Early-variant Boeing 737 operators are being instructed to replace pitot anti-icing systems and carry out related modifications, to avoid the possibility of pilots' being presented with false air data.
The US FAA states that the modifications cover 737 variants up to the -500.
Its directive is the result of notifications indicating that the pitot-heat switch on the aircraft is "not always set to 'on'," the regulator states.
Such a situation could result in "misleading" air data, it adds, potentially leading to loss of crew situational awareness and the "inability to maintain continued safe flight and landing".
The directive requires operators to replace pitot anti-icing components and install a junction box and wiring provisions.
Similar measures were previously ordered, in 2013, for newer-variant 737s, from the -600 upwards, following reports of crew failure to activate air-data probe heat.
The latest directive has a 24-month compliance time.
One operator, SF Airlines, had sought an extension to this time period, claiming that the kits and parts needed would be too difficult to obtain within the allotted interval.
It had added that the work time required, 300h, would need "creation of a detailed plan" to modify its fleet of 17 aircraft – a mix of 737-300F and -400F cargo jets – and that it could not be accomplished during a C-check.
But the FAA has rejected any extension, stating that the operator gave little evidence to substantiate the parts-availability claim, and adds that the compliance time is the same as that for the similar 2013 directive.
"We considered not only the safety implications of the identified unsafe condition," it adds, "but the average utilisation rate of the affected fleet, the availability of required parts, and the practical aspect of accomplishing the required modification within a period of time that corresponds to the normal scheduled maintenance for most affected operators."
Source: Cirium Dashboard