CFII_ASC wrote:

The effect of the condition levers is to either 1) shut off fuel to
the
engine (cut-off), or to set the idle governed speed for ground
operations (typically 25% N1), or to set the idle speed for flight
operatons (typically 50% N1). They have these two settings because
the
turbines have so much power at flight idle that on the ground you
wouldbe constantly riding the brakes to maintain a safe taxi speed,
and
scattering miscellaneous small aircraft in your prop blast as you
maneuvered on the airport. The machine is much easier to manage on
the
ground with and idle of 25% N1. However, if we used ground idle in
flight, at idle power the drag on those two huge props would be
excessive, and the airplane would develop a dangerously high sink
rate
if the pilot ever pulled back to idle, say on a slightly high
approach
on final. Compounding this problem is the long spool-up time the
engine would require if the pilot suddenly decided he wanted power
again after letting the engine spin down to 25% N1 (Remember the
Paris
Airshow crash of the Airbus?) So, in flight we keep the condition
levers in the Flight Idle stop, so that if we ever pull the power
levers back to idle on final we won't make one of those big, smoking
holes in the ground just short of the runway.

The idle open descent mode on the earliest generation of A320s did
actually set the idle speed to about 25% N1. It was also slightly
misleadingly called a 'power descent' when it was being caused by the
lack of power. Pilots are instructed to use this only during descents
up to the traffic pattern altitudes from TOD and not beyond it. Those
that remember minutiae of the Feb 90 Bangalore crash will recall that
the pilot used this on final, in violation of the recommended
procedure. As the Pratt & Whitneys were revving, the craft had already
created a huge dent on the mound of the 8th hole of the golf course
behind my house. It garnered enough legs to lift briefly beyond the
road adjacent to the golf course before flopping short of the runway.

Actually Gene, the length of the spool-up time wasn't the cause of the
Paris crash, but more so in case of the Bangalore one. In Paris, the
A320 was flying at 30 ft with the landing gear extended, while Cap'n
Michel Asseline (now in jail, incidentally) I remember asserting he'd
seen the altimeter show 100 ft! The computer interpreted this to be a
landing configuration and didn't deliver power to the engines. Asseline
insisted that his manually moving the throttle didn't elicit any
response, so it wasn't the CFM engine not responding quickly enough but
the computer having taken over control.

In any case, a gas turbine delivering 20 meg watts of power can be
expected to take about 10 secs or so to deliver thrust. You folks would
know all that better

Cheers,

Ramapriya