Hello Detlef,
Why would a mandau have been special?
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Regarding checked-in luggage, the airlines are only concerned with stuff that could be dangerous during the flight, e.g. explosives, poison, etc. You can check the international list of banned substances online and will have seen those posters at airports already. If not banned, you can check it in; actually this is the only economical way to ship long spears that are a pain with regular postal services.
The country of origin may have restrictions on what is allowed to take out of the country. Many countries don't allow export of antiques (sometimes only from their own cultures) but even if so, you can usually apply for exemptions if your piece is not a unique cultural icon (most run-of-the-mill antiques are often easy - just make sure to apply in advance with the respective authorities (if needed) before you go to the airport. In your case, the sword may not be antique?
Of course, you should know which restrictions may apply to the country of destination. Other than that, just make sure you pay any import taxes/fees upon arrival (report to the customs if unsure) and you're fine. (Since there are usually allowances, this is still cheaper than receiving a piece by mail.)
Regards,
Kai