Problems with AI aircraft and ground elevations

I am in the final stages of a version 1 detailed design of Vancouver International Airport2003 to replace my previous version. It is at my beta testers at the moment and I hope to upload it in the next week or so. The previous version added various features to the default scenery using ASD2.1. This version is completely new using Airport Build160, which I find a far superior design program. Version1 CYVR2003 covers the airfield and north terminal buildings plus the two large Air Canada hangers. Buildings and airfield are all essentially correct in location and dimension, I got maps, photos and drawings from the CYVR engineering staff a couple of years ago. The south terminal buildings, ancillary buildings and details such as apron/taxiway lines and taxiway signs will have to wait until a later version.However I have a problem!When I start a flight in Vancouver all the AI aircraft are sitting at their gates on the ground. But when I fly into Vancouver from elsewhere, the ground comes up to the wings of all the AI aircraft on the aprons and at the gates, although they are okay on the taxi-ways and runways. If I refresh my scenery library they come back up to ground level. I notice my ground elevation is about 31 feet whereas the default and my flatten command within the bgl file are at 21 feet.I ran into a similar problem (but in reverse) at Zurich Kloten Airport by Richard Mohler, where starting off at Zurich was okay, but flying into Zurich all the AI aircraft were about 5 feet above the ground! Any ideas how I can correct this?NigelVancouver