They would been drafted in to operate baggage X-ray scanners at Glasgow and Aberdeen airports during industrial action on 7 and 10 June.
The security system electronically generates images of outlawed items – including knives, guns and suspicious electronics – and projects them on to randomly-chosen items of hand luggage going through the scanners, to ensure staff are vigilant.
In two days of industrial action earlier this month, however, around 190 were missed in Glasgow and 70 in Aberdeen. The figures represent around ten times the failure rate of full-time scanner staff.
One stand-in operator alone missed more than 20 of the decoy items. Failing to notice three threats normally results in disciplinary action.
Aviation security analyst Tim Ripley said: “There is a well-known saying that terrorists only have to be lucky once. Security personnel have to be focused at all times.”
Meanwhile more walkouts are planned in Scotland tomorrow and Wednesday.
Managers have drafted in contingency workers from other airports to fill in for strikers but replacement workers at both airports have failed to spot large numbers of suspicious items on baggage scanners.
Unions have questioned the competence of stand-in staff and claim more is being spent bringing in replacements than it would cost to end the dispute.
Similar action is taking place at Southampton airport as part of a dispute over final salary pensions between unions and AGS Airports which operates the three transport hubs. To ensure vigilance, the airport’s security system beams images of prohibited items into randomly chosen bags as they are scanned.
It is understood around 100 of the threat images – which can include bomb components, guns or knives – were missed during the first eight-hour strike shift on Friday 7 June. The normal number for that period is around ten.
A Glasgow Airport insider said: “It’s deeply concerning –especially as more strike days are likely.”