Hey LML gang,
A big thanks to everyone that responded to my request for comments
and advice regarding in flight engine fire suppression. There were three
main questions raised:
1. Is Halon 1301
the best agent for an engine fire?
2. How effective
would a fire suppressant be in flight with the high air flow through the
cowling?
3. Even if extinguished,
a fire may be likely to re-ignite after the suppressant is flushed away by the
air flow.
Here’s what I’ve learned in research related to these
questions…
Is Halon 1301 the best agent for an
engine fire?
Yes. Everything I’ve found
in research indicates that for this application it is the best. It
requires a very low concentration for a very short period of time to extinguish
a fire. The FAA certification minimum is 6% concentration by volume for
at least 0.5 seconds. It actually works in milliseconds, though. It
has a high vapor pressure, about 200 psi, and disperses throughout the
atmosphere very rapidly into nooks and crannies. It is also non-corrosive
and leaves no residue – so no cleaning is required if discharged. There
is also no acute toxicity for humans below 10% concentrations.
How effective would a fire suppressant be in flight with the high
air flow through the cowling?
This was also one of my biggest
questions and I couldn’t find any analysis on the internet to answer
this. So, I worked up a simplified but conservative analysis. Below
is a screen shot of part of my spreadsheet. I talked with Safecraft’s
Don Warren and he provided three key pieces of data on Halon 1301. He
said one pound of Halon 1301 as a gas will occupy 35 cubic feet at sea level
pressure. He provided two points of data on the Halon discharge rate from
their 5 lb bottle at 200 psi. In 5 seconds 2.5 lbs of Halon will be
discharged and all 5 lbs will be discharged in 15 seconds. So with those
two points I created the table on the right of the Halon 1301 5 lb Bottle
Discharge Performance.
My basic analytical approach was to
look at the total volume of air each second that Halon would need to be
dispersed in, to see what the minimum 6% concentration volume would be vs.
airspeed. The volume required is the total airflow volume through the
inlets, for the time period being examined, plus the cowling volume. I
made conservative assumptions – for example, no cooling drag or slowing
down of the air through the cowling, I assumed larger than actual cowling
inlets, and larger than actual cowling (simplified as cube at max dimensions).
I was happy to see, that even with
these conservative assumptions, with discharge of a 5 lb bottle of Halon 1301 at
300 kts there would be 10 seconds with more than a 6% concentration of Halon to
knock down a fire (only a half second required for spec). You see this by
comparing the Halon Discharge Rate to the Required Minimum Discharge Rate vs.
Airspeed.
After doing this analysis, I found a
reference in a magazine article that pointed out that a 6 lb bottle of Halon
1301 is all that’s required on a 747’s engine to knock out an
engine fire… So with this analysis and the 747 benchmark, I’m
convinced a 5 lb bottle of Halon 1301, with proper positioning of discharge
outlets in the cowling, can knock down an in flight engine fire on a Lancair
Legacy.
Even if extinguished, a fire may be
likely to re-ignite after the suppressant is flushed away by the air flow.
Yes. And of course LML’ers
have pointed out that the proper procedure is to shut down the engine, cut off
the fuel flow, cut off electrical power, and then discharge the Halon. So
you don’t want to do it too fast – before cutting off potential fuel
and ignition sources. One interesting thing I found in research was
reference to an Air Force study that found you also don’t want to wait
too long to put out the fire because it will eventually heat things up so much
that the hot parts may serve as re-ignition sources. They said the
suppressant should be discharged within 14 seconds after the start of the fire
for the cases they were studying. So there’s too fast and waiting
too long… One advantage of going with a 5 lb bottle, which is much
more than required to knock down a fire in a Legacy, is that it will ensure
additional time to allow the airflow to cool things off to help reduce the risk
of fire hot parts re-igniting the fire.
As a passing note, Halon 1301 systems are only certified up to 130
deg F operating environments so the tank should not be installed under the
cowling – somewhere in the cabin is best. If you only have room to
put a Halon system under the cowling you should use Halon 1211 which is
certified for up to 200 deg F. The downside of 1211 is that is has a much
lower vapor pressure and will require more attention to the discharge nozzles
for good dispersion in the space. BTW, 1211 is the recommended Halon for
cabin extinguishers because it discharges in a stream before vaporizing so you
can aim it accurately at the fire.
Anyway, I wanted to share this with the community for those
considering fire suppression systems and hope this is helpful. Thanks
again for everyone’s advice. Let me know if you have any comments, questions
or find errors in my analysis I need to correct.
Fire detection is a critical part of a fire suppression system and I’m
still studying options there…
Thanks,
Valin Thorn
Legacy under construction, still
Houston, Texas