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Johan_Dees

Autopilot problems

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Hi,Playing around with the autopilot I ran in problems..Please explain this very mystery to me.When flying level, at say Fl100, and I want to climb to FL140.Speed engaged, a/t active, ALT hold on.Flying 300 knots.So, I dial in 14000 in the al window, set the alt hold to alt sel and switch the climb mode to IAS.Then nothing happens.A/T disengages, but the 747 keeps level.Same for descend. Sometimes when fiddling with the ALT hold and ALT sel suddenly a steep dive with 6000 ft p/m up or down starts without controll.Seriously lost here! JohanA LITTLE LESS CONVERSATION, AND A LITTLE MORE ACTION PLEASE..http://www.jdserver.mine.nu/johan

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Most likely a couple of things going on here. First, you have to make certain that your actual speed ( as displayed on the HSI bug) is centered when you engage IAS mode: this is critical, if you are too fast or slow, you will get the porposing that you are describing. It isn't the AT setting that is critical, it's how fast the A/C is going in relation to the HSI bug that matters.Also, make certain that you are not overweight for climbing at the speed and rate that you are trying to use. There are step climb tables available for the 747-200 which can give you the needed information.RFP uses the standard FS9 autopilot. By and large, it adjusts rate of climb and speed by changing the elevator trim of the A/C. Since this is a large A/C with a significan moment of inertia, the autopilot frequently can't keep up with changes and tends to fall behind the curve. It is possible, in fact, for the trim to be set so far out of range by the autopilot, that the angle of attack is impossible to maintain and the aircraft will stall. I have had this happen, so you have to keep an eye on what is going on and be prepared to reduce the demands if necessary. This is not a "set it and forget it" sort of aircraft.IAS mode is tricky and not very forgiving, you have to have it set up correctly and then watch it closely. Many of us will use VS at lower altitudes and speeds until we have burned off enough weight to make the aircraft less sluggish.Hope that's helpful.Ed GreenKCLT


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