Spanners
EXTERNAL INCANDESCENT lighting systems - tube or ballast????
But seriously I agree with intruder. Nav lights which are mandatory items and therefore probably most likely to cause a maintenance delay (along with anti-collision lights) would be ideally suited for LEDs.
I seem to remember reading a recent article about this in AVIONICS Magazine. Now that LEDs are available in most colours they are more likely available for more applications.
They would clearly be more reliable. For instance I have noticed how we rarely have to replace the switchlights in the B777 flightdeck (which make use of LEDs) compared to the incandescent switchlights in the B747 flightdeck.
Regards
Max