Abstract: A limited-access computer, such as an on-board flight management computer, stores a first table in a protected area of its memory defining data items in the computer memory which may be accessed, for reading and/or writing purposes, by a user such as an airline operating a ground computer. A second table, in a non-protected area of the memory, is supplied by the user and defines formats of data reading and writing messages to be transmitted by the user and identifying specific data items to be accessed for each message. The items defined by the user in messages in the second table are limited to the set of data items which are defined as accessible data items in the first table. In response to a received user message, the limited-access computer reads the second table to identify data items in the message and on the basis of data item definition information in the first table accesses the identified data item.