What is a primary concern when using ordinary cleaning compounds on aircraft?

The primary concern when using ordinary cleaning compounds on aircraft is that they can cause hydrogen embrittlement in metal structures. Hydrogen embrittlement occurs when hydrogen atoms enter the metal and make it brittle, which can significantly weaken its structural integrity. This is particularly critical for high-strength steels and certain aluminum alloys used in aircraft components.

Ordinary cleaning compounds often contain chlorinated solvents or other chemicals that can lead to hydrogen embrittlement if they interact with the metal surfaces. As a result, using appropriate and approved cleaning agents that do not have this adverse effect is vital for maintaining the safety and reliability of aircraft structures.

The other choices don't raise significant safety concerns related to aircraft materials or operations. While some cleaning compounds may be cheaper or more readily available, these factors do not account for the potential risk of damaging crucial metal components in an aircraft.