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I've heard it said many times that "everything has a backup" in commercial planes.

What about the fuel inerting system (injecting nitrogen in the fuel tank). Does it have a backup? Is it somehow failsafe all by itself?

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    $\begingroup$ Well the inerting system is itself a backup of keeping ignition sources out of the fuel tanks. $\endgroup$
    – fooot
    Feb 25, 2016 at 2:15
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    $\begingroup$ I think "everything has a backup" is an overly broad generation. It's safe to say that the redundancy of all systems are carefully considered as part of an aircraft's design, but that doesn't mean everything literally has a backup (there is no backup for, say, the wings). The incredibly rare rate of fuel tank explosions can itself be the backup for the fuel inerting system. $\endgroup$ Feb 25, 2016 at 9:17
  • $\begingroup$ read up on the Swiss Cheese Model to understand how aircraft safety works en.wikipedia.org/wiki/Swiss_cheese_model $\endgroup$
    – rbp
    Mar 26, 2016 at 21:35

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Short Answer

No, it does not have a back-up, there are some fail-safe components in the system(see below), but in general the system itself is not duplicated. The system, together with a design that avoid ignition sources within the tanks, is just a part that contributes to prevent fuel tanks to ignite. An Inerting System for commercial planes have been considered since 1996 TWA accident and first regulation arrived in may 2006.

Longe Answer

The certification CS25.981 Fuel tank ignition prevention states:

(a) No ignition source may be present at each point in the fuel tank or fuel tank system where catastrophic failure could occur due to ignition of fuel or vapors.

(c) Design precautions must be taken to achieve conditions within the fuel tanks which reduce the likelihood of flammable vapors.

So the function that should be achieved is prevent tank ignition and there are two fronts two work on, avoid ignition source, and reduce fuel vapors (similar to the two slices of cheese that rbp was mentioning in the comments).

So the fuel ignition system is a system designed to avoid fuel vapors. As can be seen in the image below, within the systems there are some redundancies (e.g.: Dual Flow Shut Off Valve), but the system itself is not duplicated.

Moreover the system is not on the Minimum Equipment List (MEL), so the aircraft can still be dispatched with the system inoperative for 10 days.

Schematic of the Fuel Tank Inerting System

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