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# When to commence descent?

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I'm having a problem with deciding when to commence descending from cruise altitude to a level to capture the Localizer. If we consider a crz. alt. of 310 down to 4000'. Even when using the FMC and starting the descent on the published VNAV TD I invariably get to a point where the aircraft goes into overspeed in an attempt to get down to the next FP altitude, this overspeed then 'trips' out the VNAV function. I seem to remember reading about a descent rate formula of 3:1 or similar. Can anyone help me to sort this out, please?KenSomerset, England

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When I use a generic formula for jets, it is number of feet to descend in thousands X 4 = number of miles out to start descent. For example, if I want to descend 20,000 ft, I multiply 20 by 4 and get 80 miles for the descent.But, if you want to be more accurate, chose how many feet per minute you want to use for your descent, then figure out how many minutes it will take to get down. Now use your desired ground speed to figure how many miles you would cover in those minutes. For example, if I want to descend 20,000 ft at 2000 FPM and 300 Kts ground speed, it will take 10 minutes which means I need 50 miles to descend. 20,000/2000=10 and 300*(10/60)=50-Gary Letona

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FlightSim has built in real world holding patterns for most approaches.I don't overspeed in a dive - I'll slow the descent. If I think I'm too high, I make sure I have the approach selected on the GPS, and throw the aircraft into the GPS mode with the NAV/GPS switch.When the aircraft hits the holding point, it will go into a holding pattern and allow me to continue with a controlled descent.When I'm down to the published altitude, the plane will level out and come out of the holding pattern at the correct point on the next circuit and continue the approach.I've yet to find a payware FMS which will do this basic function of the default GPS.However, you should be able to see the holding patterns / locations on your FMS - if it has REAL "real world procedures" loaded - and fly the holding pattern manually to descend at an acceptable rate. I've seen quite a few STARS posted for the Level D 767 which have neither enroute or missed approach holding patterns.

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Using the FS ATC I've found that it greatly underestimates how fast you can get down in airliners. So about 130-140nm out I request a level change of 10,000 ft, and then another level change down to the STAR altitude. I do a quick calculation based on my cruise altitude and where I need to be for the STAR, and then plan appropriately; but the above rule of thumb is fairly close. I'm looking forward to see how the new version of Radar Contact will handle this issue. Mike

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Hi Gary,Magnificent reply, exactly what I need. It took a few minutes to get my head around it, but OK now :) It may well be that I am programming the FMC wrongly; but now I have the accurate means to deal with the situation. Many thanks, you're a Star.Ken

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Standard Airline procedures take the descent in thousands of feet and multiply by 3 , to calculate how many miles out to begin the descent. i.e. If you are at 38,000 and your Intial approach fix is 10,000, you would begin your descent 84 miles from IAF.

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Hi Reggie,"I don't overspeed in a dive" - well I don't actually call it a dive, I am descending :), but when the VNAV flips' well then it's a dive.We do have an excellent FMC in the PMDG B737NG, generally if it 'plays up' it's down to the guy punching in the numbers. The FMC also caters for Holds and GAs very well. I'm not very conversant with Holds at the moment, though I do a very neat GA when the 'Devil' drives. What I was looking for was a way of calculating an exact descent rate that I could use when needed. Up to now some of my descents involve heading for terra firma very smartly.I'm not an 'ad man' for PMDG but they do provide a very good selection of Sids/Stars, in fact the aircraft's perfect(in my opinion).Thanks very much for your reply.Ken

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Hello Mike,Your experience with FS ATC about 'mirrors' mine, it leaves you too high, then when they want you to descend, the very next second you hear "W01", please expedite your descent" - it's as if they've just realised they are too late with the call :) - I now try to 'second guess' ATC and start my descent before they get a chance to mess it up. Often though I make things worse; Gary(post above) has given me a formula that will correct all that.In another thread of mine, Ray Proudfoot, of Radar Contact, has suggested RC vr4 to me, it looks quite interesting. However, i'm aiming for Online 'flying' when I get some of my rough edges smoothed out a little.Ken

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The PDMG works pretty well. If I could use the FMS for all my aircraft I might use it more often. But I'm the kind of guy who likes to fly a different aircraft every flight. That FMS would not be very good in my deHavilland Comet 4C or Lockheed Constellation.My first real experience with overspeed and dives came when flying the old freeware Piaggio P-180 in FS2002.That airplane has like zero drag in FS - got to talk to a P-180 pilot and see one yesterday at Million Air at KADS - like all other P-180 pilots I've talked to - that's apparently a fairly realistic representation.I've come to the conclusion that the FS ATC is reactionary in it's mode of operation rather than predictive. It responds to what happened - not what needs to happen next.Descents are a good example - You can find waypoints and altitude settings in the XML code of the approaches. If your aircraft crosses the waypoint too high, the ATC will not tell you to circle and descend - but just descend too quickly. It is hard sometimes to figure out what the next waypoint for the descent and the trigger altitude is.Charts / approach plates help - but I find I need to anticipate better than the ATC.The problem is that a system/ program routine which works for slow single engine piston props also has to work with high flying - fast jets. A threshold of 2 miles and 2,000 feet too high might be appropriate to trigger an "Expedite your descent" message for a Cessna, but not for a B737.One thing which I personally think many FS pilots tend to do is rely too much on the aircraft and systems.I am by no means a "realistic" flight simmer but in the real world one lesson I remember and use in FS is that I as the pilot are responsible.If the ATC wants me to descend too quickly, I'll hit a holding pattern, I'll make a 360. I try very hard to not let the ATC force me into what would be an unsafe position.If my aircraft if prone to the descent problems - I'll anticipate and ask for a lower altitude over 100 nm out.Ultimately it comes down to do I fly the plane, or does FS fly the plane.

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Ken:I use the 3 x cruising altitude formula as well and add a 10 mile "fudge factor" as a bit of insurance. This and remember to control your airspeed and descent rate, generally 290-300 KIAS at 1800 ft/min works fantastically, though admittedly, Gary's formulas involve no guess-work. Remember, you'll also be slowing to 250 KIAS at FL 100, so I usually have few worries when it comes time to descend. I strive for something easy to remember with the increased work load associated with any approach, especially one into crowded airspace / ATC.http://www.graphics-free.com/animations/tr...ges/plane_6.gifAlex ChristoffN562ZBaltimore, MD

PowerSpec G426 PC running Windows 10 Pro 64-bit OS, Intel Core i7-6700K processor @4.0GHz, NVIDIA GeForce GTX 1080 8GB GDDR5 VR Ready, ASRock Fatal1ty Z170 Gaming K6 motherboard, 2TB HD + 500GB SSD, Acer Predator X34 34" curved monitor (external view), Samsung U28E590D 28" Widescreen LED Backlit UHD monitor (panel view), P3Dv4.5 64 bit

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Also, try to reduce your speed BETWEEN stages of the descent rather than while descending.In level flight, you can lose 20 or 30 knots pretty quickly, but if you try to do all your slowing and descending at the same time, something is going to give somewhere!

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