The phrase "principal advisor" is correct and usable in written English.
You can use it when referring to someone who has a senior advisory role in a particular organization or project. For example: "Michelle is the principal advisor for the board of directors."
Pascoe Sawyers is a principal advisor for the leadership and localism team at the Local Government Association.
Until his retirement in 2005, he was the principal advisor on mergers and acquisitions to a number of advertising agencies.
"The whole concept of insurance is an alien thing," said Kelvin Hui, a principal advisor at GIZ, based in Dhaka, Bangladesh.
"We're potentially not measuring the equivalent of the emissions from our entire transport sector," said Mark Ogge, principal advisor at the Australia Institute.
"There are a few reasons why people might donate more to things that are beyond human control," says Michael Sanders, principal advisor and head of research at the Behavioural Insights Team.
When on duty, he said he had spent considerable time scouting Murray's future opponents, exchanging ideas with Dani Vallverdu, Murray's close friend who acted as his principal advisor before Lendl was hired.
Judith, along with her mother-in-law Anastasia, became one of her young husband's principal advisors.
In Normandy, St-Calais quickly became one of Duke Robert's principal advisors and his chief administrator.
Her maternal grandmother, Antoinette de Bourbon, was another strong influence on her childhood, and acted as one of her principal advisors.
He took refuge across the English Channel with Henry's brother Robert Curthose, where he became one of the duke's principal advisors.
He was named in Magna Carta as one of John's principal advisors, considered by a medieval chronicler to be one of King John's "evil councillors".
