Unless something has changed in the last four years, tdracer is correct, I too was surprised to find that while avionics equipment is exempt from the requirement to use lead-free solder, the use of leaded is not mandated.
The reason for this is simply that avionics are such a small market (compared to the general electronics industry) that whilst we're *allowed* to use leaded solder there are very few assembly plants that maintain the equipment to use it. They're not about to switch over to leaded solder (with the attendant clean-down, changing of fluxes and solvents, changes to speeds & temperatures) for the very limited run of avionic equipment that may be demanded.
£0.03 supplied,
PDR