Monday, June 30, 2014


One of our aircraft has an optional engine fire extinguishing system installed. The extinguishing system is optional. Engine fires are an option we don't want to exercise. The system is fairly simple, a halon bottle installed in the accessories compartment, a sensor, a couple of lights on the dashboard, and a pair of guarded switches to discharge the bottle. There's very little in the way of documentation, because it's not original to the airplane, but what's called an STC. STC stands not very helpfully for "special type certificate" and what that means is that someone has paid a whole lot of money to have this gizmo approved for this airplane.

I don't actually know how the sensor works. I'm guessing the sensor is based on a bimetallic strip that will bend and close a contact if heated to a temperature hotter than a not-on-fire engine should induce at its location. Or it could be a photocell. I've flown an airplane with a photocell-based fire detection system before. Hmm, wouldn't a photocell result in the fire light going on when maintenance did an uncowled runup in the daytime? I don't think that happens. In the event of an engine fire, I'm supposed to select the appropriate switch in after I have completed the original manufacturer's engine fire checklist.

A fire extinguisher needs to be sent out for hydrostatic testing on a periodic basis, and the ones in the engine are no exception. They have their testing due date stamped on the bottles. The director of maintenance tells me he can't find any information on the STC, can I please research and find the schematics and the maintenance instructions for the unit.  I track down the company that bought the company that bought the company that manufactured the STCed (pronounced "ess-tee-seed") system, but they don't have any information on it. They refer me to their European affiliate, which finally tells me conclusively that the product was not profitable for them, so they no longer support it. They have apparently lost or destroyed all information pertaining to the system. Charming.

We will have no choice but to remove it.


Paul said...

I'd imagine that if you had a photocell-based detector that it would detect smoke, not heat-- see this explanation of how they work.

CaptainVector said...

This sort of situation is an on-going mess for a number of aircraft owners as the GA fleet ages. In the US, the FAA now requires all STC modifications to include instructions on "Continued Airworthiness" -- basically, how to take care of the modified stuff. Sadly, a lot of existing installations, such as your fire extinguisher system, don't have any sort of documentation for maintenance or repair. The only legal choice is to undo the modification and restore the aircraft back to the manufacturer's original configuration. What a waste.

Aluwings said...

And the world becomes a little safer... riiiiight.

Unknown said...

FYI STC stands for 'Supplemental Type Certificate'. Some are pretty special though ;)

Aviatrix said...

Heh. Just goes to show that it's one of those TLAs that never gets expanded. We say something "has been Essteeceed" never that it "has a Supplemental Type Certificate."