getting down in this aircraft. You can monitor your progress with PROGress pg 2 and also look at the green arc on your ND. This is a dynamic indication of where you be at that MCP altitude at your CURRENT rate of descent and Ground speed. If you still believe that you are too high on profile then use Speed brake. If still too high then ask ATC for vectoring to assist your descent. At lower levels slow up and drop the gear (very effective speed brake that one what with 18 wheels and a load of barn doors hangin' out there in the breeze). Reconfigure with flap early as appropriate. This increases the drag and allows the aircraft too slow down and will give a steeper descent gradient. This is useful for when you find yourself say on base turn and finals and you are just that bit too high or fast. Stick with VNAV if you can as it does have some good features that can be very helpful. If it starts to "unravel" on you then hit FLCH and it will take you to what ever is set on the MCP. A great solution to the "What the F### is it doing now syndrome".For what it is worth I use all 3 modes. Boeing went to a lot of trouble to put them in there so the least I can do is use them. I normally start off the descent in VNAV and then when I get closer in to the Terminal airspace and start getting vectored around and given speed control, I use FLCH. For close in work like easing down onto an altitude that coincides with the GS descent point (to avoid flying level prior) I use V/S. You can still use FLCH and manually work the thrust levers when in THR HLD but I prefer the finer V/S mode. You will notice when using this mode that the thrust levers will move up a little. This takes out some of the Engine Fan lag and allows for quicker power applications of altering Vertical speed.Hope this helps out.CheersSteve


Cheers

Steve Hall

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Re: APP buttonNormally I am using HDG to negotiate vectors to the localizer. When I am on the final intercept heading and see the localizer indictor start moving toward center I turn the aircraft toward the inbound localizer course with HDG. When within a few degrees of the inbound course and the indicator near center I engage APP. It is important to be below the GS indicator at the correct intercept altitude so when APP is engaged the aircraft will lock on to the GS.Engaging APP too early can result in locking on to a back course and a false GS indication, especially common in FS where localizers for each end of the runway share the frequency. Just below the FMA on the PFD toward the left you should see the localizer ID. Be sure it is the correct one before engaging APP.


Ron Ginsberg
KMSP Minnesota, Land of 10,000 Puddles
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Guest JTH

Ok thanks. I tried to implement some of the above advice today.One question that I have though is what is the difference between being at 30,000ft and twisting the MCP knob down to 20,000 and pressing the centre of the MCP button or twisting the MCP knob down to 20,000 and pressing the FL CHG but? I don't understand when you use FL CHG basically.Thanks once again for the continued help! JTH

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Normally you would be cruising in VNAV and the indication on pitch would be VNAV PTH. Usually you get given a descent clearance before TOD that might be "At pilots discretion descend 20000Ft". You would set this on the MCP but not press the button. When the ac reaches the TOD point on the ND the aircraft will descend. If you press the button straight away the aircraft will cruise descend (depending on distance from TOD point )in VNAV SPD at a reduced rate. It may or may not capture the VNAV profile later on in the descent. If you have any altitude restrictions in the FMC (say cross XXXXX at FL 220) then VNAV will stop the descent at FL 220 and then go into VNAV ALT. On the other hand if you selected FLCH, when cleared to descend by ATC, the thrust would go to idle and the aircraft would descend in idle thrust. FLCH pays no attention to the VNAV profile or any of the VNAV altitude restrictions so it will go straight thru FL220 to get to FL200. FLCH can be used when you get given a descent early or late and at a speed that is different to what is the programmed descent speed in the FMC. For example at cost index 50 the VNAV descent might be at a speed of .83/275kts (look at the VNAV Descent page). If ATC said they want 250 kts then it is easier to set FLCH and just dial the speed on the MCP rather than reset the speed in the VNAV page and then try to get back on the newly computed path. Also below 10,000 ft it is better to be looking out the window for traffic rather than having your head down in the cockpit programming the FMC. I would suggest you use FLCH if you are not sure what is happening with VNAV and also when you are given speed or alt changes by ATC when you are below 10,000ft. Remember when in FLCH if the AT annunciation is HOLD then you can set the thrust manually to fine tune the rate of descent if it is coming down to quickly or not quickly enough ( if the thrust is not at idle.)If you follow the PMDG lessons (current and future) this should start clearing things up for you.CheersSteve


Cheers

Steve Hall

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