Center want me to descent too soon 737 NGX FMC VNAV

[Jetman67 517]

Hi all Love this program flying the 737 NGX eg gold coast to sydney in australia fl300 but centre wants me to descent well before my FMC TOD so i select pilot discretion which buys me some time but the controller then comes back later and tells me i must descent to fl120 with 30 miles of the approach airport so from this time on i am harassed until i start to descent. I prefer to stick to my FMC VNAV TOD and path to fly the star. Is there any way around this and will it be more flexible in RC5, thanks all Wayne

[jd 57]

you did the right thing, by asking for pd - if you want to stay higher, longer. but you are not the only plane in the sky, the controller is responsible for seperation and handing you off to approach. when you are given the crossing restriction - that is where you need to be in 30 miles or less. be there, or be harrassed. remember - the fmc is not the boss. it's a tool. it's programmed for efficiency of the plane. not for the situation with all the other planes in the area. how would you like to be on a plane, where the pilot is arguing with the controller about a descent clearance. i can just hear it now, "but the fmc says my TOD is in 20 more miles"... that pilot won't have a license in 24 hours. jd

[ronzie 21]

Adding to what JD stated RC uses the 3-in-1 TOD calculation to get you to the crossing restriction altitude, not the surface altitude which unless you program the CR as a fix into the FMC, the FMC will calculate an idle descent approach to the airport altitude. Many STARs have these crossing restrictions noted, especially by the type of aircraft. Airlines are requesting the FAA allow constant descent approaches but many area FAA control facilities are resisting this based on increased controller load. The FAA is soliciting advice from its area managers and under certain area traffic conditions the crossing restriction might be relieved. The crossing restriction represents a vertical gateway where control is transferred between center and approach as does the waypoints in a STAR for horizontal monitoring so approach can estimate getting the needed separation to get all of the "ducks in a row" for the final landing procedures. In addition vertical separation is used where approach areas may overlap for major traffic patterns negating constant descent approaches.

[Jetman67 517]

ok thanks for the explanation, is it realistic to be asked to descent so far back from top of descent, I still have about 80 mils to run, if thats the way it is in real life fair enough, many thanks Wayne

[Ray Proudfoot 8,955]

80 miles out is a long way but have you programmed a waypoint 40 miles from your arrival airport with an altitude constraint of FL110 or 120? That is a mandatory constraint in RC. I suspect once you have you'll find you're started down rather closer than 80 miles. You can of course request a PD but YOU have to have comms. The co-pilot will always accept the instruction.

[ronzie 21]

Also see this thread regarding FMC guidance: http://forum.avsim.net/topic/348911-impossible-approach/

[Jetman67 517]

thanks guysthats a good idea not really sure how to do the below though 80 miles out is a long way but have you programmed a waypoint 40 miles from your arrival airport with an altitude constraint of FL110 or 120? Wayne

[Ray Proudfoot 8,955]

Have a look through the PMDG FMC/CDU documentation on how to create a waypoint x miles from an airport or other identifiable object such as a VOR.I use Project Magenta and on the FIX page I enter the airport ICAO in LSK1, enter /40 in the scratchpad and press LSK2. This create a new waypoint 40 miles from my arrival airport positioned between the final current waypoint and the airport.Press LSK2 to enter that waypoint into the scratchpad and then go to the LEGS screen and enter it between the appropriate waypoints. Once it's entered you can enter an altitude and (optional) speed constraint for it.I appreciate this is not easy to follow but as the PMDG CDU is as sophisticated as they come it should have this feature. It's invaluable in ensuring you meet the crossing restriction.

[jimimac 19]

i have been using rc and the pmdg737(fs9),when i fly to my home airport egph i use the tweed1a star and if i follow the fmc tod i find it very difficult to get down to the correct altitude for landing, rc usually starts my descent about 40 miles before the fmc tod giving me plenty of time to set up the approach. thats why i never fly without rc.

[sluggy 3]

For years I've always programmed a waypoint 60nm (not 40nm) from the destination airport and set 280/12000 restrictions on it. Most of the time I don't actually get to the waypoint at 12000ft (ATC will step in) but the TOD (using the PMDG 737, 747, MD11 and iFly 737 FMC) will match up with ATC's descent request almost exactly. It's nice to be able to see where exactly your descent will be, enabling you to set up the aircraft correctly and not be in a blind panic (unlike my passengers in the back if I'm having an off day :) ). Cheers, SLuggy

[netshadoe 144]

What a lot of pilots don't realize is that ATC descends an aircraft to manage its energy state. This has been very well explained in the Propliner Tutorial 2008 that is available at CalClassic.com. It makes perfect sense when you wrap you head around it, and it goes for all aircraft, jet or props. Here is an excerpt from this tutorial:

