As ATP-Al among others has mentioned, there are many little things of nastiness in the -400 that have been deliberately installed due to type rating commonality between all the DH8 series. I would not be surprised if the Alt Sel was one of those items - the DH8A was designed in the late 1970s and it might well be that automation at that time required such weird (from todays point of view) solutions. Someone knowing those planes will surely be able to confirm or correct me, but I seem to remember having read about a similar system in early 747 models.
I do not know the Dash 7 either - one might think that DHC took that Alt Sel system over from this type to maintain a bit of fleet commonality. If anyone knows that type, I would be very interested to hear about this.
Operationally, this system seems to be pretty useless. The only situation I can imagine where one would not want to capture a selected altitude upon reaching it would be on approach when e. g. capturing the ILS glide slope from 5000ft with a missed approach altitude of lets say 3000ft required and dialed in on that occasion. Now while tracking the GS, an altitude capture at 3000ft will obviously be most unwanted - but Alt Sel on the -400 is inhibited when in G/S tracking mode...
Another thing just crosses my mind looking at approaches. The Localizer capturing mode is not at all perfect - it will work fine in calm wind, intercept angles of around 20° and airspeeds of not more than 180kIAS. Anything other than that - be it fairly typical speeds of around 200 or 220 knots on intercept, a moderate crosswind or an innovative intercept heading - will result in the autopilot and flight director oscillating about the localizer several times with the amplitude decreasing on each curve. In such a situation, briefly disregarding the flight director and manually setting the aircraft on the track is a proven solution.
In fact, the -400 capture logic appears well suited to -300 speeds while this type suffered from the same problems, albeit at its typically lower speeds.
Another little thing I personally dislike a bit is the aircrafts reliance on several buttons to cycle through indication modes. Be it EGPWS display vs. WX radar, TCAS AUTO vs ABV/BEL (resulting in a popup mode I will discuss later) vs. permanent display or the display of navaids and/or airfields on the ND, it all is selected via such buttons. After powering up the A/C, all those displays will default to a dedicated setting; they will do the same occasionally after generator switching common at startup or shutdown. So it is not possible to keep the displays locked to a certain setup for the duration of the duty like it would be possible, had DH installed 3-way rotary switches for example or push buttons that mechanically lock on or off. Instead, one will need to periodically build up the desired displays, usually every time after the props have unfeathered after startup.
Now to that TCAS. While it basically works like in every other plane I know, it has two little particularities on the -400. Firstly, it does not automatically display traffic in the selected altitude range - on power up, it is in the praiseworthy AUTO mode and will do its work mostly shrouded in secrecy. It only displays RA or TA traffic (which will then pop up on screen together with its dedicated aural alert), on occasion causing some surprise when one has never had the opportunity to view the intruder close in when he still was a blue square. Also, any other mode than AUTO is only displayable at ND ranges of 40NM and below - while I realize that greater ranges tend to clutter other traffic around the A/C symbol a bit, I would still find it nice to at least see that there is something around in order to select an appropriate smaller range when required...
I realize that my personal preference plays some role in what I just wrote - anyway, with some minor changes, the ergonomics of that plane would be agreeable and the workload much reduced.
Last edited by Tu.114; 1st September 2011 at 09:10.