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For the Diamond DA42 (TDI)

Learning my electric fire emergencies. Would it be wise to extend the landing gear before electric master is turned off?

Does the (hydraulic) gear power pack require the electric master to be on to extend the gear?

I presume if the electric master is switched off, could still extend manually using emergency gear extend lever?

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In general you should always follow the emergency procedures described in your POH.

The emergency procedure for an Electrical Fire in Flight does not ask to extend the landing gear:

  1. EMERGENCY SWITCH . . . . . . . . . . . . . . . . ON, if installed
  2. AVIONIC MASTER . . . . . . . . . . . . . . . . . . . . OFF
  3. ELECT. MASTER . . . . . . . . . . . . . . . . . . . . . OFF
  4. Cabin heat & defrost . . . . . . . . . . . . . . . . . . . OFF
  5. Emergency windows . . . . . . . . . . . . . . . . . . . open if required
  6. Land at the next suitable airfield

CAUTION
Switching OFF the ELECTRIC MASTER will lead to total failure of all electronic and electric equipment. [...]

(DA-42 POH - Emergency Procedures)

In case of a complete electrical failure, the electrical hydraulic pump will no longer function:

In case of a failure of the electrical pump, which is driving the landing gear actuators, the landing gear can be extended manually [...]

Even if there is still pressure left in the system, the gear lever will no longer open the valves without electrical power:

The hydraulic pressure for the landing gear operation is provided by an electrically powered hydraulic pump, which is activated by a pressure switch, when the required pressure is too low. Electrically actuated hydraulic valves, which are operated with the gear selector switch, provide the required hydraulic pressure for the movement of the landing gear.

(DA-42 POH - 7.5 Landing Gear)

The checklist for Complete Failure of the Electrical System says:

NOTE
The landing gear uplock is no longer ensured. The landing gear may slowly extend. The landing gear can be extended manually according to 3.6.2 - MANUAL EXTENSION OF THE LANDING GEAR.

So it's possible that the gear will already extend without electrical power, but you can always follow the manual extension procedure.

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Don't keep the electric master on in a fire to lower the landing gear. First off there's time: In any fire seconds matter, the sooner you eliminate the ignition source for the fire the better chance you have of controlling it and keeping it from spreading. The fumes from an electrical fire are toxic and can impair you very quickly, you really don't want an additional 5-10 seconds worth of it in your cabin.

Also, there's the chance of making the problem worse: Electrical fires are caused when malfunctions cause sparks or excessive heat which start insulation or other components on fire. Electric motors draw significant electrical power, additional load on the electrical system could create more sparks and/or heat.

So do not leave the electric master on while you extend the gear during an electrical fire. Follow the POH procedures and use the emergency gear extension lever which is by the pilot's right knee.

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  • $\begingroup$ If you are operating 10 miles from an airfield where they are giving you a Basic service (UK) and have you on radar. Surely it would be wise to issue a MAYDAY call followed by a quick ask for a QDM to airfield if required before turning off the electric and avionics master switches. I appreciate your concern about toxic fumes. But surely having some sort of plan to land, other than a farmer’s field, would be sensible? POH: emergency switch > quick plan and RT > electric master and avionics OFF > canopy position 2 > emergency windows open $\endgroup$ – ob318 Sep 7 '20 at 15:14
  • $\begingroup$ @GdB. Thanks for your answer by the way. By ‘Don’t do this’, I presume you mean don’t lower the gear $\endgroup$ – ob318 Sep 7 '20 at 15:21
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    $\begingroup$ That's exactly what I mean @ob318, and no I wouldn't make a mayday call for the exact reasons above. For all you know the radio is what's on fire! You should always have a chart to hand, and know what your position is on it, don't rely completely on your GPS. With a map and your compass you can navigate to an airfield and use radio out procedures to land. I have the UK Distress and Diversion number programmed into my phone for just in case I have an electrical failure. $\endgroup$ – GdD Sep 7 '20 at 15:34

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