Are negative values appropriate in the "new orders" data from the U.S. Census?
Yes, this dataset can legitimately contain negative values, which represent net cancellations. The Census Bureau writes:
The primary reason is due to aircraft cancellations. When cancellations occur they can be large enough to create a negative amount of new orders for the month. This goes for any industry; however it is more noticeable in aircraft.
For example, values are negative for XNONAP.US (New Orders - Nondefense Aircraft and Parts) for February and April 2009.
Manufacturers' Shipments, Inventories, and Orders (M3)