IATA filed a complaint with the European Commission in 2016 after some of its member airlines were quizzed by regulators about aviation equipment makers in the aftermarket repairs market and whether they abused their dominance.
One element of the complaint involved the CFM56, the world's best-selling jet engine, and another Honeywell's auxiliary power unit (APUs). Maintenance, repair and overhaul services are a lucrative source of revenues for engine makers.
IATA on July 31 withdrew the CFM complaint after the company, a joint venture between General Electric and Safran, agreed to open up engine maintenance services to rival engine parts and aftersales service market suppliers.
"IATA has not withdrawn the complaint against Honeywell," a spokeswoman for the body said.
IATA said it was concerned about certain business practices that its members believe limit competition in the market for APU maintenance, repair and overhaul services. Honeywell is the largest producer of gas turbine APUs found on many aircraft.
Honeywell said it was adhering to the rules.
"Honeywell believes that our practices are fair and in compliance with all relevant laws. We have and will continue to cooperate with the European Commission's preliminary inquiry," the company said in an email.
The EU competition enforcer has received the complaint and is assessing it, Commission spokesman Ricardo Cardoso said.
(Reporting by Foo Yun Chee; Editing by Mark Potter)
By Foo Yun Chee