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IS YOUR FLIGHT DIRECTOR A DUD?

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IS YOUR FLIGHT DIRECTOR A DUD? by Peter EdwardsThis is a rehash of a post I made here at Avsim some 18 months ago. Forgive me for revisiting this subject, but unfortunately nothing much has changed since then.My credentials are - retired airline pilot, having spent the last five years of my career as a captain on the B747-200 with a leading international airline, and having been a user of MS Flight Simulator in its various incarnations for some twelve years.First let's look at what a flight director's (FD) function is - it's to provide steering commands to the pilot to fly the aircraft manually just as the autopilot (AP) would in the same situation, and to this end it uses the same switching inputs as the AP. Its secondary role is to provide a monitoring function over the AP, in that if FD and AP are well integrated, the command bars stay centred throughout except in response to a new input until the aircraft catches up. An FD display is usually either a pair of "command bars" or "V-bars", the principle and display being the same whatever the aircraft type. The term "command bars" refers to the FD pitch and roll bars, not to ILS needles and such, which are situation indicators. If you use the AP all the time, you may not notice anything wrong with a bad FD as the indications can be subtle. This piece is mainly directed at people who enjoy handflying in IFR, in which case a good FD is a sine qua non.Many, if not most, of our simulated FDs are nothing of the sort. Gauge developers often seem to have little or no idea how they should function or what to aim for. Most seem to end up as situation indicators, which is pointless as we already have the raw data displays. If you treat this type as a steering aid you'll end up in real trouble. It's better to ignore them or, best of all, switchthem off. To put it another way, a bad FD tells you where an altitude/localiser/glideslope is, but a good one guides you there.How can you tell if you have a properly functioning FD? Fortunately MS, bless their hearts, got things right years ago with the FD on the default B737. It works correctly and well, so you can use it as a yardstick as well as see how a good FD should operate and what to look for in your add-ons.You need to set up a couple of situations, and you mustn't use the AP.First, fly along in FD altitude hold, manually, using your add-on panel/aircraft. Then deliberately offset yourself by, say, 1000ft from the FD held altitude. The pitch bar indicates a command to return to that altitude. But the dud bar stays deviated until you reach it regardless of the pitch you feed in. If you follow the bar, given time and space you'd probably reach the vertical. Now repeat the exercise on the default B737. The pitch bar deviates, just as before. But this time when you apply what the FD regards as an adequate control input the pitch bar will return to centre although you're still way off the preset altitude. Now all you do is keep the bar centred and it will direct you to that altitude at a reasonable rate of climb or descent and then smoothly command opposite control input as you approach it, so you can level off.The other exercise is on a manually flown ILS with the FD in approach mode. Start a fair way out to give yourself plenty of time and space - you needn't be on the glideslope. From a position of holding the centreline, offset yourself from the localiser (not the roll bar) further than full deflection of the ILS pointer or HSI display. Stay in approach mode. Your dud FD will command a turn to recover the localiser, but it won't control your roll angle which once again could exceed the vertical with the roll bar showing complete indifference until you actually reach the localiser (and zoom through it). Try the same thing in the default B737. This time, when you roll into the correcting turn, when you reach a normal bank angle (about 25 deg) the FD roll bar will come back to centre (or past, if you've over-rolled). If you keep it centred, your bank angle will be held at 25 deg until you reach a reasonable intercept angle, when the roll bar will move the opposite way, commanding you to roll out of the turn and continue straight on until approaching the localiser centreline. At this point it will direct you to turn onto the localiser and smoothly put you on the centreline.You can do the same exercise in relation to the glideslope or, if you're feeling clever, both at once. Once set up, this way you can check out your FD in five minutes or less. In my experience, though, if the altitude capture mode doesn't work properly, the rest won't either. Bear in mind that with an FD, the fact that the needles are centred or V-bars aligned doesn't necessarily mean that you're on any sort of navaid centreline (though you can be) - it just indicates that you're doing the right thing to get there.When an FD is functioning correctly, all you have to do is keep the bars centred (or the V-bars aligned with the aircraft symbol) with occasional glances at the ILS pointers to make sure things are OK. The FD makes IFR handflying far easier, but it must work properly and many panel developers aren't producing the goods. To return to what I said at the start - the FD should provide steering commands only. It is not a situation display - for that you must look at the little pointers at the edge of the ADI. At the moment many developers seem to think an FD only needs to be a repeat of the ILS pointers in a different format - there's far more to it than that. MS got it right, some five years ago, and it's high time everyone else did.For those of you who've stuck with me so far, remember - these days you're paying good money for many of these add-ons, but you can't take it for granted they'll work properly because of that - some freeware examples are fine, some payware ones amateurish, to put it mildly. Things won't change until you start complaining. So - save or copy this page, tell your friends, spread the word, and check out the FD that comes with your shiny new Cheyenne or whatever. If you think it doesn't function correctly, ask for it to be put right.A final thought - despite its importance, I've never seen this failing picked up in a website review, so perhaps reviewers should do some homework and a more searching job in this regard

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Thanks for that posting.I never truly understood the proper role of a FD till now.

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I hope this thread will be read by everyone and especially DEVELOPERSAndy

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