Altimeter setting vs Altimeter error amount?
The Helicopter wasn't landing at DCA, but, most likely would have the airports barometric setting in it's altimeter? If it was off by 0.2 inches, that would be about 200 ft?
I assume the altitude reading that the Altimeter in the aircraft displays in the cockpit is identical to the Transponded signal that ATC shows on it's screen? Is there a chance
that the Helicopter would have a different altimeter setting set? A new ATIS came out recently, or a pressure front was moving in?