L-39 hsi
The Lotussim website seems to be down again.
Mike, I chose caffeine over sleep last night and it showed in my posts.
There is something off in the HSI behavior, but I made some wrong assertions that magnified the confusion.
I am not on a "back course approach" in my screenshot, but an extended downwind.
The HSI, we both know, does not know the runway is behind you. It doesn't have to. The dumb Garmin head in this position will deviate left. It does not have a course arrow, and that is the difference. Proper pre-landing procedure is to align the course arrow with the localiser heading. So in this case, in this screenshot taken on a left downwind the runway is off the plane's left wing. The needle on the HSI will go right. But because the course arrow is upside down on the downwind leg, it will *seem* to point left. So if the pilot turns toward the needle, he will head toward runway center-line.
As the pilot turns the aircraft, the the Gyrocompass spin and the course arrow spin are linked **mechanically** (no computers involved) in such a way that you can always tell what heading off your nose is the course you have set. Finally when you turn to runway heading the course arrow will be aligned with your heading, and right side up. The HSI needle will still deviate the same way relative to the localizer. Let's say the aircraft is left of runway centerline. The plane is heading the opposite direction. The runway is off the aircraft's right wing. The HSI will detect that the runway is to the right. And it will deviate right... **Just like it did when the plane was headied away from the runway and the runway was on its opposite wing!!!*** So in either case, the pilot can turn toward the needle.
The same is true on an actual back course aproach, which I did not show a pic of. In this case you would be landing on runway 31, using the localiser beam for 13. In this case, though you are landing on 31 since it has no localizer you can tune the localizer for 13. You also set the course arrow to runway 13's heading. As you come straight in to runway 31, the needle will be right side up. And so it will magically indicate the correct way to turn to intercept runway center-line.
In this case, a back course approach; using a dumb Garmin you will have to turn the opposite way that the needle deviates. The HSI is **not** really any smarter, but you can flip the course arrow upside down so that when it points the "wrong" way it seems to point the right way...
I'll be glad to demonstrate.
On Gamespy now.
Donald
Donald