MH370 HALON
At high temperatures, halons decompose to release halogen atoms that combine readily with active hydrogen atoms, quenching flame propagation reactions even when adequate fuel, oxygen, and heat remain.
The chemical reaction in a flame proceeds as a free radical chain reaction; by sequestering the radicals which propagate the reaction, halons are able to halt the fire at much lower concentrations than are required by fire suppressants using the more traditional methods of cooling, oxygen deprivation, or fuel dilution.
For example, Halon 1301 total flooding systems are typically used at concentrations no higher than 7% by volume in air, and can suppress many fires at 2.9% v/v. By contrast, carbon dioxide fire suppression flood systems operate from 34% concentration by volume (surface-only combustion of liquid fuels) up to 75% (dust traps). Carbon dioxide can cause severe distress at concentrations of 3–6%, and has caused death by respiratory paralysis in a few minutes at 10% concentration.
Halon 1301 causes only slight giddiness at its effective concentration of 5%, and even at 15% those exposed remain conscious but impaired and suffer no long-term effects. (Experimental animals have also been exposed to 2% concentrations of Halon 1301 for 30 hours per week for 4 months, with no discernible health effects.) Halon 1211 also has low toxicity, although it is more toxic than Halon 1301, and thus considered unsuitable for flooding systems.
However, Halon 1301 fire suppression is not completely non-toxic; very high temperature flame, or contact with red-hot metal, can cause decomposition of Halon 1301 to toxic byproducts.
The presence of such byproducts is readily detected because they include hydrobromic acid and hydrofluoric acid, which are intensely irritating. Halons are very effective on Class A (organic solids), B (flammable liquids and gases) and C (electrical) fires, Halons can be used on Class K (kitchen oils and greases) fires, but offer no advantages over specialized foams.
But they are unsuitable for Class D (metal) fires, as they will not only produce toxic gas and fail to halt the fire, but in some cases pose a risk of explosion. Aluminum under the right heat burns and it is a metal.
FOWARD CARGO FIRE CONTROL PROCEDURES - BOEING MANUAL 777
OXYGEN MASKS --------------------------- ON B
CREW COMMUNICATIONS ------------- ESTABLISH B
CABIN ALTITUDE AND RATE. -----------CHECK B Confirms
pressurization problem, If cabin altitude uncontrollable:
PASSENGER OXYGEN SWITCH ---------- PUSH F/O Push and hold for 1 second.
Backs up automatic activation of the passenger oxygen system.
DESCENT
-----------------------------------------ACCOMPLISH C
Without delay, close thrust levers, extend speed brakes, and
descend at VMO/MMO. Level off at lowest safe altitude or 10,000 feet,
whichever is higher. If structural integrity is in doubt,
limit airspeed and avoid high maneuvering loads.
FIRE CARGO FWD
Condition: Smoke is detected in the forward cargo compartment.
FORWARD CARGO FIRE ARM SWITCH ------------------ARMED
CARGO FIRE DISCHARGE SWITCH------------------------PUSH Push and
hold for 1 second.
LANDING
ALTITUDE SELECTOR ----------------------------PULL, SET 8000
Minimizes extinguisher agent leakage out of the compartment.
Plan to land at the nearest suitable airport. Note: Equipment cooling normal mode is inoperative. After 30 minutes of operation at low altitude and low cabin differential pressure, electronic equipment and displays may fail.
Do not accomplish the following checklists:
EQUIP COOLING OVRD
LANDING ALTITUDE
When at top of descent: LANDING ALTITUDE SELECTOR . . . . . . . . .
. . . . . . . PUSH
I think the last thing the pilot or co-pilot did was hit the Emergency Descent Button realizing they had a fire and a course change back to dry land they were familiar with and selecting 12,000 feet as enough altitude to make dry land. I think the halon or fumes killed them before they could do anything else.