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craig_read

Speed restriction question

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Firstly I would like to know what the difference is between..Speed Transition and Speed Restriction.. it's a PMDG 747 by the way :)But my main question is this, and it's bugging me now, why is it that despite putting a speed restriction in the FMC for below FL100 (my transition altitude is 6000) of 250knts.. It STILL.. insists on flying at 252knots below FL100!?I've checked.. double checked.. 250/FL100 is in the SPEED REST box and the SPEED TRANS is blank.. and it's flying at 252knots.. I'm totally confused.. have I done something wrong or overlooked something here?Any comments or suggestions would be appreciated. Thank youCraig

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Bit tired so I may mix some of those fields up, but here goes.First of all, transition altitude has nothing to do with the 250 below 10 000 feet speed restriction.DELete the 250/FL100 field which you see in the CLIMB VNAV page and you

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Craig,1. There's a bunch of different terms and concepts going on here when you look at the VNAV CLB page:TRANS ALT is the point at which you start flying standard pressure based flight levels instead of local altimeter based thousands of feet. In the US it's usually 18,000 (it varies by country) - at this point you reset your altimeter to standard pressure (29.92 or 1013 HPa) regardless of what the real local pressure is.SPD TRANS is the altitude at which you're cleared to accelerate above whatever the local restriction is. In the US it just happens to be 250/10000.SPD REST is a global (meaning applies to the entire flight plan) speed restriction that you can input. You would use this for instance if ATC gave you an instruction to slow to a particular speed at an altitude rather than at a waypoint or at a DME from a VOR etc...It's important to understand the difference between SPD REST and just entering a restriction on the LEGS page. If you want to cross a certain waypoint in your flightplan at a certain altitude and speed, you do this on the LEGS page on the right side. SPD REST is if you have a generalized speed restriction that is soley based on altitude, not on a particular point over the ground.On a normal flight only TRANS ALT and SPD TRANS will have anything in them - SPD REST will be blank unless you set one manually. (and you shouldn't need to most of the time)2. As to why you're seeing 252 knots instead of 250 in your SPD TRANS prompt, this is because the weight you're taking off at causes the FMC to calculate a clean speed (minimum safe airspeed with flaps and slats up) that is greater than 250 knots. The 747 is sort of a special airplane in this regard because it is so heavy - the plane will often need to fly at more than 250 knots to maintain a safe amount of lift. This is absolutely NORMAL and all you need to do is advise ATC that you're heavy and need to fly faster than 250 for safe clean speed and they'll approve it. If you were to load less fuel/payload you'd see it be able to fly at 250 with no problem. The 250/10000 is not an unbreakable rule - a lot of simmers (especially a few overzealous controllers on VATSIM) seem to think it is, and it's just not the case with an airplane of this size.

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Thank you. Your post was exactly what I was looking for! I already know about transition levels etc and the differences in US and UK etc etc.. that's not a problem and the speed restriction setting, but the speed trans is new to me and thank you for explaining that. As for the 252 knots rather than 250 knots, I had no idea this aircraft would do this, I thought it would obey the restraint regardless. I've learned something else new there. I didn't know most of the time no speed is filled in here either. This is all new and improving my understanding, thank you.Yeah I knew that the view on the 250 knot rule below 10,000 being a blanket unbreakable rule in reality isn't true. I know that a lot of flights with this aircraft in particular get clearances for higher speeds at lower levels normally around 270 ish. I often ask if the speed on VATSIM is enforced for the airfield, almost 100% say yes, but you have to ask! I believe the ITVV Virgin Altantic DVD illustrates this (excellent DVD that).Thanks for the information, it's exactly what I was looking for, and also for taking the time to reply, it's appreciated.Craig

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>Yeah I knew that the view on the 250 knot rule below 10,000>being a blanket unbreakable rule in reality isn't true. I>know that a lot of flights with this aircraft in particular>get clearances for higher speeds at lower levels normally>around 270 ish. I often ask if the speed on VATSIM is>enforced for the airfield, almost 100% say yes, but you have>to ask! I believe the ITVV Virgin Altantic DVD illustrates>this (excellent DVD that).Agreed, the ITVV DVD should be mandatory viewing for all PMDG 744 pilots!The 250 knot rule is fairly strictly administered in US airspace as I understand it, in those circumstances where you are heavy and would have a clean manoevureing speed above 250 knots, you could simply delay the retraction of flaps so as to stay below 250 knots. Then, passing 10,000 feet, complete the clean up and off you go.Out of Europe on the other hand, I understand that speed restriction waivers are fairly common. For those who haven't tried it yet, there is nothing quite like taking 397 tonnes of 747 from stationary to Vmo in a matter of 4 minutes! I gather that you don't often get a speed restriction waiver in the descent though (ie you would generally not be allowed to exceed 250 knots below 10,000 on approach)One of our resident bona fide 744 jockeys could confirm or correct what I've stated here.

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the 250 below 10000 rule is only stict towards non-heavy aircraft. nobody is going to tell the crews they have to keep drag on the airplane, delaying time, causing more wake turbulance, and costing more fuel money. when a heavy needs to keep a clean speed above 250, they always do

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"I gather that you don't often get a speed restriction waiver in the descent though (ie you would generally not be allowed to exceed 250 knots below 10,000 on approach)"Just some comment from Sweden, during descent ATC often gives clearance for high speed if it doesn

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"I've checked.. double checked.. 250/FL100 is in the SPEED REST box and the SPEED TRANS is blank.. and it's flying at 252knots.. I'm totally confused.. have I done something wrong or overlooked something here?"This seems odd to me, too. I don't quite understand what the other folks are saying.If you have entered 250/FL100 at L4, then the aircraft should not exceed this speed below FL100 (at least not with VNAV engaged).The Bulfer Big Boeing FMC User's Guide says that the SPD TRANS line (L3) should blank if altitude entries are made which are greater than the SPD/TRANS altitude (not sure what happens if they are the same).If Vref+100 is greater than 250kts, this will appear automatically on the SPD TRANS line. However, your 250/FL100 should surely override this... otherwise what is the point of making an entry?Cheers.Q> (Edit) Although I haven't tried it myself in the PMDG 744, when using Aerowinx's PS1, the aircraft will not exceed the L4-selected speed rest.

