Captain,
They're the same circuits we fly anywhere else. 1500 foot circuits 'cos that's what the rules say, 30 seconds past the threshold, turn base aiming for about a 800-1000 foot gate as you turn final depending on where you're at on the profile. The final gate is that you have to be stable by 500 feet.
You're right, the Dash aint a 747, but we have these things called an FDR and EGPWS. If we make our circuits too small there's too much risk we end up getting squawked at by the EGPWS (sink rate, sink rate!), or someone gives us a call a few weeks later to say we busted stable approach parameters and for that we will be punished. It's actually really easy to bust these parameters, particularly in the Q400 because it goes a bit quicker in the circuit than the 200/300. Things like vertical speed no more than -1000 fpm, airspeed Vref to Vref plus 20 and so on. If you think about it, we can be doing sometimes up to 160 knots groundspeed on base (with a bit of tailwind), so for a 3 degree path that's about 800 fpm descent. If you're high you're going to bust the 1000 fpm restriction pretty easily so it makes sense to give yourself some room to re-intercept the vertical path gently. The other thing is that because you are going reasonably quickly around the circuit you don't have a lot of time to make lateral adjustments as required. You need to be on the money every time. Also, power: if you are off the vertical path a power adjustment might be required. With around 5000 hp per engine (on the Q400) it doesn't take much to see your airspeed go racing away or a big balloon going on, particularly with flaps fully extended.
And then, if you do have to go around for whatever reason, the performance of the aircraft means you have arms legs flying around all over the place getting re-configured while trying to stay ahead of the aircraft. The performance of the Q400 in particular is pretty awesome, and we can be back into the circuit in no time flat. But at night time, in average weather the challenge levels increase significantly. To cut a long story short, we don't want to end up in a situation where you have to go around. Don't get me wrong, a missed approach is not really a big deal but there is more risk in having to do a missed approach than there is in just getting the approach nailed the first time.
So as it is, I would suggest it's a reasonable challenge for the average pilot to land one of these things without busting the parameters. Indeed I remember my first attempt when I was training; it was a complete balls up!
Hence the large circuits.
I'm with you though; if we could I would much rather throw it around like a smaller aircraft, particularly now that I am pretty comfortable with the aircraft, but it's just not practical. It just is not a light aircraft anymore. The control responsiveness is not the same and you are dealing with much more inertia. Furthermore we have to do things in a way that is consistent and allows pilots of all experience levels and abilities a reasonable chance of achieving a safe approach. For newbies on the aircraft the circuit can still be quite a challenge.
If I haven't explained it well here I apologise. It's something I do think about often though, as I am sure people do wonder what is going on, particularly when there are other light aircraft who are trying to do circuit training. I sometimes even get the sense that our colleagues in smaller turbo-props don't quite get it. That's ok, I'll always try and fit in with what's going on, even if it does mean I have to do an orbit or whatever. I know when I used to fly smaller aircraft I never could understand what the big deal was.
Occasionally I hear stories of people who give us attitude because of their perception that we as big boys tend to push our way in. Speaking for myself, and indeed I think I can speak on behalf of my other colleagues because I have flown with nearly all of them (in NSW/VIC anyway), we do not carry some sense of being the big boys in town and everyone must get out of our way. Quite the opposite. When we are coming into a circuit and are presented with other traffic our first thought is always how we are going to work with this traffic to find a solution that works for everyone. But we have lots of other stuff to be thinking about too, so if a plan goes awry and things don't work out the way we expect and we put someone's nose out of joint, then please accept our apologies. It was not the intention to p$#ss you off.
By way of a story that demonstrates all this, I'll take the opportunity to apologise to some lighties that were doing circuits on RWY 05 at Wagga last week. We were coming in from Sydney and had the same overhead ETA as a SAAB coming in from the south. Easy. Let the SAAB go first as it would go straight in while we were doing the circuit. At about 4 miles to the northeast we were turning to join downwind at 1500', with one lighty on base, another touching down, and another on upwind about to turn crosswind (BTW the lighties were gracious in indicating that they would give way to both of us t-props while they would maneuver to stay clear). It was looking like things were going to work out fairly well. Lighty on base could land before SAAB, SAAB number 2, then us and the remaining 2 lighties. It didn't work out that way: We ended up having to extend our downwind to the limit of the circling area, while one of the lighties did (I think) 2 orbits while waiting for us to land. In retrospect it could have been done better (the orbiting lighty could have gone before us), but the R/T time and possibility of confusion made it not worth pursuing at that late stage in the game. We had too many other things to focus on.
Enough from me. I hope that explains it..