What is the advantage of not having automatic ALT SEL?
 

Once upon a time, when I was roaming the skies in the mighty ATR, I had alt preselect mode on my AFCP that automatically aremed alt hold whenever alt selector was moved from current altitude. Then I moved onto A320 and automatic alt arming was there too.

Now I'm on DHC-8 Q400 and whenever I leave altitude for some other, I have to press ALT SEL button to confirm that I really want to go to the selected altitude and no further. Being particularly unimaginative, I've struggled to come to reason behind this particular peculiarity but to no avail.

So if some good soul reveals what's the deal with manual ALT SEL, I'd be grateful.

Ah, the many mysteries of the DH8-400... This may not be the real cause, but I find there is a situation where You do not want to capture a preselected altitude. On an ILS where You capture the glide at eg. 4000ft but the G/A only takes a climb to 3000ft, a leveloff at 3000ft from the glide would be most undesired. SOP dependent, such a situation might or might not occur during non precision approaches as well. This is the only situation I can imagine that takes an alt selection without hitting ALT SEL. Now, keeping in mind the a little less sophisticated -300 that the 400 is burdened with being on a common rating, I would imagine that the reason is somewhere there. Why let the computers bother with deciding whether it is an approach or not when the pilot can just as well tell the box by pushing or not pushing the ALT SEL... This thought may be completely off though.

not contributing to the answer, but the problem is the same in the MD80's...You have to press the alt button to arm alt sel. But I think it's a software option, you can autoarm this. Lead to a near-accident to an AirLib MD83 approaching Paris during winter heavy snow weather by the end of the 90's. Aircraft go around and FDR revelead they were only 200 feet from to ground.

On our MD80'S we always had to Pull on the Altitude Selector to arm Altitude capture.


It was not a good design and easy to disarm accidentally.

before 411A gets here, on the L1011 you had to set and arm the alt. The reason was it was old and rubbish!!

Edit, just remembered that the "digital" APFDS system on later AC armed automatically.

On one of our company's King Airs you had to manually arm the altitude. In my eyes; compete bull****. The other aircraft automatically arms altitiude hold mode

And with regards to capturing a glideslope above a preset altitude; GS capture inhibits ALT ARM, so this is not a problem. (However the altitude alerter systems continues to function and would give you a prelevel warning 1000' above and when going 300 feet below).

Bfisk - you are right, it ought to be expected from a computer system to lockout the alt capture mode if on a glideslope. After all, it should not be more than one line in the program code. But the old(ish) DH8-300 is unable to do this on its own and relies on manual selection of that "ALT SEL" button, so as the -400 had to be certified to be flown on a common rating, it was kept that way.