In an unusually lucid and utterly unequivocal statement of its current musical instruments policy, Air Canada declares that it will accept violas only as checked baggage – that is, shoved in the hold.
Violins ‘may be accepted (either) as carry-on or checked baggage’ depending on various circumstaces, such as the capacity of the plane or the mood of the clerk at the check-in desk. Equally, there is no guarantee that you can buy an extra seat for an instrument.
Read the bloody rules here and weep. Be warned. Keep for reference.
In general, other airlines that accept violins will also admit violas.