The FAA, following the actions of its Canadian counterpart, is developing an order that will require inspections of Honeywell emergency locator transmitters (ELTs) on 22 aircraft types delivered or retrofitted with the units, Aviation Week has learned.

The timing of the FAA’s airworthiness directive’s (AD’s) is not clear, but once released, it will closely resemble a Transport Canada mandate issued Aug. 15. That order, quickly adopted by the European Aviation Safety Agency (EASA), gives operators until late December to inspect Honeywell RESCU 406AF and 406AFN transmitters, as well as the ELTs’ batteries and related wiring, for damage or other fire risks.

“The FAA intends to follow the Transport Canada Civil Aviation AD with an FAA AD requiring those same actions for U.S. operators of airplanes with those fixed Honeywell ELTs installed,” the U.S. agency says in a statement.

Regulators are acting on initial findings in the probe of a July 12 fire on an empty Ethiopian Airlines Boeing 787 at London Heathrow Airport. Investigators suspect the fire’s source was the 787’s RESCU 406AFN ELT or its wiring system.

The Ethiopian Airlines incident was the first of its type.

The two Honeywell ELTs have been certified as buyer- or supplier-furnished equipment (BFE/SFE) on 11 models, including most current-production Airbus and Boeing aircraft. They are available for retrofit on 18 models, including seven in the BFE/SFE category. Honeywell says it has delivered about 3,000 of the 406AFNs and 2,500 406AFs.

Despite the ELTs being available on the most popular airliners in service—including the Airbus A320 and A330, and the Boeing 737NG, 767, and 777—the inspection orders have hit Boeing operators much harder. Boeing confirms that some 1,100-1,200 of its aircraft have been delivered with one of the two ELTs, while the number of Airbus aircraft affected is understood to be fewer than 100.

Two U.K. Air Accidents Investigation Branch (AAIB) recommendations made shortly after the Ethiopian Airlines incident was the catalyst for these ADs. AAIB suggested that the FAA require checks of 787s with the ELT in question, and consider a broader mandate for other aircraft with one of the two ELT models installed.

The FAA moved first, ordering checks on 787s in late July. EASA followed with a similar mandate before adopting the broader Transport Canada measure.

The FAA’s more extensive order could differ from the Canadian directive. U.S. regulations require ELTs for many types of operations, but not scheduled airline service. This gives the FAA room to offer airlines the option of deactivating the units to mitigate ELT-related fire risks, as the agency did in its 787 AD.

U.S. operators serving countries that require ELTs cannot take advantage of the deactivation option.