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Guest meyomyx

TOGA - 60% N1 commanded at V2

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HiI've been developing some SIDSTAR files and hence departing many times from the same runway. It would seem that, if rotation and positive climb does not occur very promptly, approx 60% N1 is commanded - leading to a crash unless Auto throttle is disarmed.What seems to be happening is that I set the speed bug to V2 (which is incremented by 20kts). The aircraft is ruunning fairly heavy and if it is not rotated very promptly, the THR HOLD mode annunication changes to MCP (can't remember specific annunication here) - in other words it seem to suddenly decide to chase the speed set on the MCP (anticipating the aircraft accellerating presumably) at the point when you need max N1.I think that is what is happening.Provided I get the thing flying very sharply, we're OK. Otherwise, we visit some local housing estate!Am I doing something wrong here? I'm sure that this would not happen with the real FMS. It seems to relate to the conditions under which THR HOLD will disengage - and that should surely not be at that point.Someone is going to have an easy answer here - I can just sense it!

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by the way - it's Nick Kidd asking :)Cheers

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May it be that you have some "erratic" throttle readings?Anyway when takeing off, pitching manouver is usually initiated at V1, so that at Vr you are near liftoff angle of attack, and at V2 you are already climbing.Not that this should solve your problem anyway...

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I have a new yoke which is jitter free - no I think this is a problem of failing to achieve proper climb before arriving at V2 - which is then not increased by 20 kts just to compound the problem.V1 is point at which abort is theoretically possible. Vr is rotation speed (ie speed at which pitching manoever is made) with V2 minimum safe flying speed.I don't think rotation at V1 would normally be wise! Travelling along the runway at high alpha is not fun!..... unless PDMG program it differently! CheersNick :)

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>May it be that you have some "erratic" throttle readings?>Anyway when takeing off, pitching manouver is usually>initiated at V1, so that at Vr you are near liftoff angle of>attack, and at V2 you are already climbing.>Not that this should solve your problem anyway...Rotation is not started until Vr (the "r" stands for rotate).

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Yes Jeff,I agree with that rotation of the aircraft should not commence before Vr. But some people around this forum seems to suggest you should start pulling the yoke ever so slightly at V1 for the rotation to start at Vr.Anyone know how this is done on "the-real-thing" out there in "the-real-world"? ;-)Cheers,

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VR means rotate although sometimes V1 and VR are very close if not the same speed depending. Regardless one should not start pulling until one reaches VR. Best Wishes,[h4]Randy J. Smith[/h4][h3]P M D G's 747-400[/h3][h4]coming to a runway near you[/h4][/font color]Caution! Not a real pilot, but do play one on TV ;-)AMD 64 3200+ | ASUS KV8 DELUXE | GFORCE 5700 ULTRA @535/1000 | Maxtor 6Y080M0 SATA 80 GIG | 512 DDR 400 | Windows Xp Pro | Windows Xp Pro 64 |

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Mats (et al),It all depends. On a dry runway V1 and VR are normally only a couple of knots apart, but it's a completely different ballgame on a wet runway. Take a look a the tables on page 1 - 15 of the manual, these are for the 738, but it's the same for the other models.So, I would also hesitate against accepting the "start rotating at V1" concept being promoted by some.

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GuysIt's not about "start rotating at V1". It's about preparing the aircraft by starting to move the yoke. Nothing else. If the aircraft has started to rotate before Vr then this is of course bad.Would be glad to get some "real-world" input on this matter. ;-)Well, what do I know? I'm only a amateur desktop pilot! :-)Cheers,

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Hang on a minute ......This talk about V speeds doesn't have a lot to do with the question -Why can the FMC command circa 60% N1 at such a critical phase of flight? I'd bet that Boeing didn't make that a possibility on the 737.Shouldn't happenAny thoughts?Nick Kidd

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Nick,I would love to try to replicate this. In order for me to do that, can you give me a detailed rundown of your procedures from the time you start FS until it happens? Also please provide the state of the PMDG when you start i.e. cold and dark, engines running etc, and the initial fual and cargo/pax load.Thanks,

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Hi MatsI'm, somewhat puzzled by this. to recap - Approx 50% of the take offs I am making, MCP SPD is being annunicated somewhere between Vr and V2.VNKT - Tribhuvan is the airport - I've nearly finished the SIDSTAR file for all to use. Its at 4300 ft BUT it's a very bumpy runway. My theory is that at or around rotation, the aircraft becomes momentarily airborne which commands MCP SPD but as the aircraft is still accellerating yet level, the FMC thinks it should retard the throttles to avoid busting the MCP speed setting. If you manage to achieve climbout, N1 is gradually increased - presumably as the FMC decides we need to climb. The MCP speed is incremented by 20kts as expected.So; it seems as though the switch to MCP SPD is being allowed to happen BEFORE the aircraft is established in the climb - sometimes whilst still on the runway.Half the time, I can make the transition to climb OK - so I think it's to do with the relationship between the parameters under which MCP SPD is selectedBecause I'm repeatedly flying the same SID to test, I use a saved configuration here - all up and running 737-800. 10% flap. 59.5% fuel in wing tanks - 8.6% centre tank with a cargo of 12714.14 giving an AUW of 41857. V speeds are 128 137 147 respectively.I'll get back with more info if you're interested.Nick Kidd

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I have had this happen also. I was going down the runway at nice airport with everything set correctly, and just as I was rotating the engine spools down and I couldn't take off. I thought it was me doing something wrong, but I couldn't see what I was supposed to do wrong...glad I'm not the only one.-Mikkel

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"I thought it was me doing something wrong, but I couldn't see what I was supposed to do wrong...glad I'm not the only one."Your problem is probably something completely different, Mikkel. Did you read the first message at the top of the forum? Have you installed the interim patch?Cheers.Ian.

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Yes I did, but I must confess I don't have them 50% of the times more like 20%...hard to put a percentage on it. I have noticed though, that it usually happens when I rotate a little later than when it's called by the co-pilot.-Mikkel

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Hi MatsThe VNKT SIDSTAR file is now available for download from the Navdata site (http://www.navdata.at/php/sidstar/sidstar.php). There is some text at the beginning of the file that should be read!Feel free to try it. Problem still remains.CheersNick Kidd:)

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NickJust tried a departure from VNKT RWY20 with your SID/STAR file. Very interesting procedures to say the least. However I never managed to replicate what you have described. It is a bumpy ride down the runway but it rotates fine. And keeps N1 up at the desired level and keeps the speed to 175 knots, later 190. Started with the default Cessna 172 with engines running. Changed to 737-800 default paint and loaded fuel as per your post. Also loaded cargo and pax, but with the fuel load and cargo/pax I get a gross weight of 62 tonnes not 42 as you write.All in all. Did you install the latest patch? Have you done a torough pre-flight check to cross check all settings? Everyting entered in the FMC?Looks like something else than the thinng you suspect I'm afraid...Cheers,

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Thanks MatsI still get the problem and I will discover what is happening here.I have noticed that the problem NEVER occurs on the first take off from VNKT (Not yet sure if the location is significant) which suggests that perhaps something is not being reset from a previous departure (bearing in mind the problem occurs when repeatedly resetting to fly a SID).One thing I have been mulling is that the wether loaded is the same each time - and I haven't been setting QNH before flying the procedures which is giving a 200 ft error. It may be significant but too early to say at this time. I don't know the mode change parameters of the FMC but if pressure altitude is a factor in the algorithm .....I will of course let you knowNick:)

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"and I haven't been setting QNH before flying the procedures which is giving a 200 ft error. It may be significant but too early to say at this time. I don't know the mode change parameters of the FMC but if pressure altitude is a factor in the algorithm ....."Some FMC/APFD logic is based on Radio Altitude, but other logic is based on baro altitude. Things like thrust reduction height is, as I understand it, based on a pressure-altitude a certain height above a pressure-altitude value sampled during the takeoff roll (i.e. it is independent of pilot baro settings). It is basically the equivalent of Radio Altitude. However, things like altitude restrictions will be based on altitudes affected by pilot baro settings.Hope this makes sense.Cheers.Ian.(Edit) E.g. If your altimeter is reading 200' below what it should be (because of your error), and you've set your thrust reduction height at 3000' and an altitude restriction at 3000', the aircraft will reduce thrust at 2800' (according to your PFD) and level off at 3000' (according to your PFD).

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MatsFurther info on this. It isn't anything to do with QNH or airport or altitude or hardware etcThis only occurs following "resetting" of the flight. It never happens on the first flight. If I exit FS9 and start a new flight/load a previous flight, the problem never occurs.It's something that isn't reset by FS9 or NG. Exactly the same today - if I reset flight, twice MCP SPD was commanded before any rotation had occurred. Rapid switching off of Autothrottle and application of full throttle gets us away just about.In this way, it isn't a problem - except when I'm trying to refine SIDS when it's a bit of a pain to restart FS9 each time.Quite often following a restart, the warning klaxon sounds at rotation also - yet there is no annunication of a problem anywhere (including the master warning lamp). It lasts for a couple of seconds and then stops. All engine and OHP indications are normal. I can't help but wonder if this is related somehow to the MCP SPD. Once again, this will never happen on a new/loaded flight - only following reset.More info to follow as I discover it :)Thanks

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