One thing always leads to another... and another..

Per the discussion in another thread.. I'm going to continue with an instrument turning thread... by no means a real-world reference, but certianly in-depth enough for simming.

I'll use turning outbound during a VOR approach (runway 16R @ KPAE), per this thread : http://www.simviation.com/cgi-bin/yabb2/YaBB.pl?num=1246322113


Assuming zero wind, there are a few things to consider, when making a turn  AT  a VOR. First, your altitude. For the sake of discussion, we'll say we're at 4000agl by the time we near the VOR. If you're using a DME then keep in mind that even directly OVER the VOR, you're still ~0.8nm "from" it, in altitude alone. Next, consider your ground-speed. At 110knots, that's aproximately 2nm per minute.

OK, back to basics. ALL turns made while flying IFR (especially IMC), are made at  "Sandard Rate". This  not only keeps you from getting disoriented, it keeps you where ATC expects you to be.. and fits nicely into calculated manuevers, like we're talking about here.

Turn coordinators have markings to help a pilot maitain standard-rate turns.




With these two, common turn coordinators.. the "banking airplane" and the vertical "needle" (older version), tell the pilot how quickly he's turning. It's important to note (especially with the modern, banking airplane type), that this is  NOT  bank information like you'd get from attitude indicator. It's strictly TURNING information. You can confirm this by observing the indicator while taxiing. Obvioulsy, while in flight; turning and banking go hand-in-hand (unless you're intentionally slipping).. and in an emergency like vaccuum loss, which leaves your attitude and heading indicators useless.. a turn coordinator  can become a secondary banking instrument,, but not much more than a tool for keeping you from banking when you do not want to. That aspect to this subject gets into advanced instrument flying, beyond this thread's scope.

OK.. If you maintain a standard rate turn, you will complete 360 degrees in 2 minutes (see where it says as much under the ball ?). For this example we'll be flying at the VOR on a course that will require a 90 degree turn outbound. Now we use simple math..  We know that a 90 degree turn will take 30 seconds.. and we know our ground speed is 2 miles per minute.. a few mental gymnastics later, we determine that a turn started aproximately 0.5 miles from the VOR should work out nicely  ..  BUT REMEMBER.. even directly OVER the VOR we're more than 0.5 miles from it. A little nudge from Pythagoras, and we start the turn at  0.8 - 0.9nm out.

Now of course no pilot calculates all his instrument turns like this. It's kinda like the student pilot and his first cross-country flight. He is required to calculate an exact, magnetic heading; even though he cannot hold a heading +/- a few degrees,, let alone count on the winds aloft to be accurately forecast down to an exact degree. Going at it like this is a training thing. It reinforces the concepts. The competent instrument pilot would also have gotten a good feel for the winds, as he was tracking the VOR inbound,, that TOO is a factor when deciding to start a turn. And even our simple model has a twist. Starting a turn based on ground speed and distance FROM the VOR has to consider that the turn itself changes closure time. Our carefully calculate turn (in zero wind) would actually end up being a tad early.

Another quick note on turn coordinators. Let's say you did lose your heading indicator. You can use "timed" turns when changing headings. Say you're flying along on a 120 heading.. and you need to turn to 170. that's a 50 degree change. 50 degrees is just a tad more than 1/8 of 360   (45 degrees is 1/8 of 360)..... 1/8 of 2 minutes = 15 seconds... SO, a 16-17 second turn will put you pretty darn close.

If you don't understand why you cannot just use the compass for that turn... I'll see you in my compass-turns thread.  

For a primer.. get that C172 up in the air.. ignore (or fail) the heading indicator.. fly along at 120 and try to make a turn to 170 by compass alone