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Q,I guess we need some further clarification here, when you tried it within Aerowinx what weight did you use? If you've used a low weight on Aerowinx perhaps it's this that is ensuring the aircraft maintains the speed? If you used a higher weight maybe it would exceed it? That is what I seem to be getting from this.Although I do think it's odd I can see the logic, IF the speed rest is obviously unsafe I would expect the aircraft to exceed it automatically, and perhaps display some kind of warning message to inform the pilot is has done so? A small change from 250 to 252 is surely insignificant, and the PFD clearly showed I was outside of any buffet areas, so why did it still select this speed change?CheersCraig

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If you have entered 250/FL100 at L4, then the aircraft should not exceed this speed below FL100 (at least not with VNAV engaged). Interesting discussion. I believe you might be correct IF one sets a restriction there even though the spd trans is higher i.e. 255/10000. Wonder what the full sim shows in this regard.Best,Randy J. Smith

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I've just looked on Aerowinx, and with the same config, weight etc.. the 250 knot restriction standsin VNAV, even though it's practically sat on my flaps up speed. When I delete it.. obviously it speeds up.. but with it back on again... 250 knots stands...Where in the literature does it say it will exceed SPEED REST in some circumstances?CheersCraig

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Never been able to edit SPD TRANS (L3)... I can only delete it. Is this how it's supposed to work ?

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"IF the speed rest is obviously unsafe I would expect the aircraft to exceed it automatically"There is no unsafe speed, you as pilot will have to have the proper configuration for whatever speed/weight combination you

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"Never been able to edit SPD TRANS (L3)... I can only delete it. Is this how it's supposed to work ?"This is correct as per the FMC User's Guide. Only DELetion is possible.Cheers.Q>

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"If V2+100 results in say 270 kts and you for whatever reason have to fly at 250 kts then you

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>the 250 below 10000 rule is only stict towards non-heavy>aircraft. There are also smaller aircraft needing more than 250kts below FL100, an example here would be the MD-90 close to MTOW. Those will of course get hight speed as well if needed.Regards,Markus

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Ok I re-read what has been put here again.. and this is my problem re-stated, I think there has been some confusion..I have for SPEED REST 250/FL100 in my FMC my SPEED TRANS is blank.. AND despite this.. while in VNAV the plane is travelling with 252 knots set on the PFD and also it's doing this BELOW FL100.. and I am SURE it should not do this.RegardsCraig

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Oops, meant to put this here.. Ok I re-read what has been put here again.. and this is my problem re-stated, I think there has been some confusion..I have for SPEED REST 250/FL100 in my FMC my SPEED TRANS is blank.. AND despite this.. while in VNAV the plane is travelling with 252 knots set on the PFD and also it's doing this BELOW FL100.. and I am SURE it should not do this.RegardsCraig

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>Oops, meant to put this here.. >>>Ok I re-read what has been put here again.. and this is my>problem re-stated, I think there has been some confusion..>>I have for SPEED REST 250/FL100 in my FMC my SPEED TRANS is>blank.. AND despite this.. while in VNAV the plane is>travelling with 252 knots set on the PFD and also it's doing>this BELOW FL100.. and I am SURE it should not do this.>>Regards>>Craig>>> "Technically" I am pretty sure the AFDS should stay at what you input into the SPD RES line. In practice though, another question is why would you fly in such a manner? VREF + 100 is most likely always going to be faster than 250 knots (Of course lightly loaded might not be but I have not bothered to verify) and that is what you fly at regardless of the airport restriction. Most pilots of the 744 that I have talked to about this even stated that they go to 300 knots well below 10000' and the 250/10000 rule does not apply to 744s that I am aware of. So yes I would "guess" (and notice GUESS) that it should follow the restriction under 10000'.... Best,Randy J. Smith

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"and that is what you fly at regardless of the airport restriction."You do what ATC allow. If you ask to have the airport speed limit waivered and they refuse.... then you're not going to argue, are you? :(It depends on the country... Some countries still rigidly enforce the rule. Seems that Canada and Koread do (or did):http://www.pprune.org/forums/showthread.ph...iction+10%2C000http://www.pprune.org/forums/showthread.ph...iction+10%2C000Cheers.Q>

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>10000' and the 250/10000 rule does not apply to 744s that I am>aware of.Randy,It should apply to everybody, only those that can't hold this speed because of performance limitation would typically be exempt. A fairly light 747 after a long transoceanic crossing coming for landing should have no problem in complying with this regulation. This is a practical regulation to give pilots more reaction time when they spot each other visually in class E airspace (or even in :( where there is a high probability of encountering some VFR traffic.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

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I am going by a few pilots on the beat that have stated this. If you think about it, one flys over the Pacific and there is no need to guess about lighter weights in this regard. I don't believe ATC would hold a fully loaded or heavy loaded to some airport restriction for the restriction's sake if the pilot needs to climb faster than 250 knots which still holds true for many 744s.Best,Randy J. Smith

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Not true! Acft that must exceed 250 for safety reasons are automatically waived. Honestly, most controllers wouldn't know the differece between 250 and 280 on departures. Arrivals are a different matter. By the time you get where you're going, you should be light enough to maintain 250.Scott

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