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I am developing an application for airport fire crew and ground staff. Part of the application is to familiarize users with emergency procedures of various aircraft. I have a few questions about the procedure for discharging the engine fire extinguishers on an ATR72 that I hope someone can help me with:

  1. As I understand it, pilot must pull the red handle and twist 90 degrees. This will stop the engine. Pilot must then press the discharge squib button in order to deploy the extinguishers. is this correct?
  2. What would happen if the pilot pressed the discharge squib button before pulling the red handle?
  3. Am I correct in assuming that if the system has detected an engine fire, then the "fault off" buttons would be lit and there would be an audible alarm? If this is the case, would the alarm stop when the discharge squib button is pressed, or only when the fault off button is pressed?
  4. Above the buttons it is written: "Agent 1 Loop A" and "Loop B Agent 2". what does this mean and what is the difference between the two systems? In what circumstances would the pilot have to press both discharge squib buttons or both fault off buttons? In what circumstances would they only have to press one?

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  1. As I understand it, pilot must pull the red handle and twist 90 degrees. This will stop the engine. Pilot must then press the discharge squib button in order to deploy the extinguishers. is this correct?

Yes, this is correct, except that the handle does not need to be twisted as far as I know. When the handle is pulled, the following things will happen:

  • The propeller on this side is feathered.
  • The fuel LP (low pressure) valve is closed.
  • The bleed air and HP valves are closed.
  • The deice and shutoff valve is closed.
  • The DC and ACW generators are disconnected.
  • The hydraulic to prop brake is closed on the other engine.

Pressing the squib button will then discharge the extinguishing bottle into the engine.

  1. What would happen if the pilot pressed the discharge squib button before pulling the red handle?

Nothing, the button is not armed at this stage:

AGENT/SQUIB

armed when T handle pulled.
Discharges bottle.
(DC EMER/HOT MAIN BAT)

  1. Am I correct in assuming that if the system has detected an engine fire, then the "fault off" buttons would be lit and there would be an audible alarm? If this is the case, would the alarm stop when the discharge squib button is pressed, or only when the fault off button is pressed?

Yes, both FAULT lights would be on and the alarm is triggered, if both loops A and B detect the fire:

Fire logic

The pilots would be alerted of the fire by:

  • master WARNING light flashes
  • ENG 1/2 FIRE light illuminates on the CAP (Crew Alerting Panel)
  • FUEL SO illuminates on the respective condition lever
  • respective fire handle illuminates
  • aural alarm sounds

A single FAULT would indicate a fault in the detection system, not a fire itself:

LOOP/FAULT

change in resistance, inhibits fire signal until turned off.
Loop on CAP (DC emer)

You would press the button to turn off the respective loop such that the other loop can detect a fire alone:

OFF

takes respective loop out of parallel circuit. Allows other loop to activate fire signal alone.

  1. Above the buttons it is written: "Agent 1 Loop A" and "Loop B Agent 2". what does this mean and what is the difference between the two systems? In what circumstances would the pilot have to press both discharge squib buttons or both fault off buttons? In what circumstances would they only have to press one?

There are two fire extinguishing bottles and each can be discharged into either engine (Agent 1 refers to the bottle on the same side and Agent 2 to the one on the other side):

ATR72 extinguishing bottles

The correct procedure for an IN FLIGHT ENG FIRE is:

PL [Power Lever] affected side ... FI [Flight Idle]
CL [Condition Lever] affected side ... FTR [Feather] THEN FUEL SO
FIRE HANDLE affected side ... PULL
if condition persists after 10 seconds:
AGENT 1 affected side ... DISCH
if condition persists after 30 seconds:
AGENT 2 affected side ... DISCH
LAND ASAP

(ATR 72 QRH)

Source: ATR Systems Training Document

Also have a look at this YouTube video found by Jpe61.

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  • $\begingroup$ Excellent answer, thank you very much, and thank you for the manual reference too. Just to clarify: if the system detects an engine fire - it alerts the pilot by lighting the lights on the respective engine fire handle? Is there also an audible alarm? $\endgroup$
    – Adrian Taylor
    Jan 21, 2020 at 21:01
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    $\begingroup$ The fire alert consists of master WARNING light coming on, ENG 1/2 FIRE light on the CAP coming on, the fire light on the respective condition lever coming on, the respective fire handle illuminates and an aural alert. I haven't figured out when the aural alert stops, but typically one can press the master WARNING/CAUTION button to silence the alert (at least that is what I know from other aircraft like a Boeing 737). $\endgroup$
    – Bianfable
    Jan 21, 2020 at 21:05
  • $\begingroup$ @AdrianTaylor before to pulling the fire hande the engine bleed valve should be closed to rule out the possibility of bleed leak causing a faulty fire indication. If this does not solve the predicament, next item would be pulling the affected engine condition lewer back to cut off fuel supply to engine. If this is not done, fuel lines might collapse preventing restart (if for some weird reason you'd want to restart a burned engine). After this you may yank the fire handle. $\endgroup$
    – Jpe61
    Jan 21, 2020 at 21:46
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    $\begingroup$ @Jpe61 I included the relevant checklist from the QRH. It sounds a bit different from what you described. Do you have source for your steps? $\endgroup$
    – Bianfable
    Jan 21, 2020 at 21:55
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    $\begingroup$ @Bianfable got it from youtube, so it might not be the absolute truth :) youtu.be/mhWYIjgtRsY $\endgroup$
    – Jpe61
    Jan 21, 2020 at 21:58

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