by tdammers » Fri Jul 27, 2018 11:05 pm
It's not enough to be inside the cyan circle. Realistically, this is how you'd normally fly an ILS approach (unless the approach plate says otherwise):
1. From the IAF (Initial Approach Fix), maneuver such that you fly towards the ILS beam at an angle of about 90 degrees, at an altitude of about 2000 ft above the airport.
2. As you get closer to the beam (say 2-5 miles), turn onto a 45 degree intercept course.
3. Once you're stabilized on the 45 degree intercept course, and on the correct altitude, push the "APP" button. The aircraft will maintain heading and altitude until you start crossing the ILS beam, and then turn to follow it.
4. A while later, the glideslope indicator will start moving, and the aircraft will pitch down to descend on the glideslope. Keep an eye on your airspeed throughout, and set flaps and lower gear appropriately.
5. Disconnect the autopilot at 500ft AGL, or earlier at your discretion, and land visually.
Now, things can go wrong under some circumstances.
First, if you intercept at more extreme angles, the autopilot will have trouble turning the aircraft onto the localizer beam; especially, if the intercept angle is too sharp, you will overshoot the beam, and the autopilot needs to make an extra turn to go back onto the beam.
Second, if you're flying at the wrong altitude, you might intercept the glideslope too early, or too late, and this can mess things up - if you intercept the glideslope before you're locked on the localizer, then the glideslope needle may move all over the place, and the autopilot will try to follow it. This can easily get you outside the safe flight envelope, and if you're also banked at that moment, you could throw the aircraft into a spin.
Third, if your course intercepts the beam too close to the runway, then the above problems will be made worse: overshooting will require more extreme correction maneuvers, or might not work at all; and the glideslope errors that result from being off the localizer beam get worse the closer you are to the runway. Additionally, intercepting too close to the runway gives you less time to configure the aircraft and stabilize the approach. But even if you don't end up in a spin, just overshooting with insufficient space left to get back on the localizer will produce strange autopilot behavior: you overshoot the localizer beam to the left, so the autopilot banks right to correct; but by the time you're closing in on the localizer again, you've already flown past the runway, and so the autopilot tries to steer you back, in the direction opposite your approach path, and then it tries to intercept the localizer again, but the turn radius is too large, so it overshoots again, and the whole cycle repeats.
Fourth, if you don't watch your airspeed, you can end up flying too slow, and that of course makes you more prone to stall and spin.