Mapping the aircraft file
The following table shows the fields in each LREC in the aircraft file and the number of bytes for each field.
| Field | No. of bytes |
|---|---|
| size | 2 |
| key | 1 |
| aircraft type | 4 |
| seat range | 8 |
| seat class | 1 |
| Total | 16 |
Each LREC in the aircraft file therefore contains 16 bytes.
Because the database must be able to accommodate 50 aircraft types and three classes, you can calculate the required amounts of data as follows:
no. of aircraft types x no. of classes x LREC size = amount of data
The calculation is:
50 x 3 x 16 = 2400 bytes
This comparatively small number of bytes fits comfortably into a fixed block of size L4 (4095 bytes, including header and trailer). Because only one block is required, you can use a miscellaneous file.
Distributing the aircraft LRECs
The aircraft file contains only one block of data. Because of this, all the data can be stored in a single subfile, and you do not need to distribute the aircraft LRECs across multiple subfiles. Since the file uses a single subfile, use the single‑subfile algorithm (#TPFDB04) to distribute the LRECs.
Allowing for expansion
Although the aircraft file is small now, it may need to expand in the future. Therefore, consider how you will handle a potential data overflow.
Because the aircraft LRECs are only 16 bytes long, the current L4 block (4095 bytes) already provides space for expansion. Overflow blocks with an L2 size of 1055 bytes should be sufficient for future growth.
File structure
When deciding the structure of the LRECs, consider how the TPFDF product will interrogate the file. The aircraft file is not an index file, so the LRECs do not need to contain pointers. The TPFDF product needs only the aircraft type, so the aircraft LREC structure can be as follows:
