Main-Deck Cargo and Depressurisation Firefighting (Freighter)

The freighter (A330-243F) replaces the cabin with a vast Main Deck Cargo Compartment (MDCC) able to carry tens of tonnes. That compartment is too large for the "seal and flood with Halon" model of the passenger cargo article — there is no way to carry enough Halon to flood it. So the freighter takes the most radical design in the chapter: the MDCC carries no extinguishing system at all, and a fire is starved by depressurising the whole aeroplane and using thin, high-altitude air. This article develops that depressurisation firefighting and its coupling to the oxygen system and the diversion decision. It is required for crews who fly the freighter; passenger crews can read it as background.

1. MDCC detection — thirty detectors, same principle as the passenger hold

MDCC detection is the passenger cargo system scaled up:

"The Main Deck Cargo-Compartment (MDCC) is one area monitored by thirty optical type smoke detectors... Each cargo compartment is monitored by 15 pairs of smoke detectors which are installed in cavities in the ceiling of the cargo compartment. The detectors are of the optical type. Two CAN busses supply the communication between the smoke detectors and the CIDS-SDF."

The trigger matches the passenger hold:

"The MDCC smoke warning triggers, if: - Both smoke detectors detect smoke, or - One smoke detector detects smoke, and the other smoke detector is inoperative."

[!warning]- MDCC detection is identical to the passenger hold — the difference is entirely at the extinguishing end Same paired optical detectors, same CIDS-SDF. One has two Halon bottles; the other has no extinguishing agent whatever. So learning the freighter, the detection is free reuse; the effort goes entirely to the difference in extinguishing strategy. Warnings also reach the courier area (supernumerary/loadmaster) in addition to the cockpit.

2. First response — a large air-conditioning reconfiguration

An MDCC warning drives an extensive automatic reconfiguration of air conditioning and ventilation:

"When the main deck smoke warning triggers, the system automatically: - Closes the main deck shutoff valves, to stop the air supply to the fire area - Closes the LD FWD, LD AFT, and BULK isolation valves. In order to avoid an overpressure in the mixer unit when the main deck shutoff valves are closed, the system also automatically: - Shuts down the recirculation fans - Shuts down pack 1 (by closing the associated Pack Flow control valve) - Reduces pack 2 flow to 85 %."

[!warning]- Closing the supply valve forces closing the inlets too The packs feed the mixer unit, which distributes to each zone. Close the main-deck shutoff valve and the mixer loses an outlet; if the inlet flow (two packs + recirculation) is unchanged, the mixer over-pressures. So the system simultaneously trims the inlets: stop the recirculation fans, close pack 1, hold pack 2 at 85 %. An outlet closed means the inlets must be closed too, or the mixer over-pressures — a fire action rippling through the whole air-conditioning network (coupling into ATA-21).

3. No extinguishing — starve the fire of oxygen by depressurising

The MDCC's hardest fact:

"There is no fire extinguishing system active in the MDCC."

The FCTM gives the depressurisation-firefighting technique:

"The MDCC does not have a fire extinguishing system. Therefore, the cockpit crew must apply a specific procedure to isolate the fire/smoke source... The main steps require for the cockpit crew all of the following: - To confirm that the isolation valves of the MDCC are closed, - To set the cabin altitude at 20 000 ft to reduce immediately the amount of oxygen and limit the development of the fire/smoke source, - To descend to FL 200, - To depressurize the aircraft."

[!warning]- The most radical design in the chapter — starve the oxygen, not the fuel With no agent to carry and no way to reach the cargo, MDCC firefighting attacks the third leg of the fire triangle: descend to FL 200, set cabin altitude to 20 000 ft, depressurise — turning the compartment air into thin, high-altitude air with a low oxygen partial pressure, so the fire "cannot breathe" and its development is limited. This is not extinguishing but suppression to a non-developing state — the same aim as passenger-hold Halon, by a different means: from "release agent" to "remove oxygen". Each of the three steps has a role: cabin altitude 20 000 ft is the goal (a low-oxygen environment inside); descend FL 200 is the precondition (ambient pressure must fall to match a 20 000 ft cabin before it can be depressurised there); depressurise is the means (open the outflow so cabin equals ambient).

4. Oxygen coupling — the price of starving the fire is masks for everyone

Removing the compartment's oxygen removes the crew's too, so oxygen becomes the hard limit. The limitations chapter writes MDCC firefighting into the oxygen calculation:

"Main Deck Cargo fire suppression procedure (commanded depressurization at a cabin altitude of 20 000 ft for the duration of the diversion to the nearest suitable airport)... the oxygen chart assumes use of cockpit masks: - With 100 % oxygen for the first 15 min (climb and initial cruise at 20 000 ft) - Then with NORMAL mode (remainder of cruise, and descent) - With 100 %/EMER mode for 1 min every 7.5 min throughout the procedure."

