Dear Crossunder, when your crusty old captain insists on a 'clean aircraft' don't be so dismissive. Certainly the modern rules allow 'X' ammount of 'thin rime ice' on certain parts of the airframe, but ask yourself this....if there's an allowable ammount of 'rime' on the top of the fuselage (a non aerodynamic surface....whats the problem) then are you sure the top of the wing (the bit you can't easily see on a walkaround) is not similarily contaminated?
Well...so, what if it is?
Let me tell you a story.
Many moons ago I was a young F/O converting onto a certain turboprop. We had 3 students, a training captain, and a competent F/O on board. We did two sessions of demo general handling with my fellow students that clear winters day. Finally we landed to take a break before my session. The others were all let off the aircraft before we got airborne again.
On climb out to our safe manoeuvering alt we passed through a thin layer of stratos...the first we'd seen all day. We thought nothing of it.
We set the bugs for the approach to stall. This was the third time the exercise had been done so I was familiar wiyth the routine through observation.
We were meant to get the stall warning at a good margin above actual full stall (+10 kts?) and then recover.
At Vs + 25 the aircraft started dropping a wing. I picked it up. Then the other went. The training captain told me to quit messing with the ailerons. Then we went into a full stall! No warning. No buzzer.
We fell into the cloud layer again during the recovery. Lucky it was only a hundred feet thick.
On exiting the cloud we inspected the wing leading edge from our seats. We could see a barely visible coating of rime on the LE. It was only visible because the black boots on the LE showed its presence. They would not remove it when exercised because it was too thin.
But it caused a 15kt increase in stall speed.
I always demand a CLEAN AIRCRAFT and to hell with the books or the expense.
Safety first.