First, the bottle pressure gauge will read zero — Page 302, Lesson 375

Let's start with that thermal discharge scenario, because it's a real-world safety feature you need to recognise on the ground and in the cockpit. If a fire bottle — that's the pressurised container holding the extinguishing agent — has been subjected to excess temperature or excess pressure, a thermal discharge may take place. That means the bottle's safety relief has operated, and the contents have vented overboard, usually through a discharge line to the outside of the aircraft. You won't always get a cockpit warning for this, so you need to know the two physical indications that the bottle has emptied. First, the bottle pressure gauge will read zero. Second, look at the external discharge indicator: there's a green disc that gets ejected, and underneath it a red disc will show. So green disc in place means the bottle is intact; red disc showing means the contents have gone overboard. Now, let's move to the operational side — the fire drill itself. I want to walk you through a typical engine fire procedure, using the left engine as the example. This is the sequence you'd follow on receipt of a left engine fire warning, and I'll stress that this is an example only — you must always consult the individual aircraft checklist for the correct procedure. Step one: close the left thrust lever. That removes the power demand from that engine. Step two: close the left engine HP — that's high pressure — or engine start lever. That shuts off the fuel supply to the engine. Step three: pull the No. 1 fire handle. That's the physical handle on the overhead or pedestal panel that arms the fire extinguishing system for that engine. Step four: rotate the No. 1 engine fire handle to the left, to its mechanical limit, and hold it there for at least one second. That action discharges the left bottle into the left engine. So the left bottle is the first shot for the left engine. Step five: if after 30 seconds the fire warning remains illuminated, you rotate the No. 1 fire handle to the right, again to its mechanical limit, and hold for at least one second. That discharges the right bottle into the left engine. So you have a second bottle available as a backup shot for that same engine. Step six: land as soon as possible. That's the final instruction — once you've used your extinguishing capability, you get the aircraft on the ground. Now let's look at the Auxiliary Power Unit — the APU. The APU is a constant-speed, self-contained gas turbine. It derives its fuel supply from the aircraft system, and its services may include bleed air, hydraulic power, electrical power, or a combination of these. When certified, it can be available for airborne use. The APU is self-monitoring, and it will auto shut down in the event of four conditions: fire, oil pressure failure, overspeed, and overheat. So it protects itself automatically. Note this important point: although the APU auto shuts down, a manual control panel is normally included. And on some aircraft, in some circumstances, automatic discharge of the APU fire extinguisher may be performed. So the auto shutdown is the first line of defence, but the extinguisher discharge isn't always automatic — that depends on the aircraft design. Let me show you the external APU fire control panel on an Airbus, so you can see the layout of the controls you'd use from outside the aircraft. That's the full picture: the thermal discharge indications, the engine fire drill sequence with its two-bottle logic, and the APU's self-monitoring and shutdown conditions.