Merci de bien vouloir...

Question

Does "Merci de bien vouloir attacher vos ceintures" have the same formal register as "Veuillez attacher vos ceintures"?  Are they both polite and formal ways of making a request?

Accepted Answer

Hi T,
In my opinion -
Merci de bien vouloir attacher vos ceintures is more of a polite order . You will find the expression 'merci de ...'on 'consignes' ( formal instructions) 
Prière de ne pas stationner = Please don't park here
Prière de ne pas envoyer de fleurs = Please don't send flowers
Prière de ne pas déranger = Please do not disturb
Veuillez ... is more informal.
Veuillez trouver ci-joint = Please find enclosed 
Veuillez patienter = Please wait 
etc.
Hope this helps!

Answer

This would benefit from Cécile's input, but here is my understanding in the meantime:
Merci de bien vouloir attacher vos ceintures. -- Thank you for fastening your seat belts.Veuillez attacher vos ceintures. -- Please fasten your seat belts. 
Both are very polite and formal, to me, they belong to the same register, in my opinion.