United Airlines is in the process of temporarily grounding its Boeing 757 fleet as the carrier seeks to perform checks on modifications made to air-data computers.

A total of 96 planes are involved – almost a quarter of the total United fleet.

Thousands of passengers could by hit by delays as a result, with United seeking to comply with certain Federal Aviation Administration mandatory inspections over the next 48-hours.

The checks - which take 50 to 90 minutes to complete - are being performed on airplanes already on the ground and will continue when planes in the air land at their destinations.

Checks have presently been completed on approximately half of the aircraft involved.

However, nearly a dozen aircraft remain in locations where the checks could take longer because the required inspection aids are not available.

The computers measure air speed, air pressure and other parameters in flight, feeding them to the auto-pilot.

United uses the 172-seat 757s on domestic routes.

Its fleet is about 18 years old on average.

The 62 757s operated by Continental Airlines – which merged with United last year to create Untied Continental Holdings - are not covered by the checks.