A number of police and air ambulance helicopters have been grounded while maintenance checks are carried out.
It follows an incident involving the London Air Ambulance at Denham Airfield, in Buckinghamshire.
An internal fan system fault, which can cause a drop in height, is understood to be the cause of the scare.
Police forces in Dorset, Humberside, West Yorkshire, Sussex and Wiltshire have grounded helicopters until checks are completed.
Greater Manchester and South Yorkshire Police forces also use McDonnell Douglas (MD) 902 series helicopters.
But both said they carried out their own maintenance work and their helicopters were still operating.
GROUNDED HELICOPTERS
The 902 series does not have a tail rotor and instead uses a fan exhaust system.
It is believed that similar earlier faults with fan belts in the internal system have affected other 902 helicopters involving the police in Germany, France and the United States.
The Gloucestershire-based Specialist Aviation Services, which leases the 902s, said the fault could affect up to 30 craft throughout the UK.
Sales and marketing director Nigel Lemon said many of the helicopters could be back in service later on Monday if checks could be done quickly enough.
McDonnell Douglas has ordered the groundings until maintenance checks are carried out over the coming days.
London's Air Ambulance Helicopter Emergency Medical Service (Hems) is still out of service.
Chief executive Andrew Cameron said the craft had been on a mission when a fault forced it to land.
"The skill of the pilots brought it home to Denham safely," he said.
Air ambulance helicopters in Yorkshire, Lincolnshire and Nottinghamshire and at Durham Tees Valley Airport have also been grounded.
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