The altitude in the datasource may have been sampled with respect to Cesium World Terrain. If you add the following line to the viewer constructor…
terrainProvider: Cesium.createWorldTerrain()
…the Cesium aircraft’s altitude looks around 0.
It looks like you might also be running into this issue: Depth plane causing clipping artifact · Issue #7879 · CesiumGS/cesium · GitHub.