Boeing plane investigated after 'Dutch roll'

US regulators are investigating after an incident in which a Boeing 737 Max 8 flown by Southwest Airlines rocked side to side while in air, in a potentially dangerous movement known as a "Dutch roll".

It is one of two new inquiries involving Boeing planes that the Federal Aviation Administration (FAA) has recently confirmed. The agency is also involved after Boeing reported learning of potentially falsified documents used to certify titanium used in its planes.

The issues have emerged at a time when the safety record of Boeing planes is under intense scrutiny. Boeing referred questions about the Southwest flight to the airline, which said it was cooperating with the investigation.

Boeing said the issue involving titanium was "industry-wide" involving shipments from a limited set of suppliers. It said tests performed so far indicate that the correct alloy was used, despite the false documentation. It said it sourced the metal separately from its supplier and it believed a small number of parts had been affected.