Hagar,
I think what Zaph is pointing to is the use of a leading zero, replacing [fltsim.xx] with [fltsim.03] instead of [fltsim.3]. It does indead cause the section to go missing, the same as a wrong/out of sequence #.
Example, use [fltsim.00] and they all go missing.
Dave
Yes, I do see that. Thanks for pointing it out.
Boikat. From a quick perusal your edited Aircraft.cfg looks exactly like mine. The only possible explanation I can think of is that FS9 is not using that file. You mentioned taking a backup copy before editing it.
The only thing I can see that is different is that I kept the original aircraft.cfg (renamed aircraftorig) and I am on a Vista machine,
I have no knowledge of Vista. That might well be the cause of the problem. To be certain it's not causing a conflict*, move the original to a different location. (Better still, delete it. You can always replace it from the zipfile if necessary.) Also make sure the new one is correctly located & named Aircraft.cfg.
Note that the cargo variation is listed separately under Aircraft model.

*PS. Also check your FS9\Aircraft folder carefully to make sure you haven't got a duplicate install of the Howard 500.