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RFields5421

unstable ils final approach

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i dont know how many have experienced this but i have observed that in fs2004 if i attempt to land a fully loaded fully fuelled 747 or a 777 the autopilot takes me on a very unstable decent with wild nose pitch up and downs worsening as i get closer to the runway, ending up in a crash on it. this does not happen in the case of 737 which maintains a constant pitch and decent rate. is there any physics behind this or is it a flaw in fs2004?does this happen in the real world too?

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>i dont know how many have experienced this but i have>observed that in fs2004 if i attempt to land a fully loaded>fully fuelled 747 or a 777 the autopilot takes me on a very>unstable decent with wild nose pitch up and downs worsening as>i get closer to the runway, ending up in a crash on it. this>does not happen in the case of 737 which maintains a constant>pitch and decent rate. is there any physics behind this or is>it a flaw in fs2004?>does this happen in the real world too?in real life generally the only problem AP's have is capturing "false" glide slopes. i have seen one captured and the AP violently shoved the nose down trying to follow this. nothing you can do but disconnect the AP and reset the FCP. the false glide slopes are usually caused by a/c blocking or partially blocking the antennae while they taxi (hence the ils hold short lines).

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Trying to land a Boeing at MTOW is not permissible in the real world so I wouldn't get too het up about the auto pilot not coping.How does the auto pilot manage if you reduce the aircraft weight to the maximum landing weight?

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Also, if you're going to try to land a "fully loaded and fueled" aircraft, a rarety in an airliner in real life, than you have to add a considerable amount of speed to your Vref. If you're flying an approach at a Vref for a lower weight than you are flying very near a stall, which will cause the nose to pitch down. Try increasing your approach speed.John M

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>> Trying to land a Boeing at MTOW is not permissible in the>real world so I wouldn't get too het up about the auto pilot>not coping.>How does the auto pilot manage if you reduce the aircraft>weight to the maximum landing weight?yes, you need to jettison about 280,000 lbs of gas. i am sure the airplane lands fine at MTOW, but expect to replace all the tires.here's the weights:http://www.boeing.com/commercial/airports/acaps/777rsec2.pdf

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>yes, you need to jettison about 280,000 lbs of gas. i am sure the airplane lands fine at MTOW, but expect to replace all the tires.:-eekWow! I just couldn't let this pass without comment. :-) Do you honestly think that 280,000 lbs of fuel is dumped just to save a few lousy tires?The maximum landing weight is a STRUCTURAL limitation designed to preserve the structural integrity of the aircraft. Exceed the max landing weight at your peril as you can expect to do some very nasty structural damage to the aircraft eg. Shear the wing gear fuse bolts Bend/crease the fuselage distort the airframe symmetry write off an aircraft :-lolCheers,Roger

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In the real world the only reason to make a MTOW landing would be a dire emergency.The squadron which was my first Navy squadron had a EP-3E make an almost MTOW landing at Mombasa (HKMO) back in the 90's.Things went to heck right after rotation on a hot night with a high density altitude.Losing two engines immediately, they correctly guessed contaminated fuel. With almost certainly no efficient rescue capability, they did a 180 even though they never got above 500 ft ASL.They came in heavy and very fast - touched down a little short of the runway and took a bit over 12,000 feet to get stopped since they burned the brakes off the bird. Only the APU was running by the time they stopped.Yes, the runway is only 11,000 feet long. That early touchdown about 150 feet from the paved overrun surface and no rain for several months saved them. The plane came to rest against the airport fence off the north end of the runway.Surprisingly, the bird is still flying for VQ-1, but it was an expensive repair and recertification.

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>>>yes, you need to jettison about 280,000 lbs of gas. i am sure>the airplane lands fine at MTOW, but expect to replace all the>tires.>:-eek>>Wow! I just couldn't let this pass without comment. :-)>> Do you honestly think that 280,000 lbs of fuel is dumped just>to save a few lousy tires?>>The maximum landing weight is a STRUCTURAL limitation designed>to preserve the structural integrity of the aircraft. Exceed>the max landing weight at your peril as you can expect to do>some very nasty structural damage to the aircraft >>eg. Shear the wing gear fuse bolts> Bend/crease the fuselage> distort the airframe symmetry>> write off an aircraft :-lol>>>>Cheers,>>Roger in an emergency who cares about your list? in the CRJ anything over MLW at less than 500fpm is a NON reportable / maintenance event. most modern aircraft these days can structurally handle such items. i think the idea is to safely allow an a/c to land in an emergency, obviously not normal operations.my $0.02 (discounted.... :) )

