On Tuesday, January 3, 2017 at 2:34:59 AM UTC-5, Andy Blackburn wrote:
Most runway lights I know of are designed to break off at the base when struck.

Yes these are designed to break off at the base, but apparently not until they have done major damage to a glider wing. The yield strength must be pretty high.

I stumbled across this in AC 150/5345-46D - not sure it is for this type fixture but if true then our lights are ~1 foot tall, so maybe they are required to withstand 150lbs. They might not yield until 500lbs! That is a lot of force for a point of contact.

3.4.2.1. Yield Device.
a. Each elevated light fixture must have a yield point near the point or position where the light attaches to the base plate or mounting stake. The yield point must be no more than 1-1/2 inches (38.10 mm) above grade, must give way before any other part of the fixture is damaged, and must withstand a bending moment of 150 foot-pounds (203 Newton-meters (N-m) without failure.

(1) This yield point must also separate cleanly from the mounting system before the bending moment reaches 500 foot-pounds (678 N-m).