Please inform me as to whether altitude pre-select can be employed when flying a coupled ILS approach. I am aware that alarms and or chimes can be set, but am uncertain if it is possible or practical to set altitude pre-select so that the aircraft automatically levels out at decision height. It is my understanding that altitude pre-select does work well on non precision approaches. Any information would be appreciated.
If you’re doing a coupled ILS approach, the autopilot is not in the right mode for altitude preselect (though the alerter function will still work.) In any case, you would not want to bet your life on it pulling out at DH, as it is not sufficiently precise (and the S/Tec manual or the POH supplement prohibits autopilot use below some altitude or other.) Besides, it does not make sense to level out at DH; either you are going to continue to descend, or you need to go missed right then and there.
The electronics that sit in front of the barometric transducer to compensate for altimeter setting is only settable to .1" Hg, which is ±100 feet, give or take, and in practice I’ve found that it may be off by another 100 feet (bang.)
I’d be very careful using the preselect function on nonprecision appoaches as well, since (unless your MDA is pretty high) you are probably not operating legally, and you really don’t want to descend below MDA. You could preselect an altitude a couple hundred feet above MDA and it would be reasonably safe, but if the ceilings are that high, it’s probably no sweat anyhow.
On the up side, the preselect function is much nicer for this sort of thing than the King KFC200 that you run into in the average Bonanza or Baron, in that, being rate-based, it will automatically shallow out the descent as the target altitude approaches. The KFC200 is attitude-based, so it holds pitch attitude until it notices the target altitude, then tries to level out (but undershoots in the process.)