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Comparing a CRJ to a B-777, for example, is like comparing apples to oranges...or in this case, apples to watermelons when it comes to weights. The way I read it the difference between MTOW and MLW for a CRJ 900 is roughly 6300#, where the B-777 has a difference of 200,000#. Even with a stronger landing gear I would think that you would do considerable more damage to the Boeing when landing at MTOW. Just a matter of physics.Unless you cannot maintain altitude it's best to jetison, or burn off as much fuel as you can to get to MLW, especially in heavy aircraft. Just like trying to takeoff at a weight over MTOW, landing above the MLW will not guarentee a safe result.I am in agreement with you that in a true dire emergency to heck with the numbers and do what you got'ta do.John M

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>Comparing a CRJ to a B-777, for example, is like comparing>apples to oranges...or in this case, apples to watermelons>when it comes to weights. The way I read it the difference>between MTOW and MLW for a CRJ 900 is roughly 6300#, where the>B-777 has a difference of 200,000#. Even with a stronger>landing gear I would think that you would do considerable more>damage to the Boeing when landing at MTOW. Just a matter of>physics.>Unless you cannot maintain altitude it's best to jetison, or>burn off as much fuel as you can to get to MLW, especially in>heavy aircraft. Just like trying to takeoff at a weight over>MTOW, landing above the MLW will not guarentee a safe result.>I am in agreement with you that in a true dire emergency to>heck with the numbers and do what you got'ta do.>>John Mmost landings touch down at very little feet per minute vertical rate. if it can stand the stress of a MTOW takeoff in a crosswind (or by that measure a MTOW abort right at V1), by golly it can stand a smooth MTOW landing. its not comparing a CRJ to a 777, but rather design limits governed by the applicable FAR's.i am sure the testing of the aircraft involves landings above MLW. anybody in the know?

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>>most landings touch down at very little feet per minute>vertical rate. if it can stand the stress of a MTOW takeoff in>a crosswind (or by that measure a MTOW abort right at V1), by>golly it can stand a smooth MTOW landing. its not comparing a>CRJ to a 777, but rather design limits governed by the>applicable FAR's.>>i am sure the testing of the aircraft involves landings above>MLW. anybody in the know?I'm not in the know about testing landings above MLW, but also I wonder if they do actual flight tests to determine MTOW, or is it engineers crunching the numbers of all the parameters involved for a takeoff. I guess in the pre-computer era there must have been some "trial and error" type testing of both MTOW and MLW.For example how did they find out that an aircraft can takeoff if slightly over MTOW, but will not climb once out of ground effect.John M

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>I'm not in the know about testing landings above MLW, but also>I wonder if they do actual flight tests to determine MTOW, or>is it engineers crunching the numbers of all the parameters>involved for a takeoff. I guess in the pre-computer era there>must have been some "trial and error" type testing of both>MTOW and MLW.>>For example how did they find out that an aircraft can takeoff>if slightly over MTOW, but will not climb once out of ground>effect.sometimes, as in the CRJ 700's case, it is as simple as a scope clause. the CRJ 700 MTOW is 75,000#. it is that way due to a scope clause that the original buyer was under.obviously a 777 would not be subject to scope clauses, but i would imagine the numbers are dumbed down a little bit for error and legal protection.i know a few test pilots for hawker beechcraft. i'll see what they have to say about this.

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Today it's mostly math and number crunching. If people didn't truely understand the physics of flight, there would not be simulators worth a danged.Some of the stuff published on the A380 before the first airframe was completed showed most of those parameters and expectations.There was also lot of number crunching even back in the 40's, but there were also a lot of tests - which is why test pilots were, and still are, such heros.Ever see the landing test video of the DC-9 where the back end of the aircraft was broken off?Of course you well know that the numbers are only for certain conditions - add humidity, add temperature, add varying degrees of rain and runway adhesion......

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The FS2004 B744 reference document says at maximum weight you should be landing the B744 at 188 KIAS and you cannot use more than flaps 25.For the B777 - 157 KIAS - flaps 30Are you going slower than those speeds, have more flap?

