I went through Test Pilots Training so I could check simulators against the 
known data that was obtained during Certification Flights. 
The airplane was checked on many perameters but even so there are many areas 
that the simulator will not duplicate like the real aircraft in flight.
This is where some Accident Investigators error.  You may have heard them say
after an accident, "lets go to the simulator and see if we can duplicate what
happened"?  It would suprise me if they could ever get an answer from the 
simulator because there was just not enough data programmed into it for all 
contingencies of flight.
If the proper data has been programmed into the Simulator for a particular 
realm of flight,  yes, on this particular item it is reliable or very close.
On the flip side,  don't ask it to give you information on flight 
characterisics that are not available in it's memory,  it will duplicate 
something that was programmed for say a different altitude and it will be 
false information for the altitude you are flying at.  An example of this is 
best rate of climb airspeed.  This data is usually flown on the airplane at 
an altitude of, say 9.0 or 10.0., but let us now think about using it for a 
crossing altitude of 37.0 at our max. wght.  We are only a few miles from the 
intersection we are supposed to be at 37.0 so we go to the BEST RATE OF CLIMB 
AIRSPEED.  Well we're in for a surprise,  we will probably not get the best 
rate of climb at the altitude we are now at and even if we do get to 37.0 at 
this airspeed we will probably never be able to accelerate to get 84.0 
uise.
Now, I am using the 747 as a test bed for the above scenerio.  Other aircraft
may be able to get away with it,  I doubt it though.            
The memory of the Simulator depends entirely on the Certification Document 
that was compiled during the Certification Flights,  this is the data we 
tried to duplicate, if data was not available,  then the simulator does not 
have it.
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