In real life ATC force aircraft to descend to control their energy state. Altitude controls energy state and therefore turn rate. In real life if ATC intend to start vectoring an aircraft they will force it to descend to kill its energy state first. The canned ATC in MSFS is too dumb to do this. Because it is too dumb to control aircraft energy state it vectors aircraft over huge distances at excessive velocities in huge radius turns. Suppose in real life ATC instruct a DC-6B to maintain FL 220 and to reduce drag by ten knots from 190 KIAS to 180 KIAS. This decelerates the aircraft by 14 knots from 269 KTAS to 255 KTAS. But ATC can only tell an aircraft in the cruise to reduce profile drag (IAS) a fairly small amount before it might become unsafe. Reducing drag also potentially reduces lift. Instructing the same DC-6B to increase (drag) to 200 KIAS and descend to FL150 reduces its velocity from 269 KTAS at FL220 to 252 KTAS at FL150. Increasing drag by 10 knots while power diving 7000 feet with increasing IAS slows the aircraft 17 KTAS. The higher the IAS in a dive, the more the drag, the steeper the dive, and the faster the deceleration. On reaching FL150 the pilot can now be instructed to reduce (drag) 20 KIAS to 180 KIAS and TAS will fall by a further 25 KTAS to 227 KTAS. The aircraft will have decelerated 42 KTAS for the 10 KIAS drag reduction from the original 190 KIAS to 180 KIAS losing almost 16% of its velocity (TAS) and a quarter of its energy state. The 16% deceleration and 25% reduced energy state are mostly due to the ATC mandated descent. At any bank angle its turn radius will now be 25% reduced when (RDF or radar) vectored. In real life ATC will force it much lower and kill its energy state much further before vectoring it hard for approach sequencing else it will exit the protected airspace of the airway or terminal area when turning. That's why terminal ATC airspace has to look like a series of inverted wedding cake tiers. The sky is crowded. ATC cannot afford to do most of the early approach sequencing by dog legging high velocity aircraft all over the sky. Inbounds are selectively decelerated by instructing them to descend in the appropriate sequence. Telling a pilot to reduce altitude and drag at the same time is stupid. An aircraft can go down and slow down (reduce TAS) very easily, but it cannot easily go down and reduce drag (IAS) at the same time. A descent with drag lower than cruise drag would be very shallow. The pilot needs to target higher drag than econ cruise IAS to dive steeply to decelerate the aircraft quickly. I realise that this is entirely counter intuitive to users of terrestrial vehicles, but to succeed in flight simulation it is absolutely necessary to understand that the more we need to decelerate the harder and further we must dive. It follows that the airliner that needed to dive hardest and farthest was Concorde. It had to decelerate faster and more than any other airliner. For Concorde top of climb and top of descent were co-incident. They occurred at about FL600, Mach 2 and 1138 KTAS. Since it is unlawful to fly an airliner supersonically over land, as Concorde approached a land mass it always entered an exceptionally steep dive of more than 20,000 feet which rapidly increased the drag and allowed it to slow to just M0.9 and 515 KTAS at FL370 in just a few minutes. No other airliner could dive hard enough to shed 630 KTAS over not much more than 20,000 feet of descent. All other airliners have had tailplanes and tailplanes are too weak to allow long steep dives and the rapid deceleration they impose. In real life a pilot can bi*ch at ATC for descent in accordance with his or her airline's fuel saving policy all they like, but they get clearance according to their position in the approach sequence. At a busy airport today there are never fewer than thirty aircraft in the queue for each landing runway, often there are over fifty. In the classic era more like a dozen. Either way they are being approach sequenced by ATC before they get descent clearance from FL 220. When ATC have killed an aircraft's energy state to their satisfaction they will start to vector it hard in low radius turns that do not endanger other aircraft and don't take 2 minutes to turn 60 degrees. The canned ATC provided by Microsoft is too dumb to implement this. We must impose these real world strategies upon ourselves in order to experience realism.

So to sum it up, ATC uses the thicker air down low to:- slow aircraft down- and as a result improve an aircraft's turn rate over a given time ATC doesn't care what your FMC says...they care what their radar scope says. If you're going to fly by your FMC, why bother have an ATC simulation at all? In real life, ATC controls the air, not the pilot and not the FMC.

[ronzie 21]

Thank you Devon for that information. Listening to descent procedures at KMSP matches the conclusions of that information. The basics of that information illustrate the conflicts that have arisen regarding the airlines requesting constant descent procedures vs. controller efficiency and safety of flight and the compromises that must be met in managing traffic flow horizontal and vertical positions (and speed) against maximum aircraft efficiency. As I understand it these in the FAA jurisdiction may be implemented at specific locations and scheduled according to traffic conditions and under instances of the weather environment on a variable basis. RC has on its beta team real world current and retired controllers from FAA and other jurisdictions. In addition we now have regional airline pilots of jetliners. We also have GA pilots as well. They provide a reference base for advising on real world procedures at different airport classes and enroute.