Does ELMS inhibit cabin pressure alarms on the 777, and would the MEC Override Vent be stuck open following an electrical failure?

Question

I found this article rather interesting, but like most things on the internet, they can be wrong. I couldn't think of a better site to address this question. This user says:

when ELMS takes over power supply it shuts down the Air Cycle Machine (ACM) and the Air Supply Cabin Pressure Controller (ASCPC). Cabin pressure alarms in the cockpit also lose power.

The user follows up with this later on:

In addition to loss of electrical power from the cabin pressurization system electrical smoke in the Avionics Bay or MEC likely tripped open a vent called the MEC Override Vent, which simply allows electrical smoke to be pushed out by cabin air pressure. Once open this vent needs electrical power to close again, so if an electrical failure followed, it would be left limp and open.

I look forward to reading your answers about this, Thank you!

Answer 1

Generally, the post you quote from does not look like a credible source. For starters, it says things like:

Electrical failure shut down the Air System Cabin Pressurization Controller (ASCPC) and Air Cycle Machine (ACM) which on a Boeing 777 are all electric. Unlike most other airliners the B777 does not use engine bleed air.

This is completely wrong. The 777 uses engine (or APU) bleed air to operate the PACKs. The author might have mixed this up with the 787, which uses electrically powered compressors for cabin pressurization instead of bleed air.

Bleed air can be supplied by the engines, APU, or a ground air source.

Bleed air is used for:

• air conditioning
• pressurization [...]

^{(Boeing 777 FCOM 2.40.1 - Air Systems - Bleed Air System Description)}

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The vent in the electronics bay exists and it will indeed open automatically to vent out smoke:

The forward system reconfigures automatically to an override mode when:

• [...] smoke is detected in the forward equipment cooling system or the forward equipment ventilation system [...]

In the override mode when the FWD CARGO FIRE ARM switch is not ARMED, the vent valve opens, both supply fans shut down, and the forward cargo heat valve closes. [...] Cabin differential pressure draws air through the flight deck panels into the E & E equipment compartment to create a reverse flow of air across the equipment, then through the override valve to an overboard vent.

^{(Boeing 777 FCOM 2.20.12 - Air Systems - Air Conditioning System Description)}

With an electrical failure it is plausible that this valve could no longer close, therefore preventing cabin pressurization.

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The ELMS (Electrical Load Management System) described in the post will shed loads in this order:

The load shedding is galleys first, the utility busses. Utility busses are followed by individual equipment items powered by the main AC busses.

^{(Boeing 777 FCOM 6.20.1 - Electrical - System Description)}

The FCOM indeed does not specify the order of these "individual equipment items". Since we don't know what exact failures occurred on MH370, it is impossible to say which systems were shut down. However, during normal operation there are two AC busses (left and right) powered individually. No single point of failure could shut down both PACKs.

The post further claims that

Cabin pressure alarms in the cockpit also lose power.

This cannot be related to the ELMS, which only controls the AC power distribution. The EICAS alerts are however powered by the DC system (via the flight instrument busses), which can be powered independently of the AC busses in standby operation:

The main DC electrical system uses four transformer-rectifier units (TRUs) to produce DC power. The TRUs are powered by the AC transfer busses. [...]

The standby electrical system can supply AC and DC power to selected flight instruments, communications and navigation systems, and the flight control system, if there are primary electrical power system failures.

The standby electrical system consists of:

• the main battery
• the standby inverter
• the RAT generator and its associated generator control units
• the C1 and C2 TRUs.

^{(Boeing 777 FCOM 6.20.13 - Electrical - System Description)}

The author of the post further claims that no warning is given to the pilots because the SMOKE / FUMES REMOVAL checklist in the QRH (linked in the post) shows UNANNUNCIATED. This is however the wrong checklist for the case described earlier. The unannunciated here means that this checklist is for when the pilots recognize smoke or fumes in the flight deck before any annunciation. The correct checklist for the open vent in the electronics bay is this one (and it is annunciated on the EICAS):

[] EQUIP COOLING OVRD

Condition: The equipment cooling system is in override mode.

1. Wait 2 minutes. This allows time for any smoke in the system to clear.
2. EQUIP COOLING switch . . . . . . . . Off, then AUTO
   The EQUIP COOLING OVRD message blanks.
3. Wait 1 minute. This allows time for the system to reset.
4. Choose one:
   • EQUIP COOLING OVRD stays blank: ■ ■ ■ ■
   • EQUIP COOLING OVRD shows again:
     Note: After 30 minutes of operation at low altitude and low cabin differential pressure, electronic equipment and displays may fail.

^{(Boeing 777 QRH 2.19)}

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Another weird claim from the post is:

The brand new Boeing 777X model suffers the same vulnerability.

I wonder how the author wants to know this. The 777X is not in service yet and the author claims to be an aviation enthusiast from New Zealand. Only a Boeing engineer could possibly know about such system details of the 777X.