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yep, you got to the point rfields5421, and it does land correctly when using flaps 25 and higher speed, same is almost the case with 777; but what is the reason behind such behavior? is there 'hunting' phenomenon coming into picture due to more than required sensitivity of autothrottle as well as pitch adjustment by autopilot? because i have noticed this unstable throttling when a heavy is too light with more than required flaps.and another thing comes up: why all these problems completely absent in 737?? lands great at mtow with flaps 40 at 140kias. really curious to knowregards

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The difference in weight of 300,000 lbs and 30,000 lbs in the overweight factor of the B744 vs the B734 is the reason the bigger aircraft has such trouble.It's all weight and trying to fly too slow too heavy.The autopilot instruments and control tolerances are basically the same in the B744 and the B734.The B734 is also having issues - but they are very small compared to the issues of the bigger aircraft. FS does simulate the momentum factor which heavier aircraft would exadurates the movements and hunting.Autothrottle is a mixed blessing. It's useful for FS, but like the real world, often a hinderance on final approach because the changes can be too large, too abrupt.Have you ever heard the phrase "getting behind the aircraft" ?That is when you are reacting to what the aircraft is doing, not anticipating the aircraft and controlling what it will do. That's real good way to crash in FS, and kill yourself in the real world. You have to stay ahead of the plane - nothing it does should be a surprise.An autothrottle is always reacting, not anticipating.i.e. A real pilot, and a good FS pilot, increases the throttle slightly before/as he lowers another notch of flaps to anticipate the increased drag and keep the speed steady.An autothrottle reacts to the increased drag and add extra power to try and catch back up.I find it best to let the autothrottle establish itself on descent as after the aircraft is stabilized low, but turn it off for the final 5-10 miles.Also, the autothrottle is extremely fuel inefficient in FS.If you are trying to practice and learn approaches and landings - get the fuel weight off the aircraft - down to 25% or so.PS - we are talking FS2004 default aircraft. Addons may exhibit different behaviors due to the way their flight dynamics have been customized.

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>Ever see the landing test video of the DC-9 where the back end>of the aircraft was broken off?yes, it was a mistake. the md80 was dialed in at 1500fpm descent when it "crashed" onto the runway was what i had heard. i think it was supposed to be 500fpm (which is still very hard).it was evidently an FAA pilot who screwed the pooch on the md80 tail breaking off.http://yarchive.net/air/airliners/md80_cert_crashes.htmlhttp://www.youtube.com/watch?v=E-flw0WfjRk

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>most landings touch down at very little feet per minute>vertical rate. if it can stand the stress of a MTOW takeoff in>a crosswind (or by that measure a MTOW abort right at V1), by>golly it can stand a smooth MTOW landing. its not comparing a>CRJ to a 777, but rather design limits governed by the>applicable FAR's.Regardless of the vertical rate at touchdown an aircraft the size of the 777 or 744 requires an overweight landing check before further flight if landed above MLW. Even on the 727-200, we have to do an overweight landing check if landed above MLW. It's a two part multiple item inspection. Part 1 looks for obvious visual damage on specific items. If nothing is found then it's ok to fly. If problems are found in Part 1 then Part 2 of the inspection is carried out which is more in depth.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.sstsim.com/images/team/JR.jpgwww.SSTSIM.com

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>Regardless of the vertical rate at touchdown an aircraft the>size of the 777 or 744 requires an overweight landing check>before further flight if landed above MLW. Even on the>727-200, we have to do an overweight landing check if landed>above MLW. It's a two part multiple item inspection. Part 1>looks for obvious visual damage on specific items. If nothing>is found then it's ok to fly. If problems are found in Part 1>then Part 2 of the inspection is carried out which is more in>depth.exactly a simple check. not a write off of the airplane. the airplane CAN land above MLW in an emergency.

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>exactly a simple check. not a write off of the airplane. the>airplane CAN land above MLW in an emergency.Not exactly a simple check. They can take anywhere from 8 to 16 hours and depending on the findings, several days. Aircraft have been written off in over MLW landings that initially appeared normal. Unless the emergency is dire, it's best to dump fuel if the aircraft is equipped or burn off the fuel to get below MLW before landing.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.sstsim.com/images/team/JR.jpgwww.SSTSIM.com

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didnt hear about 'getting behind the aircraft' but surely did come to know this problem myself when flying in fltsim with autopilot and manually. i also observed the throttle hunting when the a380 had its ist touchdn at my local airport; obviously because it wasnt fully loaded. i could easily hear the engine rpm varying continously up and down, i have captured and uploaded the shortfinal-touchdn vid on google.but i still am not able to get why extending flaps aggregates the problem? any ideas??

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but i still am not able to get why extending flaps aggregates the problem?When you extend the flaps it's just like slamming on a huge speed brake.We extend flaps to allow the aircraft to fly slower speeds by having more lift - but it's a lot of extra drag.Because the autothrottle is trying to maintain the same speed but a new brake is deployed - it has to surge - and at the final flap settings - it may well go to full throttle.

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