What is the purpose of the Dash 8’s “TOUCHED RUNWAY” warning light? The DHC-8-400 (and possibly other DHC-8 variants) has (or, at least, had in late 2007, according to Appendix G of this AAR) a warning light labelled “TOUCHED RUNWAY”.
The extract of the DHC-8-400’s QRH quoted in Appendix G of the aforementioned AAR mentions this warning light at the end of the landing-gear abnormal-procedures section, and says what to do if the light illuminates, but not what the light actually indicates:

”TOUCHED RUNWAY”
(Warning Light)

Do Not Start APU
Advice [sic] ATC for Runway Inspection

Note: Continued flight operations require maintenance approval.

The landing gear of a DHC-8-400 (or, indeed, of any aircraft so equipped) routinely touches the runway, and the ramp, and the taxiways, and the hangar floor, all the time during routine operations - indeed, touching the ground (including, but not limited to, the runway) is literally the primary, and very nearly the entire, purpose of landing gear!  So what does the “TOUCHED RUNWAY” warning light indicate that is severe enough to warrant possibly grounding the airplane?
The light is a tail-strike indicator.


  Touched Runway system 
  
  The  Touched  Runway  system  provides  a  warning  light  in  the  flight  compartment.  It  illuminates when the aft fuselage makes hard contact with the runway on take off or landing. The system consists of a frangible1  switch located at the bottom of the aft fuselage, activating the warning light. The system is powered from the 28 Volt DC right main bus. Once activated, the frangible switch can not be reset but must be replaced by maintenance personnel. Source: Smart Cockpit.


1 frangible
/ˈfran(d)ʒɪb(ə)l/,
adjective


able to be broken into fragments; brittle or fragile.
"the frangible skull of an infant"
denoting ammunition designed to disintegrate into very small particles on impact.
    "frangible bullets"




Figure 1. Source: ATSB.au.



Figure 2. Source: ATSB.au.



Links:


Tail strikes during landing involving Bombardier DHC 8 402, VH-QOT and VH-QOS.
AAIB Bulletin: 10/2017 G-ECOJ EW/G2017/01/03.