The grounding of two models of Super Puma affects just over half the fleets of leading North Sea helicopter firms.
But offshore oil and gas production could still continue.
Fewer helicopters in service, however, means long delays for workers waiting to fly offshore or those returning to land.
The three main helicopter operators in the North Sea are Bristow, CHC and Bond.
Between them, they currently have 48 aircraft – 41 of them Super Pumas.
Twenty-five of them are the models subject to the new checks – AS332 L2s and EC225s.
That leaves only 16 Super Pumas AS332 Ls and seven other helicopters still flying.
Regional organiser of the RMT union Jake Molloy said: “Losing 50% of the fleet for whatever period of time would have an impact, though the scale of this would be difficult to assess.”
Mr Molloy also said offshore workers were likely to face delays with flights.
Mr Molloy said it would be possible to move offshore workers by boat, but he felt this was a non-starter as it would leave people open to “incredible” risks in transferring them from a moving vessels to the installations.
Meanwhile, the funerals of two of the victims of the North Sea helicopter crash will be held next week.
Funeral services for James Costello and Warren Mitchell will be held in Aberdeen after the two men were killed alongside 14 others on April 1.
Mr Costello, 24, of Union Grove, Aberdeen, worked for Production Services Network.
Father-of-four Mr Mitchell, 38, of Balcairn Avenue, Oldmeldrum, was an employee of Weatherford.
Mr Costello’s funeral will be held at 1pm on Wednesday at the MacIntosh and Gilchrist funeral home, followed by burial at Hazlehead Cemetery.
The ceremony for Mr Mitchell will be held on Thursday at 11.45am at Aberdeen Crematorium.
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