[!warning]- The MDCC time ruler is oxygen quantity, not Halon 260 minutes A passenger cargo fire is timed by "Halon holds for 260 minutes" (article 06); an MDCC fire's time ruler is instead how long the crew's oxygen lasts. Change the extinguishing means and you change the limiting factor — there is no agent countdown, only bottle pressure setting how long you can stay in the thin air and how far you can divert. An MDCC diversion decision therefore reads the oxygen chart (required pressure by number of persons and diversion time), which the passenger hold never needs. The chart also covers occupants, since a freighter may carry people.

5. The freighter lower deck — same as the passenger hold

The freighter's lower-deck holds keep a normal extinguishing system:

"There are two fire extinguisher bottles... Bottle 1 discharges in approximately 60 s . A flow metering system controls the discharge of bottle 2, to ensure sufficient agent concentration for 260 min ."

[!warning]- A freighter runs two firefighting logics, upper and lower The main deck starves (no agent, depressurisation); the lower deck uses Halon (two bottles, 60 s + 260 min). Same aircraft, two entirely different firefighting methods by deck — do not mix them.

6. Operations — the MD SMOKE drill and the two-stage landing

  SMOKE MD SMOKE (red) -> automatic reconfiguration (main-deck shutoff + LD isolation + recirc off + pack1 off / pack2 85%)
   1  apply ECAM + SMOKE/FUMES/AVNCS/MD SMOKE procedure, crew on oxygen masks
   2  confirm MDCC isolation valves closed
   3  CABIN ALTITUDE ... set 20 000 ft (reduce oxygen, limit development)
   4  DESCEND .......... FL 200
   5  DEPRESSURISE
   6  LAND ASAP (maintain FL 200) + manage oxygen by remaining flight time (chart)

The landing decision is two-stage:

"As long as the situation remains manageable, the cockpit crew must land at the nearest airport at which a safe landing can be made (LAND ASAP), maintaining FL 200 as long as possible. If the situation becomes unmanageable, the cockpit crew should consider an immediate landing."

[!warning]- Manageable → hold FL 200 and reach the best field; unmanageable → land immediately While FL 200 depressurisation is suppressing the fire and the oxygen lasts, hold FL 200 and press on to the nearest suitable airport; the moment it becomes unmanageable, survival takes priority over firefighting — land immediately. Unlike the passenger hold's plain LAND ASAP, the MDCC adds a FL 200 maintenance window.

The lower-deck cargo "smoke stays after discharge / temperature unreliable / do not open the door on the ground" airmanship (common to freighter and passenger) is developed in the cargo-smoke abnormal article.

Self-test

[!note]- Q1. How many MDCC detectors, and is detection different from the passenger hold? Thirty (15 pairs), optical, CIDS-SDF — the same principle; the difference is entirely at the extinguishing end (none on the MDCC).

[!note]- Q2. What does an MDCC warning shut down or reduce, and why cut pack 2 to 85 %? Close main-deck shutoff + LD isolation valves; stop recirculation fans; close pack 1; reduce pack 2 to 85 %. The inlet trim prevents mixer over-pressure once the outlet is closed.

[!note]- Q3. With no agent, how is the fire fought? What does each of the three steps do? Depressurisation firefighting. Cabin altitude 20 000 ft = the low-oxygen goal; descend FL 200 = the precondition; depressurise = the means.

[!note]- Q4. What is the MDCC time ruler, versus the passenger hold's 260 minutes? Why read the oxygen chart? Crew oxygen quantity (not Halon). The chart sets required oxygen by persons and diversion time — the passenger hold never needs it.

[!note]- Q5. Why is the MDCC landing decision two-stage? Manageable → maintain FL 200 (suppression holding, oxygen sufficient) and reach the best field; unmanageable → land immediately (survival over firefighting).

Key takeaways

References

• FCOM DSC-26-51-10/20 — MDCC trigger, automatic air-conditioning reconfiguration, no extinguishing system.
• FCOM DSC-26-52-10 — freighter lower-deck two bottles, 60 s + 260 min.
• AMM 26-19-00 — MDCC 30 optical detectors (15 pairs), CIDS-SDF/CAN, courier-area warning.
• FCTM PR-AEP-SMOKE — depressurisation firefighting (confirm isolation / cabin 20 000 ft / descend FL 200 / depressurise), two-stage LAND ASAP, oxygen management.
• FCOM LIM (oxygen) — MDCC fire-suppression oxygen assumptions (100 % first 15 min / NORMAL / EMER 1 min per 7.5 min), diversion time.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.