Assuming we're talking Air Transport rather than GA, I would still regard FDR data from the accident aircraft more meaningful than surveys even within 12 hours. To my knowledge there have been a couple of reports of bad Glideslope that did not show any evidence to a later survey aircraft. My suspicion is that it was interference caused by a cell phone tower adjacent to the beam which only was apparent during normal daytime operations but non-existent after hours when the check flight was performed.
Last edited by ICT_SLB; 13th Mar 2012 at 03:27.