The first hint to crack the puzzle "Must be fastened when the sign is lit on a plane" is:

It is a word which contains 9 letters

The second hint to crack the puzzle "Must be fastened when the sign is lit on a plane" is:

It starts with letter s

The third hint to crack the puzzle "Must be fastened when the sign is lit on a plane" is:

It ends with letter s

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The answer for the puzzle "Must be fastened when the sign is lit on a plane" is: