Among the many repercussions of the shocking revelation that Germanwings flight 9525 was deliberately crashed by co-pilot Andreas Lubitz while alone at the controls, the most immediate has been a move to require that there be two crew members in a cockpit at all times.

US regulations have long required this two-crew protocol. After the 24 March tragedy that claimed 150 lives in the French Alps, European low-cost carrier Norwegian moved quickly to change its procedures. "This means that if one of the pilots leaves the cockpit, one crew member must replace him/her during this time," Norwegian says. "Our passengers’ and crew’s safety always comes first, which is why we have decided to change our procedures, in line with US regulations."

Canadian transport authorities quickly followed the Lubitz revelations with an interim rule to ensure that two crew members are at the flight deck at all times, “to ensure the security of Canadian passengers". Early reports indicate that Air Canada and WestJet have implemented the new rule.

EASA followed suit on 27 March, with a temporary recommendation that airlines ensure at least two crew - including at least one qualified pilot - occupy the flight crew compartment at all times. Airlines, says EASA, "should re-assess the safety and security risks associated with a flight crew leaving the cockpit due to operational or physiological needs".

In Europe, national aviation safety authorities have no power to make a binding rule, which would have to come from EASA. As the UK’s CAA observes, though, individual airlines are free to modify their own operating procedures. Norwegian's adoption of a two-crew rule was followed by Germanwings parent, Lufthansa group.

The US requirement to have two crew members up front at all times was just one response to 9/11. After the attacks on New York and the Pentagon, regulations requiring that cockpit doors be sealed – and reinforced against breakage by weapons or battering devices like trolleys or fire extinguishers – were devised to prevent terrorists from taking control.

Those protocols, well-illustrated by this 2002 Airbus video, provide for an override to permit a crew member outside the cockpit to gain access should both pilots be incapacitated. However, the post-9/11 protocols also – understandably – permit the occupants of the cockpit to actively block that override. Alone in the cockpit of Germanwings 9525, Lubitz apparently employed that safeguard to keep the pilot out and ensure the success of his murderous scheme.

But could keeping both seats full at all times prevent another pilot from deliberately crashing an aircraft? The presence of a second crew member may provide some psychological deterrent to a pilot who would have acted more freely if alone, but having two crew members in the cockpit is no guarantee of safety. The NTSB’s investigation of the 1999 Egyptair 990 crash, in which all 217 people aboard died, concluded that the relief first officer intentionally flew the Boeing 767 into the Atlantic ocean – apparently "winning" a battle for control of the aircraft with the captain, who re-entered the cockpit after the descent had been initiated and whose subsequent control inputs were clearly intended to restore safe flight.

Many situations can be imagined in which a second person would be unable to stop a determined pilot from crashing an aeroplane. That second person might, for example, be a slightly-built flight attendant with little hope of physically restraining a larger, stronger pilot. In the USA, however, relative strength may be of no matter, as pilots may carry sidearms.

Moreover, the one person out, one person in protocol keeps the cabin door open for longer than a simple routine whereby one pilot exits – probably increasing the risk of a cockpit invasion.

Whether the initial reaction favoring the two-crew rule becomes regulation outside the US remains to be seen. EASA's recent stance on regulation generally has been to adopt the view that there are already too many rules, and that adding more undermines the effectiveness of those already in existence.